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Title: The Story of the Heavens

Author: Robert S. Ball

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Language: English

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THE STORY OF THE HEAVENS

PLATE I.
THE PLANET SATURN,
IN 1872.

THE

Story of the Heavens

SIR ROBERT STAWELL BALL, LL.D. D.Sc.
Author of "Star-Land"

FELLOW OF THE ROYAL SOCIETY OF LONDON, HONORARY FELLOW OF THE ROYAL SOCIETY OF
EDINBURGH, FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY, SCIENTIFIC ADVISER TO THE
COMMISSIONERS OF IRISH LIGHTS, LOWNDEAN PROFESSOR OF ASTRONOMY AND
GEOMETRY IN THE UNIVERSITY OF CAMBRIDGE, AND FORMERLY
ROYAL ASTRONOMER OF IRELAND

WITH TWENTY-FOUR COLOURED PLATES AND NUMEROUS
ILLUSTRATIONS

NEW AND REVISED EDITION

C A S S E L L a n d C O M P A N Y, L i m i t e d
LONDON, PARIS, NEW YORK & MELBOURNE
1900

ALL RIGHTS RESERVED

PREFACE TO ORIGINAL EDITION.

I have to acknowledge the kind aid which I have received in the preparation
of this book.

Mr. Nasmyth has permitted me to use some of the beautiful drawings of
the Moon, which have appeared in the well-known work published by him
in conjunction with Mr. Carpenter. To this source I am indebted for Plates
VII., VIII., IX., X., and Figs. 28, 29, 30.

Professor Pickering has allowed me to copy some of the drawings made
at Harvard College Observatory by Mr. Trouvelot, and I have availed
myself of his kindness for Plates I., IV., XII., XV.

I am indebted to Professor Langley for Plate II., to Mr. De la Rue for
Plates III. and XIV., to Mr. T.E. Key for Plate XVII., to Professor
Schiaparelli for Plate XVIII., to the late Professor C. Piazzi Smyth for Fig.
100, to Mr. Chambers for Fig. 7, which has been borrowed from his
"Handbook of Descriptive Astronomy," to Dr. Stoney for Fig. 78, and to Dr.
Copeland and Dr. Dreyer for Fig. 72. I have to acknowledge the valuable
assistance derived from Professor Newcomb's "Popular Astronomy," and
Professor Young's "Sun." In revising the volume I have had the kind aid of
the Rev. Maxwell Close.

I have also to thank Dr. Copeland and Mr. Steele for their kindness in
reading through the entire proofs; while I have also occasionally availed
myself of the help of Mr. Cathcart.

ROBERT S. BALL.

Observatory, Dunsink, Co. Dublin.
12th May, 1886.

NOTE TO THIS EDITION.
I have taken the opportunity in the present edition to revise the work in
accordance with the recent progress of astronomy. I am indebted to the
Royal Astronomical Society for the permission to reproduce some
photographs from their published series, and to Mr. Henry F. Griffiths, for
beautiful drawings of Jupiter, from which Plate XI. was prepared.

ROBERT S. BALL.

Cambridge,
1st May, 1900.

CONTENTS.
page

Introduction 1
chapter

The Astronomical
I. 9
Observatory
II. The Sun 29
III. The Moon 70
IV. The Solar System 107
V. The Law of Gravitation 122
VI. The Planet of Romance 150
VII. Mercury 155
VIII. Venus 167
IX. The Earth 192
X. Mars 208
XI. The Minor Planets 229
XII. Jupiter 245
XIII. Saturn 268
XIV. Uranus 298
XV. Neptune 315
XVI. Comets 336
XVII. Shooting Stars 372
XVIII. The Starry Heavens 409
XIX. The Distant Suns 425
XX. Double Stars 434
The Distances of the
XXI. 441
Stars
Star Clusters and
XXII. 461
Nebulæ
The Physical Nature of
XXIII. 477
the Stars

The Precession and
XXIV. Nutation of the Earth's 492
Axis
The Aberration of
XXV. 503
Light
The Astronomical
XXVI. 513
Significance of Heat
XXVII. The Tides 531
Appendix 558

LIST OF PLATES.
PLATE
I. The Planet Saturn Frontispiece
To face
II. A Typical Sun-spot 9
page
A. The Sun " " 44
Spots and Faculæ on
III. " " 37
the Sun
Solar Prominences or
IV. " " 57
Flames
V. The Solar Corona " " 62
Chart of the Moon's
VI. " " 81
Surface
B. Portion of the Moon " " 88
The Lunar Crater
VII. " " 93
Triesnecker
VIII. A Normal Lunar Crater " " 97
IX. The Lunar Crater Plato " " 102
The Lunar Crater
X. " " 106
Tycho
XI. The Planet Jupiter " " 254
XII. Coggia's Comet " " 340
Comet A., 1892, 1
C. " " 358
Swift
Spectra of the Sun and
XIII. " " 47
of three Stars
The Milky Way, near
D. " " 462
Messier II.
The Great Nebula in
XIV. " " 466
Orion

The Great Nebula in
XV. " " 468
Andromeda
E. Nebulæ in the Pleiades " " 472
F. ω Centauri " " 474
Nebulæ observed with
XVI. " " 476
Lord Rosse's Telescope
XVII. The Comet of 1882 " " 357
Schiaparelli's Map of
XVIII. " " 221
Mars

LIST OF ILLUSTRATIONS.
FIG. PAGE
1.Principle of the Refracting Telescope 11
Dome of the South Equatorial at Dunsink
2. 12
Observatory, Co. Dublin
Section of the Dome of Dunsink
3. 13
Observatory
The Telescope at Yerkes Observatory,
4. 15
Chicago
Principle of Herschel's Reflecting
5. 16
Telescope
South Front of the Yerkes Observatory,
6. 17
Chicago
7.Lord Rosse's Telescope 18
8.Meridian Circle 20
9.The Great Bear 27
Comparative Sizes of the Earth and the
10. 30
Sun
The Sun, photographed September 22,
11. 33
1870
12.Photograph of the Solar Surface 35
13.An ordinary Sun-spot 36
14.Scheiner's Observations on Sun-spots 38
Zones on the Sun's Surface in which
15. 39
Spots appear
16.Texture of the Sun and a small Spot 43
17.The Prism 45
18.Dispersion of Light by the Prism 46
19.Prominences seen in Total Eclipses 53
20.View of the Corona in a Total Eclipse 62

View of Corona during Eclipse of January
21. 63
22, 1898
22.The Zodiacal Light in 1874 69
Comparative Sizes of the Earth and the
23. 73
Moon
24.The Moon's Path around the Sun 76
25.The Phases of the Moon 76
26.The Earth's Shadow and Penumbra 78
27.Key to Chart of the Moon (Plate VI.) 81
Lunar Volcano in Activity: Nasmyth's
28. 97
Theory
Lunar Volcano: Subsequent Feeble
29. 97
Activity
Lunar Volcano: Formation of the Level
30. 98
Floor by Lava
31.Orbits of the Four Interior Planets 115
32.The Earth's Movement 116
33.Orbits of the Four Giant Planets 117
Apparent Size of the Sun from various
34. 118
Planets
35.Comparative Sizes of the Planets 119
36.Illustration of the Moon's Motion 130
37.Drawing an Ellipse 137
38.Varying Velocity of Elliptic Motion 140
39.Equal Areas in Equal Times 141
40.Transit of the Planet of Romance 153
Variations in Phase and apparent Size of
41. 160
Mercury
42.Mercury as a Crescent 161
43.Venus, May 29, 1889 170
Different Aspects of Venus in the
44. 171
Telescope
45.Venus on the Sun at the Transit of 1874 177
Paths of Venus across the Sun in the
46. 179
Transits of 1874 and 1882

A Transit of Venus, as seen from Two
47. 183
Localities
48.Orbits of the Earth and of Mars 210
49.Apparent Movements of Mars in 1877 212
50.Relative Sizes of Mars and the Earth 216
51,
Drawings of Mars 217
52.
Elevations and Depressions on the
53. 217
Terminator of Mars
54.The Southern Polar Cap on Mars 217
The Zone of Minor Planets between Mars
55. 234
and Jupiter
Relative Dimensions of Jupiter and the
56. 246
Earth
57–
The Occultation of Jupiter 255
60.
61.Jupiter and his Four Satellites 258
62.Disappearances of Jupiter's Satellites 259
63.Mode of Measuring the Velocity of Light 264
64.Saturn 270
65.Relative Sizes of Saturn and the Earth 273
Method of Measuring the Rotation of
66. 288
Saturn's Rings
Method of Measuring the Rotation of
67. 289
Saturn's Rings
68.Transit of Titan and its Shadow 295
69.Parabolic Path of a Comet 339
70.Orbit of Encke's Comet 346
71.Tail of a Comet directed from the Sun 363
72.Bredichin's Theory of Comets' Tails 366
73.Tails of the Comet of 1858 367
74.The Comet of 1744 368
The Path of the Fireball of November 6,
75. 375
1869
76.The Orbit of a Shoal of Meteors 378

77.Radiant Point of Shooting Stars 381
78.The History of the Leonids 385
79.Section of the Chaco Meteorite 398
80.The Great Bear and Pole Star 410
81.The Great Bear and Cassiopeia 411
82.The Great Square of Pegasus 413
83.Perseus and its Neighbouring Stars 415
84.The Pleiades 416
85.Orion, Sirius, and Neighbouring Stars 417
86.Castor and Pollux 418
87.The Great Bear and the Lion 419
88.Boötes and the Crown 420
89.Virgo and Neighbouring Constellations 421
90.The Constellation of Lyra 422
91.Vega, the Swan, and the Eagle 423
92.The Orbit of Sirius 426
93.The Parallactic Ellipse 444
94.61 Cygni and the Comparison Stars 447
95.Parallax in Declination of 61 Cygni 450
96.Globular Cluster in Hercules 463
97.Position of the Great Nebula in Orion 466
98.The Multiple Star θ Orionis 467
99.The Nebula N.G.C. 1499 471
Star-Map, showing Precessional
100. 493
Movement
101.Illustration of the Motion of Precession 495

THE

Story of the Heavens.

"T wondrous
Story of the Heavens" is the title of our book. We have indeed a
he
story to narrate; and could we tell it adequately it would
prove of boundless interest and of exquisite beauty. It leads to the
contemplation of grand phenomena in nature and great achievements of
human genius.

Let us enumerate a few of the questions which will be naturally asked by
one who seeks to learn something of those glorious bodies which adorn our
skies: What is the Sun—how hot, how big, and how distant? Whence comes
its heat? What is the Moon? What are its landscapes like? How does our
satellite move? How is it related to the earth? Are the planets globes like
that on which we live? How large are they, and how far off? What do we
know of the satellites of Jupiter and of the rings of Saturn? How was
Uranus discovered? What was the intellectual triumph which brought the
planet Neptune to light? Then, as to the other bodies of our system, what are
we to say of those mysterious objects, the comets? Can we discover the
laws of their seemingly capricious movements? Do we know anything of
their nature and of the marvellous tails with which they are often decorated?
What can be told about the shooting-stars which so often dash into our
atmosphere and perish in a streak of splendour? What is the nature of those
constellations of bright stars which have been recognised from all antiquity,
and of the host of smaller stars which our telescopes disclose? Can it be true
that these countless orbs are really majestic suns, sunk to an appalling depth
in the abyss of unfathomable space? What have we to tell of the different
varieties of stars—of coloured stars, of variable stars, of double stars, of
multiple stars, of stars that seem to move, and of stars that seem at rest?
What of those glorious objects, the great star clusters? What of the Milky
Way? And, lastly, what can we learn of the marvellous nebulæ which our
telescopes disclose, poised at an immeasurable distance? Such are a few of
the questions which occur when we ponder on the mysteries of the heavens.

The history of Astronomy is, in one respect, only too like many other
histories. The earliest part of it is completely and hopelessly lost. The stars
had been studied, and some great astronomical discoveries had been made,

untold ages before those to which our earliest historical records extend. For
example, the observation of the apparent movement of the sun, and the
discrimination between the planets and the fixed stars, are both to be
classed among the discoveries of prehistoric ages. Nor is it to be said that
these achievements related to matters of an obvious character. Ancient
astronomy may seem very elementary to those of the present day who have
been familiar from childhood with the great truths of nature, but, in the
infancy of science, the men who made such discoveries as we have
mentioned must have been sagacious philosophers.

Of all the phenomena of astronomy the first and the most obvious is that
of the rising and the setting of the sun. We may assume that in the dawn of
human intelligence these daily occurrences would form one of the first
problems to engage the attention of those whose thoughts rose above the
animal anxieties of everyday existence. A sun sets and disappears in the
west. The following morning a sun rises in the east, moves across the
heavens, and it too disappears in the west; the same appearances recur every
day. To us it is obvious that the sun, which appears each day, is the same
sun; but this would not seem reasonable to one who thought his senses
showed him that the earth was a flat plain of indefinite extent, and that
around the inhabited regions on all sides extended, to vast distances, either
desert wastes or trackless oceans. How could that same sun, which plunged
into the ocean at a fabulous distance in the west, reappear the next morning
at an equally great distance in the east? The old mythology asserted that
after the sun had dipped in the western ocean at sunset (the Iberians, and
other ancient nations, actually imagined that they could hear the hissing of
the waters when the glowing globe was plunged therein), it was seized by
Vulcan and placed in a golden goblet. This strange craft with its astonishing
cargo navigated the ocean by a northerly course, so as to reach the east
again in time for sunrise the following morning. Among the earlier
physicists of old it was believed that in some manner the sun was conveyed
by night across the northern regions, and that darkness was due to lofty
mountains, which screened off the sunbeams during the voyage.

In the course of time it was thought more rational to suppose that the sun
actually pursued his course below the solid earth during the course of the
night. The early astronomers had, moreover, learned to recognise the fixed
stars. It was noticed that, like the sun, many of these stars rose and set in
consequence of the diurnal movement, while the moon obviously followed
a similar law. Philosophers thus taught that the various heavenly bodies
were in the habit of actually passing beneath the solid earth.

By the acknowledgment that the whole contents of the heavens
performed these movements, an important step in comprehending the
constitution of the universe had been decidedly taken. It was clear that the
earth could not be a plane extending to an indefinitely great distance. It was
also obvious that there must be a finite depth to the earth below our feet.
Nay, more, it became certain that whatever the shape of the earth might be,
it was at all events something detached from all other bodies, and poised
without visible support in space. When this discovery was first announced it
must have appeared a very startling truth. It was so difficult to realise that
the solid earth on which we stand reposed on nothing! What was to keep it
from falling? How could it be sustained without tangible support, like the
legendary coffin of Mahomet? But difficult as it may have been to receive
this doctrine, yet its necessary truth in due time commanded assent, and the
science of Astronomy began to exist. The changes of the seasons and the
recurrence of seed-time and harvest must, from the earliest times, have been
associated with certain changes in the position of the sun. In the summer at
mid-day the sun rises high in the heavens, in the winter it is always low.
Our luminary, therefore, performs an annual movement up and down in the
heavens, as well as a diurnal movement of rising and setting. But there is a
third species of change in the sun's position, which is not quite so obvious,
though it is still capable of being detected by a few careful observations, if
combined with a philosophical habit of reflection. The very earliest
observers of the stars can hardly have failed to notice that the constellations
visible at night varied with the season of the year. For instance, the brilliant
figure of Orion, though so well seen on winter nights, is absent from the

summer skies, and the place it occupied is then taken by quite different
groups of stars. The same may be said of other constellations. Each season
of the year can thus be characterised by the sidereal objects that are
conspicuous by night. Indeed, in ancient days, the time for commencing the
cycle of agricultural occupations was sometimes indicated by the position
of the constellations in the evening.

By reflecting on these facts the early astronomers were enabled to
demonstrate the apparent annual movement of the sun. There could be no
rational explanation of the changes in the constellations with the seasons,
except by supposing that the place of the sun was altering, so as to make a
complete circuit of the heavens in the course of the year. This movement of
the sun is otherwise confirmed by looking at the west after sunset, and
watching the stars. As the season progresses, it may be noticed each
evening that the constellations seem to sink lower and lower towards the
west, until at length they become invisible from the brightness of the sky.
The disappearance is explained by the supposition that the sun appears to be
continually ascending from the west to meet the stars. This motion is, of
course, not to be confounded with the ordinary diurnal rising and setting, in
which all the heavenly bodies participate. It is to be understood that besides
being affected by the common motion our luminary has a slow independent
movement in the opposite direction; so that though the sun and a star may
set at the same time to-day, yet since by to-morrow the sun will have moved
a little towards the east, it follows that the star must then set a few minutes
before the sun.[1]

The patient observations of the early astronomers enabled the sun's track
through the heavens to be ascertained, and it was found that in its circuit
amid the stars and constellations our luminary invariably followed the same
path. This is called the ecliptic, and the constellations through which it
passes form a belt around the heavens known as the zodiac. It was anciently
divided into twelve equal portions or "signs," so that the stages on the sun's
great journey could be conveniently indicated. The duration of the year, or
the period required by the sun to run its course around the heavens, seems

to have been first ascertained by astronomers whose names are unknown.
The skill of the early Oriental geometers was further evidenced by their
determination of the position of the ecliptic with regard to the celestial
equator, and by their success in the measurement of the angle between these
two important circles on the heavens.

The principal features of the motion of the moon have also been noticed
with intelligence at an antiquity more remote than history. The attentive
observer perceives the important truth that the moon does not occupy a
fixed position in the heavens. During the course of a single night the fact
that the moon has moved from west to east across the heavens can be
perceived by noting its position relatively to adjacent stars. It is indeed
probable that the motion of the moon was a discovery prior to that of the
annual motion of the sun, inasmuch as it is the immediate consequence of a
simple observation, and involves but little exercise of any intellectual
power. In prehistoric times also, the time of revolution of the moon had
been ascertained, and the phases of our satellite had been correctly
attributed to the varying aspect under which the sun-illuminated side is
turned towards the earth.

But we are far from having exhausted the list of great discoveries which
have come down from unknown antiquity. Correct explanations had been
given of the striking phenomenon of a lunar eclipse, in which the brilliant
surface is plunged temporarily into darkness, and also of the still more
imposing spectacle of a solar eclipse, in which the sun itself undergoes a
partial or even a total obscuration. Then, too, the acuteness of the early
astronomers had detected the five wandering stars or planets: they had
traced the movements of Mercury and Venus, Mars, Jupiter, and Saturn.
They had observed with awe the various configurations of these planets:
and just as the sun, and in a lesser degree the moon, were intimately
associated with the affairs of daily life, so in the imagination of these early
investigators the movements of the planets were thought to be pregnant
with human weal or human woe. At length a certain order was perceived to
govern the apparently capricious movements of the planets. It was found

that they obeyed certain laws. The cultivation of the science of geometry
went hand in hand with the study of astronomy: and as we emerge from the
dim prehistoric ages into the historical period, we find that the theory of the
phenomena of the heavens possessed already some degree of coherence.

Ptolemy, following Pythagoras, Plato, and Aristotle, acknowledged that
the earth's figure was globular, and he demonstrated it by the same
arguments that we employ at the present day. He also discerned how this
mighty globe was isolated in space. He admitted that the diurnal movement
of the heavens could be accounted for by the revolution of the earth upon its
axis, but unfortunately he assigned reasons for the deliberate rejection of
this view. The earth, according to him, was a fixed body; it possessed
neither rotation round an axis nor translation through space, but remained
constantly at rest in what he supposed to be the centre of the universe.
According to Ptolemy's theory the sun and the moon moved in circular
orbits around the earth in the centre. The explanation of the movements of
the planets he found to be more complicated, because it was necessary to
account for the fact that a planet sometimes advanced and that it sometimes
retrograded. The ancient geometers refused to believe that any movement,
except revolution in a circle, was possible for a celestial body: accordingly
a contrivance was devised by which each planet was supposed to revolve in
a circle, of which the centre described another circle around the earth.

Although the Ptolemaic doctrine is now known to be framed on quite an
extravagant estimate of the importance of the earth in the scheme of the
heavens, yet it must be admitted that the apparent movements of the
celestial bodies can be thus accounted for with considerable accuracy. This
theory is described in the great work known as the "Almagest," which was
written in the second century of our era, and was regarded for fourteen
centuries as the final authority on all questions of astronomy.

Such was the system of Astronomy which prevailed during the Middle
Ages, and was only discredited at an epoch nearly simultaneous with that of
the discovery of the New World by Columbus. The true arrangement of the

solar system was then expounded by Copernicus in the great work to which
he devoted his life. The first principle established by these labours showed
the diurnal movement of the heavens to be due to the rotation of the earth
on its axis. Copernicus pointed out the fundamental difference between real
motions and apparent motions; he proved that the appearances presented in
the daily rising and setting of the sun and the stars could be accounted for
by the supposition that the earth rotated, just as satisfactorily as by the more
cumbrous supposition of Ptolemy. He showed, moreover, that the latter
supposition must attribute an almost infinite velocity to the stars, so that the
rotation of the entire universe around the earth was clearly a preposterous
supposition. The second great principle, which has conferred immortal
glory on Copernicus, assigned to the earth its true position in the universe.
Copernicus transferred the centre, about which all the planets revolve, from
the earth to the sun; and he established the somewhat humiliating truth, that
our earth is merely a planet pursuing a track between the paths of Venus and
of Mars, and subordinated like all the other planets to the supreme sway of
the Sun.

This great revolution swept from astronomy those distorted views of the
earth's importance which arose, perhaps not unnaturally, from the fact that
we happen to be domiciled on that particular planet. The achievements of
Copernicus were soon to be followed by the invention of the telescope, that
wonderful instrument by which the modern science of astronomy has been
created. To the consideration of this important subject we shall devote the
first chapter of our book.

PLATE II.
A TYPICAL SUN-SPOT.
(AFTER LANGLEY.)

CHAPTER I.
THE ASTRONOMICAL OBSERVATORY.
Early Astronomical Observations—The Observatory of Tycho Brahe—The Pupil of the
Eye—Vision of Faint Objects—The Telescope—The Object-Glass—Advantages of
Large Telescopes—The Equatorial—The Observatory—The Power of a Telescope—
Reflecting Telescopes—Lord Rosse's Great Reflector at Parsonstown—How the
mighty Telescope is used—Instruments of Precision—The Meridian Circle—The
Spider Lines—Delicacy of pointing a Telescope—Precautions necessary in making
Observations—The Ideal Instrument and the Practical One—The Elimination of Error
—Greenwich Observatory—The ordinary Opera-Glass as an Astronomical Instrument
—The Great Bear—Counting the Stars in the Constellation—How to become an
Observer.

The earliest rudiments of the Astronomical Observatory are as little known
as the earliest discoveries in astronomy itself. Probably the first application
of instrumental observation to the heavenly bodies consisted in the simple
operation of measuring the shadow of a post cast by the sun at noonday.
The variations in the length of this shadow enabled the primitive
astronomers to investigate the apparent movements of the sun. But even in
very early times special astronomical instruments were employed which
possessed sufficient accuracy to add to the amount of astronomical
knowledge, and displayed considerable ingenuity on the part of the
designers.

Professor Newcomb[2] thus writes: "The leader was Tycho Brahe, who
was born in 1546, three years after the death of Copernicus. His attention
was first directed to the study of astronomy by an eclipse of the sun on
August 21st, 1560, which was total in some parts of Europe. Astonished
that such a phenomenon could be predicted, he devoted himself to a study
of the methods of observation and calculation by which the prediction was
made. In 1576 the King of Denmark founded the celebrated observatory of
Uraniborg, at which Tycho spent twenty years assiduously engaged in

observations of the positions of the heavenly bodies with the best
instruments that could then be made. This was just before the invention of
the telescope, so that the astronomer could not avail himself of that
powerful instrument. Consequently, his observations were superseded by
the improved ones of the centuries following, and their celebrity and
importance are principally due to their having afforded Kepler the means of
discovering his celebrated laws of planetary motion."

The direction of the telescope to the skies by Galileo gave a wonderful
impulse to the study of the heavenly bodies. This extraordinary man is
prominent in the history of astronomy, not alone for his connection with this
supreme invention, but also for his achievements in the more abstract parts
of astronomy. He was born at Pisa in 1564, and in 1609 the first telescope
used for astronomical observation was constructed. Galileo died in 1642,
the year in which Newton was born. It was Galileo who laid with solidity
the foundations of that science of Dynamics, of which astronomy is the
most splendid illustration; and it was he who, by promulgating the doctrines
taught by Copernicus, incurred the wrath of the Inquisition.

The structure of the human eye in so far as the exquisite adaptation of
the pupil is concerned presents us with an apt illustration of the principle of
the telescope. To see an object, it is necessary that the light from it should
enter the eye. The portal through which the light is admitted is the pupil. In
daytime, when the light is brilliant, the iris decreases the size of the pupil,
and thus prevents too much light from entering. At night, or whenever the
light is scarce, the eye often requires to grasp all it can. The pupil then
expands; more and more light is admitted according as the pupil grows
larger. The illumination of the image on the retina is thus effectively
controlled in accordance with the requirements of vision.

A star transmits to us its feeble rays of light, and from those rays the
image is formed. Even with the most widely-opened pupil, it may, however,
happen that the image is not bright enough to excite the sensation of vision.
Here the telescope comes to our aid: it catches all the rays in a beam whose

original dimensions were far too great to allow of its admission through the
pupil. The action of the lenses concentrates those rays into a stream slender
enough to pass through the small opening. We thus have the brightness of
the image on the retina intensified. It is illuminated with nearly as much
light as would be collected from the same object through a pupil as large as
the great lenses of the telescope.

In astronomical observatories we employ telescopes of two entirely
different classes. The more familiar forms are those known as refractors, in
which the operation of condensing the rays of light is conducted by
refraction. The character of the refractor is shown in Fig. 1. The rays from
the star fall upon the object-glass at the end of the telescope, and on passing
through they become refracted into a converging beam, so that all intersect
at the focus. Diverging from thence, the rays encounter the eye-piece, which
has the effect of restoring them to parallelism. The large cylindrical beam
which poured down on the object-glass has been thus condensed into a
small one, which can enter the pupil. It should, however, be added that the
composite nature of light requires a more complex form of object-glass than
the simple lens here shown. In a refracting telescope we have to employ
what is known as the achromatic combination, consisting of one lens of flint
glass and one of crown glass, adjusted to suit each other with extreme care.

Fig. 1.—Principle of the
Refracting Telescope.

Fig. 2.—The Dome of the South Equatorial at
Dunsink Observatory Co Dublin.

Fig. 3.—Section of the Dome of Dunsink
Observatory.
The appearance of an astronomical observatory, designed to
accommodate an instrument of moderate dimensions, is shown in the
adjoining figures. The first (Fig. 2) represents the dome erected at Dunsink
Observatory for the equatorial telescope, the object-glass of which was
presented to the Board of Trinity College, Dublin, by the late Sir James
South. The main part of the building is a cylindrical wall, on the top of
which reposes a hemispherical roof. In this roof is a shutter, which can be
opened so as to allow the telescope in the interior to obtain a view of the
heavens. The dome is capable of revolving so that the opening may be
turned towards that part of the sky where the object happens to be situated.
The next view (Fig. 3) exhibits a section through the dome, showing the
machinery by which the attendant causes it to revolve, as well as the
telescope itself. The eye of the observer is placed at the eye-piece, and he is
represented in the act of turning a handle, which has the power of slowly
moving the telescope, in order to adjust the instrument accurately on the
celestial body which it is desired to observe. The two lenses which together

form the object-glass of this instrument are twelve inches in diameter, and
the quality of the telescope mainly depends on the accuracy with which
these lenses have been wrought. The eye-piece is a comparatively simple
matter. It consists merely of one or two small lenses; and various eye-pieces
can be employed, according to the magnifying power which may be
desired. It is to be observed that for many purposes of astronomy high
magnifying powers are not desirable. There is a limit, too, beyond which
the magnification cannot be carried with advantage. The object-glass can
only collect a certain quantity of light from the star; and if the magnifying
power be too great, this limited amount of light will be thinly dispersed
over too large a surface, and the result will be found unsatisfactory. The
unsteadiness of the atmosphere still further limits the extent to which the
image may be advantageously magnified, for every increase of power
increases in the same degree the atmospheric disturbance.

A telescope mounted in the manner here shown is called an equatorial.
The convenience of this peculiar style of supporting the instrument consists
in the ease with which the telescope can be moved so as to follow a star in
its apparent journey across the sky. The necessary movements of the tube
are given by clockwork driven by a weight, so that, once the instrument has
been correctly pointed, the star will remain in the observer's field of view,
and the effect of the apparent diurnal movement will be neutralised. The
last refinement in this direction is the application of an electrical
arrangement by which the driving of the instrument is controlled from the
standard clock of the observatory.

Fig. 4.—The Telescope at Yerkes Observatory, Chicago.
(From the Astrophysical Journal, Vol. vi., No. 1.)
The power of a refracting telescope—so far as the expression has any
definite meaning—is to be measured by the diameter of its object-glass.
There has, indeed, been some honourable rivalry between the various
civilised nations as to which should possess the greatest refracting
telescope. Among the notable instruments that have been successfully
completed is that erected in 1881 by Sir Howard Grubb, of Dublin, at the
splendid observatory at Vienna. Its dimensions may be estimated from the
fact that the object-glass is two feet and three inches in diameter. Many
ingenious contrivances help to lessen the inconvenience incident to the use
of an instrument possessing such vast proportions. Among them we may
here notice the method by which the graduated circles attached to the
telescope are brought within view of the observer. These circles are
necessarily situated at parts of the instrument which lie remote from the
eye-piece where the observer is stationed. The delicate marks and figures
are, however, easily read from a distance by a small auxiliary telescope,
which, by suitable reflectors, conducts the rays of light from the circles to
the eye of the observer.

Numerous refracting telescopes of exquisite perfection have been
produced by Messrs. Alvan Clark, of Cambridgeport, Boston, Mass. One of
their most famous telescopes is the great Lick Refractor now in use on
Mount Hamilton in California. The diameter of this object-glass is thirty-six
inches, and its focal length is fifty-six feet two inches. A still greater effort
has recently been made by the same firm in the refractor of forty inches
aperture for the Yerkes Observatory of the University of Chicago. The
telescope, which is seventy-five feet in length, is mounted under a revolving
dome ninety feet in diameter, and in order to enable the observer to reach
the eye-piece without using very large step-ladders, the floor of the room
can be raised and lowered through a range of twenty-two feet by electric
motors. This is shown in Fig. 4, while the south front of the Yerkes
Observatory is represented in Fig. 6.

Fig. 5.—Principle of
Herschel's Refracting
Telescope.

Fig. 6.—South Front of the Yerkes Observatory, Chicago.
(From the Astrophysical Journal, Vol. vi., No. 1.)

Fig. 7.—Lord Rosse's Telescope.

Within the last few years two fine telescopes have been added to the
instrumental equipment of the Royal Observatory, Greenwich, both by Sir
H. Grubb. One of these, containing a 28-inch object-glass, has been erected
on a mounting originally constructed for a smaller instrument by Sir G.
Airy. The other, presented by Sir Henry Thompson, is of 26 inches aperture,
and is adapted for photographic work.

There is a limit to the size of the refractor depending upon the material
of the object-glass. Glass manufacturers seem to experience unusual
difficulties in their attempts to form large discs of optical glass pure enough
and uniform enough to be suitable for telescopes. These difficulties are
enhanced with every increase in the size of the discs, so that the cost has a
tendency to increase at a very much greater rate. It may be mentioned in
illustration that the price paid for the object-glass of the Lick telescope
exceeded ten thousand pounds.

There is, however, an alternative method of constructing a telescope, in
which the difficulty we have just mentioned does not arise. The principle of
the simplest form of reflector is shown in Fig. 5, which represents what is
called the Herschelian instrument. The rays of light from the star under
observation fall on a mirror which is both carefully shaped and highly
polished. After reflection, the rays proceed to a focus, and diverging from
thence, fall on the eye-piece, by which they are restored to parallelism, and
thus become adapted for reception in the eye. It was essentially on this
principle (though with a secondary flat mirror at the upper end of the tube
reflecting the rays at a right angle to the side of the tube, where the eye-
piece is placed) that Sir Isaac Newton constructed the little reflecting
telescope which is now treasured by the Royal Society. A famous
instrument of the Newtonian type was built, half a century ago, by the late
Earl of Rosse, at Parsonstown. It is represented in Fig. 7. The colossal
aperture of this instrument has never been surpassed; it has, indeed, never
been rivalled. The mirror or speculum, as it is often called, is a thick
metallic disc, composed of a mixture of two parts of copper with one of tin.
This alloy is so hard and brittle as to make the necessary mechanical
operations difficult to manage. The material admits, however, of a brilliant
polish, and of receiving and retaining an accurate figure. The Rosse
speculum—six feet in diameter and three tons in weight—reposes at the
lower end of a telescope fifty-five feet long. The tube is suspended between
two massive castellated walls, which form an imposing feature on the lawn
at Birr Castle. This instrument cannot be turned about towards every part of
the sky, like the equatorials we have recently been considering. The great
tube is only capable of elevation in altitude along the meridian, and of a
small lateral movement east and west of the meridian. Every star or nebula
visible in the latitude of Parsonstown (except those very near the pole) can,
however, be observed in the great telescope, if looked for at the right time.

Before the object reaches the meridian, the telescope must be adjusted at
the right elevation. The necessary power is transmitted by a chain from a
winch at the northern end of the walls to a point near the upper end of the

tube. By this
contrivance the
telescope can be
raised or
lowered, and an
ingenious system
of counterpoises
renders the
movement
equally easy at
all altitudes. The
observer then
takes his station
in one of the
galleries which
give access to the
eye-piece; and
when the right
moment has
arrived, the star
enters the field of Fig. 8.—Meridian Circle.
view. Powerful
mechanism drives the great instrument, so as to counteract the diurnal
movement, and thus the observer can retain the object in view until he has
made his measurements or finished his drawing.

Of late years reflecting telescopes have been generally made with
mirrors of glass covered with a thin film of silver, which is capable of
reflecting much more light than the surface of a metallic mirror. Among
great reflectors of this kind we may mention two, of three and five feet
aperture respectively, with which Dr. Common has done valuable work.

We must not, however, assume that for the general work in an
observatory a colossal instrument is the most suitable. The mighty reflector,

or refractor, is chiefly of use where unusually faint objects are being
examined. For work in which accurate measurements are made of objects
not particularly difficult to see, telescopes of smaller dimensions are more
suitable. The fundamental facts about the heavenly bodies have been chiefly
learned from observations obtained with instruments of moderate optical
power, specially furnished so as to enable precise measures of position to be
secured. Indeed, in the early stages of astronomy, important determinations
of position were effected by contrivances which showed the direction of the
object without any telescopic aid.

Perhaps the most valuable measurements obtained in our modern
observatories are yielded by that instrument of precision known as the
meridian circle. It is impossible, in any adequate account of the Story of the
Heavens, to avoid some reference to this indispensable aid to astronomical
research, and therefore we shall give a brief account of one of its simpler
forms, choosing for this purpose a great instrument in the Paris
Observatory, which is represented in Fig. 8.

The telescope is attached at its centre to an axis at right angles to its
length. Pivots at each extremity of this axis rotate upon fixed bearings, so
that the movements of the telescope are completely restricted to the plane of
the meridian. Inside the eye-piece of the telescope extremely fine vertical
fibres are stretched. The observer watches the moon, or star, or planet enter
the field of view; and he notes by the clock the exact time, to the fraction of
a second, at which the object passes over each of the lines. A silver band on
the circle attached to the axis is divided into degrees and subdivisions of a
degree, and as this circle moves with the telescope, the elevation at which
the instrument is pointed will be indicated. For reading the delicately
engraved marks and figures on the silver, microscopes are necessary. These
are shown in the sketch, each one being fixed into an aperture in the wall
which supports one end of the instrument. At the opposite side is a lamp,
the light from which passes through the perforated axis of the pivot, and is
thence ingeniously deflected by mirrors so as to provide the requisite
illumination for the lines at the focus.

The fibres which the observer sees stretched over the field of view of the
telescope demand a few words of explanation. We require for this purpose a
material which shall be very fine and fairly durable, as well as somewhat
elastic, and of no appreciable weight. These conditions cannot be
completely fulfilled by any metallic wire, but they are exquisitely realised
in the beautiful thread which is spun by the spider. The delicate fibres are
stretched with nice skill across the field of view of the telescope, and
cemented in their proper places. With instruments so beautifully appointed
we can understand the precision attained in modern observations. The
telescope is directed towards a star, and the image of the star is a minute
point of light. When that point coincides with the intersection of the two
central spider lines the telescope is properly sighted. We use the word
sighted designedly, because we wish to suggest a comparison between the
sighting of a rifle at the target and the sighting of a telescope at a star.
Instead of the ordinary large bull's-eye, suppose that the target only
consisted of a watch-dial, which, of course, the rifleman could not see at the
distance of any ordinary range. But with the telescope of the meridian circle
the watch-dial would be visible even at the distance of a mile. The meridian
circle is indeed capable of such precision as a sighting instrument that it
could be pointed separately to each of two stars which subtend at the eye an
angle no greater than that subtended by an adjoining pair of the sixty minute
dots around the circumference of a watch-dial a mile distant from the
observer.

This power of directing the instrument so accurately would be of but
little avail unless it were combined with arrangements by which, when once
the telescope has been pointed correctly, the position of the star can be
ascertained and recorded. One element in the determination of the position
is secured by the astronomical clock, which gives the moment when the
object crosses the central vertical wire; the other element is given by the
graduated circle which reads the angular distance of the star from the zenith
or point directly overhead.

Superb meridian instruments adorn our great observatories, and are
nightly devoted to those measurements upon which the great truths of
astronomy are mainly based. These instruments have been constructed with
refined skill; but it is the duty of the painstaking astronomer to distrust the
accuracy of his instrument in every conceivable way. The great tube may be
as rigid a structure as mechanical engineers can produce; the graduations on
the circle may have been engraved by the most perfect of dividing
machines; but the conscientious astronomer will not be content with mere
mechanical precision. That meridian circle which, to the uninitiated, seems
a marvellous piece of workmanship, possessing almost illimitable accuracy,
is viewed in a very different light by the astronomer who makes use of it.
No one can appreciate more fully than he the skill of the artist who has
made that meridian circle, and the beautiful contrivances for illumination
and reading off which give to the instrument its perfection; but while the
astronomer recognises the beauty of the actual machine he is using, he has
always before his mind's eye an ideal instrument of absolute perfection, to
which the actual meridian circle only makes an approximation.

Contrasted with the ideal instrument, the finest meridian circle is little
more than a mass of imperfections. The ideal tube is perfectly rigid, the
actual tube is flexible; the ideal divisions of the circle are perfectly uniform,
the actual divisions are not uniform. The ideal instrument is a geometrical
embodiment of perfect circles, perfect straight lines, and perfect right
angles; the actual instrument can only show approximate circles,
approximate straight lines, and approximate right angles. Perhaps the
spider's part of the work is on the whole the best; the stretched web gives us
the nearest mechanical approach to a perfectly straight line; but we mar the
spider's work by not being able to insert those beautiful threads with perfect
uniformity, while our attempts to adjust two of them across the field of view
at right angles do not succeed in producing an angle of exactly ninety
degrees.

Nor are the difficulties encountered by the meridian observer due solely
to his instrument. He has to contend against his own imperfections; he has

often to allow for personal peculiarities of an unexpected nature; the
troubles that the atmosphere can give are notorious; while the levelling of
his instrument warns him that he cannot even rely on the solid earth itself.
We learn that the earthquakes, by which the solid ground is sometimes
disturbed, are merely the more conspicuous instances of incessant small
movements in the earth which every night in the year derange the delicate
adjustment of the instrument.

When the existence of these errors has been recognised, the first great
step has been taken. By an alliance between the astronomer and the
mathematician it is possible to measure the discrepancies between the
actual meridian circle and the instrument that is ideally perfect. Once this
has been done, we can estimate the effect which the irregularities produce
on the observations, and finally, we succeed in purging the observations
from the grosser errors by which they are contaminated. We thus obtain
results which are not indeed mathematically accurate, but are nevertheless
close approximations to those which would be obtained by a perfect
observer using an ideal instrument of geometrical accuracy, standing on an
earth of absolute rigidity, and viewing the heavens without the intervention
of the atmosphere.

In addition to instruments like those already indicated, astronomers have
other means of following the motions of the heavenly bodies. Within the
last fifteen years photography has commenced to play an important part in
practical astronomy. This beautiful art can be utilised for representing many
objects in the heavens by more faithful pictures than the pencil of even the
most skilful draughtsman can produce. Photography is also applicable for
making charts of any region in the sky which it is desired to examine. When
repeated pictures of the same region are made from time to time, their
comparison gives the means of ascertaining whether any star has moved
during the interval. The amount and direction of this motion may be
ascertained by a delicate measuring apparatus under which the photographic
plate is placed.

If a refracting telescope is to be used for taking celestial photographs,
the lenses of the object-glass must be specially designed for this purpose.
The rays of light which imprint an image on the prepared plate are not
exactly the same as those which are chiefly concerned in the production of
the image on the retina of the human eye. A reflecting mirror, however,
brings all the rays, both those which are chemically active and those which
are solely visual, to one and the same focus. The same reflecting instrument
may therefore be used either for looking at the heavens or for taking
pictures on a photographic plate which has been substituted for the
observer's eye.

A simple portrait camera has been advantageously employed for
obtaining striking photographs of larger areas of the sky than can be
grasped in a long telescope; but for purposes of accurate measurement those
taken with the latter are incomparably better.

It is needless to say that the photographic apparatus, whatever it may be,
must be driven by delicately-adjusted clockwork to counteract the apparent
daily motion of the stars caused by the rotation of the earth. The picture
would otherwise be spoiled, just as a portrait is ruined if the sitter does not
remain quiet during the exposure.

Among the observatories in the United Kingdom the Royal Observatory
at Greenwich is of course the most famous. It is specially remarkable
among all the similar institutions in the world for the continuity of its
labours for several generations. Greenwich Observatory was founded in
1675 for the promotion of astronomy and navigation, and the observations
have from the first been specially arranged with the object of determining
with the greatest accuracy the positions of the principal fixed stars, the sun,
the moon, and the planets. In recent years, however, great developments of
the work of the Observatory have been witnessed, and the most modern
branches of the science are now assiduously pursued there.

The largest equatorial at Greenwich is a refractor of twenty-eight inches
aperture and twenty-eight feet long, constructed by Sir Howard Grubb. A
remarkable composite instrument from the same celebrated workshop has
also been recently added to our national institution. It consists of a great
refractor specially constructed for photography, of twenty-six inches
aperture (presented by Sir Henry Thompson) and a reflector of thirty inches
diameter, which is the product of Dr. Common's skill. The huge volume
published annually bears witness to the assiduity with which the
Astronomer Royal and his numerous staff of assistant astronomers make
use of the splendid means at their disposal.

The southern part of the heavens, most of which cannot be seen in this
country, is watched from various observatories in the southern hemisphere.
Foremost among them is the Royal Observatory at the Cape of Good Hope,
which is furnished with first-class instruments. We may mention a great
photographic telescope, the gift of Mr. M'Clean. Astronomy has been
greatly enriched by the many researches made by Dr. Gill, the director of
the Cape Observatory.

Fig. 9.—The Great Bear.

It is not, however, necessary to use such great instruments to obtain
some idea of the aid the telescope will afford. The most suitable instrument
for commencing astronomical studies is within ordinary reach. It is the
well-known binocular that a captain uses on board ship; or if that cannot be
had, then the common opera-glass will answer nearly as well. This is, no
doubt, not so powerful as a telescope, but it has some compensating
advantages. The opera-glass will enable us to survey a large region of the
sky at one glance, while a telescope, generally speaking, presents a much
smaller field of view.

Let us suppose that the observer is provided with an opera-glass and is
about to commence his astronomical studies. The first step is to become
acquainted with the conspicuous group of seven stars represented in Fig. 9.
This group is often called the Plough, or Charles's Wain, but astronomers
prefer to regard it as a portion of the constellation of the Great Bear (Ursa
Major). There are many features of interest in this constellation, and the
beginner should learn as soon as possible to identify the seven stars which
compose it. Of these the two marked α and β, at the head of the Bear, are
generally called the "pointers." They are of special use, because they serve
to guide the eye to that most important star in the whole sky, known as the
"pole star."

Fix the attention on that region in the Great Bear, which forms a sort of
rectangle, of which the stars α β γ δ are the corners. The next fine night try
to count how many stars are visible within that rectangle. On a very fine
night, without a moon, perhaps a dozen might be perceived, or even more,
according to the keenness of the eyesight. But when the opera-glass is
directed to the same part of the constellation an astonishing sight is
witnessed. A hundred stars can now be seen with the greatest ease.

But the opera-glass will not show nearly all the stars in this region. Any
good telescope will reveal many hundreds too faint for the feebler
instrument. The greater the telescope the more numerous the stars: so that

seen through one of the colossal instruments the number would have to be
reckoned in thousands.

We have chosen the Great Bear because it is more generally known than
any other constellation. But the Great Bear is not exceptionally rich in stars.
To tell the number of the stars is a task which no man has accomplished; but
various estimates have been made. Our great telescopes can probably show
at least 50,000,000 stars.

The student who uses a good refracting telescope, having an object-glass
not less than three inches in diameter, will find occupation for many a fine
evening. It will greatly increase the interest of his work if he have the
charming handbook of the heavens known as Webb's "Celestial Objects for
Common Telescopes."

CHAPTER II.
THE SUN.
The vast Size of the Sun—Hotter than Melting Platinum—Is the Sun the Source of Heat
for the Earth?—The Sun is 92,900,000 miles distant—How to realise the magnitude
of this distance—Day and Night—Luminous and Non-Luminous Bodies—Contrast
between the Sun and the Stars—The Sun a Star—Granulated Appearance of the Sun
—The Spots on the Sun—Changes in the Form of a Spot—The Faculæ—The
Rotation of the Sun on its Axis—View of a Typical Sun-Spot—Periodicity of the Sun-
Spots—Connection between the Sun-Spots and Terrestrial Magnetism—Principles of
Spectrum Analysis—Substances present in the Sun—Spectrum of a Spot—The
Prominences surrounding the Sun—Total Eclipse of the Sun—Size and Movement of
the Prominences—Their connection with the Spots—Spectroscopic Measurement of
Motion on the Sun—The Corona surrounding the Sun—Constitution of the Sun.

In commencing our examination of the orbs which surround us, we
naturally begin with our peerless sun. His splendid brilliance gives him the
pre-eminence over all other celestial bodies.

The dimensions of our luminary are commensurate with his importance.
Astronomers have succeeded in the difficult task of ascertaining the exact
figures, but they are so gigantic that the results are hard to realise. The
diameter of the orb of day, or the length of the axis, passing through the
centre from one side to the other, is 866,000 miles. Yet this bare statement
of the dimensions of the great globe fails to convey an adequate idea of its
vastness. If a railway were laid round the sun, and if we were to start in an
express train moving sixty miles an hour, we should have to travel for five
years without intermission night or day before we had accomplished the
journey.

When the sun is compared with the earth the bulk of our luminary
becomes still more striking. Suppose his globe were cut up into one million
parts, each of these parts would appreciably exceed the bulk of our earth.
Fig. 10 exhibits a large circle and a very small one, marked S and E

respectively. These circles show the comparative sizes of the two bodies.
The mass of the sun does not, however, exceed that of the earth in the same
proportion. Were the sun placed in one pan of a mighty weighing balance,
and were 300,000 bodies as heavy as our earth placed in the other, the
luminary would turn the scale.

The sun has a
temperature far
surpassing any
that we
artificially
produce, either in
our chemical
laboratories or
our metallurgical
establishments.
We can send a
galvanic current
through a piece
of platinum wire.
The wire first
becomes red hot,
Fig. 10.—Comparative Size of the Earth and the Sun. then white hot;
then it glows
with a brilliance
almost dazzling until it fuses and breaks. The temperature of the melting
platinum wire could hardly be surpassed in the most elaborate furnaces, but
it does not attain the temperature of the sun.

It must, however, be admitted that there is an apparent discrepancy
between a fact of common experience and the statement that the sun
possesses the extremely high temperature that we have just tried to
illustrate. "If the sun were hot," it has been said, "then the nearer we
approach to him the hotter we should feel; yet this does not seem to be the

case. On the top of a high mountain we are nearer to the sun, and yet
everybody knows that it is much colder up there than in the valley beneath.
If the mountain be as high as Mont Blanc, then we are certainly two or three
miles nearer the glowing globe than we were at the sea-level; yet, instead of
additional warmth, we find eternal snow." A simple illustration may help to
lessen this difficulty. In a greenhouse on a sunshiny day the temperature is
much hotter than it is outside. The glass will permit the hot sunbeams to
enter, but it refuses to allow them out again with equal freedom, and
consequently the temperature rises. The earth may, from this point of view,
be likened to a greenhouse, only, instead of the panes of glass, our globe is
enveloped by an enormous coating of air. On the earth's surface, we stand,
as it were, inside the greenhouse, and we benefit by the interposition of the
atmosphere; but when we climb very high mountains, we gradually pass
through some of the protecting medium, and then we suffer from the cold.
If the earth were deprived of its coat of air, it seems certain that eternal frost
would reign over whole continents as well as on the tops of the mountains.

The actual distance of the sun from the earth is about 92,900,000 miles;
but by merely reciting the figures we do not receive a vivid impression of
the real magnitude. It would be necessary to count as quickly as possible for
three days and three nights before one million was completed; yet this
would have to be repeated nearly ninety-three times before we had counted
all the miles between the earth and the sun.

Every clear night we see a vast host of stars scattered over the sky. Some
are bright, some are faint, some are grouped into remarkable forms. With
regard to this multitude of brilliant points we have now to ask an important
question. Are they bodies which shine by their own light like the sun, or do
they only shine with borrowed light like the moon? The answer is easily
stated. Most of those bodies shine by their own light, and they are properly
called stars.

Suppose that the sun and the multitude of stars, properly so called, are
each and all self-luminous brilliant bodies, what is the great distinction

between the sun and the stars? There is, of course, a vast and obvious
difference between the unrivalled splendour of the sun and the feeble
twinkle of the stars. Yet this distinction does not necessarily indicate that
our luminary has an intrinsic splendour superior to that of the stars. The fact
is that we are nestled up comparatively close to the sun for the benefit of his
warmth and light, while we are separated from even the nearest of the stars
by a mighty abyss. If the sun were gradually to retreat from the earth, his
light would decrease, so that when he had penetrated the depths of space to
a distance comparable with that by which we are separated from the stars,
his glory would have utterly departed. No longer would the sun seem to be
the majestic orb with which we are familiar. No longer would he be a
source of genial heat, or a luminary to dispel the darkness of night. Our
great sun would have shrunk to the insignificance of a star, not so bright as
many of those which we see every night.

Momentous indeed is the conclusion to which we are now led. That
myriad host of stars which studs our sky every night has been elevated into
vast importance. Each one of those stars is itself a mighty sun, actually
rivalling, and in many cases surpassing, the splendour of our own luminary.
We thus open up a majestic conception of the vast dimensions of space, and
of the dignity and splendour of the myriad globes by which that space is
tenanted.

There is another aspect of the picture not without its utility. We must
from henceforth remember that our sun is only a star, and not a particularly
important star. If the sun and the earth, and all which it contains, were to
vanish, the effect in the universe would merely be that a tiny star had ceased
its twinkling. Viewed simply as a star, the sun must retire to a position of
insignificance in the mighty fabric of the universe. But it is not as a star that
we have to deal with the sun. To us his comparative proximity gives him an
importance incalculably transcending that of all the other stars. We
imagined ourselves to be withdrawn from the sun to obtain his true
perspective in the universe; let us now draw near, and give him that
attention which his supreme importance to us merits.

To the
unaided eye the
sun appears to be
a flat circle. If,
however, it be
examined with
the telescope,
taking care of
course to
interpose a piece
of dark-coloured
glass, or to
employ some
similar
precaution to
screen the eye
from injury, it
will then be
perceived that
Fig. 11.—The Sun, photographed on September 22,
the sun is not a
1870.
flat surface, but a
veritable glowing
globe.

The first question which we must attempt to answer enquires whether
the glowing matter which forms the globe is a solid mass, or, if not solid,
which is it, liquid or gaseous? At the first glance we might think that the sun
cannot be fluid, and we might naturally imagine that it was a solid ball of
some white-hot substance. But this view is not correct; for we can show that
the sun is certainly not a solid body in so far at least as its superficial parts
are concerned.

A general view of the sun as shown by a telescope of moderate
dimensions may be seen in Fig. 11, which is taken from a photograph

obtained by Mr. Rutherford at New York on the 22nd of September, 1870. It
is at once seen that the surface of the luminary is by no means of uniform
texture or brightness. It may rather be described as granulated or mottled.
This appearance is due to the luminous clouds which float suspended in a
somewhat less luminous layer of gas. It is needless to say that these solar
clouds are very different from the clouds which we know so well in our
own atmosphere. Terrestrial clouds are, of course, formed from minute
drops of water, while the clouds at the surface of the sun are composed of
drops of one or more chemical elements at an exceedingly high
temperature.

The granulated appearance of the solar surface is beautifully shown in
the remarkable photographs on a large scale which M. Janssen, of Meudon,
has succeeded in obtaining during the last twenty years. We are enabled to
reproduce one of them in Fig. 12. It will be observed that the interstices
between the luminous dots are of a greyish tint, the general effect (as
remarked by Professor Young) being much like that of rough drawing paper
seen from a little distance. We often notice places over the surface of such a
plate where the definition seems to be unsatisfactory. These are not,
however, the blemishes that might at first be supposed. They arise neither
from casual imperfections of the photographic plate nor from accidents
during the development; they plainly owe their origin to some veritable
cause in the sun itself, nor shall we find it hard to explain what that cause
must be. As we shall have occasion to mention further on, the velocities
with which the glowing gases on the sun are animated must be exceedingly
great. Even in the hundredth part of a second (which is about the duration of
the exposure of this plate) the movements of the solar clouds are
sufficiently great to produce the observed indistinctness.

Fig. 12.—Photograph of the Solar Surface.
(By Janssen.)

Irregularly dispersed over the solar surface small dark objects called sun-
spots are generally visible. These spots vary greatly both as to size and as to
number. Sun-spots were first noticed in the beginning of the seventeenth
century, shortly after the invention of the telescope. Their general
appearance is shown in Fig. 13, in which the dark central nucleus appears in
sharp contrast with the lighter margin or penumbra. Fig. 16 shows a small
spot developing out of one of the pores or interstices between the granules.

The earliest observers of these spots had
remarked that they seem to have a common
motion across the sun. In Fig. 14 we give a copy
of a remarkable drawing by Father Scheiner,
showing the motion of two spots observed by
him in March, 1627. The figure indicates the
successive positions assumed by the spots on the
several days from the 2nd to the 16th March.
Fig. 13.—An Ordinary Those marks which are merely given in outline
Sun-spot. represent the assumed positions on the 11th and
the 13th, on which days it happened that the
weather was cloudy, so that no observations could be made. It is invariably
found that these objects move in the same direction—namely, from the
eastern to the western limb[3] of the sun. They complete the journey across
the face of the sun in twelve or thirteen days, after which they remain
invisible for about the same length of time until they reappear at the eastern
limb. These early observers were quick to discern the true import of their
discovery. They deduced from these simple observations the remarkable
fact that the sun, like the earth, performs a rotation on its axis, and in the
same direction. But there is the important difference between these rotations
that whereas the earth takes only twenty-four hours to turn once round, the
solar globe takes about twenty-six days to complete one of its much more
deliberate rotations.

PLATE III.
SPOTS AND FACULÆ ON THE SUN.
(FROM A PHOTOGRAPH BY MR. WARREN DE LA RUE, 20TH
SEPT., 1861.)
If we examine sun-spots under favourable atmospheric conditions and
with a telescope of fairly large aperture, we perceive a great amount of
interesting detail which is full of information with regard to the structure of
the sun. The penumbra of a spot is often found to be made up of filaments
directed towards the middle of the spot, and generally brighter at their inner
ends, where they adjoin the nucleus. In a regularly formed spot the outline
of the penumbra is of the same general form as that of the nucleus, but
astronomers are frequently deeply interested by witnessing vast spots of
very irregular figure. In such cases the bright surface-covering of the sun
(the photosphere, as it is called) often encroaches on the nucleus and forms
a peninsula stretching out into, or even bridging across, the gloomy interior.
This is well shown in Professor Langley's fine drawing (Plate II.) of a very
irregular spot which he observed on December 23–24, 1873.

The details of a spot vary continually; changes may often be noticed
even from day to day, sometimes from hour to hour. A similar remark may
be made with respect to the bright streaks or patches which are frequently to
be observed especially in the neighbourhood of spots. These bright marks
are known by the name of faculæ (little torches). They are most distinctly
seen near the margin of the sun, where the light from its surface is not so
bright as it is nearer to the centre of the disc. The reduction of light at the
margin is due to the greater thickness of absorbing atmosphere round the
sun, through which the light emitted from the regions near the margin has to
pass in starting on its way towards us.

None of the markings on the solar disc constitute permanent features on
the sun. Some of these objects may no doubt last for weeks. It has, indeed,
occasionally happened that the same spot has marked the solar globe for
many months; but after an existence of greater or less duration those on one
part of the sun may disappear, while as frequently fresh marks of the same
kind become visible in other places. The inference from these various facts
is irresistible. They tell us that the visible surface of the sun is not a solid
mass, is not even a liquid mass, but that the globe, so far as we can see it,
consists of matter in the gaseous, or vaporous, condition.

It often happens that a large spot divides into two or more separate
portions, and these have been sometimes seen to fly apart with a velocity in
some cases not less than a thousand miles an hour. "At times, though very
rarely" (I quote here Professor Young,[4] to whom I am frequently
indebted), "a different phenomenon of the most surprising and startling
character appears in connection with these objects: patches of intense
brightness suddenly break out, remaining visible for a few minutes, moving,
while they last, with velocities as great as one hundred miles a second."

"One of these events has become classical. It occurred on the forenoon
(Greenwich time) of September 1st, 1859, and was independently witnessed
by two well-known and reliable observers—Mr. Carrington and Mr.
Hodgson—whose accounts of the matter may be found in the Monthly

Notices of the
Royal
Astronomical
Society for
November, 1859.
Mr. Carrington at
the time was
making his usual
daily
observations
upon the
position,
configuration,
and size of the
spots by means
of an image of
the solar disc
upon a screen—
being then Fig. 14.—Scheiner's Observations on Sun-spots.

engaged upon that eight years'
series of observations which lie at
the foundation of so much of our
present solar science. Mr. Hodgson,
at a distance of many miles, was at
the same time sketching details of
sun-spot structure by means of a
solar eye-piece and shade-glass.
They simultaneously saw two
luminous objects, shaped something
like two new moons, each about
eight thousand miles in length and
Fig. 15.—Zones on the Sun's Surface two thousand wide, at a distance of

in which Spots appear. some twelve thousand miles from
each other. These burst suddenly
into sight at the edge of a great sun-spot with a dazzling brightness at least
five or six times that of the neighbouring portions of the photosphere, and
moved eastward over the spot in parallel lines, growing smaller and fainter,
until in about five minutes they disappeared, after traversing a course of
nearly thirty-six thousand miles."

The sun-spots do not occur at all parts of the sun's surface indifferently.
They are mainly found in two zones (Fig. 15) on each side of the solar
equator between the latitudes of 10° and 30°. On the equator the spots are
rare except, curiously enough, near the time when there are few spots
elsewhere. In high latitudes they are never seen. Closely connected with
these peculiar principles of their distribution is the remarkable fact that
spots in different latitudes do not indicate the same values for the period of
rotation of the sun. By watching a spot near the sun's equator Carrington
found that it completed a revolution in twenty-five days and two hours. At a
latitude of 20° the period is about twenty-five days and eighteen hours, at
30° it is no less than twenty-six days and twelve hours, while the
comparatively few spots observed in the latitude of 45° require twenty-
seven and a half days to complete their circuit.

As the sun, so far at least as its outer regions are concerned, is a mass of
gas and not a solid body, there would be nothing incredible in the
supposition that spots are occasionally endowed with movements of their
own like ships on the ocean. It seems, however, from the facts before us that
the different zones on the sun, corresponding to what we call the torrid and
temperate zones on the earth, persist in rotating with velocities which
gradually decrease from the equator towards the poles. It seems probable
that the interior parts of the sun do not rotate as if the whole were a rigidly
connected mass. The mass of the sun, or at all events its greater part, is
quite unlike a rigid body, and the several portions are thus to some extent
free for independent motion. Though we cannot actually see how the
interior parts of the sun rotate, yet here the laws of dynamics enable us to

infer that the interior layers of the sun rotate more rapidly than the outer
layers, and thus some of the features of the spot movements can be
accounted for. But at present it must be confessed that there are great
difficulties in the way of accounting for the distribution of spots and the law
of rotation of the sun.

In the year 1826 Schwabe, a German astronomer, commenced to keep a
regular register of the number of spots visible on the sun. After watching
them for seventeen years he was able to announce that the number of spots
seemed to fluctuate from year to year, and that there was a period of about
ten years in their changes. Subsequent observations have confirmed this
discovery, and old books and manuscripts have been thoroughly searched
for information of early date. Thus a more or less complete record of the
state of the sun as regards spots since the beginning of the seventeenth
century has been put together. This has enabled astronomers to fix the
period of the recurring maximum with greater accuracy.

The course of one of the sun-spot cycles may be described as follows:
For two or three years the spots are both larger and more numerous than on
the average; then they begin to diminish, until in about six or seven years
from the maximum they decline to a minimum; the number of the spots
then begins to increase, and in about four and a half years the maximum is
once more attained. The length of the cycle is, on an average, about eleven
years and five weeks, but both its length and the intensity of the maxima
vary somewhat. For instance, a great maximum occurred in the summer of
1870, after which a very low minimum occurred in 1879, followed by a
feeble maximum at the end of 1883; next came an average minimum about
August, 1889, followed by the last observed maximum in January, 1894. It
is not unlikely that a second period of about sixty or eighty years affects the
regularity of the eleven-year period. Systematic observations carried on
through a great many years to come will be required to settle this question,
as the observations of sun-spots previous to 1826 are far too incomplete to
decide the issues which arise.

A curious connection seems to exist between the periodicity of the spots
and their distribution over the surface of the sun. When a minimum is about
to pass away the spots generally begin to show themselves in latitudes
about 30° north and south of the sun's equator; they then gradually break
out somewhat nearer to the equator, so that at the time of maximum
frequency most of them appear at latitudes not greater than 16°. This
distance from the sun's equator goes on decreasing till the time of
minimum. Indeed, the spots linger on very close to the equator for a couple
of years more, until the outbreak signalising the commencement of another
period has commenced in higher latitudes.

We have still to note an extraordinary feature which points to an intimate
connection between the phenomena of sun-spots and the purely terrestrial
phenomena of magnetism. It is of course well known that the needle of a
compass does not point exactly to the north, but diverges from the meridian
by an angle which is different in different places and is not even constant at
the same place. For instance, at Greenwich the needle at present points in a
direction 17° West of North, but this amount is subject to very slow and
gradual changes, as well as to very small daily oscillations. It was found
about fifty years ago by Lamont (a Bavarian astronomer, but a native of
Scotland) that the extent of this daily oscillation increases and decreases
regularly in a period which he gave as 10-1⁄3 years, but which was
subsequently found to be 11-1⁄10 years, exactly the same as the period of the
spots on the sun. From a diligent study of the records of magnetic
observations it has been found that the time of sun-spot maximum always
coincides almost exactly with that of maximum daily oscillation of the
compass needle, while the minima agree similarly. This close relationship
between the periodicity of sun-spots and the daily movements of the
magnetic needle is not the sole proof we possess that there is a connection
of some sort between solar phenomena and terrestrial magnetism. A time of
maximum sun-spots is a time of great magnetic activity, and there have
even been special cases in which a peculiar outbreak on the sun has been
associated with remarkable magnetic phenomena on the earth. A very

interesting instance of this kind is recorded by Professor Young, who, when
observing at Sherman on the 3rd August, 1872, perceived a very violent
disturbance of the sun's surface. He was told the same day by a member of
his party, who was engaged in magnetic observations and who was quite in
ignorance of what Professor Young had seen, that he had been obliged to
desist from his magnetic work in consequence of the violent motion of his
magnet. It was afterwards found from the photographic records at
Greenwich and Stonyhurst that the magnetic "storm" observed in America
had simultaneously been felt in England. A similar connection between sun-
spots and the aurora borealis has also been noticed, this fact being a natural
consequence of the well-known connection between the aurora and
magnetic disturbances. On the other hand, it must be confessed that many
striking magnetic storms have occurred without any corresponding solar
disturbance,[5] but even those who are inclined to be sceptical as to the
connection between these two classes of phenomena in particular cases can
hardly doubt the remarkable parallelism between the general rise and fall in
the number of sun-spots and the extent of the daily movements of the
compass needle.

Fig. 16.—The Texture of the Sun and a small Spot.
We have now described the principal solar phenomena with which the
telescope has made us acquainted. But there are many questions connected
with the nature of the sun which not even the most powerful telescope
would enable us to solve, but which the spectroscope has given us the
means of investigating.

What we receive from the sun is warmth and light. The intensely heated
mass of the sun radiates forth its beams in all directions with boundless
prodigality. Each beam we feel to be warm, and we see to be brilliantly
white, but a more subtle analysis than mere feeling or mere vision is
required. Each sunbeam bears marks of its origin. These marks are not
visible until a special process has been applied, but then the sunbeam can be
made to tell its story, and it will disclose to us much of the nature of the
constitution of the great luminary.

We regard the sun's light as colourless, just as we speak of water as
tasteless, but both of those expressions relate rather to our own feelings than

to anything really characteristic of water or of sunlight. We regard the
sunlight as colourless because it forms, as it were, the background on which
all other colours are depicted. The fact is, that white is so far from being
colourless that it contains every known hue blended together in certain
proportions. The sun's light is really extremely composite; Nature herself
tells us this if we will but give her the slightest attention. Whence come the
beautiful hues with which we are all familiar? Look at the lovely tints of a
garden; the red of the rose is not in the rose itself. All the rose does is to
grasp the sunbeams which fall upon it, extract from these beams the red
which they contain, and radiate that red light to our eyes. Were there not red
rays conveyed with the other rays in the sunbeam, there could be no red
rose to be seen by sunlight.

The principle here involved has many other applications; a lady will
often say that a dress which looks very well in the daylight does not answer
in the evening. The reason is that the dress is intended to show certain
colours which exist in the sunlight; but these colours are not contained to
the same degree in gaslight, and consequently the dress has a different hue.
The fault is not in the dress, the fault lies in the gas; and when the electric
light is used it sends forth beams more nearly resembling those from the
sun, and the colours of the dress appear with all their intended beauty.

The most glorious natural indication of the nature of the sunlight is seen
in the rainbow. Here the sunbeams are refracted and reflected from tiny
globes of water in the clouds; these convey to us the sunlight, and in doing
so decompose the white beams into the seven primary hues—red, orange,
yellow, green, blue, indigo, and violet.

PLATE A.
THE SUN.
Royal Observatory, Greenwich, July 8, 1892.

The bow set in the cloud is typical of that great department of modern
science of which we shall now set forth the principles. The globes of water
decompose the solar beams; and we follow the course suggested by the
rainbow, and analyse the sunlight into its constituents. We are enabled to do
this with scientific accuracy when we employ that remarkable key to
Nature's secrets known as the spectroscope. The beams of white sunlight

consist of innumerable beams of
every hue in intimate association.
Every shade of red, of yellow, of
blue, and of green, can be found in
a sunbeam. The magician's wand,
with which we strike the sunbeam
and sort the tangled skein into
Fig. 17.—The Prism.
perfect order, is the simple
instrument known as the glass
prism. We have represented this instrument in its simplest form in the
adjoining figure (Fig. 17). It is a piece of pure and homogeneous glass in
the shape of a wedge. When a ray of light from the sun or from any source
falls upon the prism, it passes through the transparent glass and emerges on
the other side; a remarkable change is, however, impressed on the ray by the
influence of the glass. It is bent by refraction from the path it originally
pursued, and is compelled to follow a different path. If, however, the prism
bent all rays of light equally, then it would be of no service in the analysis
of light; but it fortunately happens that the prism acts with varying
efficiency on the rays of different hues. A red ray is not refracted so much
as a yellow ray; a yellow ray is not refracted so much as a blue one. It
consequently happens that when the composite beam of sunlight, in which
all the different rays are blended, passes through the prism, they emerge in
the manner shown in the annexed figure (Fig. 18). Here then we have the
source of the analysing power of the prism; it bends the different hues
unequally and consequently the beam of composite sunlight, after passing
through the prism, no longer shows mere white light, but is expanded into a
coloured band of light, with hues like the rainbow, passing from deep red at
one end through every intermediate grade to the violet.

Fig. 18.—Dispersion of Light by the Prism.
We have in the prism the means of decomposing the light from the sun,
or the light from any other source, into its component parts. The
examination of the quality of the light when analysed enables us to learn
something of the constitution of the body from which this light has
emanated. Indeed, in some simple cases the mere colour of a light will be
sufficient to indicate the source from which it has come. There is, for
instance, a splendid red light sometimes seen in displays of fireworks, due
to the metal strontium. The eye can identify the element by the mere colour
of the flame. There is also a characteristic yellow light produced by the
flame of common salt burned with spirits of wine. Sodium is the important
constituent of salt, so here we recognise another substance merely by the
colour it emits when burning. We may also mention a third substance,
magnesium, which burns with a brilliant white light, eminently
characteristic of the metal.

PLATE XIII.
SPECTRA OF THE SUN AND STARS.
I. SUN.
II. SIRIUS.
III. ALDEBARAN.
IV. BETELGEUZE.
The three metals, strontium, sodium, and magnesium, may thus be
identified by the colours they produce when incandescent. In this simple
observation lies the germ of the modern method of research known as
spectrum analysis. We may now examine with the prism the colours of the
sun and the colours of the stars, and from this examination we can learn
something of the materials which enter into their composition. We are not
restricted to the use of merely a single prism, but we may arrange that the
light which it is desired to analyse shall pass through several prisms in
succession in order to increase the dispersion or the spreading out of the
different colours. To enter the spectroscope the light first passes through a
narrow slit, and the rays are then rendered parallel by passing through a
lens; these parallel rays next pass through one or more prisms, and are
finally viewed through a small telescope, or they may be intercepted by a
photographic plate on which a picture will then be made. If the beam of
light passing through the slit has radiated from an incandescent solid or

liquid body, or from a gas under high pressure, the coloured band or
spectrum is found to contain all the colours indicated on Plate XIII., without
any interruption between the colours. This is known as a continuous
spectrum. But if we examine light from a gas under low pressure, as can be
done by placing a small quantity of the gas in a glass tube and making it
glow by an electric current, we find that it does not emit rays of all colours,
but only rays of certain distinct colours which are different for different
gases. The spectrum of a gas, therefore, consists of a number of detached
luminous lines.

When we study the sunlight through the prism, it is found that the
spectrum does not extend quite continuously from one end to the other, but
is shaded over by a multitude of dark lines, only a few of which are shown
in the adjoining plate. (Plate XIII.) These lines are a permanent feature in
the solar spectrum. They are as characteristic of the sunlight as the
prismatic colours themselves, and are full of interest and information with
regard to the sun. These lines are the characters in which the history and the
nature of the sun are written. Viewed through an instrument of adequate
power, dark lines are to be found crossing the solar spectrum in hundreds
and in thousands. They are of every variety of strength and faintness; their
distribution seems guided by no simple law. At some parts of the spectrum
there are but few lines; in other regions they are crowded so closely
together that it is difficult to separate them. They are in some places
exquisitely fine and delicate, and they never fail to excite the admiration of
every one who looks at this interesting spectacle in a good instrument.

There can be no better method of expounding the rather difficult subject
of spectrum analysis than by actually following the steps of the original
discovery which first gave a clear demonstration of the significance of the
dark "Fraunhofer" lines. Let us concentrate our attention specially upon that
line of the solar spectrum marked d. This, when seen in the spectroscope, is
found to consist of two lines, very delicately separated by a minute interval,
one of these lines being slightly thicker than the other. Suppose that while
the attention is concentrated on these lines the flame of an ordinary spirit-

lamp coloured by common salt be held in front of the instrument, so that the
ray of direct solar light passes through the flame before entering the
spectroscope. The observer sees at once the two lines known as d flash out
with a greatly increased blackness and vividness, while there is no other
perceptible effect on the spectrum. A few trials show that this intensification
of the d lines is due to the vapour of sodium arising from the salt burning in
the lamp through which the sunlight has passed.

It is quite impossible that this marvellous connection between sodium
and the d lines of the spectrum can be merely casual. Even if there were
only a single line concerned, it would be in the highest degree unlikely that
the coincidence should arise by accident; but when we find the sodium
affecting both of the two close lines which form d, our conviction that there
must be some profound connection between these lines and sodium rises to
absolute certainty. Suppose that the sunlight be cut off, and that all other
light is excluded save that emanating from the glowing vapour of sodium in
the spirit flame. We shall then find, on looking through the spectroscope,
that we no longer obtain all the colours of the rainbow; the light from the
sodium is concentrated into two bright yellow lines, filling precisely the
position which the dark d lines occupied in the solar spectrum, and the
darkness of which the sodium flame seemed to intensify.

We must here endeavour to remove what may at first sight appear to be a
paradox. How is it, that though the sodium flame produces two bright lines
when viewed in the absence of other light, yet it actually appears to
intensify the two dark lines in the sun's spectrum? The explanation of this
leads us at once to the cardinal doctrine of spectrum analysis. The so-called
dark lines in the solar spectrum are only dark by contrast with the brilliant
illumination of the rest of the spectrum. A good deal of solar light really lies
in the dark lines, though not enough to be seen when the eye is dazzled by
the brilliancy around. When the flame of the spirit-lamp charged with
sodium intervenes, it sends out a certain amount of light, which is entirely
localised in these two lines. So far it would seem that the influence of the
sodium flame ought to be manifested in diminishing the darkness of the

lines and rendering them less conspicuous. As a matter of fact, they are far
more conspicuous with the sodium flame than without it. This arises from
the fact that the sodium flame possesses the remarkable property of cutting
off the sunlight which was on its way to those particular lines; so that,
though the sodium contributes some light to the lines, yet it intercepts a far
greater quantity of the light that would otherwise have illuminated those
lines, and hence they became darker with the sodium flame than without it.

We are thus conducted to a remarkable principle, which has led to the
interpretation of the dark lines in the spectrum of the sun. We find that when
the sodium vapour is heated, it gives out light of a very particular type,
which, viewed through the prism, is concentrated in two lines. But the
sodium vapour possesses also this property, that light from the sun can pass
through it without any perceptible absorption, except of those particular
rays which are of the same characters as the two lines in question. In other
words, we say that if the heated vapour of a substance gives a spectrum of
bright lines, corresponding to lights of various kinds, this same vapour will
act as an opaque screen to lights of those special kinds, while remaining
transparent to light of every other description.

This principle is of such importance in the theory of spectrum analysis
that we add a further example. Let us take the element iron, which in a very
striking degree illustrates the law in question. In the solar spectrum some
hundreds of the dark lines are known to correspond with the spectrum of
iron. This correspondence is exhibited in a vivid manner when, by a suitable
contrivance, the light of an electric spark from poles of iron is examined in
the spectroscope side by side with the solar spectrum. The iron lines in the
sun are identical in position with the lines in the spectrum of glowing iron
vapour. But the spectrum of iron, as here described, consists of bright lines;
while those with which it is compared in the sun are dark on a bright
background. They can be completely understood if we suppose the vapour
arising from intensely heated iron to be present in the atmosphere which
surrounds the luminous strata on the sun. This vapour would absorb or stop
precisely the same rays as it emits when incandescent, and hence we learn

the important fact that iron, no less than sodium, must, in one form or
another, be a constituent of the sun.

Such is, in brief outline, the celebrated discovery of modern times which
has given an interpretation to the dark lines of the solar spectrum. The
spectra of a large number of terrestrial substances have been examined in
comparison with the solar spectrum, and thus it has been established that
many of the elements known on the earth are present in the sun. We may
mention calcium, iron, hydrogen, sodium, carbon, nickel, magnesium,
cobalt, aluminium, chromium, strontium, manganese, copper, zinc,
cadmium, silver, tin, lead, potassium. Some of the elements which are of
the greatest importance on the earth would appear to be missing from the
sun. Sulphur, phosphorus, mercury, gold, nitrogen may be mentioned
among the elements which have hitherto given no indication of their being
solar constituents.

It is also possible that the lines of a substance in the sun's atmosphere
may be so very bright that the light of the continuous spectrum, on which
they are superposed, is not able to "reverse" them—i.e. turn them into dark
lines. We know, for instance, that the bright lines of sodium vapour may be
made so intensely bright that the spectrum of an incandescent lime-cylinder
placed behind the sodium vapour does not reverse these lines. If, then, we
make the sodium lines fainter, they may be reduced to exactly the intensity
prevailing in that part of the spectrum of the lime-light, in which case the
lines, of course, could not be distinguished. The question as to what
elements are really missing from the sun must therefore, like many other
questions concerning our great luminary, at present be considered an open
one. We shall shortly see that an element previously unknown has actually
been discovered by means of a line representing it in the solar spectrum.

Let us now return to the sun-spots and see what the spectroscope can
teach us as to their nature. We attach a powerful spectroscope to the eye-end
of a telescope in order to get as much light as possible concentrated on the
slit; the latter has therefore to be placed exactly at the focus of the object-

glass. The instrument is then pointed to a spot, so that its image falls on the
slit, and the presence of the dark central part called the umbra reveals itself
by a darkish stripe which traverses the ordinary sun-spectrum from end to
end. It is bordered on both sides by the spectrum of the penumbra, which is
much brighter than that of the umbra, but fainter than that of the adjoining
regions of the sun.

From the fact that the spectrum is darkened we learn that there is
considerable general absorption of light in the umbra. This absorption is
not, however, such as would be caused by the presence of volumes of
minute solid or liquid particles like those which constitute smoke or cloud.
This is indicated by the fact, first discovered by Young in 1883, that the
spectrum is not uniformly darkened as it would be if the absorption were
caused by floating particles. In the course of examination of many large and
quiescent spots, he perceived that the middle green part of the spectrum was
crossed by countless fine, dark lines, generally touching each other, but here
and there separated by bright intervals. Each line is thicker in the middle
(corresponding to the centre of the spot) and tapers to a fine thread at each
end; indeed, most of these lines can be traced across the spectrum of the
penumbra and out on to that of the solar surface. The absorption would
therefore seem to be caused by gases at a much lower temperature than that
of the gases present outside the spot.

In the red and yellow parts of the spot-spectrum, which have been
specially studied for many years by Sir Norman Lockyer at the South
Kensington Observatory, interesting details are found which confirm this
conclusion. Many of the dark lines are not thicker and darker in the spot
than they are in the ordinary sun-spectrum, while others are very much
thickened in the spot-spectrum, such as the lines of iron, calcium, and
sodium. The sodium lines are sometimes both widened and doubly reversed
—that is, on the thick dark line a bright line is superposed. The same
peculiarity is not seldom seen in the notable calcium lines h and k at the
violet end of the spectrum. These facts indicate the presence of great masses
of the vapours of sodium and calcium over the nucleus. The observations at

South Kensington have also brought to light another interesting peculiarity
of the spot-spectra. At the time of minimum frequency of spots the lines of
iron and other terrestrial elements are prominent among the most widened
lines; at the maxima these almost vanish, and the widening is found only
amongst lines of unknown origin.

The spectroscope has given us the means of studying other interesting
features on the sun, which are so faint that in the full blaze of sunlight they
cannot be readily observed with a mere telescope. We can, however, see
them easily enough when the brilliant body of the sun is obscured during
the rare occurrence of a total eclipse. The conditions necessary for the
occurrence of an eclipse will be more fully considered in the next chapter.
For the present it will be sufficient to observe that by the movement of the
moon it may so happen that the moon completely hides the sun, and thus for
certain parts of the earth produces what we call a total eclipse. The few
minutes during which a total eclipse lasts are of much interest to the
astronomer. Darkness reigns over the landscape, and in that darkness rare
and beautiful sights are witnessed.

We have in Fig. 19 a
diagram of a total
eclipse, showing some
of the remarkable
objects known as
prominences (a, b, c, d,
e) which project from
behind the dark body
of the moon. That they
do not belong to the
moon, but are solar
appendages of some
sort, is easily
demonstrated. They
first appear on the
eastern limb at the Fig. 19.—Prominences seen in Total Eclipse.
commencement of
totality. Those first seen are gradually more or less covered by the
advancing moon, while others peep out behind the western limb of the
moon, until totality is over and the sunlight bursts out again, when they all
instantly vanish.

The first total eclipse which occurred after the spectroscope had been
placed in the hands of astronomers was in 1868. On the 18th August in that
year a total eclipse was visible in India. Several observers, armed with
spectroscopes, were on the look-out for the prominences, and were able to
announce that their spectrum consisted of detached bright lines, thus
demonstrating that these objects were masses of glowing gas. On the
following day the illustrious astronomer, Janssen, one of the observers of
the eclipse, succeeded in seeing the lines in full sunlight, as he now knew
exactly where to look for them. Many months before the eclipse Sir
Norman Lockyer had been preparing to search for the prominences, as he
expected them to yield a line spectrum which would be readily visible, if

only the sun's ordinary light could be sufficiently winnowed away. He
proposed to effect this by using a spectroscope of great dispersion, which
would spread out the continuous spectrum considerably and make it fainter.
The effect of the great dispersion on the isolated bright lines he expected to
see would be only to widen the intervals between them without interfering
with their brightness. The new spectroscope, which he ordered to be
constructed for this purpose, was not completed until some weeks after the
eclipse was over, though before the news of Janssen's achievement reached
Europe from India. When that news did arrive Sir N. Lockyer had already
found the spectrum of unseen prominences at the sun's limb. The honour of
the practical application of a method of observing solar prominences
without the help of an eclipse must therefore be shared between the two
astronomers.

When a spectroscope is pointed to the margin of the sun so that the slit is
radial, certain short luminous lines become visible which lie exactly in the
prolongation of the corresponding dark lines in the solar spectrum. From
due consideration of the circumstances it can be shown that the gases which
form the prominences are also present as a comparatively shallow
atmospheric layer all round the great luminary. This layer is about five or
six thousand miles deep, and is situated immediately above the dense layer
of luminous clouds which forms the visible surface of the sun and which we
call the photosphere. The gaseous envelope from which the prominences
spring has been called the chromosphere on account of the coloured lines
displayed in its spectrum. Such lines are very numerous, but those
pertaining to the single substance, hydrogen, predominate so greatly that we
may say the chromosphere consists chiefly of this element. It is, however,
to be noted that calcium and one other element are also invariably present,
while iron, manganese and magnesium are often apparent. The remarkable
element, of which we have not yet mentioned the name, has had an
astonishing history.

During the eclipse of 1868 a fine yellow line was noticed among the
lines of the prominence spectrum, and it was not unnaturally at first

assumed that it must be the yellow sodium line. But when careful
observations were afterwards made without hurry in full sunshine, and
accurate measures were obtained, it was at once remarked that this line was
not identical with either of the components of the double sodium line. The
new line was, no doubt, quite close to the sodium lines, but slightly towards
the green part of the spectrum. It was also noticed there was not generally
any corresponding line to be seen among the dark lines in the ordinary solar
spectrum, though a fine dark one has now and then been detected,
especially near a sun-spot. Sir Norman Lockyer and Sir Edward Frankland
showed that this was not produced by any known terrestrial element. It was,
therefore, supposed to be caused by some hitherto unknown body to which
the name of helium, or the sun element, was given. About a dozen less
conspicuous lines were gradually identified in the spectrum of the
prominences and the chromosphere, which appeared also to be caused by
this same mysterious helium. These same remarkable lines have in more
recent years also been detected in the spectra of various stars.

This gas so long known in the heavens was at last detected on earth. In
April, 1895, Professor Ramsay, who with Lord Rayleigh had discovered the
new element argon, detected the presence of the famous helium line in the
spectrum of the gas liberated by heating the rare mineral known as cleveite,
found in Norway. Thus this element, the existence of which had first been
detected on the sun, ninety-three million miles away, has at last been proved
to be a terrestrial element also.

When it was announced by Runge that the principal line in the spectrum
of the terrestrial helium had a faint and very close companion line on the
red-ward side, some doubt seemed at first to be cast on the identity of the
new terrestrial gas discovered by Ramsay with the helium of the
chromosphere. The helium line of the latter had never been noticed to be
double. Subsequently, however, several observers provided with very
powerful instruments found that the famous line in the chromosphere really
had a very faint companion line. Thus the identity between the celestial
helium and the gas found on our globe was established in the most

remarkable manner. Certain circumstances have seemed to indicate that the
new gas might possibly be a mixture of two gases of different densities, but
up to the present this has not been proved to be the case.

After it had been found possible to see the spectra of prominences
without waiting for an eclipse, Sir W. Huggins, in an observation on the
13th of February, 1869, successfully applied a method for viewing the
remarkable solar objects themselves instead of their mere spectra in full
sunshine. It is only necessary to adjust the spectroscope so that one of the
brightest lines—e.g. the red hydrogen line—is in the middle of the field of
the viewing telescope, and then to open wide the slit of the spectroscope. A
red image of the prominence will then be displayed instead of the mere line.
In fact, when the slit is opened wide, the prisms produce a series of
detached images of the prominence under observation, one for each kind of
light which the object emits.

We have spoken of the spectroscope as depending upon the action of
glass prisms. It remains to be added that in the highest class of
spectroscopes the prisms are replaced by ruled gratings from which the light
is reflected. The effect of the ruling is to produce by what is known as
diffraction the required breaking up of the beam of light into its constituent
parts.

PLATE IV.

SOLAR PROMINENCES.
(DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN 1872.)

Majestic indeed are the proportions of some of those mighty
prominences which leap from the luminous surface; yet they flicker, as do
our terrestrial flames, when we allow them time comparable to their
gigantic dimensions. Drawings of the same prominence made at intervals of
a few hours, or even less, often show great changes. The magnitude of the
displacements that have been noticed sometimes attains many thousands of
miles, and the actual velocity with which such masses move frequently
exceeds 100 miles a second. Still more violent are the convulsions when,
from the surface of the chromosphere, as from a mighty furnace, vast
incandescent masses of gas are projected upwards. Plate IV. gives a view of
a number of prominences as seen by Trouvelot at Harvard College
Observatory, Cambridge, U.S.A. Trouvelot has succeeded in exhibiting in
the different pictures the wondrous variety of aspect which these objects
assume. The dimensions of the prominences may be inferred from the scale
appended to the plate. The largest of those here shown is fully 80,000 miles
high; and trustworthy observers have recorded prominences of an altitude
even much greater. The rapid changes which these objects sometimes
undergo are well illustrated in the two sketches on the left of the lowest
line, which were drawn on April 27th, 1872. These are both drawings of the
same prominence taken at an interval no greater than twenty minutes. This
mighty flame is so vast that its length is ten times as great as the diameter of
the earth, yet in this brief period it has completely changed its aspect; the
upper part of the flame has, indeed, broken away, and is now shown in that
part of the drawing between the two figures on the line above. The same
plate also shows various instances of the remarkable spike-like objects,
taken, however, at different times and at various parts of the sun. These
spikes attain altitudes not generally greater than 20,000 miles, though
sometimes they soar aloft to stupendous distances.

We may refer to one special object of this kind, the remarkable history of
which has been chronicled by Professor Young. On October 7th, 1880, a

prominence was seen, at about 10.30 a.m., on the south-east limb of the sun.
It was then about 40,000 miles high, and attracted no special attention. Half
an hour later a marvellous transformation had taken place. During that brief
interval the prominence became very brilliant and doubled its length. For
another hour the mighty flame still soared upwards, until it attained the
unprecedented elevation of 350,000 miles—a distance more than one-third
the diameter of the great luminary itself. At this climax the energy of the
mighty outbreak seems to have at last become exhausted: the flame broke
up into fragments, and by 12.30—an interval of only two hours from the
time when it was first noticed—the phenomenon had completely faded
away.

No doubt this particular eruption was exceptional in its vehemence, and
in the vastness of the changes of which it was an indication. The velocity of
upheaval must have been at least 200,000 miles an hour, or, to put it in
another form, more than fifty miles a second. This mighty flame leaped
from the sun with a velocity more than 100 times as great as that of the
swiftest bullet ever fired from a rifle.

The prominences may be generally divided into two classes. We have
first those which are comparatively quiescent, and in form somewhat
resemble the clouds which float in our earth's atmosphere. The second class
of prominences are best described as eruptive. They are, in fact, thrown up
from the chromosphere like gigantic jets of incandescent material. These
two classes of objects differ not only in appearance but also in the gases of
which they are composed. The cloud-like prominences consist mainly of
hydrogen, with helium and calcium, while many metals are present in the
eruptive discharges. The latter are never seen in the neighbourhood of the
sun's poles, but generally appear close to a sun-spot, thus confirming the
conclusion that the spots are associated with violent disturbances on the
surface of the sun. When a spot has reached the limb of the sun it is
frequently found to be surrounded by prominences. It has even been
possible in a few instances to detect powerful gaseous eruptions in the
neighbourhood of a spot, the spectroscope rendering them visible against

the background of the solar surface just as the prominences are observed at
the limb against the background of the sky.

In order to photograph a prominence we have, of course, to substitute a
photographic plate for the observer's eye. Owing, however, to the difficulty
of preventing the feeble light from the prominence from being overpowered
by extraneous light, the photography of these bodies was not very
successful until Professor Hale, of Chicago, designed his spectro-
heliograph. In this instrument there is (in addition to the usual slit through
which the light falls on the prisms, or grating,) a second slit immediately in
front of the photographic plate through which the light of a given wave-
length can be permitted to pass to the exclusion of all the rest. The light
chosen for producing an image of the prominences is that radiated in the
remarkable "k line," due to calcium. This lies at the extreme end of the
violet. The light from that part of the spectrum, though it is invisible to the
eye, is much more active photographically than the light from the red,
yellow, or green parts of the spectrum. The front slit is adjusted so that the k
line falls upon the second slit, and as the front slit is slowly swept by
clockwork over the whole of a prominence, the second slit keeps pace with
it by a mechanical contrivance.

If the image of the solar disc is hidden by a screen of exactly the proper
size, the slits may be made to sweep over the whole sun, thus giving us at
one exposure a picture of the chromospheric ring round the sun's limb with
its prominences. The screen may now be withdrawn, and the slits may be
made to sweep rapidly over the disc itself. They reveal the existence of
glowing calcium vapours in many parts of the surface of the sun. Thus we
get a striking picture of the sun as drawn by this particular light. In this
manner Professor Hale confirmed the observation made long before by
Professor Young, that the spectra of faculæ always show the two great
calcium bands.

The velocity with which a prominence shoots upward from the sun's
limb can, of course, be measured directly by observations of the ordinary

kind with a micrometer. The spectroscope, however, enables us to estimate
the speed with which disturbances at the surface of the sun travel in the
direction towards the earth or from the earth. We can measure this speed by
watching the peculiar behaviour of the spectral lines representing the
rapidly moving masses. This opens up a remarkable line of investigation
with important applications in many branches of astronomy.

It is, of course, now generally understood that the sensation of light is
caused by waves or undulations which impinge on the retina of the eye after
having been transmitted through that medium which we call the ether. To
the different colours correspond different wave-lengths—that is to say,
different distances between two successive waves. A beam of white light is
formed by the union of innumerable different waves whose lengths have
almost every possible value lying between certain limits. The wave-length
of red light is such that there are 33,000 waves in an inch, while that of
violet light is but little more than half that of red light. The position of a line
in the spectrum depends solely on the wave-length of the light to which it is
due. Suppose that the source of light is approaching directly towards the
observer; obviously the waves follow each other more closely than if the
source were at rest, and the number of undulations which his eye receives in
a second must be proportionately increased. Thus the distance between two
successive ether waves will be very slightly diminished. A well-known
phenomenon of a similar character is the change of pitch of the whistle of a
locomotive engine as it rushes past. This is particularly noticeable if the
observer happens to be in a train which is moving rapidly in the opposite
direction. In the case of sound, of course, the vibrations or waves take place
in the air and not in the ether. But the effect of motion to or from the
observer is strictly analogous in the two cases. As, however, light travels
186,000 miles a second, the source of light will also have to travel with a
very high velocity in order to produce even the smallest perceptible change
in the position of a spectral line.

We have already seen that enormously high velocities are by no means
uncommon in some of these mighty disturbances on the sun; accordingly,

when we examine the spectrum of a sun-spot, we often see that some of the
lines are shifted a little towards one end of the spectrum and sometimes
towards the other, while in other cases the lines are seen to be distorted or
twisted in the most fantastic manner, indicating very violent local
commotions. If the spot happens to be near the centre of the sun's disc, the
gases must be shooting upwards or downwards to produce these changes in
the lines. The velocities indicated in observations of this class sometimes
amount to as much as two or even three hundred miles per second. We find
it difficult to conceive the enormous internal pressures which are required
to impel such mighty masses of gases aloft from the photosphere with
speeds so terrific, or the conditions which bring about the downrush of such
gigantic masses of vapour from above. In the spectra of the prominences on
the sun's limb also we often see the bright lines bent or shifted to one side.
In such cases what we witness is evidently caused by movements along the
surface of the chromosphere, conveying materials towards us or away from
us.

An interesting application of this beautiful method of measuring the
speed of moving bodies has been made in various attempts to determine the
period of rotation of the sun spectroscopically. As the sun turns round on its
axis, a point on the eastern limb is moving towards the observer and a point
on the western limb is moving away from him. In each case the velocity is a
little over a mile per second. At the eastern limb the lines in the solar
spectrum are very slightly shifted towards the violet end of the spectrum,
while the lines in the spectrum of the western limb are equally shifted
towards the red end. By an ingenious optical contrivance it is possible to
place the spectra from the two limbs side by side, which doubles the
apparent displacement, and thus makes it much more easy to measure. Even
with this contrivance the visual quantities to be measured remain
exceedingly minute. All the parts of the instrument have to be most
accurately adjusted, and the observations are correspondingly delicate. They
have been attempted by various observers. Among the most successful
investigations of this kind we may mention that of the Swedish astronomer,

Dunér, who, by pointing his instrument to a number of places on the limb,
found values in good agreement with the peculiar law of rotation which has
been deduced from the motion of sun-spots. This result is specially
interesting, as it shows that the atmospheric layers, in which that absorption
takes place which produces the dark lines in the spectrum, shares in the
motion of the photosphere at the same latitude.

Fig. 20.—View of the Corona (and a Comet) in a Total
Eclipse.

PLATE V.
TOTAL SOLAR ECLIPSE, JULY 29TH, 1878.

THE CORONA FROM THE PHOTOGRAPHS.
(HARKNESS.)

We have yet to mention one other striking phenomenon which is among
the chief attractions to observers of total eclipses, and which it has hitherto
not been found possible to see in full daylight. This is the corona or aureole
of light which is suddenly seen to surround the sun in an eclipse when the
moon has completely covered the last remaining crescent of the sun. A
general idea of the appearance of the corona is given in Fig. 20, and we
further present in Plate V. the drawing of the corona made by Professor
Harkness from a comparison of a large number of photographs obtained at
different places in the United States during the total eclipse of July 29th,
1878. In Fig. 21 we are permitted by the kindness of Mr. and Mrs. Maunder
to reproduce the remarkable photograph of the corona which they obtained
in India during the eclipse of January 22nd, 1898.

Fig. 21.—View of Corona during the Eclipse of Jan. 22nd, 1898
(Reproduced by kind permission of Mr. and Mrs. Maunder and of the
proprietors of "Knowledge.")

The part of the corona nearest the sun is very bright, though not so
brilliant as the prominences, which (as Professor Young says) blaze through
it like carbuncles. This inner portion is generally of fairly regular outline,
forming a white ring about a tenth part of the solar diameter in width. The

outer parts of the corona are usually very irregular and very extensive. They
are often interrupted by narrow "rifts," or narrow dark bands, which reach
from the limb of the sun through the entire corona. On the other hand, there
are also sometimes narrow bright streamers, inclined at various angles to
the limb of the sun and not seldom curved. In the eclipses of 1867, 1878,
and 1889, all of which occurred at periods of sun-spot minimum, the corona
showed long and faint streamers nearly in the direction of the sun's equator,
and short but distinct brushes of light near the poles. In the eclipses of 1870,
1882, and 1893, near sun-spot maxima, the corona was more regularly
circular, and chiefly developed over the spot zones. We have here another
proof (if one were necessary) of the intimate connection between the
periodicity of the spots and the development of all other solar phenomena.

In the spectrum of the corona there is a mysterious line in the green, as
to the origin of which nothing is at present certainly known. It is best seen
during eclipses occurring near the time of sun-spot maximum. It is
presented in the ordinary solar spectrum as a very thin, dark line, which
generally remains undisturbed even when lines of hydrogen and other
substances are twisted and distorted by the violent rush of disturbed
elements. The line is always present among the bright lines of the
chromosphere spectrum. In addition to it the corona shows a few other
bright lines, belonging, no doubt, to the same unknown element
("coronium"), and also a faint continuous spectrum, in which even a few of
the more prominent dark lines of the solar spectrum have been sometimes
detected. This shows that in addition to glowing gas (represented by the
bright lines) the corona also contains a great deal of matter like dust, or fog,
the minute particles of which are capable of reflecting the sunlight and
thereby producing a feeble continuous spectrum. This matter seems to form
the principal constituent of the long coronal rays and streamers, as the latter
are not visible in the detached images of the corona which appear instead of
the bright lines when the corona is viewed, or photographed, during an
eclipse, in a spectroscope without a slit. If the long rays were composed of
the gas or gases which constitute the inner corona, it is evident that they

ought to appear in these detached images. As to the nature of the forces
which are continually engaged in shooting out these enormously long
streamers, we have at present but little information. It is, however, certain
that the extensive atmospheric envelope round the sun, which shows itself
as the inner corona, must be extremely attenuated. Comets have on several
occasions been known to rush through this coronal atmosphere without
evincing the slightest appreciable diminution in their speed from the
resistance to which they were exposed.

We have accumulated by observation a great number of facts concerning
the sun, but when we try to draw from these facts conclusions as to the
physical constitution of that great body, it cannot be denied that the
difficulties seem to be very great indeed. We find that the best authorities
differ considerably in the opinions they entertain as to its nature. We shall
here set forth the principal conclusions as to which there is little or no
controversy.

We shall see in a following chapter that astronomers have been able to
determine the relative densities of the bodies in the solar system; in other
words, they have found the relation between the quantities of matter
contained in an equally large volume of each. It has thus been ascertained
that the average density of the sun is about a quarter that of the earth. If we
compare the weight of the sun with that of an equally great globe of water,
we find that the luminary would be barely one and a half times as heavy as
the water. Of course, the actual mass of the sun is very enormous; it is no
less than 330,000 times as great as that of the earth. The solar material itself
is, however, relatively light, so that the sun is four times as big as it would
have to be if, while its weight remained the same, its density equalled that
of the earth. Bearing in mind this lightness of the sun, and also the
exceedingly high temperature which we know to prevail there, no other
conclusion seems possible than that the body of the sun must be in a
gaseous state. The conditions under which such gases exist in the sun are,
no doubt, altogether different from those with which we are acquainted on
the earth. At the surface of the sun the force of gravity is more than twenty-

seven times as great as it is on the earth. A person who on the earth could
just lift twenty-seven equal pieces of metal would, if he were transferred to
the sun, only be able to lift one of the pieces at a time. The pressure of the
gases below the surface must therefore be very great, and it might be
supposed that they would become liquefied in consequence. It was,
however, discovered by Andrews that so long as a gas is kept at a
temperature higher than a certain point, known as the "critical temperature"
(which is different for different gases), the gas will not be turned into a
liquid however great be the pressure to which it is submitted. The
temperature on the sun cannot be lower than the critical temperatures of the
gases there existing; so it would seem that even the enormous pressure can
hardly reduce the gases in the great luminary to the liquid form.

Of the interior of the sun we can, of course, expect to learn little or
nothing. What we observe is the surface-layer, the so-called photosphere, in
which the cold of space produces the condensation of the gases into those
luminous clouds which we see in our drawings and photographs as "rice
grains" or "willow leaves." It has been suggested by Dr. Johnstone Stoney
(and afterwards by Professor Hastings, of Baltimore) that these luminous
clouds are mainly composed of carbon with those of the related elements
silicon and boron, the boiling points of which are much higher than those of
other elements which might be considered likely to form the photospheric
clouds. The low atomic weight of carbon must also have the effect of giving
the molecules of this element a very high velocity, and thereby enabling
them to work their way into the upper regions, where the temperature has so
fallen that the vapour becomes chilled into cloud. A necessary consequence
of the rapid cooling of these clouds, and the consequent radiation of heat on
a large scale, would be the formation of what we may perhaps describe as
smoke, which settles by degrees through the intervals between the clouds
(making these intervals appear darker) until it is again volatilised on
reaching a level of greater heat below the clouds. This same smoke is
probably the cause of the well-known fact that the solar limb is
considerably fainter than the middle of the disc. This seems to arise from

the greater absorption caused by the longer distance which a ray of light
from a point near the limb has to travel through this layer of smoke before
reaching the earth. It is shown that this absorption cannot be attributed to a
gaseous atmosphere, since this would have the effect of producing more
dark absorption lines in the spectrum. There would thus be a marked
difference between the solar spectrum from a part near the middle of the
disc and the spectrum from a part near the limb. This, however, we do not
find to be the case.

With regard to the nature of sun-spots, the idea first suggested by Secchi
and Lockyer, that they represent down rushes of cooler vapours into the
photosphere (or to its surface), seems on the whole to accord best with the
observed phenomena. We have already mentioned that the spots are
generally accompanied by faculæ and eruptive prominences in their
immediate neighbourhood, but whether these eruptions are caused by the
downfall of the vapour which makes the photospheric matter "splash up" in
the vicinity, or whether the eruptions come first, and by diminishing the
upward pressure from below form a "sink," into which overlying cooler
vapour descends, are problems as to which opinions are still much divided.

A remarkable appendage to the sun, which extends to a distance very
much greater than that of the corona, produces the phenomenon of the
zodiacal light. A pearly glow is sometimes seen in the spring to spread over
a part of the sky in the vicinity of the point where the sun has disappeared
after sunset. The same spectacle may also be witnessed before sunrise in the
autumn, and it would seem as if the material producing the zodiacal light,
whatever it may be, had a lens-shaped form with the sun in the centre. The
nature of this object is still a matter of uncertainty, but it is probably
composed of a kind of dust, as the faint spectrum it affords is of a
continuous type. A view of the zodiacal light is shown in Fig. 22.

In all directions the sun pours forth, with the most prodigal liberality, its
torrents of light and of heat. The earth can only grasp the merest fraction,
less than the 2,000,000,000th part of the whole. Our fellow planets and the

moon also intercept a trifle; but how small is the portion of the mighty flood
which they can utilise! The sip that a flying swallow takes from a river is as
far from exhausting the water in the river as are the planets from using all
the heat which streams from the sun.

The sun's gracious beams supply the magic power that enables the corn
to grow and ripen. It is the heat of the sun which raises water from the
ocean in the form of vapour, and then sends down that vapour as rain to
refresh the earth and to fill the rivers which bear our ships down to the
ocean. It is the heat of the sun beating on the large continents which gives
rise to the breezes and winds that waft our vessels across the deep; and
when on a winter's evening we draw around the fire and feel its invigorating
rays, we are only enjoying sunbeams which shone on the earth countless
ages ago. The heat in those ancient sunbeams developed the mighty
vegetation of the coal period, and in the form of coal that heat has
slumbered for millions of years, till we now call it again into activity. It is
the power of the sun stored up in coal that urges on our steam-engines. It is
the light of the sun stored up in coal that beams from every gaslight in our
cities.

For the power to live and move, for the plenty with which we are
surrounded, for the beauty with which nature is adorned, we are
immediately indebted to one body in the countless hosts of space, and that
body is the sun.

Fig. 22.—The Zodiacal Light in 1874.

CHAPTER III.
THE MOON.
The Moon and the Tides—The Use of the Moon in Navigation—The Changes of the Moon
—The Moon and the Poets—Whence the Light of the Moon?—Sizes of the Earth and
the Moon—Weight of the Moon—Changes in Apparent Size—Variations in its
Distance—Influence of the Earth on the Moon—The Path of the Moon—Explanation
of the Moon's Phases—Lunar Eclipses—Eclipses of the Sun, how produced—
Visibility of the Moon in a Total Eclipse—How Eclipses are Predicted—Uses of the
Moon in finding Longitude—The Moon not connected with the Weather—
Topography of the Moon—Nasmyth's Drawing of Triesnecker—Volcanoes on the
Moon—Normal Lunar Crater—Plato—The Shadows of Lunar Mountains—The
Micrometer—Lunar Heights—Former Activity on the Moon—Nasmyth's View of the
Formation of Craters—Gravitation on the Moon—Varied Sizes of the Lunar Craters—
Other Features of the Moon—Is there Life on the Moon?—Absence of Water and of
Air—Dr. Stoney's Theory—Explanation of the Rugged Character of Lunar Scenery—
Possibility of Life on Distant Bodies in Space.

If the moon were suddenly struck out of existence, we should be
immediately apprised of the fact by a wail from every seaport in the
kingdom. From London and from Liverpool we should hear the same story
—the rise and fall of the tide had almost ceased. The ships in dock could
not get out; the ships outside could not get in; and the maritime commerce
of the world would be thrown into dire confusion.

The moon is the principal agent in causing the daily ebb and flow of the
tide, and this is the most important work which our satellite has to do. The
fleets of fishing boats around the coasts time their daily movements by the
tide, and are largely indebted to the moon for bringing them in and out of
harbour. Experienced sailors assure us that the tides are of the utmost
service to navigation. The question as to how the moon causes the tides is
postponed to a future chapter, in which we shall also sketch the marvellous
part which the tides seem to have played in the early history of our earth.

Who is there that has not watched, with admiration, the beautiful series
of changes through which the moon passes every month? We first see her as
an exquisite crescent of pale light in the western sky after sunset. If the
night is fine, the rest of the moon is visible inside the crescent, being faintly
illumined by light reflected from our own earth. Night after night she moves
further and further to the east, until she becomes full, and rises about the
same time that the sun sets. From the time of the full the disc of light begins
to diminish until the last quarter is reached. Then it is that the moon is seen
high in the heavens in the morning. As the days pass by, the crescent shape
is again assumed. The crescent wanes thinner and thinner as the satellite
draws closer to the sun. Finally she becomes lost in the overpowering light
of the sun, again to emerge as the new moon, and again to go through the
same cycle of changes.

The brilliance of the moon arises solely from the light of the sun, which
falls on the not self-luminous substance of the moon. Out of the vast flood
of light which the sun pours forth with such prodigality into space the dark
body of the moon intercepts a little, and of that little it reflects a small
fraction to illuminate the earth. The moon sheds so much light, and seems
so bright, that it is often difficult at night to remember that the moon has no
light except what falls on it from the sun. Nevertheless, the actual surface of
the brightest full moon is perhaps not much brighter than the streets of
London on a clear sunshiny day. A very simple observation will suffice to
show that the moon's light is only sunlight. Look some morning at the moon
in daylight, and compare the moon with the clouds. The brightness of the
moon and of the clouds are directly comparable, and then it can be readily
comprehended how the sun which illuminates the clouds has also illumined
the moon. An attempt has been made to form a comparative estimate of the
brightness of the sun and the full moon. If 600,000 full moons were shining
at once, their collective brilliancy would equal that of the sun.

The beautiful crescent moon has furnished a theme for many a poet.
Indeed, if we may venture to say so, it would seem that some poets have
forgotten that the moon is not to be seen every night. A poetical description

of evening is almost certain to be associated with the appearance of the
moon in some phase or other. We may cite one notable instance in which a
poet, describing an historical event, has enshrined in exquisite verse a
statement which cannot be correct. Every child who speaks our language
has been taught that the burial of Sir John Moore took place

"By the struggling moonbeams' misty light."

There is an appearance of detail in this statement which wears the garb
of truth. We are not inclined to doubt that the night was misty, nor as to
whether the moonbeams had to struggle into visibility; the question at issue
is a much more fundamental one. We do not know who was the first to raise
the point as to whether any moon shone on that memorable event at all or
not; but the question having been raised, the Nautical Almanac immediately
supplies an answer. From it we learn in language, whose truthfulness
constitutes its only claim to be poetry, that the moon was new at one o'clock
in the morning of the day of the battle of Corunna (16th January, 1809). The
ballad evidently implies that the funeral took place on the night following
the battle. We are therefore assured that the moon can hardly have been a
day old when the hero was consigned to his grave. But the moon in such a
case is practically invisible, and yields no appreciable moonbeams at all,
misty or otherwise. Indeed, if the funeral took place at the "dead of night,"
as the poet asserts, then the moon must have been far below the horizon at
the time.[6]

In alluding to this and similar instances, Mr. Nasmyth gives a word of
advice to authors or to artists who desire to bring the moon on a scene
without knowing as a matter of fact that our satellite was actually present.
He recommends them to follow the example of Bottom in A Midsummer's
Night's Dream, and consult "a calendar, a calendar! Look in the almanac;
find out moonshine, find out moonshine!"

Among the countless host of celestial bodies—the sun, the moon, the
planets, and the stars—our satellite enjoys one special claim on our

attention. The
moon is our
nearest
permanent
neighbour. It is
just possible that
a comet may
occasionally
approach the
earth more
closely than the
moon but with
this exception
Fig. 23.—Comparative Sizes of the Earth and the the other
Moon. celestial bodies
are all many
hundreds or thousands, or even many millions, of times further from us than
the moon.

It is also to be observed that the moon is one of the smallest visible
objects which the heavens contain. Every one of the thousands of stars that
can be seen with the unaided eye is enormously larger than our satellite.
The brilliance and apparent vast proportions of the moon arise from the fact
that it is only 240,000 miles away, which is a distance almost immeasurably
small when compared with the distances between the earth and the stars.

Fig. 23 exhibits the relative sizes of the earth and its attendant. The small
globe shows the moon, while the larger globe represents the earth. When we
measure the actual diameters of the two globes, we find that of the earth to
be 7,918 miles and of the moon 2,160 miles, so that the diameter of the
earth is nearly four times greater than the diameter of the moon. If the earth
were cut into fifty pieces, all equally large, then one of these pieces rolled
into a globe would equal the size of the moon. The superficial extent of the
moon is equal to about one thirteenth part of the surface of the earth. The

hemisphere our neighbour turns towards us exhibits an area equal to about
one twenty-seventh part of the area of the earth. This, to speak
approximately, is about double the actual extent of the continent of Europe.
The average materials of the earth are, however, much heavier than those
contained in the moon. It would take more than eighty globes, each as
ponderous as the moon, to weigh down the earth.

Amid the changes which the moon presents to us, one obvious fact
stands prominently forth. Whether our satellite be new or full, at first
quarter or at last, whether it be high in the heavens or low near the horizon,
whether it be in process of eclipse by the sun, or whether the sun himself is
being eclipsed by the moon, the apparent size of the latter is nearly
constant. We can express the matter numerically. A globe one foot in
diameter, at a distance of 111 feet from the observer, would under ordinary
circumstances be just sufficient to hide the disc of the moon; occasionally,
however, the globe would have to be brought in to a distance of only 103
feet, or occasionally it might have to be moved out to so much as 118 feet,
if the moon is to be exactly hidden. It is unusual for the moon to approach
either of its extreme limits of position, so that the distance from the eye at
which the globe must be situated so as to exactly cover the moon is usually
more than 105 feet, and less than 117 feet. These fluctuations in the
apparent size of our satellite are contained within such narrow limits that in
the first glance at the subject they may be overlooked. It will be easily seen
that the apparent size of the moon must be connected with its real distance
from the earth. Suppose, for the sake of illustration, that the moon were to
recede into space, its size would seem to dwindle, and long ere it had
reached the distance of even the very nearest of the other celestial bodies it
would have shrunk into insignificance. On the other hand, if the moon were
to come nearer to the earth, its apparent size would gradually increase until,
when close to our globe, it would seem like a mighty continent stretching
over the sky. We find that the apparent size of the moon is nearly constant,
and hence we infer that the average distance of the same body is also nearly
constant. The average value of that distance is 239,000 miles. In rare

circumstances it may approach to a distance but little more than 221,000
miles, or recede to a distance hardly less than 253,000 miles, but the
ordinary fluctuations do not exceed more than about 13,000 miles on either
side of its mean value.

From the moon's incessant changes we perceive that she is in constant
motion, and we now further see that whatever these movements may be, the
earth and the moon must at present remain at nearly the same distance
apart. If we further add that the path pursued by the moon around the
heavens lies nearly in a plane, then we are forced to the conclusion that our
satellite must be revolving in a nearly circular path around the earth at the
centre. It can, indeed, be shown that the constant distance of the two bodies
involves as a necessary condition the revolution of the moon around the
earth. The attraction between the moon and the earth tends to bring the two
bodies together. The only way by which such a catastrophe can be
permanently avoided is by making the satellite move as we actually find it
to do. The attraction between the earth and the moon still exists, but its
effect is not then shown in bringing the moon in towards the earth. The
attraction has now to exert its whole power in restraining the moon in its
circular path; were the attraction to cease, the moon would start off in a
straight line, and recede never to return.

Fig. 24.—The Moon's Path around the Sun.
The fact of the moon's revolution around the earth is easily demonstrated
by observations of the stars. The rising and setting of our satellite is, of
course, due to the rotation of the earth, and this apparent diurnal movement
the moon possesses in common with the sun and with the stars. It will,

however, be noticed that the moon is continually changing its place among
the stars. Even in the course of a single night the displacement will be
conspicuous to a careful observer without the aid of a telescope. The moon
completes each revolution around the earth in a period of 27·3 days.

Fig. 25.—The Phases of the Moon.

In Fig. 24 we have a view of the relative positions of the earth, the sun,
and the moon, but it is to be observed that, for the convenience of
illustration, we have been obliged to represent the orbit of the moon on a
much larger scale than it ought to be in comparison with the distance of the
sun. That half of the moon which is turned towards the sun is brilliantly
illuminated, and, according as we see more or less of that brilliant half, we
say that the moon is more or less full, the several "phases" being visible in
the succession shown by the numbers in Fig. 25. A beginner sometimes
finds considerable difficulty in understanding how the light on the full
moon at night can have been derived from the sun. "Is not," he will say, "the
earth in the way? and must it not intercept the sunlight from every object on
the other side of the earth to the sun?" A study of Fig. 24 will explain the
difficulty. The plane in which the moon revolves does not coincide with the
plane in which the earth revolves around the sun. The line in which the
plane of the earth's motion is intersected by that of the moon divides the
moon's path into two semicircles. We must imagine the moon's path to be
tilted a little, so that the upper semicircle is somewhat above the plane of
the paper, and the other semicircle below. It thus follows that when the
moon is in the position marked full, under the circumstances shown in the
figure, the moon will be just above the line joining the earth and the sun;
the sunlight will thus pass over the earth to the moon, and the moon will be

illuminated. At new moon, the moon will be under the line joining the earth
and the sun.

As the relative positions of the earth and the sun are changing, it happens
twice in each revolution that the sun comes into the position of the line of
intersection of the two planes. If this occurs at the time of full moon, the
earth lies directly between the moon and the sun; the moon is thus plunged
into the shadow of the earth, the light from the sun is intercepted, and we
say that the moon is eclipsed. The moon sometimes only partially enters the
earth's shadow, in which case the eclipse is a partial one. When, on the other
hand, the sun is situated on the line of intersection at the time of new moon,
the moon lies directly between the earth and the sun, and the dark body of
the moon will then cut off the sunlight from the earth, producing a solar
eclipse. Usually only a part of the sun is thus obscured, forming the well-
known partial eclipse; if, however, the moon pass centrally over the sun,
then we must have one or other of two very remarkable kinds of eclipse.
Sometimes the moon entirely blots out the sun, and thus is produced the
sublime spectacle of a total eclipse, which tells us so much as to the nature
of the sun, and to which we have already referred in the last chapter. Even
when the moon is placed centrally over the sun, a thin rim of sunlight is
occasionally seen round the margin of the moon. We then have what is
known as an annular eclipse.

It is remarkable that the moon is sometimes able to hide the sun
completely, while on other occasions it fails to do so. It happens that the
average apparent size of the moon is nearly equal to the average apparent
size of the sun, but, owing to the fluctuations in their distances, the actual
apparent sizes of both bodies undergo certain changes. On certain occasions
the apparent size of the moon is greater than that of the sun. In this case a
central passage produces a total eclipse; but it may also happen that the
apparent size of the sun exceeds that of the moon, in which case a central
passage can only produce an annular eclipse.

Fig. 26.—Form of the Earth's Shadow, showing the Penumbra, or partially
shaded region. Within the Penumbra, the Moon is visible; in the Shadow it
is nearly invisible.

There are hardly any more interesting celestial phenomena than the
different descriptions of eclipses. The almanac will always give timely
notice of the occurrence, and the more striking features can be observed
without a telescope. In an eclipse of the moon (Fig. 26) it is interesting to
note the moment when the black shadow is first detected, to watch its
gradual encroachment over the bright surface of the moon, to follow it, in
case the eclipse is total, until there is only a thin crescent of moonlight left,
and to watch the final extinction of that crescent when the whole moon is
plunged into the shadow. But now a spectacle of great interest and beauty is
often manifested; for though the moon is so hidden behind the earth that not
a single direct ray of the sunlight could reach its surface, yet we often find
that the moon remains visible, and, indeed, actually glows with a copper-
coloured hue bright enough to permit several of the markings on the surface
to be discerned.

This illumination of the moon even in the depth of a total eclipse is due
to the sunbeams which have just grazed the edge of the earth. In doing so
they have become bent by the refraction of the atmosphere, and have thus
been turned inwards into the shadow. Such beams have passed through a
prodigious thickness of the earth's atmosphere, and in this long journey
through hundreds of miles of air they have become tinged with a ruddy or
copper-like hue. Nor is this property of our atmosphere an unfamiliar one.
The sun both at sunrise and at sunset glows with a light which is much more
ruddy than the beams it dispenses at noonday. But at sunset or at sunrise the
rays which reach our eyes have much more of our atmosphere to penetrate
than they have at noon, and accordingly the atmosphere imparts to them

that ruddy colour so characteristic and often so lovely. If the spectrum of
the sun when close to the horizon is examined it is seen to be filled with
numerous dark lines and bands situated chiefly towards the blue and violet
end. These are caused by the increased absorption which the light suffers in
the atmosphere, and give rise to the preponderating red light on the sun
under such conditions. In the case of the eclipsed moon, the sunbeams have
to take an atmospheric journey more than double as long as that at sunrise
or sunset, and hence the ruddy glow of the eclipsed moon may be accounted
for.

The almanacs give the full particulars of each eclipse that happens in the
corresponding year. These predictions are reliable, because astronomers
have been carefully observing the moon for ages, and have learned from
these observations not only how the moon moves at present, but also how it
will move for ages to come. The actual calculations are so complicated that
we cannot here discuss them. There is, however, one leading principle about
eclipses which is so simple that we must refer to it. The eclipses occurring
this year have no very obvious relation to the eclipses that occurred last
year, or to those that will occur next year. Yet, when we take a more
extended view of the sequence of these phenomena, a very definite
principle becomes manifest. If we observe all the eclipses in a period of
eighteen years, or nineteen years, then we can predict, with at least an
approximation to the truth, all the future eclipses for many years. It is only
necessary to recollect that in 6,585-1⁄3 days after one eclipse a nearly similar
eclipse follows. For instance, a beautiful eclipse of the moon occurred on
the 5th of December, 1881. If we count back 6,585 days from that date, or,
that is, eighteen years and eleven days, we come to November 24th, 1863,
and a similar eclipse of the moon took place then. Again, there were four
eclipses in the year 1881. If we add 6,585-1⁄3 days to the date of each
eclipse, it will give the dates of all the four eclipses in the year 1899. It was
this rule which enabled the ancient astronomers to predict the recurrence of
eclipses, at a time when the motions of the moon were not understood
nearly so well as they now are.

During a long voyage, and perhaps in critical circumstances, the moon
will often render invaluable information to the sailor. To navigate a ship,
suppose from Liverpool to China, the captain must frequently determine the
precise position which his ship then occupies. If he could not do this, he
would never find his way across the trackless ocean. Observations of the
sun give him his latitude and tell him his local time, but the captain further
requires to know the Greenwich time before he can place his finger at a
point of the chart and say, "My ship is here." To ascertain the Greenwich
time the ship carries a chronometer which has been carefully rated before
starting, and, as a precaution, two or three chronometers are usually
provided to guard against the risk of error. An unknown error of a minute in
the chronometer might perhaps lead the vessel fifteen miles from its proper
course.

PLATE VI.
CHART OF THE MOON'S SURFACE.

Fig. 27.—Key to Chart of the Moon (Plate VI.).

It is important to have the means of testing the chronometers during the
progress of the voyage; and it would be a great convenience if every
captain, when he wished, could actually consult some infallible standard of
Greenwich time. We want, in fact, a Greenwich clock which may be visible
over the whole globe. There is such a clock; and, like any other clock, it has
a face on which certain marks are made, and a hand which travels round
that face. The great clock at Westminster shrinks into insignificance when
compared with the mighty clock which the captain uses for setting his
chronometer. The face of this stupendous dial is the face of the heavens.
The numbers engraved on the face of a clock are replaced by the twinkling
stars; while the hand which moves over the dial is the beautiful moon
herself. When the captain desires to test his chronometer, he measures the

distance of the moon from a neighbouring star. For example, he may see
that the moon is three degrees from the star Regulus. In the Nautical
Almanac he finds the Greenwich time at which the moon was three degrees
from Regulus. Comparing this with the indications of the chronometer, he
finds the required correction.

There is one widely-credited myth about the moon which must be
regarded as devoid of foundation. The idea that our satellite and the weather
bear some relation has no doubt been entertained by high authority, and
appears to be an article in the belief of many an excellent mariner. Careful
comparison between the state of the weather and the phases of the moon
has, however, quite discredited the notion that any connection of the kind
does really exist.

We often notice large blank spaces on maps of Africa and of Australia
which indicate our ignorance of parts of the interior of those great
continents. We can find no such blank spaces in the map of the moon.
Astronomers know the surface of the moon better than geographers know
the interior of Africa. Every spot on the face of the moon which is as large
as an English parish has been mapped, and all the more important objects
have been named.

A general map of the moon is shown in Plate VI. It has been based upon
drawings made with small telescopes, and it gives an entire view of that
side of our satellite which is presented towards us. The moon is shown as it
appears in an astronomical telescope, which inverts everything, so that the
south is at the top and the north at the bottom (to show objects upright a
telescope requires an additional pair of lenses in the eye-piece, and as this
diminishes the amount of light reaching the eye they are dispensed with in
astronomical telescopes). We can see on the map some of the characteristic
features of lunar scenery. Those dark regions so conspicuous in the ordinary
full moon are easily recognised on the map. They were thought to be seas
by astronomers before the days of telescopes, and indeed the name "Mare"
is still retained, though it is obvious that they contain no water at present.

The map also shows certain ridges or elevated portions, and when we apply
measurement to these objects we learn that they must be mighty mountain
ranges. But the most striking features on the moon are those ring-like
objects which are scattered over the surface in profusion. These are known
as the lunar craters.

To facilitate reference to the chief points of interest we have arranged an
index map (Fig. 27) which will give a clue to the names of the several
objects depicted upon the plate. The so-called seas are represented by
capital letters; so that A is the Mare Crisium, and H the Oceanus
Procellarum. The ranges of mountains are indicated by small letters; thus a
on the index is the site of the so-called Caucasus mountains, and similarly
the Apennines are denoted by c. The numerous craters are distinguished by
numbers; for example, the feature on the map corresponding to 20 on the
index is the crater designated Ptolemy.

A. Mare Crisium.
B. Mare Fœcunditatis.
C. Mare Tranquillitatis.
D. Mare Serenitatis.
E. Mare Imbrium.
F. Sinus Iridum.
G. Mare Vaporum.
H. Oceanus Procellarum.
I. Mare Humorum.
J. Mare Nubium.
K. Mare Nectaris.

a. Caucasus.
b. Alps.
c. Apennines.
d. Carpathians.
f. Cordilleras & D'Alembert mountains.
g. Rook mountains.

h. Dœrfel mountains.
i. Leibnitz mountains.

1. Posidonius.
2. Linné.
3. Aristotle.
4. Great Valley of the Alps.
5. Aristillus.
6. Autolycus.
7. Archimedes.
8. Plato.
9. Eratosthenes.
10. Copernicus.
11. Kepler.
12. Aristarchus.
13. Grimaldi.
14. Gassendi.
15. Schickard.
16. Wargentin.
17. Clavius.
18. Tycho.
19. Alphonsus.
20. Ptolemy.
21. Catharina.
22. Cyrillus.
23. Theophilus.
24. Petavius.
25. Hyginus.
26. Triesnecker.

In every geographical atlas there is a map showing the two hemispheres
of the earth, the eastern and the western. In the case of the moon we can
only give a map of one hemisphere, for the simple reason that the moon

always turns the same side towards us, and accordingly we never get a view
of the other side. This is caused by the interesting circumstance that the
moon takes exactly the same time to turn once round its own axis as it takes
to go once round the earth. The rotation is, however, performed with
uniform speed, while the moon does not move in its orbit with a perfectly
uniform velocity (see Chapter IV.). The consequence is that we now get a
slight glimpse round the east limb, and now a similar glimpse round the
west limb, as if the moon were shaking its head very gently at us. But it is
only an insignificant margin of the far side of the moon which this libration
permits us to examine.

Lunar objects are well suited for observation when the sunlight falls
upon them in such a manner as to exhibit strongly contrasted lights and
shadows. It is impossible to observe the moon satisfactorily when it is full,
for then no conspicuous shadows are cast. The most opportune moment for
seeing any particular lunar object is when it lies just at the illuminated side
of the boundary between light and shade, for then the features are brought
out with exquisite distinctness.

Plate VII.[7] gives an illustration of lunar scenery, the object represented
being known to astronomers by the name of Triesnecker. The district
included is only a very small fraction of the entire surface of the moon, yet
the actual area is very considerable, embracing as it does many hundreds of
square miles. We see in it various ranges of lunar mountains, while the
central object in the picture is one of those remarkable lunar craters which
we meet with so frequently in every lunar landscape. This crater is about
twenty miles in diameter, and it has a lofty mountain in the centre, the peak
of which is just illuminated by the rising sun in that phase of our satellite
which is represented in the picture.

A typical view of a lunar crater is shown in Plate VIII. This is, no doubt,
a somewhat imaginary sketch. The point of view from which the artist is
supposed to have taken the picture is one quite unattainable by terrestrial
astronomers, yet there can be little doubt that it is a fair representation of

objects on the moon. We should, however, recollect the scale on which it is
drawn. The vast crater must be many miles across, and the mountain at its
centre must be thousands of feet high. The telescope will, even at its best,
only show the moon as well as we could see it with the unaided eye if it
were 250 miles away instead of being 240,000. We must not, therefore,
expect to see any details on the moon even with the finest telescopes, unless
they were coarse enough to be visible at a distance of 250 miles. England
from such a point of view would only show London as a coloured spot, in
contrast with the general surface of the country.

We return, however, from a somewhat fancy sketch to a more prosaic
examination of what the telescope does actually reveal. Plate IX. represents
the large crater Plato, so well known to everyone who uses a telescope. The
floor of this remarkable object is nearly flat, and the central mountain, so
often seen in other craters, is entirely wanting. We describe it more fully in
the general list of lunar objects.

The mountain peaks on the moon throw long, well-defined shadows,
characterised by a sharpness which we do not find in the shadows of
terrestrial objects. The difference between the two cases arises from the
absence of air from the moon. Our atmosphere diffuses a certain amount of
light, which mitigates the blackness of terrestrial shadows and tends to
soften their outline. No such influences are at work on the moon, and the
sharpness of the shadows is taken advantage of in our attempts to measure
the heights of the lunar mountains.

It is often easy to compute the altitude of a church steeple, a lofty
chimney, or any similar object, from the length of its shadow. The simplest
and the most accurate process is to measure at noon the number of feet from
the base of the object to the end of the shadow. The elevation of the sun at
noon on the day in question can be obtained from the almanac, and then the
height of the object follows by a simple calculation. Indeed, if the
observations can be made either on the 6th of April or the 6th of September,
at or near the latitude of London, then calculations would be unnecessary.

The noonday length of the shadow on either of the dates named is equal to
the altitude of the object. In summer the length of the noontide shadow is
less than the altitude; in winter the length of the shadow exceeds the
altitude. At sunrise or sunset the shadows are, of course, much longer than
at noon, and it is shadows of this kind that we observe on the moon. The
necessary measurements are made by that indispensable adjunct to the
equatorial telescope known as the micrometer.

This word denotes an instrument for measuring small distances. In one
sense the term is not a happy one. The objects to which the astronomer
applies the micrometer are usually anything but small. They are generally of
the most transcendent dimensions, far exceeding the moon or the sun, or
even our whole system. Still, the name is not altogether inappropriate, for,
vast though the objects may be, they generally seem minute, even in the
telescope, on account of their great distance.

We require for such measurements an instrument capable of the greatest
nicety. Here, again, we invoke the aid of the spider, to whose assistance in
another department we have already referred. In the filar micrometer two
spider lines are parallel, and one intersects them at right angles. One or both
of the parallel lines can be moved by means of screws, the threads of which
have been shaped by consummate workmanship. The distance through
which the line has been moved is accurately indicated by noting the number
of revolutions and parts of a revolution of the screw. Suppose the two lines
be first brought into coincidence, and then separated until the apparent
length of the shadow of the mountain on the moon is equal to the distance
between the lines: we then know the number of revolutions of the
micrometer screw which is equivalent to the length of the shadow. The
number of miles on the moon which correspond to one revolution of the
screw has been previously ascertained by other observations, and hence the
length of the shadow can be determined. The elevation of the sun, as it
would have appeared to an observer at this point of the moon, at the
moment when the measures were being made, is also obtainable, and hence
the actual elevation of the mountain can be calculated. By measurements of

this kind the altitudes of other lunar objects, such, for example, as the
height of the rampart surrounding a circular-walled plane, can be
determined.

The beauty and interest of the moon as a telescopic object induces us to
give to the student a somewhat detailed account of the more remarkable
features which it presents. Most of the objects we are to describe can be
effectively exhibited with very moderate telescopic power. It is, however, to
be remembered that all of them cannot be well seen at one time. The region
most distinctly shown is the boundary between light and darkness. The
student will, therefore, select for observation such objects as may happen to
lie near that boundary at the time when he is observing.

1. Posidonius.—The diameter of this large crater is nearly 60 miles.
Although its surrounding wall is comparatively slender, it is so distinctly
marked as to make the object very conspicuous. As so frequently happens in
lunar volcanoes, the bottom of the crater is below the level of the
surrounding plain, in the present instance to the extent of nearly 2,500 feet.

2. Linné.—This small crater lies in the Mare Serenitatis. About sixty
years ago it was described as being about 6-1⁄2 miles in diameter, and seems
to have been sufficiently conspicuous. In 1866 Schmidt, of Athens,
announced that the crater had disappeared. Since then an exceedingly small
shallow depression has been visible, but the whole object is now very
inconsiderable. This seems to be the most clearly attested case of change in
a lunar object. Apparently the walls of the crater have tumbled into the
interior and partly filled it up, but many astronomers doubt that a change
has really taken place, as Schröter, a Hanoverian observer at the end of the
eighteenth century, appears not to have seen any conspicuous crater in the
place, though it must be admitted that his observations are rather
incomplete. To give some idea of Schmidt's amazing industry in lunar
researches, it may be mentioned that in six years he made nearly 57,000
individual settings of his micrometer in the measurement of lunar altitudes.
His great chart of the mountains in the moon is based on no less than 2,731

drawings and sketches, if those are counted twice that may have been used
for two divisions of the map.

3. Aristotle.—This great philosopher's name has been attached to a grand
crater 50 miles in diameter, the interior of which, although very hilly, shows
no decidedly marked central cone. But the lofty wall of the crater,
exceeding 10,500 feet in height, overshadows the floor so continuously that
its features are never seen to advantage.

4. The Great Valley of the Alps.—A wonderfully straight valley, with a
width ranging from 3-1⁄2 to 6 miles, runs right through the lunar Alps. It is,
according to Mädler, at least 11,500 feet deep, and over 80 miles in length.
A few low ridges which are parallel to the sides of the valley may possibly
be the result of landslips.

5. Aristillus.—Under favourable conditions Lord Rosse's great telescope
has shown the exterior of this magnificent crater to be scored with deep
gullies radiating from its centre. Aristillus is about 34 miles wide and
10,000 feet in depth.

6. Autolycus is somewhat smaller than the foregoing, to which it forms a
companion in accordance with what Mädler thought a well-defined relation
amongst lunar craters, by which they frequently occurred in pairs, with the
smaller one more usually to the south. Towards the edge this arrangement is
generally rather apparent than real, and is merely a result of foreshortening.

7. Archimedes.—This large plain, about 50 miles in diameter, has its vast
smooth interior divided by unequally bright streaks into seven distinct
zones, running east and west. There is no central mountain or other obvious
internal sign of former activity, but its irregular wall rises into abrupt
towers, and is marked outside by decided terraces.

PLATE B.
PORTION OF THE MOON.
(ALPS, ARCHIMEDES, APENNINES.)
Messrs. Loewy & Puiseux.

8. Plato.—We have already referred to this extensive circular plain,
which is noticeable with the smallest telescope. The average height of the
rampart is about 3,800 feet on the eastern side; the western side is

somewhat lower, but there is one peak rising to the height of nearly 7,300
feet. The plain girdled by this vast rampart is of ample proportions. It is a
somewhat irregular circle, about 60 miles in diameter, and containing an
area of 2,700 square miles. On its floor the shadows of the western wall are
shown in Plate IX., as are also three of the small craters, of which a large
number have been detected by persevering observers. The narrow sharp line
leading from the crater to the left is one of those remarkable "clefts" which
traverse the moon in so many directions. Another may be seen further to the
left. Above Plato are several detached mountains, the loftiest of which is
Pico, about 8,000 feet in height. Its long and pointed shadow would at first
sight lead one to suppose that it must be very steep; but Schmidt, who
specially studied the inclinations of the lunar slopes, is of opinion that it
cannot be nearly so steep as many of the Swiss mountains that are
frequently ascended. As many as thirty minute craters have been carefully
observed on the floor of Plato, and variations have been thought by Mr.
W.H. Pickering to be perceptible.

9. Eratosthenes.—This profound crater, upwards of 37 miles in diameter,
lies at the end of the gigantic range of the Apennines. Not improbably,
Eratosthenes once formed the volcanic vent for the stupendous forces that
elevated the comparatively craterless peaks of these great mountains.

10. Copernicus.—Of all the lunar craters this is one of the grandest and
best known. The region to the west is dotted over with innumerable minute
craterlets. It has a central many-peaked mountain about 2,400 feet in height.
There is good reason to believe that the terracing shown in its interior is
mainly due to the repeated alternate rise, partial congelation, and
subsequent retreat of a vast sea of lava. At full moon the crater of
Copernicus is seen to be surrounded by radiating streaks.

11. Kepler.—Although the internal depth of this crater is scarcely less
than 10,000 feet, it has but a very low surrounding wall, which is
remarkable for being covered with the same glistening substance that also
forms a system of bright rays not unlike those surrounding the last object.

12. Aristarchus is the most brilliant of the lunar craters, being specially
vivid with a low power in a large telescope. So bright is it, indeed, that it
has often been seen on the dark side just after new moon, and has thus
given rise to marvellous stories of active lunar volcanoes. To the south-east
lies another smaller crater, Herodotus, north of which is a narrow, deep
valley, nowhere more than 2-1⁄2 miles broad, which makes a remarkable
zigzag. It is one of the largest of the lunar "clefts."

13. Grimaldi calls for notice as the darkest object of its size in the moon.
Under very exceptional circumstances it has been seen with the naked eye,
and as its area has been estimated at nearly 14,000 square miles, it gives an
idea of how little unaided vision can discern in the moon; it must, however,
be added that we always see Grimaldi considerably foreshortened.

14. The great crater Gassendi has been very frequently mapped on
account of its elaborate system of "clefts." At its northern end it
communicates with a smaller but much deeper crater, that is often filled
with black shadow after the whole floor of Gassendi has been illuminated.

15. Schickard is one of the largest walled plains on the moon, about 134
miles in breadth. Within its vast expanse Mädler detected 23 minor craters.
With regard to this object Chacornac pointed out that, owing to the
curvature of the surface of the moon, a spectator at the centre of the floor
"would think himself in a boundless desert," because the surrounding wall,
although in one place nearly 10,000 feet high, would lie entirely beneath his
horizon.

16. Close to the foregoing is Wargentin. There can be little doubt that
this is really a huge crater almost filled with congealed lava, as there is
scarcely any fall towards the interior.

17. Clavius.—Near the 60th parallel of lunar south latitude lies this
enormous enclosure, the area of which is not less than 16,500 square miles.
Both in its interior and on its walls are many peaks and secondary craters.
The telescopic view of a sunrise upon the surface of Clavius is truly said by

Mädler to be indescribably magnificent. One of the peaks rises to a height
of 24,000 feet above the bottom of one of the included craters. Mädler even
expressed the opinion that in this wild neighbourhood there are craters so
profound that no ray of sunlight ever penetrated their lowest depths, while,
as if in compensation, there are peaks whose summits enjoy a mean day
almost twice as long as their night.

18. If the full moon be viewed through an opera-glass or any small hand-
telescope, one crater is immediately seen to be conspicuous beyond all
others, by reason of the brilliant rays or streaks that radiate from it. This is
the majestic Tycho, 17,000 feet in depth and 50 miles in diameter (Plate X.).
A peak 6,000 feet in height rises in the centre of its floor, while a series of
terraces diversity its interior slopes; but it is the mysterious bright rays that
chiefly surprise us. When the sun rises on Tycho, these streaks are utterly
invisible; indeed, the whole object is then so obscure that it requires a
practised eye to recognise Tycho amidst its mountainous surroundings. But
as soon as the sun has attained a height of about 30° above its horizon, the
rays emerge from their obscurity and gradually increase in brightness until
the moon becomes full, when they are the most conspicuous objects on her
surface. They vary in length, from a few hundred miles to two or, in one
instance, nearly three thousand miles. They extend indifferently across vast
plains, into the deepest craters, or over the loftiest elevations. We know of
nothing on our earth to which they can be compared. As these rays are only
seen about the time of full moon, their visibility obviously depends on the
light falling more or less closely in the line of sight, quite regardless of the
inclination of the surfaces, mountains or valleys, on which they appear.
Each small portion of the surface of the streak must therefore be of a form
which is symmetrical to the spectator from whatever point it is seen. The
sphere alone appears to fulfil this condition, and Professor Copeland
therefore suggests that the material constituting the surface of the streak
must be made up of a large number of more or less completely spherical
globules. The streaks must represent parts of the lunar surface either pitted

with minute cavities of spherical figure, or strewn over with minute
transparent spheres.[8]

Near the centre of the moon's disc is a fine range of ring plains fully
open to our view under all illuminations. Of these, two may be mentioned
—Alphonsus (19), the floor of which is strangely characterised by two
bright and several dark markings which cannot be explained by
irregularities in the surface.—Ptolemy (20). Besides several small enclosed
craters, its floor is crossed by numerous low ridges, visible when the sun is
rising or setting.

21, 22, 23.—When the moon is five or six days old this beautiful group
of three craters will be favourably placed for observation. They are named
Catharina, Cyrillus, and Theophilus. Catharina, the most southerly of the
group, is more than 16,000 feet deep, and connected with Cyrillus by a
wide valley; but between Cyrillus and Theophilus there is no such
connection. Indeed, Cyrillus looks as if its huge surrounding ramparts, as
high as Mont Blanc, had been completely finished before the volcanic
forces commenced the formation of Theophilus, the rampart of which
encroaches considerably on its older neighbour. Theophilus stands as a
well-defined circular crater about 64 miles in diameter, with an internal
depth of 14,000 to 18,000 feet, and a beautiful central group of mountains,
one-third of that height, on its floor. Although Theophilus is the deepest
crater we can see in the moon, it has suffered little or no deformation from
secondary eruptions, while the floor and wall of Catharina show complete
sequences of lesser craters of various sizes that have broken in upon and
partly destroyed each other. In the spring of the year, when the moon is
somewhat before the first quarter, this instructive group of extinct volcanoes
can be seen to great advantage at a convenient hour in the evening.

PLATE VII.
TRIESNECKER.
(AFTER NASMYTH.)

24. Petavius is remarkable not only for its great size, but also for the rare
feature of having a double rampart. It is a beautiful object soon after new
moon, or just after full moon, but disappears absolutely when the sun is
more than 45° above its horizon. The crater floor is remarkably convex,
culminating in a central group of hills intersected by a deep cleft.

25. Hyginus is a small crater near the centre of the moon's disc. One of
the largest of the lunar chasms passes right through it, making an abrupt
turn as it does so.

26. Triesnecker.—This fine crater has been already described, but is
again alluded to in order to draw attention to the elaborate system of chasms
so conspicuously shown in Plate VII. That these chasms are depressions is
abundantly evident by the shadows inside. Very often their margins are
appreciably raised. They seem to be fractures in the moon's surface.

Of the various mountains that are occasionally seen as projections on the
actual edge of the moon, those called after Leibnitz (i) seem to be the
highest. Schmidt found the highest peak to be upwards of 41,900 feet above
a neighbouring valley. In comparing these altitudes with those of mountains
on our earth, we must for the latter add the depth of the sea to the height of
the land. Reckoned in this way, our highest mountains are still higher than
any we know of in the moon.

We must now discuss the important question as to the origin of these
remarkable features on the surface of the moon. We shall admit at the outset
that our evidence on this subject is only indirect. To establish by
unimpeachable evidence the volcanic origin of the remarkable lunar craters,
it would seem almost necessary that volcanic outbursts should have been
witnessed on the moon, and that such outbursts should have been seen to
result in the formation of the well-known ring, with or without the mountain
rising from the centre. To say that nothing of the kind has ever been
witnessed would be rather too emphatic a statement. On certain occasions
careful observers have reported the occurrence of minute local changes on

the moon. As we have already remarked, a crater named Linné, of
dimensions respectable, no doubt, to a lunar inhabitant, but forming a very
inconsiderable telescopic object, was thought to have undergone some
change. On another occasion a minute crater was thought to have arisen
near the well-known object named Hyginus. The mere enumeration of such
instances gives real emphasis to the statement that there is at the present
time no appreciable source of disturbance of the moon's surface. Even were
these trifling cases of suspected change really established—and this is
perhaps rather farther than many astronomers would be willing to go—they
are still insignificant when compared with the mighty phenomena that gave
rise to the host of great craters which cover so large a portion of the moon's
surface.

We are led inevitably to the conclusion that our satellite must have once
possessed much greater activity than it now displays. We can also give a
reasonable, or, at all events, a plausible, explanation of the cessation of that
activity in recent times. Let us glance at two other bodies of our system, the
earth and the sun, and compare them with the moon. Of the three bodies,
the sun is enormously the largest, while the moon is much less than the
earth. We have also seen that though the sun must have a very high
temperature, there can be no doubt that it is gradually parting with its heat.
The surface of the earth, formed as it is of solid rocks and clay, or covered
in great part by the vast expanse of ocean, bears but few obvious traces of a
high temperature. Nevertheless, it is highly probable from ordinary volcanic
phenomena that the interior of the earth still possesses a temperature of
incandescence.

A large body when heated takes a longer time to cool than does a small
body raised to the same temperature. A large iron casting will take days to
cool; a small casting will become cold in a few hours. Whatever may have
been the original source of heat in our system—a question which we are not
now discussing—it seems demonstrable that the different bodies were all
originally heated, and have now for ages been gradually cooling. The sun is
so vast that he has not yet had time to cool; the earth, of intermediate bulk,

has become cold on the outside, while still retaining vast stores of internal
heat; while the moon, the smallest body of all, has lost its heat to such an
extent that changes of importance on its surface can no longer be originated
by internal fires.

We are thus led to refer the origin of the lunar craters to some ancient
epoch in the moon's history. We have no moans of knowing the remoteness
of that epoch, but it is reasonable to surmise that the antiquity of the lunar
volcanoes must be extremely great. At the time when the moon was
sufficiently heated to originate those convulsions, of which the mighty
craters are the survivals, the earth must also have been much hotter than it is
at present. When the moon possessed sufficient heat for its volcanoes to be
active, the earth was probably so hot that life was impossible on its surface.
This supposition would point to an antiquity for the lunar craters far too
great to be estimated by the centuries and the thousands of years which are
adequate for such periods as those with which the history of human events
is concerned. It seems not unlikely that millions of years may have elapsed
since the mighty craters of Plato or of Copernicus consolidated into their
present form.

We shall now attempt to account for the formation of the lunar craters.
The most probable views on the subject seem to be those which have been
set forth by Mr. Nasmyth, though it must be admitted that his doctrines are
by no means free from difficulty. According to his theory we can explain
how the rampart around the lunar crater has been formed, and how the great
mountain arose which so often adorns the centre of the plain. The view in
Fig. 28 contains an imaginary sketch of a volcanic vent on the moon in the
days when the craters were active. The eruption is here shown in the fulness
of its energy, when the internal forces are hurling forth ashes or stones
which fall at a considerable distance from the vent. The materials thus
accumulated constitute the rampart surrounding the crater.

The second picture (Fig. 29) depicts the crater in a later stage of its
history. The prodigious explosive power has now been exhausted, and has

perhaps been intermitted for some time. Again, the volcano bursts into
activity, but this time with only a small part of its original energy. A
comparatively feeble eruption now issues from the same vent, deposits
materials close around the orifice, and raises a mountain in the centre.
Finally, when the activity has subsided, and the volcano is silent and still,
we find the evidence of the early energy testified to by the rampart which
surrounds the ancient crater, and by the mountain which adorns the interior.
The flat floor which is found in some of the craters may not improbably
have arisen from an outflow of lava which has afterwards consolidated.
Subsequent outbreaks have also occurred in many cases.

One of the principal difficulties attending this method of accounting for
the structure of a crater arises from the great size which some of these
objects attain. There are ancient volcanoes on the moon forty or fifty miles
in diameter; indeed, there is one well-formed ring, with a mountain rising in
the centre, the diameter of which is no less than seventy-eight miles
(Petavius). It seems difficult to conceive how a blowing cone at the centre
could convey the materials to such a distance as the thirty-nine miles
between the centre of Petavius and the rampart. The explanation is,
however, facilitated when it is borne in mind that the force of gravitation is
much less on the moon than on the earth.

PLATE VIII.
A NORMAL LUNAR CRATER.

Fig. 28.—Volcano in Activity.

Fig. 29.—Subsequent Feeble Activity.
Have we not already seen that our satellite is so much smaller than the
earth that eighty moons rolled into one would not weigh as much as the
earth? On the earth an ounce weighs an ounce and a pound weighs a pound;
but a weight of six ounces here would only weigh one ounce on the moon,
and a weight of six pounds here would only weigh one pound on the moon.
A labourer who can carry one sack of corn on the earth could, with the same
exertion, carry six sacks of corn on the moon. A cricketer who can throw a
ball 100 yards on the earth could with precisely the same exertion throw the
same ball 600 yards on the moon. Hiawatha could shoot ten arrows into the
air one after the other before the first reached the ground; on the moon he
might have emptied his whole quiver. The volcano, which on the moon
drove projectiles to the distance of thirty-nine miles, need only possess the
same explosive power as would have been sufficient to drive the missiles
six or seven miles on the earth. A modern cannon properly elevated would
easily achieve this feat.

Fig. 30.—Formation of the Level Floor by Lava.

It must also be borne in mind that there are innumerable craters on the
moon of the same general type but of the most varied dimensions; from a
tiny telescopic object two or three miles in diameter, we can point out
gradually ascending stages until we reach the mighty Petavius just
considered. With regard to the smaller craters, there is obviously little or no
difficulty in attributing to them a volcanic origin, and as the continuity from
the smallest to the largest craters is unbroken, it seems quite reasonable to
suppose that even the greatest has arisen in the same way.

It should, however, be remarked that some lunar features might be
explained by actions from without rather than from within. Mr. G.K. Gilbert
has marshalled the evidence in support of the belief that lunar sculptures
arise from the impact of bodies falling on the moon. The Mare Imbrium,
according to this view, has been the seat of a collision to which the
surrounding lunar scenery is due. Mr. Gilbert explains the furrows as hewn
out by mighty projectiles moving with such velocities as meteors possess.

The lunar landscapes are excessively weird and rugged. They always
remind us of sterile deserts, and we cannot fail to notice the absence of
grassy plains or green forests such as we are familiar with on our globe. In
some respects the moon is not very differently circumstanced from the
earth. Like it, the moon has the pleasing alternations of day and night,
though the day in the moon is as long as twenty-nine of our days, and the
night of the moon is as long as twenty-nine of our nights. We are warmed
by the rays of the sun; so, too, is the moon; but, whatever may be the

temperature during the long day on the moon, it seems certain that the cold
of the lunar night would transcend that known in the bleakest regions of our
earth. The amount of heat radiated to us by the moon has been investigated
by Lord Rosse, and more recently by Professor Langley. Though every
point on the moon's surface is exposed to the sunlight for a fortnight
without any interruption, the actual temperature to which the soil is raised
cannot be a high one. The moon does not, like the earth, possess a warm
blanket, in the shape of an atmosphere, which can keep in and accumulate
the heat received.

Even our largest telescopes can tell nothing directly as to whether life
can exist on the moon. The mammoth trees of California might be growing
on the lunar mountains, and elephants might be walking about on the plains,
but our telescopes could not show them. The smallest object that we can see
on the moon must be about as large as a good-sized cathedral, so that
organised beings resembling in size any that we are familiar with, if they
existed, could not make themselves visible as telescopic objects.

We are therefore compelled to resort to indirect evidence as to whether
life would be possible on the moon. We may say at once that astronomers
believe that life, as we know it, could not exist. Among the necessary
conditions of life, water is one of the first. Take every form of vegetable
life, from the lichen which grows on the rock to the giant tree of the forest,
and we find the substance of every plant contains water, and could not exist
without it. Nor is water less necessary to the existence of animal life.
Deprived of this element, all organic life, the life of man himself, would be
inconceivable.

Unless, therefore, water be present in the moon, we shall be bound to
conclude that life, as we know it, is impossible. If anyone stationed on the
moon were to look at the earth through a telescope, would he be able to see
any water here? Most undoubtedly he would. He would see the clouds and
he would notice their incessant changes, and the clouds alone would be
almost conclusive evidence of the existence of water. An astronomer on the

moon would also see our oceans as coloured surfaces, remarkably
contrasted with the land, and he would perhaps frequently see an image of
the sun, like a brilliant star, reflected from some smooth portion of the sea.
In fact, considering that much more than half of our globe is covered with
oceans, and that most of the remainder is liable to be obscured by clouds,
the lunar astronomer in looking at our earth would often see hardly
anything but water in one form or other. Very likely he would come to the
conclusion that our globe was only fitted to be a residence for amphibious
animals.

But when we look at the moon with our telescopes we see no direct
evidence of water. Close inspection shows that the so-called lunar seas are
deserts, often marked with small craters and rocks. The telescope reveals no
seas and no oceans, no lakes and no rivers. Nor is the grandeur of the
moon's scenery ever impaired by clouds over her surface. Whenever the
moon is above our horizon, and terrestrial clouds are out of the way, we can
see the features of our satellite's surface with distinctness. There are no
clouds in the moon; there are not even the mists or the vapours which
invariably arise wherever water is present, and therefore astronomers have
been led to the conclusion that the surface of the globe which attends the
earth is a sterile and a waterless desert.

Another essential element of organic life is also absent from the moon.
Our globe is surrounded with a deep clothing of air resting on the surface,
and extending above our heads to the height of about 200 or 300 miles. We
need hardly say how necessary air is to life, and therefore we turn with
interest to the question as to whether the moon can be surrounded with an
atmosphere. Let us clearly understand the problem we are about to consider.
Imagine that a traveller started from the earth on a journey to the moon; as
he proceeded, the air would gradually become more and more rarefied, until
at length, when he was a few hundred miles above the earth's surface, he
would have left the last perceptible traces of the earth's envelope behind
him. By the time he had passed completely through the atmosphere he
would have advanced only a very small fraction of the whole journey of

240,000 miles, and there would still remain a vast void to be traversed
before the moon would be reached. If the moon were enveloped in the same
way as the earth, then, as the traveller approached the end of his journey,
and came within a few hundred miles of the moon's surface, he would meet
again with traces of an atmosphere, which would gradually increase in
density until he arrived at the moon's surface. The traveller would thus have
passed through one stratum of air at the beginning of his journey, and
through another at the end, while the main portion of the voyage would
have been through space more void than that to be found in the exhausted
receiver of an air-pump.

Such would be the case if the moon were coated with an atmosphere like
that surrounding our earth. But what are the facts? The traveller as he drew
near the moon would seek in vain for air to breathe at all resembling ours. It
is possible that close to the surface there are faint traces of some gaseous
material surrounding the moon, but it can only be equal to a very small
fractional part of the ample clothing which the earth now enjoys. For all
purposes of respiration, as we understand the term, we may say that there is
no air on the moon, and an inhabitant of our earth transferred thereto would
be as certainly suffocated as he would be in the middle of space.

It may, however, be asked how we learn this. Is not air transparent, and
how, therefore, could our telescopes be expected to show whether the moon
really possessed such an envelope? It is by indirect, but thoroughly reliable,
methods of observation that we learn the destitute condition of our satellite.
There are various arguments to be adduced; but the most conclusive is that
obtained on the occurrence of what is called an "occultation." It sometimes
happens that the moon comes directly between the earth and a star, and the
temporary extinction of the latter is an "occultation." We can observe the
moment when the phenomenon takes place, and the suddenness of the
disappearance of the star is generally remarked. If the moon were
enveloped in a copious atmosphere, the interposition of this gaseous mass
by the movement of the moon would produce a gradual evanescence of the
star wholly wanting the abruptness which marks the obscuration.[9]

Let us consider how we can account for the absence of an atmosphere
from the moon. What we call a gas has been found by modern research to
be a collection of an immense number of molecules, each of which is in
exceedingly rapid motion. This motion is only pursued for a short distance
in one direction before a molecule comes into collision with some other
molecule, whereby the directions and velocities of the individual molecules
are continually changed. There is a certain average speed for each gas
which is peculiar to the molecules of that gas at a certain temperature.
When several gases are mixed, as oxygen and nitrogen are in our
atmosphere, the molecules of each gas continue to move with their own
characteristic velocities. So far as we can estimate the temperature at the
boundary of the earth's atmosphere, we may assume that the average of the
velocities of the oxygen molecules there found is about a quarter of a mile
per second. The velocities for nitrogen are much the same, while the
average speed of a molecule of hydrogen is about one mile per second,
being, in fact, by far the greatest molecular velocity possessed by any gas.

PLATE IX.
PLATO.
(AFTER NASMYTH.)

A stone thrown into the air soon regains the earth. A rifle bullet fired
vertically upwards will ascend higher and higher, until at length its motion
ceases, it begins to return, and falls to the ground. Let us for the moment
suppose that we had a rifle of infinite strength and gunpowder of unlimited
power. As we increase the charge we find that the bullet will ascend higher
and higher, and each time it will take a longer period before it returns to the
ground. The descent of the bullet is due to the attraction of the earth.
Gravitation must necessarily act on the projectile throughout its career, and
it gradually lessens the velocity, overcomes the upward motion, and brings

the bullet back. It must be remembered that the efficiency of the attraction
decreases when the height is increased. Consequently when the body has a
prodigiously great initial velocity, in consequence of which it ascends to an
enormous height, its return is retarded by a twofold cause. In the first place,
the distance through which it has to be recalled is greatly increased, and in
the second place the efficiency of gravitation in effecting its recall has
decreased. The greater the velocity, the feebler must be the capacity of
gravitation for bringing back the body. We can conceive the speed to be
increased to that point at which the gravitation, constantly declining as the
body ascends, is never quite able to neutralise the velocity, and hence we
have the remarkable case of a body projected away never to return.

It is possible to exhibit this reasoning in a numerical form, and to show
that a velocity of six or seven miles a second directed upwards would
suffice to convey a body entirely away from the gravitation of the earth.
This speed is far beyond the utmost limits of our artillery. It is, indeed, at
least a dozen times as swift as a cannon shot; and even if we could produce
it, the resistance of the air would present an insuperable difficulty. Such
reflections, however, do not affect the conclusion that there is for each
planet a certain specific velocity appropriate to that body, and depending
solely upon its size and mass, with which we should have to discharge a
projectile, in order to prevent the attraction of that body from pulling the
projectile back again.

It is a simple matter of calculation to determine this "critical velocity"
for any celestial body. The greater the body the greater in general must be
the initial speed which will enable the projectile to forsake for ever the
globe from which it has been discharged. As we have already indicated, this
speed is about seven miles per second on the earth. It would be three on the
planet Mercury, three and a half on Mars, twenty-two on Saturn, and thirty-
seven on Jupiter; while for a missile to depart from the sun without prospect
of return, it must leave the brilliant surface at a speed not less than 391
miles per second.

Supposing that a quantity of free hydrogen was present in our
atmosphere, its molecules would move with an average velocity of about
one mile per second. It would occasionally happen by a combination of
circumstances that a molecule would attain a speed which exceeded seven
miles a second. If this happened on the confines of the atmosphere where it
escaped collision with other molecules, the latter object would fly off into
space, and would not be recaptured by the earth. By incessant repetitions of
this process, in the course of countless ages, all the molecules of hydrogen
gas would escape from the earth, and in this manner we may explain the
fact that there is no free hydrogen present in the earth's atmosphere.[10]

The velocities which can be attained by the molecules of gases other
than hydrogen are far too small to permit of their escape from the attraction
of the earth. We therefore find oxygen, nitrogen, water vapour, and carbon
dioxide remaining as permanent components of our air. On the other hand,
the enormous mass of the sun makes the "critical velocity" at the surface of
that body to be so great (391 miles per second) that not even the molecules
of hydrogen can possibly emulate it. Consequently, as we have seen,
hydrogen is a most important component of the sun's atmospheric envelope.

If we now apply this reasoning to the moon, the critical velocity is found
by calculation to be only a mile and a half per second. This seems to be well
within the maximum velocities attainable by the molecules of oxygen,
nitrogen, and other gases. It therefore follows that none of these gases could
remain permanently to form an atmosphere at the surface of so small a body
as the moon. This seems to be the reason why there are no present traces of
any distinct gaseous surroundings to our satellite.

The absence of air and of water from the moon explains the sublime
ruggedness of the lunar scenery. We know that on the earth the action of
wind and of rain, of frost and of snow, is constantly tending to wear down
our mountains and reduce their asperities. No such agents are at work on
the moon. Volcanoes sculptured the surface into its present condition, and,
though they have ceased to operate for ages, the traces of their handiwork

seem nearly as fresh to-day as they were when the mighty fires were
extinguished.

"The cloud-capped towers, the gorgeous palaces, the solemn temples"
have but a brief career on earth. It is chiefly the incessant action of water
and of air that makes them vanish like the "baseless fabric of a vision." On
the moon these causes of disintegration and of decay are all absent, though
perhaps the changes of temperature in the transition from lunar day to lunar
night would be attended with expansions and contractions that might
compensate in some slight degree for the absence of more potent agents of
dissolution.

It seems probable that a building on the moon would remain for century
after century just as it was left by the builders. There need be no glass in the
windows, for there is no wind and no rain to keep out. There need not be
fireplaces in the rooms, for fuel cannot burn without air. Dwellers in a lunar
city would find that no dust could rise, no odours be perceived, no sounds
be heard.

Man is a creature adapted for life under circumstances which are very
narrowly limited. A few degrees of temperature more or less, a slight
variation in the composition of air, the precise suitability of food, make all
the difference between health and sickness, between life and death. Looking
beyond the moon, into the length and breadth of the universe, we find
countless celestial globes with every conceivable variety of temperature and
of constitution. Amid this vast number of worlds with which space is
tenanted, are there any inhabited by living beings? To this great question
science can make no response: we cannot tell. Yet it is impossible to resist a
conjecture. We find our earth teeming with life in every part. We find life
under the most varied conditions that can be conceived. It is met with under
the burning heat of the tropics and in the everlasting frost at the poles. We
find life in caves where not a ray of light ever penetrates. Nor is it wanting
in the depths of the ocean, at the pressure of tons on the square inch.

Whatever may be the external circumstances, Nature generally provides
some form of life to which those circumstances are congenial.

It is not at all probable that among the million spheres of the universe
there is a single one exactly like our earth—like it in the possession of air
and of water, like it in size and in composition. It does not seem probable
that a man could live for one hour on any body in the universe except the
earth, or that an oak-tree could live in any other sphere for a single season.
Men can dwell on the earth, and oak-trees can thrive therein, because the
constitutions of the man and of the oak are specially adapted to the
particular circumstances of the earth.

Could we obtain a closer view of some of the celestial bodies, we should
probably find that they, too, teem with life, but with life specially adapted to
the environment—life in forms strange and weird; life far stranger to us
than Columbus found it to be in the New World when he first landed there.
Life, it may be, stranger than ever Dante described or Doré sketched.
Intelligence may also have a home among those spheres no less than on the
earth. There are globes greater and globes less—atmospheres greater and
atmospheres less. The truest philosophy on this subject is crystallised in the
language of Tennyson:—

"This truth within thy mind rehearse,
That in a boundless universe
Is boundless better, boundless worse.

"Think you this mould of hopes and fears
Could find no statelier than his peers
In yonder hundred million spheres?"

PLATE X.
TYCHO AND ITS SURROUNDINGS.
(AFTER NASMYTH.)

CHAPTER IV.
THE SOLAR SYSTEM.
Exceptional Importance of the Sun and Moon—The Course to be pursued—The Order of
Distance—The Neighbouring Orbs—How are they to be discriminated?—The Planets
Venus and Jupiter attract Notice by their Brilliancy—Sirius not a Neighbour—The
Planets Saturn and Mercury—Telescopic Planets—The Criterion as to whether a
Body is to be ranked as a Neighbour—Meaning of the word Planet—Uranus and
Neptune—Comets—The Planets are illuminated by the Sun—The Stars are not—The
Earth is really a Planet—The Four Inner Planets, Mercury, Venus, the Earth, and Mars
—Velocity of the Earth—The Outer Planets, Jupiter, Saturn, Uranus, Neptune—Light
and Heat received by the Planets from the Sun—Comparative Sizes of the Planets—
The Minor Planets—The Planets all revolve in the same Direction—The Solar System
—An Island Group in Space.

In the two preceding chapters of this work we have endeavoured to describe
the heavenly bodies in the order of their relative importance to mankind.
Could we doubt for a moment as to which of the many orbs in the universe
should be the first to receive our attention? We do not now allude to the
intrinsic significance of the sun when compared with other bodies or groups
of bodies scattered through space. It may be that numerous globes rival the
sun in real splendour, in bulk, and in mass. We shall, in fact, show later on
in this volume that this is the case; and we shall then be in a position to
indicate the true rank of the sun amid the countless hosts of heaven. But
whatever may be the importance of the sun, viewed merely as one of the
bodies which teem through space, there can be no hesitation in asserting
how immeasurably his influence on the earth surpasses that of all other
bodies in the universe together. It was therefore natural—indeed inevitable
—that our first examination of the orbs of heaven should be directed to that
mighty body which is the source of our life itself.

Nor could there be much hesitation as to the second step which ought to
be taken. The intrinsic importance of the moon, when compared with other

celestial bodies, may be small; it is, indeed, as we shall afterwards see,
almost infinitesimal. But in the economy of our earth the moon has played,
and still plays, a part second only in importance to that of the sun himself.
The moon is so close to us that her brilliant rays pale to invisibility
countless orbs of a size and an intrinsic splendour incomparably greater
than her own. The moon also occupies an exceptional position in the history
of astronomy; for the law of gravitation, the greatest discovery that science
has yet witnessed, was chiefly accomplished by observations of the moon.
It was therefore natural that an early chapter in our Story of the Heavens
should be devoted to a body the interest of which approximated so closely
to that of the sun himself.

But the sun and the moon having been partly described (we shall
afterwards have to refer to them again), some hesitation is natural in the
choice of the next step. The two great luminaries being abstracted from our
view, there remains no other celestial body of such exceptional interest and
significance as to make it quite clear what course to pursue; we desire to
unfold the story of the heavens in the most natural manner. If we made the
attempt to describe the celestial bodies in the order of their actual
magnitude, our ignorance must at once pronounce the task to be impossible.
We cannot even make a conjecture as to which body in the heavens is to
stand first on the list. Even if that mightiest body be within reach of our
telescopes (in itself a highly improbable supposition), we have not the least
idea in what part of the heavens it is to be sought. And even if this were
possible—if we were able to arrange all the visible bodies rank by rank in
the order of their magnitude and their splendour—still the scheme would be
impracticable, for of most of them we know little or nothing.

We are therefore compelled to adopt a different method of procedure,
and the simplest, as well as the most natural, will be to follow as far as
possible the order of distance of the different bodies. We have already
spoken of the moon as the nearest neighbour to the earth; we shall next
consider some of the other celestial bodies which are comparatively near to
us; then, as the subject unfolds, we shall discuss the objects further and

further away, until towards the close of the volume we shall be engaged in
considering the most distant bodies in the universe which the telescope has
yet revealed to us.

Even when we have decided on this principle, our course is still not free
from ambiguity. Many of the bodies in the heavens are in motion, so that
their relative distances from the earth are in continual change; this is,
however, a difficulty which need not detain us. We shall make no attempt to
adhere closely to the principle in all details. It will be sufficient if we first
describe those great bodies—not a very numerous class—which are,
comparatively speaking, in our vicinity, though still at varied distances; and
then we shall pass on to the uncounted bodies which are separated from us
by distances so vast that the imagination is baffled in the attempt to realise
them.

Let us, then, scan the heavens to discover those orbs which lie in our
neighbourhood. The sun has set, the moon has not risen; a cloudless sky
discloses a heaven glittering with countless gems of light. Some are
grouped together into well-marked constellations; others seem scattered
promiscuously, with every degree of lustre, from the very brightest down to
the faintest point that the eye can just glimpse. Amid all this host of objects,
how are we to identify those which lie nearest to the earth? Look to the
west: and there, over the spot where the departing sunbeams still linger, we
often see the lovely evening star shining forth. This is the planet Venus—a
beauteous orb, twin-sister to the earth. The brilliancy of this planet, its rapid
changes both in position and in lustre, would suggest at once that it was
much nearer to the earth than other star-like objects. This presumption has
been amply confirmed by careful measurements, and therefore Venus is to
be included in the list of the orbs which constitute our neighbours.

Another conspicuous planet—almost rivalling Venus in lustre, and vastly
surpassing Venus in the magnificence of its proportions and its retinue—has
borne from antiquity the majestic name of Jupiter. No doubt Jupiter is much
more distant from us than Venus. Indeed, he is always at least twice as far,

and sometimes as much as ten times. But still we must include Jupiter
among our neighbours. Compared with the host of stars which glitter on the
heavens, Jupiter must be regarded as quite contiguous. The distance of the
great planet requires, it is true, hundreds of millions of miles for its
expression; yet, vast as is that distance, it would have to be multiplied by
tens of thousands, or hundreds of thousands, before it would be long
enough to span the abyss which intervenes between the earth and the
nearest of the stars.

Venus and Jupiter have invited our attention by their exceptional
brilliancy. We should, however, fall into error if we assumed generally that
the brightest objects were those nearest to the earth. An observer
unacquainted with astronomy might not improbably point to the Dog Star—
or Sirius, as astronomers more generally know it—as an object whose
exceptional lustre showed it to be one of our neighbours. This, however,
would be a mistake. We shall afterwards have occasion to refer more
particularly to this gem of our southern skies, and then it will appear that
Sirius is a mighty globe far transcending our own sun in size as well as in
splendour, but plunged into the depths of space to such an appalling
distance that his enfeebled rays, when they reach the earth, give us the
impression, not of a mighty sun, but only of a brilliant star.

The principle of selection, by which the earth's neighbours can be
discriminated, will be explained presently; in the meantime, it will be
sufficient to observe that our list is to be augmented first by the addition of
the unique object known as Saturn, though its brightness is far surpassed by
that of Sirius, as well as by a few other stars. Then we add Mars, an object
which occasionally approaches so close to the earth that it shines with a
fiery radiance which would hardly prepare us for the truth that this planet is
intrinsically one of the smallest of the celestial bodies. Besides the objects
we have mentioned, the ancient astronomers had detected a fifth, known as
Mercury—a planet which is usually invisible amid the light surrounding the
sun. Mercury, however, occasionally wanders far enough from our luminary
to be seen before sunrise or after sunset. These five—Mercury, Venus,

Mars, Jupiter, and Saturn—comprised the planets known from remote
antiquity.

We can, however, now extend the list somewhat further by adding to it
the telescopic objects which have in modern times been found to be among
our neighbours. Here we must no longer postpone the introduction of the
criterion by which we can detect whether a body is near the earth or not.
The brighter planets can be recognised by the steady radiance of their light
as contrasted with the incessant twinkling of the stars. A little attention
devoted to any of the bodies we have named will, however, point out a
more definite contrast between the planets and the stars.

Observe, for instance, Jupiter, on any clear night when the heavens can
be well seen, and note his position with regard to the constellations in his
neighbourhood—how he is to the right of this star, or to the left of that;
directly between this pair, or directly pointed to by that. We then mark
down the place of Jupiter on a celestial map, or we make a sketch of the
stars in the neighbourhood showing the position of the planet. After a month
or two, when the observations are repeated, the place of Jupiter is to be
compared again with those stars by which it was defined. It will be found
that, while the stars have preserved their relative positions, the place of
Jupiter has changed. Hence this body is with propriety called a planet, or a
wanderer, because it is incessantly moving from one part of the starry
heavens to another. By similar comparisons it can be shown that the other
bodies we have mentioned—Venus and Mercury, Saturn and Mars—are
also wanderers, and belong to that group of heavenly bodies known as
planets. Here, then, we have the simple criterion by which the earth's
neighbours are readily to be discriminated from the stars. Each of the bodies
near the earth is a planet, or a wanderer, and the mere fact that a body is a
wanderer is alone sufficient to prove it to be one of the class which we are
now studying.

Provided with this test, we can at once make an addition to our list of
neighbours. Amid the myriad orbs which the telescope reveals, we

occasionally find one which is a wanderer. Two other mighty planets,
known as Uranus and Neptune, must thus be added to the five already
mentioned, making in all a group of seven great planets. A vastly greater
number may also be reckoned when we admit to our view bodies which not
only seem to be minute telescopic objects, but really are small globes when
compared with the mighty bulk of our earth. These lesser planets, to the
number of more than four hundred, are also among the earth's neighbours.

We should remark that another class of heavenly bodies widely differing
from the planets must also be included in our system. These are the comets,
and, indeed, it may happen that one of these erratic bodies will sometimes
draw nearer to the earth than even the closest approach ever made by a
planet. These mysterious visitors will necessarily engage a good deal of our
attention later on. For the present we confine our attention to those more
substantial globes, whether large or small, which are always termed planets.

Imagine for a moment that some opaque covering could be clasped
around our sun so that all his beams were extinguished. That our earth
would be plunged into the darkness of midnight is of course an obvious
consequence. A moment's consideration will show that the moon, shining as
it does by the reflected rays of the sun, would become totally invisible. But
would this extinction of the sunlight have any other effect? Would it
influence the countless brilliant points that stud the heavens at midnight?
Such an obscuration of the sun would indeed produce a remarkable effect
on the sky at night, which a little attention would disclose. The stars, no
doubt, would not exhibit the slightest change in brilliancy. Each star shines
by its own light and is not indebted to the sun. The constellations would
thus twinkle on as before, but a wonderful change would come over the
planets. Were the sun to be obscured, the planets would also disappear from
view. The midnight sky would thus experience the effacement of the planets
one by one, while the stars would remain unaltered. It may seem difficult to
realise how the brilliancy of Venus or the lustre of Jupiter have their origin
solely in the beams which fall upon these bodies from the distant sun. The
evidence is, however, conclusive on the question; and it will be placed

before the reader more fully when we come to discuss the several planets in
detail.

Suppose that we are looking at Jupiter high in mid-heavens on a winter's
night, it might be contended that, as the earth lies between Jupiter and the
sun, it must be impossible for the rays of the sun to fall upon the planet.
This is, perhaps, not an unnatural view for an inhabitant of this earth to
adopt until he has become acquainted with the relative sizes of the various
bodies concerned, and with the distances by which those bodies are
separated. But the question would appear in a widely different form to an
inhabitant of the planet Jupiter. If such a being were asked whether he
suffered much inconvenience by the intrusion of the earth between himself
and the sun, his answer would be something of this kind:—"No doubt such
an event as the passage of the earth between me and the sun is possible, and
has occurred on rare occasions separated by long intervals; but so far from
the transit being the cause of any inconvenience, the whole earth, of which
you think so much, is really so minute, that when it did come in front of the
sun it was merely seen as a small telescopic point, and the amount of
sunlight which it intercepted was quite inappreciable."

The fact that the planets shine by the sun's light points at once to the
similarity between them and our earth. We are thus led to regard our sun as
a central fervid globe associated with a number of much smaller bodies,
each of which, being dark itself, is indebted to the sun both for light and for
heat.

That was, indeed, a grand step in astronomy which demonstrated the
nature of the solar system. The discovery that our earth must be a globe
isolated in space was in itself a mighty exertion of human intellect; but
when it came to be recognised that this globe was but one of a whole group
of similar objects, some smaller, no doubt, but others very much larger, and
when it was further ascertained that these bodies were subordinated to the
supreme control of the sun, we have a chain of discoveries that wrought a
fundamental transformation in human knowledge.

We thus see that the sun presides over a numerous family. The members
of that family are dependent upon the sun, and their dimensions are suitably
proportioned to their subordinate position. Even Jupiter, the largest member
of that family, does not contain one-thousandth part of the material which
forms the vast bulk of the sun. Yet the bulk of Jupiter alone would exceed
that of the rest of the planets were they all rolled together.

Around the central luminary in Fig. 31 we have drawn four circles in
dotted lines which sufficiently illustrate the orbits in which the different
bodies move. The innermost of these four paths represents the orbit of the
planet Mercury. The planet moves around the sun in this path, and regains
the place from which it started in eighty-eight days.

The next orbit, proceeding outwards from the sun, is that of the planet
Venus, which we have already referred to as the well-known Evening Star.
Venus completes the circuit of its path in 225 days. One step further from
the sun and we come to the orbit of another planet. This body is almost the
same size as Venus, and is therefore much larger than Mercury. The planet
now under consideration accomplishes each revolution in 365 days. This
period sounds familiar to our ears. It is the length of the year; and the planet
is the earth on which we stand. There is an impressive way in which to
realise the length of the road along which the earth has to travel in each
annual journey. The circumference of a circle is about three and one-
seventh times the diameter of the same figure; so that taking the distance
from the earth to the centre of the sun as 92,900,000 miles, the diameter of
the circle which the earth describes around the sun will be 185,800,000
miles, and consequently the circumference of the mighty circle in which the
earth moves round the sun is fully 583,000,000 miles. The earth has to
travel this distance every year. It is merely a sum in division to find how far
we have to move each second in order to accomplish this long journey in a
twelvemonth. It will appear that the earth must actually complete eighteen
miles every second, as otherwise it would not finish its journey within the
allotted time.

Fig. 31.—The Orbits of the Four Interior Planets.
Pause for a moment to think what a velocity of eighteen miles a second
really implies. Can we realise a speed so tremendous? Let us compare it
with our ordinary types of rapid movement. Look at that express train how
it crashes under the bridge, how, in another moment, it is lost to view! Can
any velocity be greater than that? Let us try it by figures. The train moves a
mile a minute; multiply that velocity by eighteen and it becomes eighteen
miles a minute, but we must further multiply it by sixty to make it eighteen
miles a second. The velocity of the express train is not even the thousandth
part of the velocity of the earth. Let us take another illustration. We stand at
the rifle ranges to see a rifle fired at a target 1,000 feet away, and we find

that a second or two is sufficient to carry the bullet over that distance. The
earth moves nearly one hundred times as fast as the rifle bullet.

Fig. 32.—The Earth's Movement.
Viewed in another way, the stupendous speed of the earth does not seem
immoderate. The earth is a mighty globe, so great indeed that even when
moving at this speed it takes almost eight minutes to pass over its own
diameter. If a steamer required eight minutes to traverse a distance equal to
its own length, its pace would be less than a mile an hour. To illustrate this
method of considering the subject, we show here a view of the progress
made by the earth (Fig. 32). The distance between the centres of these
circles is about six times the diameter; and, accordingly, if they be taken to
represent the earth, the time required to pass from one position to the other
is about forty-eight minutes.

Outside the path of the earth, we come to the orbit of the fourth planet,
Mars, which requires 687 days, or nearly two years, to complete its circuit
round the sun. With our arrival at Mars we have gained the limit to the inner
portion of the solar system.

The four planets we have mentioned form a group in themselves,
distinguished by their comparative nearness to the sun. They are all bodies
of moderate dimensions. Venus and the Earth are globes of about the same
size. Mercury and Mars are both smaller objects which lie, so far as bulk is
concerned, between the earth and the moon. The four planets which come
nearest to the sun are vastly surpassed in bulk and weight by the giant
bodies of our system—the stately group of Jupiter and Saturn, Uranus and
Neptune.

Fig. 33.—The Orbits of the Four Giant Planets.

These giant planets enjoy the sun's guidance equally with their weaker
brethren. In the diagram on this page (Fig. 33) parts of the orbits of the
great outer planets are represented. The sun, as before, presides at the
centre, but the inner planets would on this scale be so close to the sun that it
is only possible to represent the orbit of Mars. After the orbit of Mars comes
a considerable interval, not, however, devoid of planetary activity, and then
follow the orbits of Jupiter and Saturn; further still, we have Uranus, a great
globe on the verge of unassisted vision; and, lastly, the whole system is
bounded by the grand orbit of Neptune—a planet of which we shall have a
marvellous story to narrate.

The various circles in Fig. 34 show the apparent sizes of the sun as seen
from the different planets. Taking the circle corresponding to the earth to

represent the amount of heat and light which the earth derives from the sun
then the other circles indicate the heat and the light enjoyed by the
corresponding planets. The next outer planet to the earth is Mars, whose
share of solar blessings is not so very inferior to that of the earth; but we
fail to see how bodies so remote as Jupiter or Saturn can enjoy climates at
all comparable with those of the planets which are more favourably
situated.

Fig. 34.—Comparative Apparent Size of the Sun as seen from
the Various Planets.
Fig. 35 shows a picture of the whole family of planets surrounding the
sun—represented on the same scale, so as to exhibit their comparative sizes.

Measured by bulk, Jupiter is more than 1,200 times as great as the earth, so
that it would take at least 1,200 earths rolled into one to form a globe equal
to the globe of Jupiter. Measured by weight, the disparity between the earth
and Jupiter, though still enormous, is not quite so great; but this is a matter
to be discussed more fully in a later chapter.

Fig. 35.—Comparative Sizes of the Planets.

Even in this preliminary survey of the solar system we must not omit to
refer to the planets which attract our attention, not by their bulk, but by their
multitude. In the ample zone bounded on the inside by the orbit of Mars and
on the outside by the orbit of Jupiter it was thought at one time that no
planet revolved. Modern research has shown that this region is tenanted, not
by one planet, but by hundreds. The discovery of these planets is a charge
which has been undertaken by various diligent astronomers of the present
day, while the discussion of their movements affords labour to other men of
science. We shall find something to learn from the study of these tiny

bodies, and especially from another small planet called Eros, which lies
nearer to the earth than the limit above indicated. A chapter will be devoted
to these objects.

But we do not propose to enter deeply into the mere statistics of the
planetary system at present. Were such our intention, the tables at the end of
the volume would show that ample materials are available. Astronomers
have taken an inventory of each of the planets. They have measured their
distances, the shapes of their orbits and the positions of those orbits, their
times of revolution, and, in the case of all the larger planets, their sizes and
their weights. Such results are of interest for many purposes. It is, however,
the more general features of the science which at present claim our
attention.

Let us, in conclusion, note one or two important truths with reference to
our planetary system. We have seen that all the planets revolve in nearly
circular paths around the sun. We have now to add another fact possessing
much significance. Each of the planets pursues its path in the same
direction. It thus happens that one such body may overtake another, but it
can never happen that two planets pass by each other as do the trains on
adjacent lines of railway. We shall subsequently find that the whole welfare
of our system, nay, its continuous existence, is dependent upon this
remarkable uniformity taken in conjunction with other features of the
system.

Such is our solar system; a mighty organised group of planets circulating
under the control of the sun, and completely isolated from all external
interference. No star, no constellation, has any appreciable influence on our
solar system. We constitute a little island group, separated from the nearest
stars by the most amazing distances. It may be that as the other stars are
suns, so they too may have systems of planets circulating around them; but
of this we know nothing. Of the stars we can only say that they appear to us
as points of light, and any planets they may possess must for ever remain
invisible to us, even if they were many times larger than Jupiter.

We need not repine at this limitation to our possible knowledge, for just
as we find in the solar system all that is necessary for our daily bodily
wants, so shall we find ample occupation for whatever faculties we may
possess in endeavouring to understand those mysteries of the heavens
which lie within our reach.

CHAPTER V.
THE LAW OF GRAVITATION.
Gravitation—The Falling of a Stone to the Ground—All Bodies fall equally, Sixteen Feet
in a Second—Is this true at Great Heights?—Fall of a Body at a Height of a Quarter
of a Million Miles—How Newton obtained an Answer from the Moon—His Great
Discovery—Statement of the Law of Gravitation—Illustrations of the Law—How is it
that all the Bodies in the Universe do not rush Together?—The Effect of Motion—
How a Circular Path can be produced by Attraction—General Account of the Moon's
Motion—Is Gravitation a Force of Great Intensity?—Two Weights of 50 lbs.—Two
Iron Globes, 53 Yards in Diameter, and a Mile apart, attract with a Force of 1 lb.—
Characteristics of Gravitation—Orbits of the Planets not strictly Circles—The
Discoveries of Kepler—Construction of an Ellipse—Kepler's First Law—Does a
Planet move Uniformly?—Law of the Changes of Velocity—Kepler's Second Law—
The Relation between the Distances and the Periodic Times—Kepler's Third Law—
Kepler's Laws and the Law of Gravitation—Movement in a Straight Line—A Body
unacted on by Disturbing Forces would move in a Straight Line with Constant
Velocity—Application to the Earth and the Planets—The Law of Gravitation deduced
from Kepler's Laws—Universal Gravitation.

Our description of the heavenly bodies must undergo a slight interruption,
while we illustrate with appropriate detail an important principle, known as
the law of gravitation, which underlies the whole of astronomy. By this law
we can explain the movements of the moon around the earth, and of the
planets around the sun. It is accordingly incumbent upon us to discuss this
subject before we proceed to the more particular account of the separate
planets. We shall find, too, that the law of gravitation sheds some much-
needed light on the nature of the stars situated at the remotest distances in
space. It also enables us to cast a glance through the vistas of time past, and
to trace with plausibility, if not with certainty, certain early phases in the
history of our system. The sun and the moon, the planets and the comets,
the stars and the nebulæ, all alike are subject to this universal law, which is
now to engage our attention.

What is more familiar than the fact that when a stone is dropped it will
fall to the ground? No one at first thinks the matter even worthy of remark.
People are often surprised at seeing a piece of iron drawn to a magnet. Yet
the fall of a stone to the ground is the manifestation of a force quite as
interesting as the force of magnetism. It is the earth which draws the stone,
just as the magnet draws the iron. In each case the force is one of attraction;
but while the magnetic attraction is confined to a few substances, and is of
comparatively limited importance, the attraction of gravitation is significant
throughout the universe.

Let us commence with a few very simple experiments upon the force of
gravitation. Hold in the hand a small piece of lead, and then allow it to drop
upon a cushion. The lead requires a certain time to move from the fingers to
the cushion, but that time is always the same when the height is the same.
Take now a larger piece of lead, and hold one piece in each hand at the
same height. If both are released at the same moment, they will both reach
the cushion simultaneously. It might have been thought that the heavy body
would fall more quickly than the light body; but when the experiment is
tried, it is seen that this is not the case. Repeat the experiment with various
other substances. An ordinary marble will be found to fall in the same time
as the piece of lead. With a piece of cork we again try the experiment, and
again obtain the same result. At first it seems to fail when we compare a
feather with the piece of lead; but that is solely on account of the air, which
resists the feather more than it resists the lead. If, however, the feather be
placed upon the top of a penny, and the penny be horizontal when dropped,
it will clear the air out of the way of the feather in its descent, and then the
feather will fall as quickly as the penny, as quickly as the marble, or as
quickly as the lead.

If the observer were in a gallery when trying these experiments, and if
the cushion were sixteen feet below his hands, then the time the marble
would take to fall through the sixteen feet would be one second. The time
occupied by the cork or by the lead would be the same; and even the feather
itself would fall through sixteen feet in one second, if it could be screened

from the interference of the air. Try this experiment where we like, in
London, or in any other city, in any island or continent, on board a ship at
sea, at the North Pole, or the South Pole, or the equator, it will always be
found that any body, of any size or any material, will fall about sixteen feet
in one second of time.

Lest any erroneous impression should arise, we may just mention that
the distance traversed in one second does vary slightly at different parts of
the earth, but from causes which need not at this moment detain us. We
shall for the present regard sixteen feet as the distance through which any
body, free from interference, would fall in one second at any part of the
earth's surface. But now let us extend our view above the earth's surface,
and enquire how far this law of sixteen feet in a second may find obedience
elsewhere. Let us, for instance, ascend to the top of a mountain and try the
experiment there. It would be found that at the top of the mountain a marble
would take a little longer to fall through sixteen feet than the same marble
would if let fall at its base. The difference would be very small; but yet it
would be measurable, and would suffice to show that the power of the earth
to pull the marble to the ground becomes somewhat weakened at a point
high above the earth's surface. Whatever be the elevation to which we
ascend, be it either the top of a high mountain, or the still greater altitudes
that have been reached in balloon ascents, we shall never find that the
tendency of bodies to fall to the ground ceases, though no doubt the higher
we go the more is that tendency weakened. It would be of great interest to
find how far this power of the earth to draw bodies towards it can really
extend. We cannot attain more than about five or six miles above the earth's
surface in a balloon; yet we want to know what would happen if we could
ascend 500 miles, or 5,000 miles, or still further, into the regions of space.

Conceive that a traveller were endowed with some means of soaring
aloft for miles and thousands of miles, still up and up, until at length he had
attained the awful height of nearly a quarter of a million of miles above the
ground. Glancing down at the surface of that earth, which is at such a
stupendous depth beneath, he would be able to see a wonderful bird's-eye

view. He would lose, no doubt, the details of towns and villages; the
features in such a landscape would be whole continents and whole oceans,
in so far as the openings between the clouds would permit the earth's
surface to be exposed.

At this stupendous elevation he could try one of the most interesting
experiments that was ever in the power of a philosopher. He could test
whether the earth's attraction was felt at such a height, and he could
measure the amount of that attraction. Take for the experiment a cork, a
marble, or any other object, large or small; hold it between the fingers, and
let it go. Everyone knows what would happen in such a case down here; but
it required Sir Isaac Newton to tell what would happen in such a case up
there. Newton asserts that the power of the earth to attract bodies extends
even to this great height, and that the marble would fall. This is the doctrine
that we can now test. We are ready for the experiment. The marble is
released, and, lo! our first exclamation is one of wonder. Instead of
dropping instantly, the little object appears to remain suspended. We are on
the point of exclaiming that we must have gone beyond the earth's
attraction, and that Newton is wrong, when our attention is arrested; the
marble is beginning to move, so slowly that at first we have to watch it
carefully. But the pace gradually improves, so that the attraction is beyond
all doubt, until, gradually acquiring more and more velocity, the marble
speeds on its long journey of a quarter of a million of miles to the earth.

But surely, it will be said, such an experiment must be entirely
impossible; and no doubt it cannot be performed in the way described. The
bold idea occurred to Newton of making use of the moon itself, which is
almost a quarter of a million of miles above the earth, for the purpose of
answering the question. Never was our satellite put to such noble use
before. It is actually at each moment falling in towards the earth. We can
calculate how much it is deflected towards the earth in each second, and
thus obtain a measure of the earth's attractive power. From such enquiries
Newton was able to learn that a body released at the distance of 240,000
miles above the surface of the earth would still be attracted by the earth,

that in virtue of the attraction the body would commence to move off
towards the earth—not, indeed, with the velocity with which a body falls in
experiments on the surface, but with a very much lesser speed. A body
dropped down from the distance of the moon would commence its long
journey so slowly that a minute, instead of a second, would have elapsed
before the distance of sixteen feet had been accomplished.[11]

It was by pondering on information thus won from the moon that
Newton made his immortal discovery. The gravitation of the earth is a force
which extends far and wide through space. The more distant the body, the
weaker the gravitation becomes; here Newton found the means of
determining the great problem as to the law according to which the intensity
of the gravitation decreased. The information derived from the moon, that a
body 240,000 miles away requires a minute to fall through a space equal to
that through which it would fall in a second down here, was of paramount
importance. In the first place, it shows that the attractive power of the earth,
by which it draws all bodies earthwards, becomes weaker at a distance. This
might, indeed, have been anticipated. It is as reasonable to suppose that as
we retreated further and further into the depths of space the power of
attraction should diminish, as that the lustre of light should diminish as we
recede from it; and it is remarkable that the law according to which the
attraction of gravitation decreases with the increase of distance is precisely
the same as the law according to which the brilliancy of a light decreases as
its distance increases.

The law of nature, stated in its simplest form, asserts that the intensity of
gravitation varies inversely as the square of the distance. Let me endeavour
to elucidate this somewhat abstract statement by one or two simple
illustrations. Suppose a body were raised above the surface of the earth to a
height of nearly 4,000 miles, so as to be at an altitude equal to the radius of
the earth. In other words, a body so situated would be twice as far from the
centre of the earth as a body which lay on the surface. The law of
gravitation says that the intensity of the attraction is then to be decreased to
one-fourth part, so that the pull of the earth on a body 4,000 miles high is

only one quarter of the pull of the earth on that body so long as it lies on the
ground. We may imagine the effect of this pull to be shown in different
ways. Allow the body to fall, and in the interval of one second it will only
drop through four feet, a mere quarter of the distance that gravity would
cause near the earth's surface.

We may consider the matter in another way by supposing that the
attraction of the earth is measured by one of those little weighing machines
known as a spring balance. If a weight of four pounds be hung on such a
contrivance, at the earth's surface, the index of course shows a weight of
four pounds; but conceive this balance, still bearing the weight appended
thereto, were to be carried up and up, the indicated strain would become
less and less, until by the time the balance reached 4,000 miles high, where
it was twice as far away from the earth's centre as at first, the indicated
strain would be reduced to the fourth part, and the balance would only show
one pound. If we could imagine the instrument to be carried still further into
the depths of space, the indication of the scale would steadily continue to
decline. By the time the apparatus had reached a distance of 8,000 miles
high, being then three times as far from the earth's centre as at first, the law
of gravitation tells us that the attraction must have decreased to one-ninth
part. The strain thus shown on the balance would be only the ninth part of
four pounds, or less than half a pound. But let the voyage be once again
resumed, and let not a halt be made this time until the balance and its four-
pound weight have retreated to that orbit which the moon traverses in its
monthly course around the earth. The distance thus attained is about sixty
times the radius of the earth, and consequently the attraction of gravitation
is diminished in the proportion of one to the square of sixty; the spring will
then only be strained by the inappreciable fraction of 1-3,600 part of four
pounds. It therefore appears that a weight which on the earth weighed a ton
and a half would, if raised 240,000 miles, weigh less than a pound. But
even at this vast distance we are not to halt; imagine that we retreat still
further and further; the strain shown by the balance will ever decrease, but
it will still exist, no matter how far we go. Astronomy appears to teach us

that the attraction of gravitation can extend, with suitably enfeebled
intensity, across the most profound gulfs of space.

The principle of gravitation is of far wider scope than we have yet
indicated. We have spoken merely of the attraction of the earth, and we
have stated that this force extends throughout space. But the law of
gravitation is not so limited. Not only does the earth attract every other
body, and every other body attract the earth, but each of these bodies
attracts the other; so that in its more complete shape the law of gravitation
announces that "every body in the universe attracts every other body with a
force which varies inversely as the square of the distance."

It is impossible for us to over-estimate the importance of this law. It
supplies the clue by which we can unravel the complicated movements of
the planets. It has led to marvellous discoveries, in which the law of
gravitation has enabled us to anticipate the telescope, and to feel the
existence of bodies before those bodies have even been seen.

An objection which may be raised at this point must first be dealt with. It
seems to be, indeed, a plausible one. If the earth attracts the moon, why
does not the moon tumble down on the earth? If the earth is attracted by the
sun, why does it not tumble into the sun? If the sun is attracted by other
stars, why do they not rush together with a frightful collision? It may not
unreasonably be urged that if all these bodies in the heavens are attracting
each other, it would seem that they must all rush together in consequence of
that attraction, and thus weld the whole material universe into a single
mighty mass. We know, as a matter of fact, that these collisions do not often
happen, and that there is extremely little likelihood of their taking place. We
see that although our earth is said to have been attracted by the sun for
countless ages, yet the earth is just as far from the sun as ever it was. Is not
this in conflict with the doctrine of universal gravitation? In the early days
of astronomy such objections would be regarded, and doubtless were
regarded, as well-nigh insuperable; even still we occasionally hear them
raised, and it is therefore the more incumbent on us to explain how it

happens that the solar system has been able to escape from the catastrophe
by which it seems to be threatened.

There can be no doubt that if the moon and the earth had been initially
placed at rest, they would have been drawn together by their mutual
attraction. So, too, if the system of planets surrounding the sun had been left
initially at rest they would have dashed into the sun, and the system would
have been annihilated. It is the fact that the planets are moving, and that the
moon is moving, which has enabled these bodies successfully to resist the
attraction in so far, at least, as that they are not drawn thereby to total
destruction.

It is so desirable that the student should understand clearly how a central
attraction is compatible with revolution in a nearly circular path, that we
give an illustration to show how the moon pursues its monthly orbit under
the guidance and the control of the attracting earth.

The imaginary sketch in Fig. 36 denotes a section of the earth with a
high mountain thereon.[12] If a cannon were stationed on the top of the
mountain at C, and if the cannonball were fired off in the direction C E with
a moderate charge of powder, the ball would move down along the first
curved path. If it be fired a second time with a heavier charge, the path will
be along the second curved line, and the ball would again fall to the ground.
But let us try next time with a charge still further increased, and, indeed,
with a far stronger cannon than any piece of ordnance ever yet made. The
velocity of the projectile must now be assumed to be some miles per
second, but we can conceive that the speed shall be so adjusted that the ball
shall move along the path C D, always at the same height above the earth,
though still curving, as every projectile must curve, from the horizontal line
in which it moved at the first moment. Arrived at D, the ball will still be at
the same height above the surface, and its velocity must be unabated. It will
therefore continue in its path and move round another quadrant of the circle
without getting nearer to the surface. In this manner the projectile will
travel completely round the whole globe, coming back again to C and then

taking
another
start in the
same path.
If we
could
abolish the
mountain
and the
cannon at
the top, we
should
have a
body
revolving
for ever
around the
earth in
consequen
ce of the
attraction
of
Fig. 36.—Illustration of the Moon's Motion.
gravitation
.

Make now a bold stretch of the imagination. Conceive a terrific cannon
capable of receiving a round bullet not less than 2,000 miles in diameter.
Discharge this enormous bullet with a velocity of about 3,000 feet per
second, which is two or three times as great as the velocity actually
attainable in modern artillery. Let this notable bullet be fired horizontally
from some station nearly a quarter of a million miles above the surface of
the earth. That fearful missile would sweep right round the earth in a nearly
circular orbit, and return to where it started in about four weeks. It would

then commence another revolution, four weeks more would find it again at
the starting point, and this motion would go on for ages.

Do not suppose that we are entirely romancing. We cannot indeed show
the cannon, but we can point to a great projectile. We see it every month; it
is the beautiful moon herself. No one asserts that the moon was ever shot
from such a cannon; but it must be admitted that she moves as if she had
been. In a later chapter we shall enquire into the history of the moon, and
show how she came to revolve in this wonderful manner.

As with the moon around the earth, so with the earth around the sun. The
illustration shows that a circular or nearly circular motion harmonises with
the conception of the law of universal gravitation.

We are accustomed to regard gravitation as a force of stupendous
magnitude. Does not gravitation control the moon in its revolution around
the earth? Is not even the mighty earth itself retained in its path around the
sun by the surpassing power of the sun's attraction? No doubt the actual
force which keeps the earth in its path, as well as that which retains the
moon in our neighbourhood, is of vast intensity, but that is because
gravitation is in such cases associated with bodies of enormous mass. No
one can deny that all bodies accessible to our observation appear to attract
each other in accordance with the law of gravitation; but it must be
confessed that, unless one or both of the attracting bodies is of gigantic
dimensions, the intensity is almost immeasurably small.

Let us attempt to illustrate how feeble is the gravitation between masses
of easily manageable dimensions. Take, for instance, two iron weights, each
weighing about 50lb., and separated by a distance of one foot from centre to
centre. There is a certain attraction of gravitation between these weights.
The two weights are drawn together, yet they do not move. The attraction
between them, though it certainly exists, is an extremely minute force, not
at all comparable as to intensity with magnetic attraction. Everyone knows
that a magnet will draw a piece of iron with considerable vigour, but the

intensity of gravitation is very much less on masses of equal amount. The
attraction between these two 50lb. weights is less than the ten-millionth part
of a single pound. Such a force is utterly infinitesimal in comparison with
the friction between the weights and the table on which they stand, and
hence there is no response to the attraction by even the slightest movement.
Yet, if we can conceive each of these weights mounted on wheels absolutely
devoid of friction, and running on absolutely perfect horizontal rails, then
there is no doubt that the bodies would slowly commence to draw together,
and in the course of time would arrive in actual contact.

If we desire to conceive gravitation as a force of measurable intensity,
we must employ masses immensely more ponderous than those 50lb.
weights. Imagine a pair of globes, each composed of 417,000 tons of cast
iron, and each, if solid, being about 53 yards in diameter. Imagine these
globes placed at a distance of one mile apart. Each globe attracts the other
by the force of gravitation. It does not matter that buildings and obstacles of
every description intervene; gravitation will pass through such impediments
as easily as light passes through glass. No screen can be devised dense
enough to intercept the passage of this force. Each of these iron globes will
therefore under all circumstances attract the other; but, notwithstanding
their ample proportions, the intensity of that attraction is still very small,
though appreciable. The attraction between these two globes is a force no
greater than the pressure exerted by a single pound weight. A child could
hold back one of these massive globes from its attraction by the other.
Suppose that all was clear, and that friction could be so neutralised as to
permit the globes to follow the impulse of their mutual attractions. The two
globes will then commence to approach, but the masses are so large, while
the attraction is so small, that the speed will be accelerated very slowly. A
microscope would be necessary to show when the motion has actually
commenced. An hour and a half must elapse before the distance is
diminished by a single foot; and although the pace improves subsequently,
yet three or four days must elapse before the two globes will come together.

The most remarkable characteristic of the force of gravitation must be
here specially alluded to. The intensity appears to depend only on the
quantity of matter in the bodies, and not at all on the nature of the
substances of which these bodies are composed. We have described the two
globes as made of cast iron, but if either or both were composed of lead or
copper, of wood or stone, of air or water, the attractive power would still be
the same, provided only that the masses remain unaltered. In this we
observe a profound difference between the attraction of gravitation and
magnetic attraction. In the latter case the attraction is not perceptible at all
in the great majority of substances, and is only considerable in the case of
iron.

In our account of the solar system we have represented the moon as
revolving around the earth in a nearly circular path, and the planets as
revolving around the sun in orbits which are also approximately circular. It
is now our duty to give a more minute description of these remarkable
paths; and, instead of dismissing them as being nearly circles, we must
ascertain precisely in what respects they differ therefrom.

If a planet revolved around the sun in a truly circular path, of which the
sun was always at the centre, it is then obvious that the distance from the
sun to the planet, being always equal to the radius of the circle, must be of
constant magnitude. Now, there can be no doubt that the distance from the
sun to each planet is approximately constant; but when accurate
observations are made, it becomes clear that the distance is not absolutely
so. The variations in distance may amount to many millions of miles, but,
even in extreme cases, the variation in the distance of the planet is only a
small fraction—usually a very small fraction—of the total amount of that
distance. The circumstances vary in the case of each of the planets. The
orbit of the earth itself is such that the distance from the earth to the sun
departs but little from its mean value. Venus makes even a closer approach
to perfectly circular movement; while, on the other hand, the path of Mars,
and much more the path of Mercury, show considerable relative fluctuations
in the distance from the planet to the sun.

It has often been noticed that many of the great discoveries in science
have their origin in the nice observation and explanation of minute
departures from some law approximately true. We have in this department
of astronomy an excellent illustration of this principle. The orbits of the
planets are nearly circles, but they are not exactly circles. Now, why is this?
There must be some natural reason. That reason has been ascertained, and it
has led to several of the grandest discoveries that the mind of man has ever
achieved in the realms of Nature.

In the first place, let us see the inferences to be drawn from the fact that
the distance of a planet from the sun is not constant. The motion in a circle
is one of such beauty and simplicity that we are reluctant to abandon it,
unless the necessity for doing so be made clearly apparent. Can we not
devise any way by which the circular motion might be preserved, and yet be
compatible with the fluctuations in the distance from the planet to the sun?
This is clearly impossible with the sun at the centre of the circle. But
suppose the sun did not occupy the centre, while the planet, as before,
revolved around the sun. The distance between the two bodies would then
necessarily fluctuate. The more eccentric the position of the sun, the larger
would be the proportionate variation in the distance of the planet when at
the different parts of its orbit. It might further be supposed that by placing a
series of circles around the sun the various planetary orbits could be
accounted for. The centre of the circle belonging to Venus is to coincide
very nearly with the centre of the sun, and the centres of the orbits of all the
other planets are to be placed at such suitable distances from the sun as will
render a satisfactory explanation of the gradual increase and decrease of the
distance between the two bodies.

There can be no doubt that the movements of the moon and of the
planets would be, to a large extent, explained by such a system of circular
orbits; but the spirit of astronomical enquiry is not satisfied with
approximate results. Again and again the planets are observed, and again
and again the observations are compared with the places which the planets
would occupy if they moved in accordance with the system here indicated.

The centres of the circles are moved hither and thither, their radii are
adjusted with greater care; but it is all of no avail. The observations of the
planets are minutely examined to see if they can be in error; but of errors
there are none at all sufficient to account for the discrepancies. The
conclusion is thus inevitable—astronomers are forced to abandon the
circular motion, which was thought to possess such unrivalled symmetry
and beauty, and are compelled to admit that the orbits of the planets are not
circular.

Then if these orbits be not circles, what are they? Such was the great
problem which Kepler proposed to solve, and which, to his immortal glory,
he succeeded in solving and in proving to demonstration. The great
discovery of the true shape of the planetary orbits stands out as one of the
most conspicuous events in the history of astronomy. It may, in fact, be
doubted whether any other discovery in the whole range of science has led
to results of such far-reaching interest.

We must here adventure for a while into the field of science known as
geometry, and study therein the nature of that curve which the discovery of
Kepler has raised to such unparalleled importance. The subject, no doubt, is
a difficult one, and to pursue it with any detail would involve us in many
abstruse calculations which would be out of place in this volume; but a
general sketch of the subject is indispensable, and we must attempt to
render it such justice as may be compatible with our limits.

The curve which represents with perfect fidelity the movements of a
planet in its revolution around the sun belongs to that well-known group of
curves which mathematicians describe as the conic sections. The particular
form of conic section which denotes the orbit of a planet is known by the
name of the ellipse: it is spoken of somewhat less accurately as an oval. The
ellipse is a curve which can be readily constructed. There is no simpler
method of doing so than that which is familiar to draughtsmen, and which
we shall here briefly describe.

We represent on the next page (Fig. 37) two pins passing through a sheet
of paper. A loop of twine passes over the two pins in the manner here
indicated, and is stretched by the point of a pencil. With a little care the
pencil can be guided so as to keep the string stretched, and its point will
then describe a curve completely round the pins, returning to the point from
which it started. We thus produce that celebrated geometrical figure which
is called an ellipse.

It will be instructive to draw a number of ellipses, varying in each case
the circumstances under which they are formed. If, for instance, the pins
remain placed as before, while the length of the loop is increased, so that
the pencil is farther away from the pins, then it will be observed that the
ellipse has lost some of its elongation, and approaches more closely to a
circle. On the other hand, if the length of the cord in the loop be lessened,
while the pins remain as before, the ellipse will be found more oval, or, as a
mathematician would say, its eccentricity is increased. It is also useful to
study the changes which the form of the ellipse undergoes when one of the
pins is altered, while the length of the loop remains unchanged. If the two
pins be brought nearer together the eccentricity will decrease, and the
ellipse will approximate more closely to the shape of a circle. If the pins be
separated more widely the eccentricity of the ellipse will be increased. That
the circle is an extreme form of ellipse will be evident, if we suppose the
two pins to draw in so close together that they become coincident; the point
will then simply trace out a circle as the pencil moves round the figure.

The points marked by the pins obviously possess very remarkable
relations with respect to the curve. Each one is called a focus, and an ellipse
can only have one pair of foci. In other words, there is but a single pair of
positions possible for the two pins, when an ellipse of specified size, shape,
and position is to be constructed.

The ellipse differs principally from a circle in the circumstance that it
possesses variety of form. We can have large and small ellipses just as we
can have large and small circles, but we can also have ellipses of greater or

less
eccentricit
y. If the
ellipse has
not the
perfect
simplicity
of the
circle it
has, at
least, the
charm of
variety
which the
circle has
not. The
Fig. 37.—Drawing an Ellipse.
oval curve
has also
the beauty derived from an outline of perfect grace and an association with
ennobling conceptions.

The ancient geometricians had studied the ellipse: they had noticed its
foci; they were acquainted with its geometrical relations; and thus Kepler
was familiar with the ellipse at the time when he undertook his celebrated
researches on the movements of the planets. He had found, as we have
already indicated, that the movements of the planets could not be reconciled
with circular orbits. What shape of orbit should next be tried? The ellipse
was ready to hand, its properties were known, and the comparison could be
made; memorable, indeed, was the consequence of this comparison. Kepler
found that the movement of the planets could be explained, by supposing
that the path in which each one revolved was an ellipse. This in itself was a
discovery of the most commanding importance. On the one hand it reduced
to order the movements of the great globes which circulate round the sun;

while on the other, it took that beautiful class of curves which had exercised
the geometrical talents of the ancients, and assigned to them the dignity of
defining the highways of the universe.

But we have as yet only partly enunciated the first discovery of Kepler.
We have seen that a planet revolves in an ellipse around the sun, and that
the sun is, therefore, at some point in the interior of the ellipse—but at what
point? Interesting, indeed, is the answer to this question. We have pointed
out how the foci possess a geometrical significance which no other points
enjoy. Kepler showed that the sun must be situated in one of the foci of the
ellipse in which each planet revolves. We thus enunciate the first law of
planetary motion in the following words:—
Each planet revolves around the sun in an elliptic path, having the sun at one of
the foci.

We are now enabled to form a clear picture of the orbits of the planets,
be they ever so numerous, as they revolve around the sun. In the first place,
we observe that the ellipse is a plane curve; that is to say, each planet must,
in the course of its long journey, confine its movements to one plane. Each
planet has thus a certain plane appropriated to it. It is true that all these
planes are very nearly coincident, at least in so far as the great planets are
concerned; but still they are distinct, and the only feature in which they all
agree is that each one of them passes through the sun. All the elliptic orbits
of the planets have one focus in common, and that focus lies at the centre of
the sun.

It is well to illustrate this remarkable law by considering the
circumstances of two or three different planets. Take first the case of the
earth, the path of which, though really an ellipse, is very nearly circular. In
fact, if it were drawn accurately to scale on a sheet of paper, the difference
between the elliptic orbit and the circle would hardly be detected without
careful measurement. In the case of Venus the ellipse is still more nearly a
circle, and the two foci of the ellipse are very nearly coincident with the
centre of the circle. On the other hand, in the case of Mercury, we have an

ellipse which departs from the circle to a very marked extent, while in the
orbits of some of the minor planets the eccentricity is still greater. It is
extremely remarkable that every planet, no matter how far from the sun,
should be found to move in an ellipse of some shape or other. We shall
presently show that necessity compels each planet to pursue an elliptic path,
and that no other form of path is possible.

Started on its elliptic path, the planet pursues its stately course, and after
a certain duration, known as the periodic time, regains the position from
which its departure was taken. Again the planet traces out anew the same
elliptic path, and thus, revolution after revolution, an identical track is
traversed around the sun. Let us now attempt to follow the body in its
course, and observe the history of its motion during the time requisite for
the completion of one of its circuits. The dimensions of a planetary orbit are
so stupendous that the planet must run its course very rapidly in order to
finish the journey within the allotted time. The earth, as we have already
seen, has to move eighteen miles a second to accomplish one of its voyages
round the sun in the lapse of 365-1⁄4 days. The question then arises as to
whether the rate at which a planet moves is uniform or not. Does the earth,
for instance, actually move at all times with the velocity of eighteen miles a
second, or does our planet sometimes move more rapidly and sometimes
more slowly, so that the average of eighteen miles a second is still
maintained? This is a question of very great importance, and we are able to
answer it in the clearest and most emphatic manner. The velocity of a planet
is not uniform, and the variations of that velocity can be explained by the
adjoining figure (Fig. 38).

Fig. 38.—Varying Velocity of Elliptic Motion.

Let us first of all imagine the planet to be situated at that part of its path
most distant from the sun towards the right of the figure. In this position the
body's velocity is at its lowest; as the planet begins to approach the sun the
speed gradually improves until it attains its mean value. After this point has
been passed, and the planet is now rapidly hurrying on towards the sun, the
velocity with which it moves becomes gradually greater and greater, until at
length, as it dashes round the sun, its speed attains a maximum. After
passing the sun, the distance of the planet from the luminary increases, and
the velocity of the motion begins to abate; gradually it declines until the
mean value is again reached, and then it falls still lower, until the body
recedes to its greatest distance from the sun, by which time the velocity has
abated to the value from which we supposed it to commence. We thus
observe that the nearer the planet is to the sun the quicker it moves. We can,
however, give numerical definiteness to the principle according to which
the velocity of the planet varies. The adjoining figure (Fig. 39) shows a
planetary orbit, with, of course, the sun at the focus S. We have taken two

portions, A B and C D, round the ellipse, and joined their extremities to the
focus. Kepler's second law may be stated in these words:—
"Every planet moves round the sun with such a velocity at every point, that a
straight line drawn from it to the sun passes over equal areas in equal times."

Fig. 39.—Equal Areas in Equal Times.
For example, if the two shaded portions, A B S and D C S, are equal in
area, then the times occupied by the planet in travelling over the portions of
the ellipse, A B and C D, are equal. If the one area be greater than the other,
then the times required are in the proportion of the areas.

This law being admitted, the reason of the increase in the planet's
velocity when it approaches the sun is at once apparent. To accomplish a
definite area when near the sun, a larger arc is obviously necessary than at
other parts of the path. At the opposite extremity, a small arc suffices for a
large area, and the velocity is accordingly less.

These two laws completely prescribe the motion of a planet round the
sun. The first defines the path which the planet pursues; the second
describes how the velocity of the body varies at different points along its

path. But Kepler added to these a third law, which enables us to compare
the movements of two different planets revolving round the same sun.
Before stating this law, it is necessary to explain exactly what is meant by
the mean distance of a planet. In its elliptic path the distance from the sun to
the planet is constantly changing; but it is nevertheless easy to attach a
distinct meaning to that distance which is an average of all the distances.
This average is called the mean distance. The simplest way of finding the
mean distance is to add the greatest of these quantities to the least, and take
half the sum. We have already defined the periodic time of the planet; it is
the number of days which the planet requires for the completion of a
journey round its path. Kepler's third law establishes a relation between the
mean distances and the periodic times of the various planets. That relation
is stated in the following words:—
"The squares of the periodic times are proportional to the cubes of the mean
distances."

Kepler knew that the different planets had different periodic times; he
also saw that the greater the mean distance of the planet the greater was its
periodic time, and he was determined to find out the connection between
the two. It was easily found that it would not be true to say that the periodic
time is merely proportional to the mean distance. Were this the case, then if
one planet had a distance twice as great as another, the periodic time of the
former would have been double that of the latter; observation showed,
however, that the periodic time of the more distant planet exceeded twice,
and was indeed nearly three times, that of the other. By repeated trials,
which would have exhausted the patience of one less confident in his own
sagacity, and less assured of the accuracy of the observations which he
sought to interpret, Kepler at length discovered the true law, and expressed
it in the form we have stated.

To illustrate the nature of this law, we shall take for comparison the earth
and the planet Venus. If we denote the mean distance of the earth from the
sun by unity then the mean distance of Venus from the sun is 0·7233.
Omitting decimals beyond the first place, we can represent the periodic

time of the earth as 365·3 days, and the periodic time of Venus as 224·7
days. Now the law which Kepler asserts is that the square of 365·3 is to the
square of 224·7 in the same proportion as unity is to the cube of 0·7233.
The reader can easily verify the truth of this identity by actual
multiplication. It is, however, to be remembered that, as only four figures
have been retained in the expressions of the periodic times, so only four
figures are to be considered significant in making the calculations.

The most striking manner of making the verification will be to regard the
time of the revolution of Venus as an unknown quantity, and deduce it from
the known revolution of the earth and the mean distance of Venus. In this
way, by assuming Kepler's law, we deduce the cube of the periodic time by
a simple proportion, and the resulting value of 224·7 days can then be
obtained. As a matter of fact, in the calculations of astronomy, the distances
of the planets are usually ascertained from Kepler's law. The periodic time
of the planet is an element which can be measured with great accuracy; and
once it is known, then the square of the mean distance, and consequently
the mean distance itself, is determined.

Such are the three celebrated laws of Planetary Motion, which have
always been associated with the name of their discoverer. The profound
skill by which these laws were elicited from the mass of observations, the
intrinsic beauty of the laws themselves, their widespread generality, and the
bond of union which they have established between the various members of
the solar system, have given them quite an exceptional position in
astronomy.

As established by Kepler, these planetary laws were merely the results of
observation. It was found, as a matter of fact, that the planets did move in
ellipses, but Kepler assigned no reason why they should adopt this curve
rather than any other. Still less was he able to offer a reason why these
bodies should sweep over equal areas in equal times, or why that third law
was invariably obeyed. The laws as they came from Kepler's hands stood
out as three independent truths; thoroughly established, no doubt, but

unsupported by any arguments as to why these movements rather than any
others should be appropriate for the revolutions of the planets.

It was the crowning triumph of the great law of universal gravitation to
remove this empirical character from Kepler's laws. Newton's grand
discovery bound together the three isolated laws of Kepler into one
beautiful doctrine. He showed not only that those laws are true, but he
showed why they must be true, and why no other laws could have been
true. He proved to demonstration in his immortal work, the "Principia," that
the explanation of the famous planetary laws was to be sought in the
attraction of gravitation. Newton set forth that a power of attraction resided
in the sun, and as a necessary consequence of that attraction every planet
must revolve in an elliptic orbit round the sun, having the sun as one focus;
the radius of the planet's orbit must sweep over equal areas in equal times;
and in comparing the movements of two planets, it was necessary to have
the squares of the periodic times proportional to the cubes of the mean
distances.

As this is not a mathematical treatise, it will be impossible for us to
discuss the proofs which Newton has given, and which have commanded
the immediate and universal acquiescence of all who have taken the trouble
to understand them. We must here confine ourselves only to a very brief and
general survey of the subject, which will indicate the character of the
reasoning employed, without introducing details of a technical character.

Let us, in the first place, endeavour to think of a globe freely poised in
space, and completely isolated from the influence of every other body in the
universe. Let us imagine that this globe is set in motion by some impulse
which starts it forward on a rapid voyage through the realms of space.
When the impulse ceases the globe is in motion, and continues to move
onwards. But what will be the path which it pursues? We are so accustomed
to see a stone thrown into the air moving in a curved path, that we might
naturally think a body projected into free space will also move in a curve. A
little consideration will, however, show that the cases are very different. In

the realms of free space we find no conception of upwards or downwards;
all paths are alike; there is no reason why the body should swerve to the
right or to the left; and hence we are led to surmise that in these
circumstances a body, once started and freed from all interference, would
move in a straight line. It is true that this statement is one which can never
be submitted to the test of direct experiment. Circumstanced as we are on
the surface of the earth, we have no means of isolating a body from external
forces. The resistance of the air, as well as friction in various other forms,
no less than the gravitation towards the earth itself, interfere with our
experiments. A stone thrown along a sheet of ice will be exposed to but
little resistance, and in this case we see that the stone will take a straight
course along the frozen surface. A stone similarly cast into empty space
would pursue a course absolutely rectilinear. This we demonstrate, not by
any attempts at an experiment which would necessarily be futile, but by
indirect reasoning. The truth of this principle can never for a moment be
doubted by one who has duly weighed the arguments which have been
produced in its behalf.

Admitting, then, the rectilinear path of the body, the next question which
arises relates to the velocity with which that movement is performed. The
stone gliding over the smooth ice on a frozen lake will, as everyone has
observed, travel a long distance before it comes to rest. There is but little
friction between the ice and the stone, but still even on ice friction is not
altogether absent; and as that friction always tends to stop the motion, the
stone will at length be brought to rest. In a voyage through the solitudes of
space, a body experiences no friction; there is no tendency for the velocity
to be reduced, and consequently we believe that the body could journey on
for ever with unabated speed. No doubt such a statement seems at variance
with our ordinary experience. A sailing ship makes no progress on the sea
when the wind dies away. A train will gradually lose its velocity when the
steam has been turned off. A humming-top will slowly expend its rotation
and come to rest. From such instances it might be plausibly argued that
when the force has ceased to act, the motion that the force generated

gradually wanes, and ultimately vanishes. But in all these cases it will be
found, on reflection, that the decline of the motion is to be attributed to the
action of resisting forces. The sailing ship is retarded by the rubbing of the
water on its sides; the train is checked by the friction of the wheels, and by
the fact that it has to force its way through the air; and the atmospheric
resistance is mainly the cause of the stopping of the humming-top, for if the
air be withdrawn, by making the experiment in a vacuum, the top will
continue to spin for a greatly lengthened period. We are thus led to admit
that a body, once projected freely in space and acted upon by no external
resistance, will continue to move on for ever in a straight line, and will
preserve unabated to the end of time the velocity with which it originally
started. This principle is known as the first law of motion.

Let us apply this principle to the important question of the movement of
the planets. Take, for instance, the case of our earth, and let us discuss the
consequences of the first law of motion. We know that the earth is moving
each moment with a velocity of about eighteen miles a second, and the first
law of motion assures us that if this globe were submitted to no external
force, it would for ever pursue a straight track through the universe, nor
would it depart from the precise velocity which it possesses at the present
moment. But is the earth moving in this manner? Obviously not. We have
already found that our globe is moving round the sun, and the
comprehensive laws of Kepler have given to that motion the most perfect
distinctness and precision. The consequence is irresistible. The earth cannot
be free from external force. Some potent influence on our globe must be in
ceaseless action. That influence, whatever it may be, constantly deflects the
earth from the rectilinear path which it tends to pursue, and constrains it to
trace out an ellipse instead of a straight line.

The great problem to be solved is now easily stated. There must be some
external agent constantly influencing the earth. What is that agent, whence
does it proceed, and to what laws is it submitted? Nor is the question
confined to the earth. Mercury and Venus, Mars, Jupiter, and Saturn,
unmistakably show that, as they are not moving in rectilinear paths, they

must be exposed to some force. What is this force which guides the planets
in their paths? Before the time of Newton this question might have been
asked in vain. It was the splendid genius of Newton which supplied the
answer, and thus revolutionised the whole of modern science.

The data from which the question is to be answered must be obtained
from observation. We have here no problem which can be solved by mere
mathematical meditation. Mathematics is no doubt a useful, indeed, an
indispensable, instrument in the enquiry; but we must not attribute to
mathematics a potency which it does not possess. In a case of this kind, all
that mathematics can do is to interpret the results obtained by observation.
The data from which Newton proceeded were the observed phenomena in
the movement of the earth and the other planets. Those facts had found a
succinct expression by the aid of Kepler's laws. It was, accordingly, the
laws of Kepler which Newton took as the basis of his labours, and it was for
the interpretation of Kepler's laws that Newton invoked the aid of that
celebrated mathematical reasoning which he created.

The question is then to be approached in this way: A planet being subject
to some external influence, we have to determine what that influence is,
from our knowledge that the path of each planet is an ellipse, and that each
planet sweeps round the sun over equal areas in equal times. The influence
on each planet is what a mathematician would call a force, and a force must
have a line of direction. The most simple conception of a force is that of a
pull communicated along a rope, and the direction of the rope is in this case
the direction of the force. Let us imagine that the force exerted on each
planet is imparted by an invisible rope. Kepler's laws will inform us with
regard to the direction of this rope and the intensity of the strain transmitted
through it.

The mathematical analysis of Kepler's laws would be beyond the scope
of this volume. We must, therefore, confine ourselves to the results to which
they lead, and omit the details of the reasoning. Newton first took the law
which asserted that the planet moved over equal areas in equal times, and

he showed by unimpeachable logic that this at once gave the direction in
which the force acted on the planet. He showed that the imaginary rope by
which the planet is controlled must be invariably directed towards the sun.
In other words, the force exerted on each planet was at all times pointed
from the planet towards the sun.

It still remained to explain the intensity of the force, and to show how
the intensity of that force varied when the planet was at different points of
its path. Kepler's first law enables this question to be answered. If the
planet's path be elliptic, and if the force be always directed towards the sun
at one focus of that ellipse, then mathematical analysis obliges us to say that
the intensity of the force must vary inversely as the square of the distance
from the planet to the sun.

The movements of the planets, in conformity with Kepler's laws, would
thus be accounted for even in their minutest details, if we admit that an
attractive power draws the planet towards the sun, and that the intensity of
this attraction varies inversely as the square of the distance. Can we hesitate
to say that such an attraction does exist? We have seen how the earth
attracts a falling body; we have seen how the earth's attraction extends to
the moon, and explains the revolution of the moon around the earth. We
have now learned that the movement of the planets round the sun can also
be explained as a consequence of this law of attraction. But the evidence in
support of the law of universal gravitation is, in truth, much stronger than
any we have yet presented. We shall have occasion to dwell on this matter
further on. We shall show not only how the sun attracts the planets, but how
the planets attract each other; and we shall find how this mutual attraction
of the planets has led to remarkable discoveries, which have elevated the
law of gravitation beyond the possibility of doubt.

Admitting the existence of this law, we can show that the planets must
revolve around the sun in elliptic paths with the sun in the common focus.
We can show that they must sweep over equal areas in equal times. We can
prove that the squares of the periodic times must be proportional to the

cubes of their mean distances. Still further, we can show how the
mysterious movements of comets can be accounted for. By the same great
law we can explain the revolutions of the satellites. We can account for the
tides, and for other phenomena throughout the Solar System. Finally, we
shall show that when we extend our view beyond the limits of our Solar
System to the beautiful starry systems scattered through space, we find even
there evidence of the great law of universal gravitation.

CHAPTER VI.
THE PLANET OF ROMANCE.
Outline of the Subject—Is Mercury the Planet nearest the Sun?—Transit of an Interior
Planet across the Sun—Has a Transit of Vulcan ever been seen?—Visibility of Planets
during a Total Eclipse of the Sun—Professor Watson's Researches in 1878.

Provided with a general survey of the Solar System, and with such an
outline of the law of universal gravitation as the last chapter has afforded
us, we commence the more detailed examination of the planets and their
satellites. We shall begin with the planets nearest to the sun, and then we
shall gradually proceed outwards to one planet after another, until we reach
the confines of the system. We shall find much to occupy our attention.
Each planet is itself a globe, and it will be for us to describe as much as is
known of it. The satellites by which so many of the planets are
accompanied possess many points of interest. The circumstances of their
discovery, their sizes, their movements, and their distances must be duly
considered. It will also be found that the movements of the planets present
much matter for reflection and examination. We shall have occasion to
show how the planets mutually disturb each other, and what remarkable
consequences have arisen from these influences. We must also occasionally
refer to the important problems of celestial measuring and celestial
weighing. We must show how the sizes, the weights, and the distances of
the various members of our system are to be discovered. The greater part of
our task will fortunately lead us over ground which is thoroughly certain,
and where the results have been confirmed by frequent observation. It
happens, however, that at the very outset of our course we are obliged to
deal with observations which are far from certain. The existence of a planet
much closer to the sun than those hitherto known has been asserted by
competent authority. The question is still unsettled, but the planet cannot at

present be found. Hence it is that we have called the subject of this chapter,
The Planet of Romance.

It had often been thought that Mercury, long supposed to be the nearest
planet to the sun, was perhaps not really the body entitled to that distinction.
Mercury revolves round the sun at an average distance of about 36,000,000
miles. In the interval between it and the sun there might have been one or
many other planets. There might have been one revolving at ten million
miles, another at fifteen, and so on. But did such planets exist? Did even
one planet revolve inside the orbit of Mercury? There were certain reasons
for believing in such a planet. In the movements of Mercury indications
were perceptible of an influence that it was at one time thought might have
been accounted for by the supposition of an interior planet.[13] But there
was necessarily a great difficulty about seeing this object. It must always be
close to the sun, and even in the best telescope it is generally impossible to
see a star-like point in that position. Nor could such a planet be seen after
sunset, for under the most favourable conditions it would set almost
immediately after the sun, and a like difficulty would make it invisible at
sunrise.

Our ordinary means of observing a planet have therefore completely
failed. We are compelled to resort to extraordinary methods if we would
seek to settle the great question as to the existence of the intra-Mercurial
planets. There are at least two lines of observation which might be expected
to answer our purpose.

An opportunity for the first would arise when it happened that the
unknown planet came directly between the earth and the sun. In the diagram
(Fig. 40) we show the sun at the centre; the internal dotted circle denotes
the orbit of the unknown planet, which has received the name of Vulcan
before even its very existence has been at all satisfactorily established. The
outer circle denotes the orbit of the earth. As Vulcan moves more rapidly
than the earth, it will frequently happen that the planet will overtake the
earth, so that the three bodies will have the positions represented in the

diagram. It would not, however, necessarily follow that Vulcan was exactly
between the earth and the luminary. The path of the planet may be tilted, so
that, as seen from the earth, Vulcan would be over or under the sun,
according to circumstances.

If, however, Vulcan really does exist, we might expect that sometimes
the three bodies will be directly in line, and this would then give the desired
opportunity of making the telescopic discovery of the planet. We should
expect on such an occasion to observe the planet as a dark spot, moving
slowly across the face of the sun. The two other planets interior to the earth,
namely, Mercury and Venus, are occasionally seen in the act of transit; and
there cannot be a doubt that if Vulcan exists, its transits across the sun must
be more numerous than those of Mercury, and far more numerous than
those of Venus. On the other hand, it may reasonably be anticipated that
Vulcan is a small globe, and as it will be much more distant from us than
Mercury at the time of its transit, we could not expect that the transit of the
planet of romance would be at all comparable as a spectacle with those of
either of the two other bodies we have named.

The question arises as to whether telescopic research has ever disclosed
anything which can be regarded as a transit of Vulcan. On this point it is not
possible to speak with any certainty. It has, on more than one occasion,
been asserted by observers that a spot has been seen traversing the sun, and
from its shape and general appearance they have presumed it to have been
an intra-Mercurial planet. But a close examination of the circumstances in
which such observations have been made has not tended to increase
confidence in this presumption. Such discoveries have usually been made
by persons little familiar with telescopic observations. It is certainly a
significant fact that, notwithstanding the diligent scrutiny to which the sun
has been subjected during the past century by astronomers who have
specially devoted themselves to this branch of research, no telescopic
discovery of Vulcan on the sun has been announced by any really
experienced astronomer. The last announcement of a planet having crossed
the sun dates from 1876, and was made by a German amateur, but what he

thought to have been a planet was promptly shown to have been a small
sun-spot, which had been photographed at Greenwich in the course of the
daily routine work, and had also been observed at Madrid. From an
examination of the whole subject, we are inclined to believe that there is not
at this moment any reliable telescopic evidence of the transit of an intra-
Mercurial planet over the face of the central luminary.

Fig. 40.—The Transit of the Planet of Romance.
But there is still another method by which we might reasonably hope to
detect new planets in the vicinity of the sun. This method is, however, but
seldom available. It is only possible when the sun is totally eclipsed.

When the moon is interposed directly between the earth and the sun, the
brightness of day is temporarily exchanged for the gloom of night. If the

sky be free from clouds the stars spring forth, and can be seen around the
obscured sun. Even if a planet were quite close to the luminary it would be
visible on such an occasion if its magnitude were comparable with that of
Mercury. Careful preparation is necessary when it is proposed to make a
trial of this kind. The danger to be specially avoided is that of confounding
the planet with the ordinary stars, which it will probably resemble. The late
distinguished American astronomer, Professor Watson, specially prepared to
devote himself to this research during the notable total eclipse in 1878.
When the eclipse occurred the light of the sun vanished and the stars burst
forth. Among them Professor Watson saw an object which to him seemed to
be the long-sought intra-Mercurial planet. We should add that this zealous
observer saw another object which he at first took to be the star known as
Zeta in the constellation Cancer. When he afterwards found that the
recorded place of this object did not agree so well as he expected with the
known position of this star, he came to the conclusion that it could not be
Zeta but must be some other unknown planet. The relative positions of the
two objects which he took to be planets agree, however, sufficiently well,
considering the difficulties of the observation, with the relative positions of
the stars Theta and Zeta Cancri, and it can now hardly be doubted that
Watson merely saw these two stars. He maintained, however, that he had
noticed Theta Cancri as well as the two planets, but without recording its
position. There is, however, a third star, known as 20 Cancri, near the same
place, and this Watson probably mistook for Theta. It is necessary to record
that Vulcan has not been observed, though specially looked for, during the
eclipses which have occurred since 1878, and it is accordingly the general
belief among astronomers that no planet has yet been detected within the
orbit of Mercury.

CHAPTER VII.
MERCURY.
The Ancient Astronomical Discoveries—How Mercury was first found—Not easily seen—
Mercury was known from the earliest ages—Skill necessary in the Discovery—The
Distinction of Mercury from a Star—Mercury in the East and in the West—The
Prediction—How to Observe Mercury—Its Telescopic Appearance—Difficulty of
Observing its Appearance—Orbit of Mercury—Velocity of the Planet—Can there be
Life on the Planet?—Changes in its Temperature—Transit of Mercury over the Sun—
Gassendi's Observations—Rotation of Mercury—The Weight of Mercury.

Long and glorious is the record of astronomical discovery. The discoveries
of modern days have succeeded each other with such rapidity, they have so
often dazzled our imaginations with their brilliancy, that we are sometimes
apt to think that astronomical discovery is a purely modern product. But no
idea could be more fundamentally wrong. While we appreciate to the
utmost the achievements of modern times, let us endeavour to do justice to
the labours of the astronomers of antiquity.

And when we speak of the astronomers of antiquity, let us understand
clearly what is meant. The science is now growing so rapidly that each
century witnesses a surprising advance; each generation, each decade, each
year, has its own rewards for those diligent astronomers by whom the
heavens are so carefully scanned. We must, however, project our glance to a
remote epoch in time past, if we would view the memorable discovery of
Mercury. Compared with it, the discoveries of Newton are to be regarded as
very modern achievements; even the announcement of the Copernican
system of the heavens is itself a recent event in comparison with the
detection of this planet now to be discussed.

By whom was this great discovery made? Let us see if the question can
be answered by the examination of astronomical records. At the close of his
memorable life Copernicus was heard to express his sincere regret that he

never enjoyed an opportunity of beholding the planet Mercury. He had
specially longed to see this body, the movements of which were to such a
marked extent illustrative of the theory of the celestial motions which it was
his immortal glory to have established, but he had never been successful.
Mercury is not generally to be seen so easily as are some of the other
planets, and it may well have been that the vapours from the immense
lagoon at the mouth of the Vistula obscured the horizon at Frauenburg,
where Copernicus dwelt, and thus his opportunities of viewing Mercury
were probably even rarer than they are at other places.

The existence of Mercury was certainly quite a familiar fact in the time
of Copernicus, and therefore we must look to some earlier epoch for its
discovery. In the scanty astronomical literature of the Middle Ages we find
occasional references to the existence of this object. We can trace
observations of Mercury through remote centuries to the commencement of
our era. Records from dates still earlier are not wanting, until at length we
come on an observation which has descended to us for more than 2,000
years, having been made in the year 265 before the Christian era. It is not
pretended, however, that this observation records the discovery of the
planet. Earlier still we find the chief of the astronomers at Nineveh alluding
to Mercury in a report which he made to Assurbanipal, the King of Assyria.
It does not appear in the least degree likely that the discovery was even then
a recent one. It may have been that the planet was independently discovered
in two or more localities, but all records of such discoveries are totally
wanting; and we are ignorant alike of the names of the discoverers, of the
nations to which they belonged, and of the epochs at which they lived.

Although this discovery is of such vast antiquity, although it was made
before correct notions were entertained as to the true system of the universe,
and, it is needless to add, long before the invention of the telescope, yet it
must not be assumed that the detection of Mercury was by any means a
simple or obvious matter. This will be manifest when we try to conceive the
manner in which the discovery must probably have been made.

Some primæval astronomer, long familiar with the heavens, had learned
to recognise the various stars and constellations. Experience had impressed
upon him the permanence of these objects; he had seen that Sirius
invariably appeared at the same seasons of the year, and he had noticed how
it was placed with regard to Orion and the other neighbouring
constellations. In the same manner each of the other bright stars was to him
a familiar object always to be found in a particular region of the heavens.
He saw how the stars rose and set in such a way, that though each star
appeared to move, yet the relative positions of the stars were incapable of
alteration. No doubt this ancient astronomer was acquainted with Venus; he
knew the evening star; he knew the morning star; and he may have
concluded that Venus was a body which oscillated from one side of the sun
to the other.

We can easily imagine how the discovery of Mercury was made in the
clear skies over an Eastern desert. The sun has set, the brief twilight has
almost ceased, when lo, near that part of the horizon where the glow of the
setting sun still illuminates the sky, a bright star is seen. The primæval
astronomer knows that there is no bright star at this place in the heavens. If
the object of his attention be not a star, what, then, can it be? Eager to
examine this question, the heavens are watched next night, and there again,
higher above the horizon, and more brilliant still, is the object seen the night
before. Each successive night the body gains more and more lustre, until at
length it becomes a conspicuous gem. Perhaps it will rise still higher and
higher; perhaps it will increase till it attains the brilliancy of Venus itself.
Such were the surmises not improbably made by those who first watched
this object; but they were not realised. After a few nights of exceptional
splendour the lustre of this mysterious orb declines. The planet again draws
near the horizon at sunset, until at length it sets so soon after the sun that it
has become invisible. Is it lost for ever? Years may elapse before another
opportunity of observing the object after sunset may be available; but then
again it will be seen to run through the same series of changes, though,

perhaps, under very different circumstances. The greatest height above the
horizon and the greatest brightness both vary considerably.

Long and careful observations must have been made before the
primæval astronomer could assure himself that the various appearances
might all be attributed to a single body. In the Eastern deserts the
phenomena of sunrise must have been nearly as familiar as those of sunset,
and in the clear skies, at the point where the sunbeams were commencing to
dawn above the horizon, a bright star-like point might sometimes be
perceived. Each successive day this object rose higher and higher above the
horizon before the moment of sunrise, and its lustre increased with the
distance; then again it would draw in towards the sun, and return for many
months to invisibility. Such were the data which were presented to the mind
of the primitive astronomer. One body was seen after sunset, another body
was seen before sunrise. To us it may seem an obvious inference from the
observed facts that the two bodies were identical. The inference is a correct
one, but it is in no sense an obvious one. Long and patient observation
established the remarkable law that one of these bodies was never seen until
the other had disappeared. Hence it was inferred that the phenomena, both
at sunrise and at sunset, were due to the same body, which oscillated to and
fro about the sun.

We can easily imagine that the announcement of the identity of these
two objects was one which would have to be carefully tested before it could
be accepted. How are the tests to be applied in a case of this kind? There
can hardly be a doubt that the most complete and convincing demonstration
of scientific truth is found in the fulfilment of prediction. When Mercury
had been observed for years, a certain regularity in the recurrence of its
visibility was noticed. Once a periodicity had been fully established,
prediction became possible. The time when Mercury would be seen after
sunset, the time when it would be seen before sunrise, could be foretold
with accuracy! When it was found that these predictions were obeyed to the
letter—that the planet was always seen when looked for in accordance with
the predictions—it was impossible to refuse assent to the hypothesis on

which these predictions were based. Underlying that hypothesis was the
assumption that all the various appearances arose from the oscillations of a
single body, and hence the discovery of Mercury was established on a basis
as firm as the discovery of Jupiter or of Venus.

In the latitudes of the British Islands it is generally possible to see
Mercury some time during the course of the year. It is not practicable to lay
down, within reasonable limits, any general rule for finding the dates at
which the search should be made; but the student who is determined to see
the planet will generally succeed with a little patience. He must first consult
an almanac which gives the positions of the body, and select an occasion
when Mercury is stated to be an evening or a morning star. Such an
occasion during the spring months is especially suitable, as the elevation of
Mercury above the horizon is usually greater then than at other seasons; and
in the evening twilight, about three-quarters of an hour after sunset, a view
of this shy but beautiful object will reward the observer's attention.

To those astronomers who are provided with equatorial telescopes such
instructions are unnecessary. To enjoy a telescopic view of Mercury, we
first turn to the Nautical Almanac, and find the position in which the planet
lies. If it happen to be above the horizon, we can at once direct the telescope
to the place, and even in broad daylight the planet will very often be seen.
The telescopic appearance of Mercury is, however, disappointing. Though it
is much larger than the moon, yet it is sufficiently far off to seem
insignificant. There is, however, one feature in a view of this planet which
would immediately attract attention. Mercury is not usually observed to be
a circular object, but more or less crescent-shaped, like a miniature moon.
The phases of the planet are also to be accounted for on exactly the same
principles as the phases of the moon. Mercury is a globe composed, like our
earth, of materials possessing in themselves no source of illumination. One
hemisphere of the planet must necessarily be turned towards the sun, and
this side is accordingly lighted up brilliantly by the solar rays. When we
look at Mercury we see nothing of the non-illuminated side, and the
crescent is due to the foreshortened view which we obtain of the

illuminated part. The planet is such a small object that, in the glitter of the
naked-eye view, the shape of the luminous body cannot be defined. Indeed,
even in the much larger crescent of Venus, the aid of the telescope has to be
invoked before the characteristic form can be observed. Beyond, however,
the fact that Mercury is a crescent, and that it undergoes varying phases in
correspondence with the changes in its relative position to the earth and the
sun, we cannot see much of the planet. It is too small and too bright to
admit of easy delineation of details on its surface. No doubt attempts have
been made, and observations have been recorded, as to certain very faint
and indistinct markings on the planet, but such statements must be received
with great hesitation.

Fig. 41.—The Movement of Mercury, showing the
Variations in Phase and in apparent size.

The facts which have been thoroughly established with regard to
Mercury are mainly numerical statements as to the path it describes around
the sun. The time taken by the planet to complete one of its revolutions is
eighty-eight days nearly. The average distance from the sun is about
36,000,000 miles, and the mean velocity with which the body moves is over
twenty-nine miles a second. We have already alluded to the most
characteristic and remarkable feature of the orbit of Mercury. That orbit
differs from the paths of all the other large planets by its much greater

departure from
the circular form.
In the majority of
cases the
planetary orbits
are so little
elliptic that a
diagram of the
orbit drawn
accurately to
scale would not
be perceived to
differ from a
circle unless
Fig. 42.—Mercury as a Crescent. careful
measurements
were made. In the case of Mercury the circumstances are different. The
elliptic form of the path would be quite unmistakable by the most casual
observer. The distance from the sun to the planet fluctuates between very
considerable limits. The lowest value it can attain is about 30,000,000
miles; the highest value is about 43,000,000 miles. In accordance with
Kepler's second law, the velocity of the planet must exhibit corresponding
changes. It must sweep rapidly around that part of his path near the sun, and
more slowly round the remote parts of his path. The greatest velocity is
about thirty-five miles a second, and the least is twenty-three miles a
second.

For an adequate conception of the movements of Mercury we ought not
to dissociate the velocity from the true dimensions of the body by which it
is performed. No doubt a speed of twenty-nine miles a second is enormous
when compared with the velocities with which daily life makes us familiar.
The speed of the planet is not less than a hundred times as great as the
velocity of the rifle bullet. But when we compare the sizes of the bodies

with their velocities, the velocity of Mercury seems relatively much less
than that of the bullet. A rifle bullet traverses a distance equal to its own
diameter many thousands of times in a second. But even though Mercury is
moving so much faster, yet the dimensions of the planet are so considerable
that a period of two minutes will be required for it to move through a
distance equal to its diameter. Viewing the globe of the planet as a whole,
the velocity of its movement is but a stately and dignified progress
appropriate to its dimensions.

As we can learn little or nothing of the true surface of Mercury, it is
utterly impossible for us to say whether life can exist on the surface of that
planet. We may, however, reasonably conclude that there cannot be life on
Mercury in any respect analogous to the life which we know on the earth.
The heat of the sun and the light of the sun beat down on Mercury with an
intensity many times greater than that which we experience. When this
planet is at its utmost distance from the sun the intensity of solar radiation is
even then more than four times greater than the greatest heat which ever
reaches the earth. But when Mercury, in the course of its remarkable
changes of distance, draws in to the warmest part of its orbit, it is exposed
to a terrific scorching. The intensity of the sun's heat must then be not less
than nine times as great as the greatest radiation to which we are exposed.

These tremendous climatic changes succeed each other much more
rapidly than do the variations of our seasons. On Mercury the interval
between midsummer and midwinter is only forty-four days, while the whole
year is only eighty-eight days. Such rapid variations in solar heat must in
themselves exercise a profound effect on the habitability of Mercury. Mr.
Ledger well remarks, in his interesting work,[14] that if there be inhabitants
on Mercury the notions of "perihelion" and "aphelion," which are here often
regarded as expressing ideas of an intricate or recondite character, must on
the surface of that planet be familiar to everybody. The words imply "near
the sun," and "away from the sun;" but we do not associate these
expressions with any obvious phenomena, because the changes in the
distance of the earth from the sun are inconsiderable. But on Mercury,

where in six weeks the sun rises to more than double his apparent size, and
gives more than double the quantity of light and of heat, such changes as
are signified by perihelion and aphelion embody ideas obviously and
intimately connected with the whole economy of the planet.

It is nevertheless rash to found any inferences as to climate merely upon
the proximity or the remoteness of the sun. Climate depends upon other
matters besides the sun's distance. The atmosphere surrounding the earth
has a profound influence on heat and cold, and if Mercury have an
atmosphere—as has often been supposed—its climate may be thereby
modified to any necessary extent. It seems, however, hardly possible to
suppose that any atmosphere could form an adequate protection for the
inhabitants from the violent and rapid fluctuations of solar radiation. All we
can say is, that the problem of life in Mercury belongs to the class of
unsolved, and perhaps unsolvable, mysteries.

It was in the year 1629 that Kepler made an important announcement as
to impending astronomical events. He had been studying profoundly the
movements of the planets; and from his study of the past he had ventured to
predict the future. Kepler announced that in the year 1631 the planets Venus
and Mercury would both make a transit across the sun, and he assigned the
dates to be November 7th for Mercury, and December 6th for Venus. This
was at the time a very remarkable prediction. We are so accustomed to turn
to our almanacs and learn from them all the astronomical phenomena which
are anticipated during the year, that we are apt to forget how in early times
this was impossible. It has only been by slow degrees that astronomy has
been rendered so perfect as to enable us to foretell, with accuracy, the
occurrence of the more delicate phenomena. The prediction of those transits
by Kepler, some years before they occurred, was justly regarded at the time
as a most remarkable achievement.

The illustrious Gassendi prepared to apply the test of actual observation
to the announcements of Kepler. We can now assign the time of the transit
accurately to within a few minutes, but in those early attempts equal

precision was not practicable. Gassendi considered it necessary to
commence watching for the transit of Mercury two whole days before the
time indicated by Kepler, and he had arranged an ingenious plan for making
his observations. The light of the sun was admitted into a darkened room
through a hole in the shutter, and an image of the sun was formed on a
white screen by a lens. This is, indeed, an admirable and a very pleasing
way of studying the surface of the sun, and even at the present day, with our
best telescopes, one of the methods of viewing our luminary is founded on
the same principle.

Gassendi commenced his watch on the 5th of November, and carefully
studied the sun's image at every available opportunity. It was not, however,
until five hours after the time assigned by Kepler that the transit of Mercury
actually commenced. Gassendi's preparations had been made with all the
resources which he could command, but these resources seem very
imperfect when compared with the appliances of our modern observatories.
He was anxious to note the time when the planet appeared, and for this
purpose he had stationed an assistant in the room beneath, who was to
observe the altitude of the sun at the moment indicated by Gassendi. The
signal to the assistant was to be conveyed by a very primitive apparatus.
Gassendi was to stamp on the floor when the critical moment had arrived.
In spite of the long delay, which exhausted the patience of the assistant,
some valuable observations were obtained, and thus the first passage of a
planet across the sun was observed.

The transits of Mercury are not rare phenomena (there have been
thirteen of them during the nineteenth century), and they are chiefly of
importance on account of the accuracy which their observation infuses into
our calculations of the movements of the planet. It has often been hoped
that the opportunities afforded by a transit would be available for procuring
information as to the physical character of the globe of Mercury, but these
hopes have not been realised.

Spectroscopic observations of Mercury are but scanty. They seem to
indicate that water vapour is a probable constituent in the atmosphere of
Mercury, as it is in our own.

A distinguished Italian astronomer, Professor Schiaparelli, some years
ago announced a remarkable discovery with respect to the rotation of the
planet Mercury. He found that the planet rotates on its axis in the same
period as it revolves around the sun. The practical consequence of the
identity between these two periods is that Mercury always turns the same
face to the sun. If our earth were to rotate in a similar fashion, then the
hemisphere directed to the sun would enjoy eternal day, while the opposite
hemisphere would be relegated to perpetual night. According to this
discovery, Mercury revolves around the sun in the same way as the moon
revolves around the earth. As the velocity with which Mercury travels
round the sun is very variable, owing to the highly elliptic shape of its orbit,
while the rotation about its axis is performed with uniform speed, it follows
that rather more than a hemisphere (about five-eighths of the surface)
enjoys more or less the light of the sun in the course of a Mercurial year.

This important discovery of Schiaparelli has lately been confirmed by an
American astronomer, Mr. Lowell, of Arizona, U.S.A., who observed the
planet under very favourable conditions with a refractor of twenty-four
inches aperture. He has detected on the globe of Mercury certain narrow,
dark lines, the very slow shifting of which points to a period of rotation
about its axis exactly coincident with the period of revolution round the sun.
The same observer shows that the axis of rotation of Mercury is
perpendicular to the plane of the orbit. Mr. Lowell has perceived no sign of
clouds or obscurations, and indeed no indication of any atmospheric
envelope; the surface of Mercury is colourless, "a geography in black and
white."

We may assert that, there is a strong à priori probability in favour of the
reality of Schiaparelli's discovery. Mercury, being one of the planets devoid
of a moon, will be solely influenced by the sun in so far as tidal phenomena

are concerned. Owing, moreover, to the proximity of Mercury to the sun,
the solar tides on that planet possess an especial vehemence. As the
tendency of tides is to make Mercury present a constant face to the sun,
there need be little hesitation in accepting testimony that tides have wrought
exactly the result that we know they were competent to perform.

Here we take leave of the planet Mercury—an interesting and beautiful
object, which stimulates our intellectual curiosity, while at the same time it
eludes our attempts to make a closer acquaintance. There is, however, one
point of attainable knowledge which we must mention in conclusion. It is a
difficult, but not by any means an impossible, task to weigh Mercury in the
celestial balance, and determine his mass in comparison with the other
globes of our system. This is a delicate operation, but it leads us through
some of the most interesting paths of astronomical discovery. The weight of
the planet, as recently determined by Von Asten, is about one twenty-fourth
part of the weight of the earth, but the result is more uncertain than the
determinations of the mass of any of the other larger planets.

CHAPTER VIII.
VENUS.
Interest attaching to this Planet—The Unexpectedness of its Appearance—The Evening
Star—Visibility in Daylight—Lighted only by the Sun—The Phases of Venus—Why
the Crescent is not Visible to the Unaided Eye—Variations in the Apparent Size of the
Planet—The Rotation of Venus—Resemblance of Venus to the Earth—The Transit of
Venus—Why of such Especial Interest—The Scale of the Solar System—Orbits of the
Earth and Venus not in the same Plane—Recurrence of the Transits in Pairs—
Appearance of Venus in Transit—Transits of 1874 and 1882—The Early Transits of
1631 and 1639—The Observations of Horrocks and Crabtree—The Announcement of
Halley—How the Track of the Planet differs from Different Places—Illustrations of
Parallax—Voyage to Otaheite—The Result of Encke—Probable Value of the Sun's
Distance—Observations at Dunsink of the Last Transit of Venus—The Question of an
Atmosphere to Venus—Other Determinations of the Sun's Distance—Statistics about
Venus.

It might, for one reason, have been not inappropriate to have commenced
our review of the planetary system by the description of the planet Venus.
This body is not especially remarkable for its size, for there are other
planets hundreds of times larger. The orbit of Venus is no doubt larger than
that of Mercury, but it is much smaller than that of the outer planets. Venus
has not even the splendid retinue of minor attendants which gives such
dignity and such interest to the mighty planets of our system. Yet the fact
still remains that Venus is peerless among the planetary host. We speak not
now of celestial bodies only seen in the telescope; we refer to the ordinary
observation which detected Venus ages before telescopes were invented.

Who has not been delighted with the view of this glorious object? It is
not to be seen at all times. For months together the star of evening is hidden
from mortal gaze. Its beauties are even enhanced by the caprice and the
uncertainty which attend its appearance. We do not say that there is any
caprice in the movements of Venus, as known to those who diligently
consult their almanacs. The movements of the lovely planet are there

prescribed with a prosaic detail hardly in harmony with the character
usually ascribed to the Goddess of Love. But to those who do not devote
particular attention to the stars, the very unexpectedness of its appearance is
one of its greatest charms. Venus has not been noticed, not been thought of,
for many months. It is a beautifully clear evening; the sun has just set. The
lover of nature turns to admire the sunset, as every lover of nature will. In
the golden glory of the west a beauteous gem is seen to glitter; it is the
evening star—the planet Venus. A few weeks later another beautiful sunset
is seen, and now the planet is no longer a point low down in the western
glow; it has risen high above the horizon, and continues a brilliant object
long after the shades of night have descended. Again, a little later, and
Venus has gained its full brilliancy and splendour. All the heavenly host—
even Sirius and even Jupiter—must pale before the splendid lustre of Venus,
the unrivalled queen of the firmament.

After weeks of splendour, the height of Venus at sunset diminishes, and
its lustre begins gradually to decline. It sinks to invisibility, and is forgotten
by the great majority of mankind; but the capricious goddess has only
moved from one side of the sky to the other. Ere the sun rises, the morning
star will be seen in the east. Its splendour gradually augments until it rivals
the beauty of the evening star. Then again the planet draws near to the sun,
and remains lost to view for many months, until the same cycle of changes
recommences, after an interval of a year and seven months.

When Venus is at its brightest it can be easily seen in broad daylight with
the unaided eye. This striking spectacle proclaims in an unmistakable
manner the unrivalled supremacy of this planet as compared with its fellow-
planets and with the fixed stars. Indeed, at this time Venus is from forty to
sixty times more brilliant than any stellar object in the northern heavens.

The beautiful evening star is often such a very conspicuous object that it
may seem difficult at first to realise that the body is not self-luminous. Yet it
is impossible to doubt that the planet is really only a dark globe, and to that
extent resembles our own earth. The brilliance of the planet is not so very

much greater than that of the earth on a sunshiny day. The splendour of
Venus entirely arises from the reflected light of the sun, in the manner
already explained with respect to the moon.

We cannot distinguish the characteristic crescent shape of the planet with
the unaided eye, which merely shows a brilliant point too small to possess
sensible form. This is to be explained on physiological grounds. The optical
contrivances in the eye form an image of the planet on the retina which is
necessarily very small. Even when Venus is nearest to the earth the diameter
of the planet subtends an angle not much more than one minute of arc. On
the delicate membrane a picture of Venus is thus drawn about one six-
thousandth part of an inch in diameter. Great as may be the delicacy of the
retina, it is not adequate to the perception of form in a picture so minute.
The nervous structure, which has been described as the source of vision,
forms too coarse a canvas for the reception of the details of this tiny picture.
Hence it is that to the unaided eye the brilliant Venus appears merely as a
bright spot. Ordinary vision cannot tell what shape it has; still less can it
reveal the true beauty of the crescent.

If the diameter of Venus were several times as great as it actually is;
were this body, for instance, as large as Jupiter or some of the other great
planets, then its crescent could be readily discerned by the unaided eye. It is
curious to speculate on what might have been the history of astronomy had
Venus only been as large as Jupiter. Were everyone able to see the crescent
form without a telescope, it would then have been an elementary and almost
obvious truth that Venus must be a dark body revolving round the sun. The
analogy between Venus and our earth would have been at once perceived;
and the doctrine which was left to be discovered by Copernicus in
comparatively modern times might not improbably have been handed down
to us with the other discoveries which have come from the ancient nations
of the East.

Fig. 43. Venus, May 29th, 1889.
Perhaps the most perfect drawing of Venus that has been hitherto
obtained is that made (Fig. 43) by Professor E.E. Barnard, on 29th May,
1889, with a 12-inch equatorial, at the Lick Observatory, which for this
purpose and on this occasion Professor Barnard found to be superior to the
36-inch. The markings shown seem undoubtedly to exist on the planet, and
in 1897 Professor Barnard writes: "The circumstances under which this
drawing was made are memorable with me, for I never afterwards had such
perfect conditions to observe Venus."

In Fig. 44 we show three views of Venus under different aspects. The
planet is so much closer to the earth when the crescent is seen, that it
appears to be part of a much larger circle than that made by Venus when
more nearly full. This drawing shows the different aspects of the globe in
their true relative proportions. It is very difficult to perceive distinctly any

markings on the brilliantly lighted surface. Sometimes observers have seen
spots or other features, and occasionally the pointed extremities of the horns
have been irregular, as if to show that the surface of Venus is not smooth.
Some observers report having seen white spots at the poles of Venus, in
some degree resembling the more conspicuous features of the same
character to be seen on Mars.

As it is
so very
difficult to
see any
markings
on Venus,
we are
hardly yet
able to
give a
definite
answer to
Fig. 44.—Different Aspects of Venus in the Telescope.
the
important
question as to the period of rotation of this planet round its axis. Various
observers during the last two hundred years have from very insufficient data
concluded that Venus rotated in about twenty-three hours. Schiaparelli, of
Milan, turned his attention to this planet in 1877 and noticed a dark shade
and two bright spots, all situated not far from the southern end of the
crescent. This most painstaking astronomer watched these markings for
three months, and found that there was no change perceptible in the position
which they occupied. This was particularly the case when he continued his
watch for some consecutive hours. This fact seemed to show conclusively
that Venus could not rotate in twenty-three hours nor in any other short
period. Week after week the spots remained unaltered, until Schiaparelli felt
convinced that his observations could only be reconciled with a period of

rotation between six and nine months. He naturally concluded that the
period was 225 days—that is to say, the period which Venus takes to
complete one revolution round the sun; in other words, Venus always turns
the same face to the sun.

This remarkable result was confirmed by observations made at Nice; but
it has been vigorously assailed by several observers, who maintain that their
own drawings can only agree with a period about equal to that of the
rotation of our own earth. Schiaparelli's result is, however, well supported
by the letters of Mr. Lowell. He has published a number of drawings of
Venus made with his 24-inch refractor, and he finds that the rotation is
performed in the same time as the planet's orbital revolution, the axis of
rotation being perpendicular to the plane of the orbit. The markings seen by
Mr. Lowell were long and streaky, and they were always visible whenever
his own atmospheric conditions were fairly good.

We have seen that the moon revolves so as to keep the same face always
turned towards the earth. We have now seen that the planets Venus and
Mercury each appear to revolve in such a way that they keep the same face
towards the sun. All these phenomena are of profound interest in the higher
departments of astronomical research. They are not mere coincidences.
They arise from the operation of the tides, in a manner that will be
explained in a later chapter.

It happens that our earth and Venus are very nearly equal in bulk. The
difference is hardly perceptible, but the earth has a diameter a few miles
greater than that of Venus. There are indications of the existence of an
atmosphere around Venus, and the evidence of the spectroscope shows that
water vapour is there present.

If there be oxygen in the atmosphere of Venus, then it would seem
possible that there might be life on that globe not essentially different in
character from some forms of life on the earth. No doubt the sun's heat on
Venus is greatly in excess of the sun's heat with which we are acquainted,

but this is not an insuperable difficulty. We see at present on the earth, life
in very hot regions and life in very cold regions. Indeed, with each approach
to the Equator we find life more and more exuberant; so that, if water be
present on the surface of Venus and if oxygen be a constituent of its
atmosphere, we might expect to find in that planet a luxuriant tropical life,
of a kind perhaps analogous in some respects to life on the earth.

In our account of the planet Mercury, as well as in the brief description
of the hypothetical planet Vulcan, it has been necessary to allude to the
phenomena presented by the transit of a planet over the face of the sun.
Such an event is always of interest to astronomers, and especially so in the
case of Venus. We have in recent years had the opportunity of witnessing
two of these rare occurrences. It is perhaps not too much to assert that the
transits of 1874 and 1882 have received a degree of attention never before
accorded to any astronomical phenomenon.

The transit of Venus cannot be described as a very striking or beautiful
spectacle. It is not nearly so fine a sight as a great comet or a shower of
shooting stars. Why is it, then, that it is regarded as of so much scientific
importance? It is because the phenomenon helps us to solve one of the
greatest problems which has ever engaged the mind of man. By the transit
of Venus we may determine the scale on which our solar system is
constructed. Truly this is a noble problem. Let us dwell upon it for a
moment. In the centre of our system we have the sun—a majestic globe
more than a million times as large as the earth. Circling round the sun we
have the planets, of which our earth is but one. There are hundreds of small
planets. There are a few comparable with our earth; there are others vastly
surpassing the earth. Besides the planets there are other bodies in our
system. Many of the planets are accompanied by systems of revolving
moons. There are hundreds, perhaps thousands, of comets. Each member of
this stupendous host moves in a prescribed orbit around the sun, and
collectively they form the solar system.

It is comparatively easy to learn the proportions of this system, to
measure the relative distances of the planets from the sun, and even the
relative sizes of the planets themselves. Peculiar difficulties are, however,
experienced when we seek to ascertain the actual size of the system as well
as its shape. It is this latter question which the transit of Venus offers us a
method of solving.

Look, for instance, at an ordinary map of Europe. We see the various
countries laid down with precision; we can tell the courses of the rivers; we
can say that France is larger than England, and Russia larger than France;
but no matter how perfectly the map be constructed, something else is
necessary before we can have a complete conception of the dimensions of
the country. We must know the scale on which the map is drawn. The map
contains a reference line with certain marks upon it. This line is to give the
scale of the map. Its duty is to tell us that an inch on the map corresponds
with so many miles on the actual surface. Unless it be supplemented by the
scale, the map would be quite useless for many purposes. Suppose that we
consulted it in order to choose a route from London to Vienna, we can see at
once the direction to be taken and the various towns and countries to be
traversed; but unless we refer to the little scale in the corner, the map will
not tell how many miles long the journey is to be.

A map of the solar system can be readily constructed. We can draw on it
the orbits of some of the planets and of their satellites, and we can include
many of the comets. We can assign to the planets and to the orbits their
proper proportions. But to render the map quite efficient something more is
necessary. We must have the scale which is to tell us how many millions of
miles on the heavens correspond to one inch of the map. It is at this point
we encounter a difficulty. There are, however, several ways of solving the
problem, though they are all difficult and laborious. The most celebrated
method (though far from the best) is that presented on an occasion of the
transit of Venus. Herein, then, lies the importance of this rare event. It is one
of the best-known means of finding the actual scale on which our system is
constructed. Observe the full importance of the problem. Once the scale has

been determined, then all is known. We know the size of the sun; we know
his distance; we know the bulk of Jupiter, and the distances at which his
satellites revolve; we know the dimensions of the comets, and the number
of miles to which they recede in their wanderings; we know the velocity of
the shooting stars; and we learn the important lesson that our earth is but
one of the minor members of the sun's family.

As the path of Venus lies inside that of the earth, and as Venus moves
more quickly than the earth, it follows that the earth is frequently passed by
the planet, and just at the critical moment it will sometimes happen that the
earth, the planet, and the sun lie in the same straight line. We can then see
Venus on the face of the sun, and this is the phenomenon which we call the
transit of Venus. It is, indeed, quite plain that if the three bodies were
exactly in a line, an observer on the earth, looking at the planet, would see it
brought out vividly against the brilliant background of the sun.

Considering that the earth is overtaken by Venus once every nineteen
months, it might seem that the transits of the planet should occur with
corresponding frequency. This is not the case; the transit of Venus is an
exceedingly rare occurrence, and a hundred years or more will often elapse
without a single one taking place. The rarity of these phenomena arises
from the fact that the path of the planet is inclined to the plane of the earth's
orbit; so that for half of its path Venus is above the plane of the earth's orbit,
and in the other half it is below. When Venus overtakes the earth, the line
from the earth to Venus will therefore usually pass over or under the sun. If,
however, it should happen that Venus overtakes the earth at or near either of
the points in which the plane of the orbit of Venus passes through that of the
earth, then the three bodies will be in line, and a transit of Venus will be the
consequence. The rarity of the occurrence of a transit need no longer be a
mystery. The earth passes through one of the critical parts every December,
and through the other every June. If it happens that the conjunction of
Venus occurs on, or close to, June 6th or December 7th, then a transit of
Venus will occur at that conjunction, but in no other circumstances.

The most remarkable law with reference to the repetition of the
phenomenon is the well-known eight-year interval. The transits may be all
grouped together into pairs, the two transits of any single pair being
separated by an interval of eight years. For instance, a transit of Venus took
place in 1761, and again in 1769. No further transits occurred until those
witnessed in 1874 and in 1882. Then, again, comes a long interval, for
another transit will not occur until 2004, but it will be followed by another
in 2012.

This arrangement of the transits in pairs admits of a very simple
explanation. It happens that the periodic time of Venus bears a remarkable
relation to the periodic time of the earth. The planet accomplishes thirteen
revolutions around the sun in very nearly the same time that the earth
requires for eight revolutions. If, therefore, Venus and the earth were in line
with the sun in 1874, then in eight years more the earth will again be found
in the same place; and so will Venus, for it has just been able to accomplish
thirteen revolutions. A transit of Venus having occurred on the first
occasion, a transit must also occur on the second.

It is not, however, to be supposed that every eight years the planets will
again resume the same position with sufficient precision for a regular eight-
year transit interval. It is only approximately true that thirteen revolutions
of Venus are coincident with eight revolutions of the earth. Each recurrence
of conjunction takes place at a slightly different position of the planets, so
that when the two planets came together again in the year 1890 the point of
conjunction was so far removed from the critical point that the line from the
earth to Venus did not intersect the sun, and thus, although Venus passed
very near the sun, yet no transit took place.

Fig. 45 represents the transit of Venus in 1874. It is taken from a
photograph obtained, during the occurrence, by M. Janssen. His telescope
was directed towards the sun during the eventful minutes while it lasted,
and thus an image of the sun was depicted on the photographic plate placed
in the telescope. The lighter circle represents the disc of the sun. On that

disc we
see the
round,
sharp
image of
Venus,
showing
the
characteris
tic
appearance
of the
planet
during the
progress of
the transit.
The only
other
features to
be noticed
Fig. 45.—Venus on the Sun at the Transit of 1874. are a few
of the solar
spots, rather dimly shown, and a network of lines which were marked on a
glass plate across the field of view of the telescope to facilitate
measurements.

The adjoining sketch (Fig. 46) exhibits the course which the planet
pursued in its passage across the sun on the two occasions in 1874 and
1882. Our generation has had the good fortune to witness the two
occurrences indicated on this picture. The white circle denotes the disc of
the sun; the planet encroaches on the white surface, and at first is like a bite
out of the sun's margin. Gradually the black spot steals in front of the sun,
until, after nearly half an hour, the black disc is entirely visible. Slowly the

planet wends its way across, followed by hundreds of telescopes from every
accessible part of the globe whence the phenomenon is visible, until at
length, in the course of a few hours, it emerges at the other side.

It will be useful to take a brief retrospect of the different transits of
Venus of which there is any historical record. They are not numerous.
Hundreds of such phenomena have occurred since man first came on the
earth. It was not until the approach of the year 1631 that attention began to
be directed to the matter, though the transit which undoubtedly occurred in
that year was not noticed by anyone. The success of Gassendi in observing
the transit of Mercury, to which we have referred in the last chapter, led him
to hope that he would be equally fortunate in observing the transit of Venus,
which Kepler had also foretold. Gassendi looked at the sun on the 4th, 5th,
and 6th December. He looked at it again on the 7th, but he saw no sign of
the planet. We now know the reason. The transit of Venus took place during
the night, between the 6th and the 7th, and must therefore have been
invisible to European observers.

Kepler had not noticed that another transit would occur in 1639. This
discovery was made by another astronomer, and it is the one with which the
history of the subject may be said to commence. It was the first occasion on
which the phenomenon was ever actually witnessed; nor was it then seen by
many. So far as is known, it was witnessed by only two persons.

A young and ardent English astronomer, named Horrocks, had
undertaken some computations about the motions of Venus. He made the
discovery that the transit of Venus would be repeated in 1639, and he
prepared to verify the fact. The sun rose bright on the morning of the day—
which happened to be a Sunday. The clerical profession, which Horrocks
followed, here came into collision with his desires as an astronomer. He
tells us that at nine he was called away by business of the highest
importance—referring, no doubt, to his official duties; but the service was
quickly performed, and a little before ten he was again on the watch, only to
find the brilliant face of the sun without any unusual feature. It was marked

with a spot, but
nothing that
could be
mistaken for a
planet. Again, at
noon, came an
interruption; he
went to church,
but he was back
by one. Nor were
these the only
impediments to
his observations.
The sun was also
more or less
clouded over
during part of the
day. However, at
a quarter past Fig. 46.—The Path of Venus across the Sun in the
three in the Transits of 1874 and 1882.
afternoon his
clerical work was over; the clouds had dispersed, and he once more
resumed his observations. To his intense delight he then saw on the sun the
round, dark spot, which was at once identified as the planet Venus. The
observations could not last long; it was the depth of winter, and the sun was
rapidly setting. Only half an hour was available, but he had made such
careful preparations beforehand that it sufficed to enable him to secure
some valuable measurements.

Horrocks had previously acquainted his friend, William Crabtree, with
the impending occurrence. Crabtree was therefore on the watch, and
succeeded in seeing the transit; a striking picture of Crabtree's famous
observation is shown in one of the beautiful frescoes in the Town Hall at

Manchester. But to no one else had Horrocks communicated the
intelligence; as he says, "I hope to be excused for not informing other of my
friends of the expected phenomenon, but most of them care little for trifles
of this kind, rather preferring their hawks and hounds, to say no worse; and
although England is not without votaries of astronomy, with some of whom
I am acquainted, I was unable to convey to them the agreeable tidings,
having myself had so little notice."

It was not till long afterwards that the full importance of the transit of
Venus was appreciated. Nearly a century had rolled away when the great
astronomer, Halley (1656–1742), drew attention to the subject. The next
transit was to occur in 1761, and forty-five years before that event Halley
explained his celebrated method of finding the distance of the sun by means
of the transit of Venus.[15] He was then a man sixty years of age; he could
have no expectation that he would live to witness the event; but in noble
language he commends the problem to the notice of the learned, and thus
addresses the Royal Society of London:—"And this is what I am now
desirous to lay before this illustrious Society, which I foretell will continue
for ages, that I may explain beforehand to young astronomers, who may,
perhaps, live to observe these things, a method by which the immense
distance of the sun may be truly obtained.... I recommend it, therefore,
again and again to those curious astronomers who, when I am dead, will
have an opportunity of observing these things, that they would remember
this my admonition, and diligently apply themselves with all their might in
making the observations, and I earnestly wish them all imaginable success
—in the first place, that they may not by the unseasonable obscurity of a
cloudy sky be deprived of this most desirable sight, and then that, having
ascertained with more exactness the magnitudes of the planetary orbits, it
may redound to their immortal fame and glory." Halley lived to a good old
age, but he died nineteen years before the transit occurred.

The student of astronomy who desires to learn how the transit of Venus
will tell the distance from the sun must prepare to encounter a geometrical
problem of no little complexity. We cannot give to the subject the detail that

would be requisite for a full explanation. All we can attempt is to render a
general account of the method, sufficient to enable the reader to see that the
transit of Venus really does contain all the elements necessary for the
solution of the problem.

We must first explain clearly the conception which is known to
astronomers by the name of parallax; for it is by parallax that the distance
of the sun, or, indeed, the distance of any other celestial body, must be
determined. Let us take a simple illustration. Stand near a window whence
you can look at buildings, or the trees, the clouds, or any distant objects.
Place on the glass a thin strip of paper vertically in the middle of one of the
panes. Close the right eye, and note with the left eye the position of the strip
of paper relatively to the objects in the background. Then, while still
remaining in the same position, close the left eye and again observe the
position of the strip of paper with the right eye. You will find that the
position of the paper on the background has changed. As I sit in my study
and look out of the window I see a strip of paper, with my right eye, in front
of a certain bough on a tree a couple of hundred yards away; with my left
eye the paper is no longer in front of that bough, it has moved to a position
near the outline of the tree. This apparent displacement of the strip of paper,
relatively to the distant background, is what is called parallax.

Move closer to the window, and repeat the observation, and you find that
the apparent displacement of the strip increases. Move away from the
window, and the displacement decreases. Move to the other side of the
room, the displacement is much less, though probably still visible. We thus
see that the change in the apparent place of the strip of paper, as viewed
with the right eye or the left eye, varies in amount as the distance changes;
but it varies in the opposite way to the distance, for as either becomes
greater the other becomes less. We can thus associate with each particular
distance a corresponding particular displacement. From this it will be easy
to infer that if we have the means of measuring the amount of displacement,
then we have the means of calculating the distance from the observer to the
window.

It is this principle, applied on a gigantic scale, which enables us to
measure the distances of the heavenly bodies. Look, for instance, at the
planet Venus; let this correspond to the strip of paper, and let the sun, on
which Venus is seen in the act of transit, be the background. Instead of the
two eyes of the observer, we now place two observatories in distant regions
of the earth; we look at Venus from one observatory, we look at it from the
other; we measure the amount of the displacement, and from that we
calculate the distance of the planet. All depends, then, on the means which
we have of measuring the displacement of Venus as viewed from the two
different stations. There are various ways of accomplishing this, but the
most simple is that originally proposed by Halley.

From the observatory at a Venus seems to pursue the upper of the two
tracks shown in the adjoining figure (Fig. 47). From the observatory at b it
follows the lower track, and it is for us to measure the distance between the
two tracks. This can be accomplished in several ways. Suppose the observer
at a notes the time that Venus has occupied in crossing the disc, and that
similar observations be made at b. As the track seen from b is the larger, it
must follow that the time observed at b will be greater than that at a. When
the observations from the different hemispheres are compared, the times
observed will enable the lengths of the tracks to be calculated. The lengths
being known, their places on the circular disc of the sun are determined, and
hence the amount of displacement of Venus in transit is ascertained. Thus it
is that the distance of Venus is measured, and the scale of the solar system is
known.

The two transits to which Halley's memorable researches referred
occurred in the years 1761 and 1769. The results of the first were not very
successful, in spite of the arduous labours of those who undertook the
observations. The transit of 1769 is of particular interest, not only for the
determination of the sun's distance, but also because it gave rise to the first
of the celebrated voyages of Captain Cook. It was to see the transit of Venus
that Captain Cook was commissioned to sail to Otaheite, and there, on the
3rd of June, on a splendid day in that exquisite climate, the phenomenon

was carefully
observed and
measured by
different
observers.
Simultaneously
with these
observations
others were
obtained in
Europe and
elsewhere, and
from the
combination of
all the
observations an
approximate
knowledge of the
Fig. 47.—To Illustrate the Observation of the Transit of sun's distance
Venus from Two Localities, a and b, on the Earth. was gained. The
most complete
discussion of these observations did not, however, take place for some time.
It was not until the year 1824 that the illustrious Encke computed the
distance of the sun, and gave as the definite result 95,000,000 miles.

For many years this number was invariably adopted, and many of the
present generation will remember how they were taught in their school-days
that the sun was 95,000,000 miles away. At length doubts began to be
whispered as to the accuracy of this result. The doubts arose in different
quarters, and were presented with different degrees of importance; but they
all pointed in one direction, they all indicated that the distance of the sun
was not really so great as the result which Encke had obtained. It must be
remembered that there are several ways of finding the distance of the sun,

and it will be our duty to allude to some other methods later on. It has been
ascertained that the result obtained by Encke from the observations made in
1761 and 1769, with instruments inferior to our modern ones, was too great,
and that the distance of the sun may probably be now stated at 92,000,000
miles.

I venture to record our personal experience of the last transit of Venus,
which we had the good fortune to view from Dunsink Observatory on the
afternoon of the 6th of December, 1882.

The morning of the eventful day appeared to be about as unfavourable
for a grand astronomical spectacle as could well be imagined. Snow, a
couple of inches thick, covered the ground, and more was falling, with but
little intermission, all the forenoon. It seemed almost hopeless that a view
of the phenomenon could be obtained from that observatory; but it is well in
such cases to bear in mind the injunction given to the observers on a
celebrated eclipse expedition. They were instructed, no matter what the day
should be like, that they were to make all their preparations precisely as
they would have done were the sun shining with undimmed splendour. By
this advice no doubt many observers have profited; and we acted upon it
with very considerable success.

There were at that time at the observatory two equatorials, one of them
an old, but tolerably good, instrument, of about six inches aperture; the
other the great South equatorial, of twelve inches aperture, already referred
to. At eleven o'clock the day looked worse than ever; but we at once
proceeded to make all ready. I stationed Mr. Rambaut at the small
equatorial, while I myself took charge of the South instrument. The snow
was still falling when the domes were opened; but, according to our
prearranged scheme, the telescopes were directed, not indeed upon the sun,
but to the place where we knew the sun was, and the clockwork was set in
motion which carried round the telescopes, still constantly pointing towards
the invisible sun. The predicted time of the transit had not yet arrived.

The eye-piece employed on the South equatorial must also receive a
brief notice. It will, of course, be obvious that the full glare of the sun has to
be greatly mitigated before the eye can view it with impunity. The light
from the sun falls upon a piece of transparent glass inclined at a certain
angle, and the chief portion of the sun's heat, as well as a certain amount of
its light, pass through the glass and are lost. A certain fraction of the light is,
however, reflected from the glass, and enters the eye-piece. This light is
already much reduced in intensity, but it undergoes as much further
reduction as we please by an ingenious contrivance. The glass which
reflects the light does so at what is called the polarising angle, and between
the eye-piece and the eye is a plate of tourmaline. This plate of tourmaline
can be turned round by the observer. In one position it hardly interferes with
the polarised light at all, while in the position at right angles thereto it cuts
off nearly the whole of it. By simply adjusting the position of the
tourmaline, the observer has it in his power to render the image of any
brightness that may be convenient, and thus the observations of the sun can
be conducted with the appropriate degree of illumination.

But such appliances seemed on this occasion to be a mere mockery. The
tourmaline was all ready, but up to one o'clock not a trace of the sun could
be seen. Shortly after one o'clock, however, we noticed that the day was
getting lighter; and, on looking to the north, whence the wind and the snow
were coming, we saw, to our inexpressible delight, that the clouds were
clearing. At length, the sky towards the south began to improve, and at last,
as the critical moment approached, we could detect the spot where the sun
was becoming visible. But the predicted moment arrived and passed, and
still the sun had not broken through the clouds, though every moment the
certainty that it would do so became more apparent. The external contact
was therefore missed. We tried to console ourselves by the reflection that
this was not, after all, a very important phase, and hoped that the internal
contact would be more successful.

At length the struggling beams pierced the obstruction, and I saw the
round, sharp disc of the sun in the finder, and eagerly glanced at the point

on which attention was concentrated. Some minutes had now elapsed since
the predicted moment of first contact, and, to my delight, I saw the small
notch in the margin of the sun showing that the transit had commenced, and
that the planet was then one-third on the sun. But the critical moment had
not yet arrived. By the expression "first internal contact" we are to
understand the moment when the planet has completely entered on the sun.
This first contact was timed to occur twenty-one minutes later than the
external contact already referred to. But the clouds again disappointed our
hope of seeing the internal contact. While steadily looking at the exquisitely
beautiful sight of the gradual advance of the planet, I became aware that
there were other objects besides Venus between me and the sun. They were
the snowflakes, which again began to fall rapidly. I must admit the
phenomenon was singularly beautiful. The telescopic effect of a snowstorm
with the sun as a background I had never before seen. It reminded me of the
golden rain which is sometimes seen falling from a flight of sky-rockets
during pyrotechnic displays; I would gladly have dispensed with the
spectacle, for it necessarily followed that the sun and Venus again
disappeared from view. The clouds gathered, the snowstorm descended as
heavily as ever, and we hardly dared to hope that we should see anything
more; 1 hr. 57 min. came and passed, the first internal contact was over, and
Venus had fully entered on the sun. We had only obtained a brief view, and
we had not yet been able to make any measurements or other observations
that could be of service. Still, to have seen even a part of a transit of Venus
is an event to remember for a lifetime, and we felt more delight than can be
easily expressed at even this slight gleam of success.

But better things were in store. My assistant came over with the report
that he had also been successful in seeing Venus in the same phase as I had.
We both resumed our posts, and at half-past two the clouds began to
disperse, and the prospect of seeing the sun began to improve. It was now
no question of the observations of contact. Venus by this time was well on
the sun, and we therefore prepared to make observations with the
micrometer attached to the eye-piece. The clouds at length dispersed, and at

this time Venus had so completely entered on the sun that the distance from
the edge of the planet to the edge of the sun was about twice the diameter of
the planet. We measured the distance of the inner edge of Venus from the
nearest limb of the sun. These observations were repeated as frequently as
possible, but it should be added that they were only made with very
considerable difficulty. The sun was now very low, and the edges of the sun
and of Venus were by no means of that steady character which is suitable
for micrometrical measurement. The margin of the luminary was quivering,
and Venus, though no doubt it was sometimes circular, was very often
distorted to such a degree as to make the measures very uncertain.

We succeeded in obtaining sixteen measures altogether; but the sun was
now getting low, the clouds began again to interfere, and we saw that the
pursuit of the transit must be left to the thousands of astronomers in happier
climes who had been eagerly awaiting it. But before the phenomena had
ceased I spared a few minutes from the somewhat mechanical work at the
micrometer to take a view of the transit in the more picturesque form which
the large field of the finder presented. The sun was already beginning to put
on the ruddy hues of sunset, and there, far in on its face, was the sharp,
round, black disc of Venus. It was then easy to sympathise with the supreme
joy of Horrocks, when, in 1639, he for the first time witnessed this
spectacle. The intrinsic interest of the phenomenon, its rarity, the fulfilment
of the prediction, the noble problem which the transit of Venus helps us to
solve, are all present to our thoughts when we look at this pleasing picture,
a repetition of which will not occur again until the flowers are blooming in
the June of a.d. 2004.

The occasion of a transit of Venus also affords an opportunity of
studying the physical nature of the planet, and we may here briefly indicate
the results that have been obtained. In the first place, a transit will throw
some light on the question as to whether Venus is accompanied by a
satellite. If Venus were attended by a small body in close proximity, it
would be conceivable that in ordinary circumstances the brilliancy of the
planet would obliterate the feeble beam of rays from the minute companion,

and thus the satellite would remain undiscovered. It was therefore a matter
of great interest to scrutinise the vicinity of the planet while in the act of
transit. If a satellite existed—and the existence of one or more of such
bodies has often been suspected—then it would be capable of detection
against the brilliant background of the sun. Special attention was directed to
this point during the recent transits, but no satellite of Venus was to be
found. It seems, therefore, to be very unlikely that Venus can be attended by
any companion globe of appreciable dimensions.

The observations directed to the investigation of the atmosphere
surrounding Venus have been more successful. If the planet were devoid of
an atmosphere, then it would be totally invisible just before commencing to
enter on the sun, and would relapse into total invisibility as soon as it had
left the sun. The observations made during the transits are not in conformity
with such suppositions. Special attention has been directed to this point
during the recent transits. The result has been very remarkable, and has
proved in the most conclusive manner the existence of an atmosphere
around Venus. As the planet gradually moved off the sun, the circular edge
of the planet extending out into the darkness was seen to be bounded by a
circular arc of light, and Dr. Copeland, who observed this transit in very
favourable circumstances, was actually able to follow the planet until it had
passed entirely away from the sun, at which time the globe, though itself
invisible, was distinctly marked by the girdle of light by which it was
surrounded. This luminous circle is inexplicable save by the supposition
that the globe of Venus is surrounded by an atmospheric shell in the same
way as the earth.

It may be asked, what is the advantage of devoting so much time and
labour to a celestial phenomenon like the transit of Venus which has so little
bearing on practical affairs? What does it matter whether the sun be
95,000,000 miles off, or whether it be only 93,000,000, or any other
distance? We must admit at once that the enquiry has but a slender bearing
on matters of practical utility. No doubt a fanciful person might contend that
to compute our nautical almanacs with perfect accuracy we require a

precise knowledge of the distance of the sun. Our vast commerce depends
on skilful navigation, and one factor necessary for success is the reliability
of the "Nautical Almanac." The increased perfection of the almanac must
therefore bear some relation to increased perfection in navigation. Now, as
good authorities tell us that in running for a harbour on a tempestuous night,
or in other critical emergencies, even a yard of sea-room is often of great
consequence, so it may conceivably happen that to the infinitesimal
influence of the transit of Venus on the "Nautical Almanac" is due the safety
of a gallant vessel.

But the time, the labour, and the money expended in observing the
transit of Venus are really to be defended on quite different grounds. We see
in it a fruitful source of information. It tells us the distance of the sun,
which is the foundation of all the great measurements of the universe. It
gratifies the intellectual curiosity of man by a view of the true dimensions
of the majestic solar system, in which the earth is seen to play a dignified,
though still subordinate, part; and it leads us to a conception of the
stupendous scale on which the universe is constructed.

It is not possible for us, with a due regard to the limits of this volume, to
protract any longer our discussion of the transit of Venus. When we begin to
study the details of the observations, we are immediately confronted with a
multitude of technical and intricate matters. Unfortunately, there are very
great difficulties in making the observations with the necessary precision.
The moments when Venus enters on and leaves the solar disc cannot be
very accurately observed, partly owing to a peculiar optical illusion known
as "the black drop," whereby Venus seems to cling to the sun's limb for
many seconds, partly owing to the influence of the planet's atmosphere,
which helps to make the observed time of contact uncertain. These
circumstances make it difficult to determine the distance of the sun from
observations of transits of Venus with the accuracy which modern science
requires. It seems therefore likely that the final determination of the sun's
distance will be obtained in quite a different manner. This will be explained

in Chapter XI., and hence we feel the less reluctance in passing any from
the consideration of the transit of Venus as a method of celestial surveying.

We must now close our description of this lovely planet; but before
doing so, let us add—or in some cases repeat—a few statistical facts as to
the size and the dimensions of the planet and its orbit.

The diameter of Venus is about 7,660 miles, and the planet shows no
measurable departure from the globular form, though we can hardly doubt
that its polar diameter must really be somewhat shorter than the equatorial
diameter. This diameter is only about 258 miles less than that of the earth.
The mass of Venus is about three-quarters of the mass of the earth; or if, as
is more usual, we compare the mass of Venus with the sun, it is to be
represented by the fraction 1 divided by 425,000. It is to be observed that
the mass of Venus is not quite so great in comparison with its bulk as might
have been expected. The density of this planet is about 0·850 of that of the
earth. Venus would weigh 4·81 times as much as a globe of water of equal
size. The gravitation at its surface will, to a slight extent, be less than the
gravitation at the surface of the earth. A body here falls sixteen feet in a
second; a body let fall at the surface of Venus would fall about three feet
less. It seems not unlikely that the time of rotation of Venus may be equal to
the period of its revolution around the sun.

The orbit of Venus is remarkable for the close approach which it makes
to a circle. The greatest distance of this planet from the sun does not exceed
the least distance by one per cent. Its mean distance from the sun is about
67,000,000 miles, and the movement in the orbit amounts to a mean
velocity of nearly 22 miles per second, the entire journey being
accomplished in 224·70 days.

CHAPTER IX.
THE EARTH.

The Earth is a great Globe—How the Size of the Earth is Measured—The Base Line—The
Latitude found by the Elevation of the Pole—A Degree of the Meridian—The Earth
not a Sphere—The Pendulum Experiment—Is the Motion of the Earth slow or fast?—
Coincidence of the Axis of Rotation and the Axis of Figure—The Existence of Heat in
the Earth—The Earth once in a Soft Condition—Effects of Centrifugal Force—
Comparison with the Sun and Jupiter—The Protuberance of the Equator—The
Weighing of the Earth—Comparison between the Weight of the Earth and an equal
Globe of Water—Comparison of the Earth with a Leaden Globe—The Pendulum—
Use of the Pendulum in Measuring the Intensity of Gravitation—The Principle of
Isochronism—Shape of the Earth measured by the Pendulum.

That the earth must be a round body is a truth immediately suggested by
simple astronomical considerations. The sun is round, the moon is round,
and telescopes show that the planets are round. No doubt comets are not
round, but then a comet seems to be in no sense a solid body. We can see
right through one of these frail objects, and its weight is too small for our
methods of measurement to appreciate. If, then, all the solid bodies we can
see are round globes, is it not likely that the earth is a globe also? But we
have far more direct information than mere surmise.

There is no better way of actually seeing that the surface of the ocean is
curved than by watching a distant ship on the open sea. When the ship is a
long way off and is still receding, its hull will gradually disappear, while the
masts will remain visible. On a fine summer's day we can often see the top
of the funnel of a steamer appearing above the sea, while the body of the
steamer is below. To see this best the eye should be brought as close as
possible to the surface of the sea. If the sea were perfectly flat, there would
be nothing to obscure the body of the vessel, and it would therefore be
visible so long as the funnel remains visible. If the sea be really curved, the
protuberant part intercepts the view of the hull, while the funnel is still to be
seen.

We thus learn how the sea is curved at every part, and therefore it is
natural to suppose that the earth is a sphere. When we make more careful
measurements we find that the globe is not perfectly round. It is flattened to
some extent at each of the poles. This may be easily illustrated by an

indiarubber ball, which can be compressed on two opposite sides so as to
bulge out at the centre. The earth is similarly flattened at the poles, and
bulged out at the equator. The divergence of the earth from the truly
globular form is, however, not very great, and would not be noticed without
very careful measurements.

The determination of the size of the earth involves operations of no little
delicacy. Very much skill and very much labour have been devoted to the
work, and the dimensions of the earth are known with a high degree of
accuracy, though perhaps not with all the precision that we may ultimately
hope to attain. The scientific importance of an accurate measurement of the
earth can hardly be over-estimated. The radius of the earth is itself the unit
in which many other astronomical magnitudes are expressed. For example,
when observations are made with the view of finding the distance of the
moon, the observations, when discussed and reduced, tell us that the
distance of the moon is equal to fifty-nine times the equatorial radius of the
earth. If we want to find the distance of the moon in miles, we require to
know the number of miles in the earth's radius.

A level part of the earth's surface having been chosen, a line a few miles
long is measured. This is called the base, and as all the subsequent measures
depend ultimately on the base, it is necessary that this measurement shall be
made with scrupulous accuracy. To measure a line four or five miles long
with such precision as to exclude any errors greater than a few inches
demands the most minute precautions. We do not now enter upon a
description of the operations that are necessary. It is a most laborious piece
of work, and many ponderous volumes have been devoted to the discussion
of the results. But when a few base lines have been obtained in different
places on the earth's surface, the measuring rods are to be laid aside, and the
subsequent task of the survey of the earth is to be conducted by the
measurement of angles from one station to another and trigonometrical
calculations based thereon. Starting from a base line a few miles long,
distances of greater length are calculated, until at length stretches 100 miles

long, or even more, can be accomplished. It is thus possible to find the
length of a long line running due north and south.

So far the work has been merely that of the terrestrial surveyor. The
distance thus ascertained is handed over to the astronomer to deduce from it
the dimensions of the earth. The astronomer fixes his observatory at the
northern end of the long line, and proceeds to determine his latitude by
observation. There are various ways by which this can be accomplished.
They will be found fully described in works on practical astronomy. We
shall here only indicate in a very brief manner the principle on which such
observations are to be made.

Everyone ought to be familiar with the Pole Star, which, though by no
means the most brilliant, is probably the most important star in the whole
heavens. In these latitudes we are accustomed to find the Pole Star at a
considerable elevation, and there we can invariably find it, always in the
same place in the northern sky. But suppose we start on a voyage to the
southern hemisphere: as we approach the equator we find, night after night,
the Pole Star coming closer to the horizon. At the equator it is on the
horizon; while if we cross the line, we find on entering the southern
hemisphere that this useful celestial body has become invisible. This is in
itself sufficient to show us that the earth cannot be the flat surface that
untutored experience seems to indicate.

On the other hand, a traveller leaving England for Norway observes that
the Pole Star is every night higher in the heavens than he has been
accustomed to see it. If he extend his journey farther north, the same object
will gradually rise higher and higher, until at length, when approaching the
pole of the earth, the Pole Star is high up over his head. We are thus led to
perceive that the higher our latitude, the higher, in general, is the elevation
of the Pole Star. But we cannot use precise language until we replace the
twinkling point by the pole of the heavens itself. The pole of the heavens is
near the Pole Star, which itself revolves around the pole of the heavens, as
all the other stars do, once every day. The circle described by the Pole Star

is, however, so small that, unless we give it special attention, the motion
will not be perceived. The true pole is not a visible point, but it is capable of
being accurately defined, and it enables us to state with the utmost precision
the relation between the pole and the latitude. The statement is, that the
elevation of the pole above the horizon is equal to the latitude of the place.

The astronomer stationed at one end of the long line measures the
elevation of the pole above the horizon. This is an operation of some
delicacy. In the first place, as the pole is invisible, he has to obtain its
position indirectly. He measures the altitude of the Pole Star when that
altitude is greatest, and repeats the operation twelve hours later, when the
altitude of the Pole Star is least; the mean between the two, when corrected
in various ways which it is not necessary for us now to discuss, gives the
true altitude of the pole. Suffice it to say that by such operations the latitude
of one end of the line is determined. The astronomer then, with all his
equipment of instruments, moves to the other end of the line. He there
repeats the process, and he finds that the pole has now a different elevation,
corresponding to the different latitude. The difference of the two elevations
thus gives him an accurate measure of the number of degrees and fractional
parts of a degree between the latitudes of the two stations. This can be
compared with the actual distance in miles between the two stations, which
has been ascertained by the trigonometrical survey. A simple calculation
will then show the number of miles and fractional parts of a mile
corresponding to one degree of latitude—or, as it is more usually expressed,
the length of a degree of the meridian.

This operation has to be repeated in different parts of the earth—in the
northern hemisphere and in the southern, in high latitudes and in low. If the
sea-level over the entire earth were a perfect sphere, an important
consequence would follow—the length of a degree of the meridian would
be everywhere the same. It would be the same in Peru as in Sweden, the
same in India as in England. But the lengths of the degrees are not all the
same, and hence we learn that our earth is not really a sphere. The measured
lengths of the degrees enable us to see to what extent the shape of the earth

departs from a perfect sphere. Near the pole the length of a degree is longer
than near the equator. This shows that the earth is flattened at the poles and
protuberant at the equator, and it provides the means by which we may
calculate the actual lengths of the polar and the equatorial axes. In this way
the equatorial diameter has been found equal to 7,927 miles, while the polar
diameter is 27 miles shorter.

The polar axis of the earth may be defined as the diameter about which
the earth rotates. This axis intersects the surface at the north and south
poles. The time which the earth occupies in making a complete rotation
around this axis is called a sidereal day. The sidereal day is a little shorter
than the ordinary day, being only 23 hours, 56 minutes, and 4 seconds. The
rotation is performed just as if a rigid axis passed through the centre of the
earth; or, to use the old and homely illustration, the earth rotates just as a
ball of worsted may be made to rotate around a knitting-needle thrust
through its centre.

It is a noteworthy circumstance that the axis about which the earth
rotates occupies a position identical with that of the shortest diameter of the
earth as found by actual surveying. This is a coincidence which would be
utterly inconceivable if the shape of the earth was not in some way
physically connected with the fact that the earth is rotating. What
connection can then be traced? Let us enquire into the subject, and we shall
find that the shape of the earth is a consequence of its rotation.

The earth at the present time is subject, at various localities, to
occasional volcanic outbreaks. The phenomena of such eruptions, the allied
occurrence of earthquakes, the well-known fact that the heat increases the
deeper we descend into the earth, the existence of hot springs, the geysers
found in Iceland and elsewhere, all testify to the fact that heat exists in the
interior of the earth. Whether that heat be, as some suppose, universal in the
interior of the earth, or whether it be merely local at the several places
where its manifestations are felt, is a matter which need not now concern

us. All that is necessary for our present purpose is the admission that heat is
present to some extent.

This internal heat, be it much or little, has obviously a different origin
from the heat which we know on the surface. The heat we enjoy is derived
from the sun. The internal heat cannot have been derived from the sun; its
intensity is far too great, and there are other insuperable difficulties
attending the supposition that it has come from the sun. Where, then, has
this heat come from? This is a question which at present we can hardly
answer—nor, indeed, does it much concern our argument that we should
answer it. The fact being admitted that the heat is there, all that we require
is to apply one or two of the well-known thermal laws to the interpretation
of the facts. We have first to consider the general principle by which heat
tends to diffuse itself and spread away from its original source. The heat,
deep-seated in the interior of the earth, is transmitted through the
superincumbent rocks, and slowly reaches the surface. It is true that the
rocks and materials with which our earth is covered are not good
conductors of heat; most of them are, indeed, extremely inefficient in this
way; but, good or bad, they are in some shape conductors, and through
them the heat must creep to the surface.

It cannot be urged against this conclusion that we do not feel this heat. A
few feet of brickwork will so confine the heat of a mighty blast furnace that
but little will escape through the bricks; but some heat does escape, and the
bricks have never been made, and never could be made, which would
absolutely intercept all the heat. If a few feet of brickwork can thus nearly
mask the heat of a furnace, cannot some scores of miles of rock nearly mask
the heat in the depths of the earth, even though that heat were seven times
hotter than the mightiest furnace that ever existed? The heat would escape
slowly, and perhaps imperceptibly, but, unless all our knowledge of nature
is a delusion, no rocks, however thick, can prevent, in the course of time,
the leakage of the heat to the surface. When this heat arrives at the surface
of the earth it must, in virtue of another thermal law, gradually radiate away
and be lost to the earth.

It would lead us too far to discuss fully the objections which may
perhaps be raised against what we have here stated. It is often said that the
heat in the interior of the earth is being produced by chemical combination
or by mechanical process, and thus that the heat may be constantly renewed
as fast or even faster than it escapes. This, however, is more a difference in
form than in substance. If heat be produced in the way just supposed (and
there can be no doubt that there may be such an origin for some of the heat
in the interior of the globe) there must be a certain expenditure of chemical
or mechanical energies that produces a certain exhaustion. For every unit of
heat which escapes there will either be a loss of an unit of heat from the
globe, or, what comes nearly to the same thing, a loss of an unit of heat-
making power from the chemical or the mechanical energies. The
substantial result is the same; the heat, actual or potential, of the earth must
be decreasing. It should, of course, be observed that a great part of the
thermal losses experienced by the earth is of an obvious character, and not
dependent upon the slow processes of conduction. Each outburst of a
volcano discharges a stupendous quantity of heat, which disappears very
speedily from the earth; while in the hot springs found in so many places
there is a perennial discharge of the same kind, which in the course of years
attains enormous proportions.

The earth is thus losing heat, while it never acquires any fresh supplies
of the same kind to replace the losses. The consequence is obvious; the
interior of the earth must be growing colder. No doubt this is an extremely
slow process; the life of an individual, the life of a nation, perhaps the life
of the human race itself, has not been long enough to witness any
pronounced change in the store of terrestrial heat. But the law is inevitable,
and though the decline in heat may be slow, yet it is continuous, and in the
lapse of ages must necessarily produce great and important results.

It is not our present purpose to offer any forecast as to the changes which
must necessarily arise from this process. We wish at present rather to look
back into past time and see what consequences we may legitimately infer.
Such intervals of time as we are familiar with in ordinary life, or even in

ordinary history, are for our present purpose quite inappreciable. As our
earth is daily losing internal heat, or the equivalent of heat, it must have
contained more heat yesterday than it does to-day, more last year than this
year, more twenty years ago than ten years ago. The effect has not been
appreciable in historic time; but when we rise from hundreds of years to
thousands of years, from thousands of years to hundreds of thousands of
years, and from hundreds of thousands of years to millions of years, the
effect is not only appreciable, but even of startling magnitude.

There must have been a time when the earth contained much more heat
than at present. There must have been a time when the surface of the earth
was sensibly hot from this source. We cannot pretend to say how many
thousands or millions of years ago this epoch must have been; but we may
be sure that earlier still the earth was even hotter, until at length we seem to
see the temperature increase to a red heat, from a red heat we look back to a
still earlier age when the earth was white hot, back further till we find the
surface of our now solid globe was actually molten. We need not push the
retrospect any further at present, still less is it necessary for us to attempt to
assign the probable origin of that heat. This, it will be observed, is not
required in our argument. We find heat now, and we know that heat is being
lost every day. From this the conclusion that we have already drawn seems
inevitable, and thus we are conducted back to some remote epoch in the
abyss of time past when our solid earth was a globe molten and soft
throughout.

A dewdrop on the petal of a flower is nearly globular; but it is not quite a
globe, because the gravitation presses it against the flower and somewhat
distorts the shape. A falling drop of rain is a globe; a drop of oil suspended
in a liquid with which it does not mix forms a globe. Passing from small
things to great things, let us endeavour to conceive a stupendous globe of
molten matter. Let that globe be as large as the earth, and let its materials be
so soft as to obey the forces of attraction exerted by each part of the globe
on all the other parts. There can be no doubt as to the effect of these
attractions; they would tend to smooth down any irregularities on the

surface just in the same way as the surface of the ocean is smooth when
freed from the disturbing influences of the wind. We might, therefore,
expect that our molten globe, isolated from all external interference, would
assume the form of a sphere.

But now suppose that this great sphere, which we have hitherto assumed
to be at rest, is made to rotate round an axis passing through its centre. We
need not suppose that this axis is a material object, nor are we concerned
with any supposition as to how the velocity of rotation was caused. We can,
however, easily see what the consequence of the rotation would be. The
sphere would become deformed, the centrifugal force would make the
molten body bulge out at the equator and flatten down at the poles. The
greater the velocity of rotation the greater would be the bulging. To each
velocity of rotation a certain degree of bulging would be appropriate. The
molten earth thus bulged out to an extent which was dependent upon the
fact that it turned round once a day. Now suppose that the earth, while still
rotating, commences to pass from the liquid to the solid state. The form
which the earth would assume on consolidation would, no doubt, be very
irregular on the surface; it would be irregular in consequence of the
upheavals and the outbursts incident to the transformation of so mighty a
mass of matter; but irregular though it be, we can be sure that, on the whole,
the form of the earth's surface would coincide with the shape which it had
assumed by the movement of rotation. Hence we can explain the
protuberant form of the equator of the earth, and we can appeal to that form
in corroboration of the view that this globe was once in a soft or molten
condition.

The argument may be supported and illustrated by comparing the shape
of our earth with the shapes of some of the other celestial bodies. The sun,
for instance, seems to be almost a perfect globe. No measures that we can
make show that the polar diameter of the sun is shorter than the equatorial
diameter. But this is what we might have expected. No doubt the sun is
rotating on its axis, and, as it is the rotation that causes the protuberance,
why should not the rotation have deformed the sun like the earth? The

probability is that a difference really does exist between the two diameters
of the sun, but that the difference is too small for us to measure. It is
impossible not to connect this with the slowness of the sun's rotation. The
sun takes twenty-five days to complete a rotation, and the protuberance
appropriate to so low a velocity is not appreciable.

On the other hand, when we look at one of the quickly-rotating planets,
we obtain a very different result. Let us take the very striking instance
which is presented in the great planet Jupiter. Viewed in the telescope,
Jupiter is at once seen not to be a globe. The difference is so conspicuous
that accurate measures are not necessary to show that the polar diameter of
Jupiter is shorter than the equatorial diameter. The departure of Jupiter from
the truly spherical shape is indeed much greater than the departure of the
earth. It is impossible not to connect this with the much more rapid rotation
of Jupiter. We shall presently have to devote a chapter to the consideration
of this splendid orb. We may, however, so far anticipate what we shall then
say as to state that the time of Jupiter's rotation is under ten hours, and this
notwithstanding the fact that Jupiter is more than one thousand times
greater than the earth. His enormously rapid rotation has caused him to
bulge out at the equator to a remarkable extent.

The survey of our earth and the measurement of its dimensions having
been accomplished, the next operation for the astronomer is the
determination of its weight. Here, indeed, is a problem which taxes the
resources of science to the very uttermost. Of the interior of the earth we
know little—I might almost say we know nothing. No doubt we sink deep
mines into the earth. These mines enable us to penetrate half a mile, or even
a whole mile, into the depths of the interior. But this is, after all, only a
most insignificant attempt to explore the interior of the earth. What is an
advance of one mile in comparison with the distance to the centre of the
earth? It is only about one four-thousandth part of the whole. Our
knowledge of the earth merely reaches to an utterly insignificant depth
below the surface, and we have not a conception of what may be the nature
of our globe only a few miles below where we are standing. Seeing, then,

our almost complete ignorance of the solid contents of the earth, does it not
seem a hopeless task to attempt to weigh the entire globe? Yet that problem
has been solved, and the result is known—not, indeed, with the accuracy
attained in other astronomical researches, but still with tolerable
approximation.

It is needless to enunciate the weight of the earth in our ordinary units.
The enumeration of billions of tons does not convey any distinct
impression. It is a far more natural course to compare the mass of the earth
with that of an equal globe of water. We should be prepared to find that our
earth was heavier than a like volume of water. The rocks which form its
surface are heavier, bulk for bulk, than the oceans which repose on those
rocks. The abundance of metals in the earth, the gradual increase in the
density of the earth, which must arise from the enormous pressure at great
depths—all these considerations will prepare us to learn that the earth is
very much heavier than a globe of water of equal size.

Newton supposed that the earth was between five and six times as heavy
as an equal bulk of water. Nor is it hard to see that such a suggestion is
plausible. The rocks and materials on the surface are usually about two or
three times as heavy as water, but the density of the interior must be much
greater. There is good reason to believe that down in the remote depths of
the earth there is a very large proportion of iron. An iron earth would weigh
about seven times as much as an equal globe of water. We are thus led to
see that the earth's weight must be probably more than three, and probably
less than seven, times an equal globe of water; and hence, in fixing the
density between five and six, Newton adopted a result plausible at the
moment, and since shown to be probably correct. Several methods have
been proposed by which this important question can be solved with
accuracy. Of all these methods we shall here only describe one, because it
illustrates, in a very remarkable manner, the law of universal gravitation.

In the chapter on Gravitation it was pointed out that the intensity of this
force between two masses of moderate dimensions was extremely minute,

and the difficulty in weighing the earth arises from this cause. The practical
application of the process is encumbered by multitudinous details, which it
will be unnecessary for us to consider at present. The principle of the
process is simple enough. To give definiteness to our description, let us
conceive a large globe about two feet in diameter; and as it is desirable for
this globe to be as heavy as possible, let us suppose it to be made of lead. A
small globe brought near the large one is attracted by the force of
gravitation. The amount of this attraction is extremely small, but,
nevertheless, it can be measured by a refined process which renders
extremely small forces sensible. The intensity of the attraction depends both
on the masses of the globes and on their distance apart, as well as on the
force of gravitation. We can also readily measure the attraction of the earth
upon the small globe. This is, in fact, nothing more nor less than the weight
of the small globe in the ordinary acceptation of the word. We can thus
compare the attraction exerted by the leaden globe with the attraction
exerted by the earth.

If the centre of the earth and the centre of the leaden globe were at the
same distance from the attracted body, then the intensity of their attractions
would give at once the ratio of their masses by simple proportion. In this
case, however, matters are not so simple: the leaden ball is only distant by a
few inches from the attracted ball, while the centre of the earth's attraction
is nearly 4,000 miles away at the centre of the earth. Allowance has to be
made for this difference, and the attraction of the leaden sphere has to be
reduced to what it would be were it removed to a distance of 4,000 miles.
This can fortunately be effected by a simple calculation depending upon the
general law that the intensity of gravitation varies inversely as the square of
the distance. We can thus, partly by calculation and partly by experiment,
compare the intensity of the attraction of the leaden sphere with the
attraction of the earth. It is known that the attractions are proportional to the
masses, so that the comparative masses of the earth and of the leaden sphere
have been measured; and it has been ascertained that the earth is about half
as heavy as a globe of lead of equal size would be. We may thus state

finally that the mass of the earth is about five and a half times as great as
the mass of a globe of water equal to it in bulk.

In the chapter on Gravitation we have mentioned the fact that a body let
fall near the surface of the earth drops through sixteen feet in the first
second. This distance varies slightly at different parts of the earth. If the
earth were a perfect sphere, then the attraction would be the same at every
part, and the body would fall through the same distance everywhere. The
earth is not round, so the distance which the body falls in one second differs
slightly at different places. At the pole the radius of the earth is shorter than
at the equator, and accordingly the attraction of the earth at the pole is
greater than at the equator. Had we accurate measurements showing the
distance a body would fall in one second both at the pole and at the equator,
we should have the means of ascertaining the shape of the earth.

It is, however, difficult to measure correctly the distance a body will fall
in one second. We have, therefore, been obliged to resort to other means for
determining the force of attraction of the earth at the equator and other
accessible parts of its surface. The methods adopted are founded on the
pendulum, which is, perhaps, the simplest and certainly one of the most
useful of philosophical instruments. The ideal pendulum is a small and
heavy weight suspended from a fixed point by a fine and flexible wire. If
we draw the pendulum aside from its vertical position and then release it,
the weight will swing to and fro.

For its journey to and fro the pendulum requires a small period of time.
It is very remarkable that this period does not depend appreciably on the
length of the circular arc through which the pendulum swings. To verify this
law we suspend another pendulum beside the first, both being of the same
length. If we draw both pendulums aside and then release them, they swing
together and return together. This might have been expected. But if we draw
one pendulum a great deal to one side, and the other only a little, the two
pendulums still swing sympathetically. This, perhaps, would not have been
expected. Try it again, with even a still greater difference in the arc of

vibration, and still we see the two weights occupy the same time for the
swing.

We can vary the experiment in another way. Let us change the weights
on the pendulums, so that they are of unequal size, though both of iron.
Shall we find any difference in the periods of vibration? We try again: the
period is the same as before; swing them through different arcs, large or
small, the period is still the same. But it may be said that this is due to the
fact that both weights are of the same material. Try it again, using a leaden
weight instead of one of the iron weights; the result is identical. Even with a
ball of wood the period of oscillation is the same as that of the ball of iron,
and this is true no matter what be the arc through which the vibration takes
place.

If, however, we change the length of the wire by which the weight is
supported, then the period will not remain unchanged. This can be very
easily illustrated. Take a short pendulum with a wire only one-fourth of the
length of that of the long one; suspend the two close together, and compare
the periods of vibration of the short pendulum with that of the long one, and
we find that the former has a period only half that of the latter. We may state
the result generally, and say that the time of vibration of a pendulum is
proportional to the square root of its length. If we quadruple the length of
the suspending cord we double the time of its vibration; if we increase the
length of the pendulum ninefold, we increase its period of vibration
threefold.

It is the gravitation of the earth which makes the pendulum swing. The
greater the attraction, the more rapidly will the pendulum oscillate. This
may be easily accounted for. If the earth pulls the weight down very
vigorously, the time will be short; if the power of the earth's attraction be
lessened, then it cannot pull the weight down so quickly, and the period will
be lengthened.

The time of vibration of the pendulum can be determined with great
accuracy. Let it swing for 10,000 oscillations, and measure the time that
these oscillations have consumed. The arc through which the pendulum
swings may not have remained quite constant, but this does not appreciably
affect the time of its oscillation. Suppose that an error of a second is made
in the determination of the time of 10,000 oscillations; this will only entail
an error of the ten-thousandth part of the second in the time of a single
oscillation, and will afford a correspondingly accurate determination of the
force of gravity at the place where the experiment was made.

Take a pendulum to the equator. Let it perform 10,000 oscillations, and
determine carefully the time that these oscillations have required. Bring the
same pendulum to another part of the earth, and repeat the experiment. We
have thus a means of comparing the gravitation at the two places. There are,
no doubt, a multitude of precautions to be observed which need not here
concern us. It is not necessary to enter into details as to the manner in which
the motion of the pendulum is to be sustained, nor as to the effect of
changes of temperature in the alteration of its length. It will suffice for us to
see how the time of the pendulum's swing can be measured accurately, and
how from that measurement the intensity of gravitation can be calculated.

The pendulum thus enables us to make a gravitational survey of the
surface of the earth with the highest degree of accuracy. We cannot,
however, infer that gravity alone affects the oscillations of the pendulum.
We have seen how the earth rotates on its axis, and we have attributed the
bulging of the earth at the equator to this influence. But the centrifugal
force arising from the rotation has the effect of decreasing the apparent
weight of bodies, and the change is greatest at the equator, and lessens
gradually as we approach the poles. From this cause alone the attraction of
the pendulum at the equator is less than elsewhere, and therefore the
oscillations of the pendulum will take a longer time there than at other
localities. A part of the apparent change in gravitation is accordingly due to
the centrifugal force; but there is, in addition, a real alteration.

In a work on astronomy it does not come within our scope to enter into
further detail on the subject of our planet. The surface of the earth, its
contour and its oceans, its mountain chains and its rivers, are for the
physical geographer; while its rocks and their contents, its volcanoes and its
earthquakes, are to be studied by the geologists and the physicists.

CHAPTER X.
MARS.
Our nearer Neighbours in the Heavens—Surface of Mars can be Examined in the
Telescope—Remarkable Orbit of Mars—Resemblance of Mars to a Star—Meaning of
Opposition—The Eccentricity of the Orbit of Mars—Different Oppositions of Mars—
Apparent Movements of the Planet—Effect of the Earth's Movement—Measurement
of the Distance of Mars—Theoretical Investigation of the Sun's Distance—Drawings
of the Planet—Is there Snow on Mars?—The Rotation of the Planet—Gravitation on
Mars—Has Mars any Satellites?—Prof. Asaph Hall's great Discovery—The
Revolutions of the Satellites—Deimos and Phobos—"Gulliver's Travels."

The special relation in which we stand to one planet of our system has
necessitated a somewhat different treatment of that globe from the treatment
appropriate to the others. We discussed Mercury and Venus as distant
objects known chiefly by telescopic research, and by calculations of which
astronomical observations were the foundation. Our knowledge of the earth
is of a different character, and attained in a different way. Yet it was
necessary for symmetry that we should discuss the earth after the planet
Venus, in order to give to the earth its true position in the solar system. But
now that the earth has been passed in our outward progress from the sun,
we come to the planet Mars; and here again we resume, though in a
somewhat modified form, the methods that were appropriate to Venus and
to Mercury.

Venus and Mars have, from one point of view, quite peculiar claims on
our attention. They are our nearest planetary neighbours, on either side. We
may naturally expect to learn more of them than of the other planets farther
off. In the case of Venus, however, this anticipation can hardly be realised,
for, as we have already pointed out, its dense atmosphere prevents us from
making a satisfactory telescopic examination. When we turn to our other
planetary neighbour, Mars, we are enabled to learn a good deal with regard

to his appearance. Indeed, with the exception of the moon, we are better
acquainted with the details of the surface of Mars than with those of any
other celestial body.

This beautiful planet offers many features for consideration besides
those presented by its physical structure. The orbit of Mars is one of
remarkable proportions, and it was by the observations of this orbit that the
celebrated laws of Kepler were discovered. During the occasional
approaches of Mars to the earth it has been possible to measure its distance
with accuracy, and thus another method of finding the sun's distance has
arisen which, to say the least, may compete in precision with that afforded
by the transit of Venus. It must also be observed that the greatest
achievement in pure telescopic research which this century has witnessed
was that of the discovery of the satellites of Mars.

To the unaided eye this planet generally appears like a star of the first
magnitude. It is usually to be distinguished by its ruddy colour, but the
beginner in astronomy cannot rely on its colour only for the identification
of Mars. There are several stars nearly, if not quite, as ruddy as this globe.
The bright star Aldebaran, the brightest star in the constellation of the Bull,
has often been mistaken for the planet. It often resembles Betelgeuze, a
brilliant point in the constellation of Orion. Mistakes of this kind will be
impossible if the learner has first studied the principal constellations and the
more brilliant stars. He will then find great interest in tracing out the
positions of the planets, and in watching their ceaseless movements.

The position of each orb can always be ascertained from the almanac.
Sometimes the planet will be too near the sun to be visible. It will rise with
the sun and set with the sun, and consequently will not be above the horizon
during the night. The best time for seeing one of the planets situated like
Mars will be during what is called its opposition. This state of things occurs
when the earth intervenes directly between the planet and the sun. In this
case, the distance from Mars to the earth is less than at any other time.
There is also another advantage in viewing Mars during opposition. The

planet is
then at one
side of the
earth and
the sun at
the
opposite
side, so
that when
Mars is
high in the
heavens
the sun is
directly
beneath
the earth;
in other
words, the
planet is
then at its
greatest
Fig. 48.—The Orbits of the Earth and of Mars, showing the
elevation Favourable Opposition of 1877.
above the
horizon at
midnight. Some oppositions of Mars are, however, much more favourable
than others. This is distinctly shown in Fig. 48, which represents the orbit of
Mars and the orbit of the Earth accurately drawn to scale. It will be seen
that while the orbit of the earth is very nearly circular, the orbit of Mars has
a very decided degree of eccentricity; indeed, with the exception of the orbit
of Mercury, that of Mars has the greatest eccentricity of any orbit of the
larger planets in our system.

The value of an opposition of Mars for telescopic purposes will vary
greatly according to circumstances. The favourable oppositions will be
those which occur as near as possible to the 26th of August. The other
extreme will be found in an opposition which occurs near the 22nd of
February. In the latter case the distance between the planet and the earth is
nearly twice as great as the former. The last opposition which was suitable
for the highest class of work took place in the year 1877. Mars was then a
magnificent object, and received much, and deserved, attention. The
favourable oppositions follow each other at somewhat irregular intervals;
the last occurred in the year 1892, and another will take place in the year
1909.

The apparent movements of Mars are by no means simple. We can
imagine the embarrassment of the early astronomer who first undertook the
task of attempting to decipher these movements. The planet is seen to be a
brilliant and conspicuous object. It attracts the astronomer's attention; he
looks carefully, and he sees how it lies among the constellations with which
he is familiar. A few nights later he observes the same body again; but is it
exactly in the same place? He thinks not. He notes more carefully than
before the place of the planet. He sees how it is situated with regard to the
stars. Again, in a few days, his observations are repeated. There is no longer
a trace of doubt about the matter—Mars has decidedly changed his position.
It is veritably a wanderer.

Night after night the primitive astronomer is at his post. He notes the
changes of Mars. He sees that it is now moving even more rapidly than it
was at first. Is it going to complete the circuit of the heavens? The
astronomer determines to watch the orb and see whether this surmise is
justified. He pursues his task night after night, and at length he begins to
think that the body is not moving quite so rapidly as at first. A few nights
more, and he is sure of the fact: the planet is moving more slowly. Again a
few nights more, and he begins to surmise that the motion may cease; after
a short time the motion does cease, and the object seems to rest; but is it
going to remain at rest for ever? Has its long journey been finished? For

many nights this seems to be the case, but at length the astronomer suspects
that the planet must be commencing to move backwards. A few nights
more, and the fact is confirmed beyond possibility of doubt, and the
extraordinary discovery of the direct and the retrograde movement of Mars
has been accomplished.

In the
greater
part of its
journey
around the
heavens
Mars
seems to
move
steadily
from the
west to the
east. It
moves
backwards,
in fact, as
the moon
Fig. 49.—The Apparent Movements of Mars In 1877. moves and
as the sun
moves. It is only during a comparatively small part of its path that those
elaborate movements are accomplished which presented such an enigma to
the primitive observer. We show in the adjoining picture (Fig. 49) the track
of the actual journey which Mars accomplished in the opposition of 1877.
The figure only shows that part of its path which presents the anomalous
features; the rest of the orbit is pursued, not indeed with uniform velocity,
but with unaltered direction.

This complexity of the apparent movements of Mars seems at first sight
fatal to the acceptance of any simple and elementary explanation of the
planetary motion. If the motion of Mars were purely elliptic, how, it may
well be said, could it perform this extraordinary evolution? The elucidation
is to be found in the fact that the earth on which we stand is itself in motion.
Even if Mars were at rest, the fact that the earth moves would make the
planet appear to move. The apparent movements of Mars are thus combined
with the real movements. This circumstance will not embarrass the
geometer. He is able to disentangle the true movement of the planet from its
association with the apparent movement, and to account completely for the
complicated evolutions exhibited by Mars. Could we transfer our point of
view from the ever-shifting earth to an immovable standpoint, we should
then see that the shape of the orbit of Mars was an ellipse, described around
the sun in conformity with the laws which Kepler discovered by
observations of this planet.

Mars takes 687 days to travel round the sun, its average distance from
that body being 141,500,000 miles. Under the most favourable
circumstances the planet, at the time of opposition, may approach the earth
to a distance not greater than about 35,500,000 miles. No doubt this seems
an enormous distance, when estimated by any standard adapted for
terrestrial measurements; it is, however, hardly greater than the distance of
Venus when nearest, and it is much less than the distance from the earth to
the sun.

We have explained how the form of the solar system is known from
Kepler's laws, and how the absolute size of the system and of its various
parts can be known when the direct measurement of any one part has been
accomplished. A close approach of Mars affords a favourable opportunity
for measuring his distance, and thus, in a different way, solving the same
problem as that investigated by the transit of Venus. We are thus led a
second time to a knowledge of the distance of the sun and the distances of
the planets generally, and to many other numerical facts about the solar
system.

On the occasion of the opposition of Mars in 1877 a successful attempt
was made to apply this refined process to the solution of the problem of
celestial measurement. It cannot be said to have been the first occasion on
which this method was suggested, or even practically attempted. The
observations of 1877 were, however, conducted with such skill and with
such minute attention to the necessary precautions as to render them an
important contribution to astronomy. Dr. David Gill, now her Majesty's
Astronomer at the Cape of Good Hope, undertook a journey to the Island of
Ascension for the purpose of observing the parallax of Mars in 1877. On
this occasion Mars approached to the earth so closely as to afford an
admirable opportunity for the application of the method. Dr. Gill succeeded
in obtaining a valuable series of measurements, and from them he
concluded the distance of the sun with an accuracy somewhat superior to
that attainable by the transit of Venus.

There is yet another method by which Mars can be made to give us
information as to the distance of the sun. This method is one of some
delicacy, and is interesting from its connection with the loftiest enquiries in
mathematical astronomy. It was foreshadowed in the Dynamical theory of
Newton, and was wrought to perfection by Le Verrier. It is based upon the
great law of gravitation, and is intimately associated with the splendid
discoveries in planetary perturbation which form so striking a chapter in
modern astronomical discovery.

There is a certain relation between two quantities which at first sight
seems quite independent. These quantities are the mass of the earth and the
distance of the sun. The distance of the sun bears to a certain distance
(which can be calculated when we know the intensity of gravitation at the
earth's surface, the size of the earth and the length of the year) the same
proportion that the cube root of the sun's mass bears to the cube root of that
of the earth. There is no uncertainty about this result, and the consequence
is obvious. If we have the means of weighing the earth in comparison with
the sun, then the distance of the sun can be immediately deduced. How are
we to place our great earth in the weighing scales? This is the problem

which Le Verrier has shown us how to solve, and he does so by invoking
the aid of the planet Mars.

If Mars in his revolution around the sun were solely swayed by the
attraction of the sun, he would, in accordance with the well-known laws of
planetary motion, follow for ever the same elliptic path. At the end of one
century, or even of many centuries, the shape, the size, and the position of
that ellipse would remain unaltered. Fortunately for our present purpose, a
disturbance in the orbit of Mars is produced by the earth. Although the mass
of our globe is so much less than that of the sun, yet the earth is still large
enough to exercise an appreciable attraction on Mars. The ellipse described
by the planet is consequently not invariable. The shape of that ellipse and its
position gradually change, so that the position of the planet depends to
some extent upon the mass of the earth. The place in which the planet is
found can be determined by observation; the place which the planet would
have had if the earth were absent can be found by calculation. The
difference between the two is due to the attraction of the earth, and, when it
has been measured, the mass of the earth can be ascertained. The amount of
displacement increases from one century to another, but as the rate of
growth is small, ancient observations are necessary to enable the measures
to be made with accuracy.

A remarkable occurrence which took place more than two centuries ago
fortunately enables the place of Mars to be determined with great precision
at that date. On the 1st of October, 1672, three independent observers
witnessed the occultation of a star in Aquarius by the ruddy planet. The
place of the star is known with accuracy, and hence we are provided with
the means of indicating the exact point in the heavens occupied by Mars on
the day in question. From this result, combined with the modern meridian
observations, we learn that the displacement of Mars by the attraction of the
earth has, in the lapse of two centuries, grown to about five minutes of arc
(294 seconds). It has been maintained that this cannot be erroneous to the
extent of more than a second, and hence it would follow that the earth's
mass is determined to about one three-hundredth part of its amount. If no

other error were present, this would give the sun's distance to about one
nine-hundredth part.

Fig. 50.—Relative Sizes of Mars and the Earth.

Notwithstanding the intrinsic beauty of this method, and the very high
auspices under which it has been introduced, it is, we think, at present
hardly worthy of reliance in comparison with some of the other methods. As
the displacement of Mars, due to the perturbing influence of the earth, goes
on increasing continually, it will ultimately attain sufficient magnitude to
give a very exact value of the earth's mass, and then this method will give
us the distance of the sun with great precision. But interesting and beautiful
though this method may be, we must as yet rather regard it as a striking
confirmation of the law of gravitation than as affording an accurate means
of measuring the sun's distance.

Fig. 51.—Drawing of Mars (July 30th, 1894).

Fig. 54.—The Southern Polar Cap on Mars (July
1, 1894).

The close approaches of Mars to the earth afford us opportunities for
making a careful telescopic scrutiny of his surface. It must not be expected
that the details on Mars could be inspected with the same minuteness as

those on the moon.
Even under the most
favourable
circumstances, Mars is
still more than a
hundred times as far as
the moon, and,
therefore, the features
of the planet have to be
at least one hundred
times as large if they
are to be seen as
distinctly as the
features on the moon.
Mars is much smaller
than the earth. The
diameter of the planet
is 4,200 miles, but little Fig. 52.—Drawing of Mars (August 16th, 1894).
more than half that of
the earth. Fig. 50 shows the comparative sizes of the two bodies. We here
reproduce two of the remarkable drawings[16] of Mars made by Professor
William H. Pickering at the Lowell Observatory, Flagstaff A.T. Fig. 51 was
taken on the 30th of July, 1894, and Fig. 52 on the 16th of August, 1894.

The southern polar cap on Mars, as seen by Professor William H.
Pickering at Lowell Observatory on the 1st of July, 1894, is represented in
Fig. 54.[17] The remarkable black mark intruding into the polar area will be
noticed. In Fig. 53 are shown a series of unusually marked elevations and
depressions upon the "terminator" of the planet, drawn as accurately as
possible to scale by the same skilful hand on the 24th of August, 1894.

In making an examination of the planet it is to be observed that it does
not, like the moon, always present the same face towards the observer. Mars

rotates upon an
axis in exactly
the same manner
as the earth. It is
not a little
remarkable that
the period
required by Mars
for the
completion of
one rotation
should be only
about half an
hour greater than
the period of
Fig. 53.—Elevations and Depressions on the rotation of the
"Terminator" of Mars (August 24th, 1894). earth. The exact
period is 24
hours, 37 minutes, 22-3⁄4 seconds. It therefore follows that the aspect of the
planet changes from hour to hour. The western side gradually sinks from
view, the eastern side gradually assumes prominence. In twelve hours the
aspect of the planet is completely changed. These changes, together with
the inevitable effects of foreshortening, render it often difficult to correlate
the objects on the planet with those on the maps. The latter, it must be
confessed, fall short of the maps of the moon in definiteness and in
certainty; yet there is no doubt that the main features of the planet are to be
regarded as thoroughly established, and some astronomers have given
names to all the prominent objects.

The markings on the surface of Mars are of two classes. Some of them
are of an iron-grey hue verging on green, while the others are generally dark
yellow or orange, occasionally verging on white. The former have usually
been supposed to represent the tracts of ocean, the latter the continental

masses on the ruddy planet. We possess a great number of drawings of
Mars, the earliest being taken in the middle of the seventeenth century.
Though these early sketches are very rough, and are not of much value for
the solution of questions of topography, they have been found very useful in
aiding us to fix the period of rotation of the planet on its axis by comparison
with our modern drawings.

Early observers had already noticed that each of the poles of Mars is
distinguished by a white spot. It is, however, to William Herschel that we
owe the first systematic study of these remarkable polar caps. This
illustrious astronomer was rewarded by a very interesting discovery. He
found that these arctic tracts on Mars vary both in extent and distinctness
with the seasons of the hemisphere on which they are situated. They attain a
maximum development from three to six months after the winter solstice on
that planet, and then diminish until they are smallest about three to six
months after the summer solstice. The analogy with the behaviour of the
masses of snow and ice which surround our own poles is complete, and
there has until lately been hardly any doubt that the white polar spots of
Mars are somewhat similarly constituted.

As the period of revolution of Mars around the sun is so much longer
than our year, 687 days instead of 365, the seasons of the planet are, of
course, also much longer than the terrestrial seasons. In the northern
hemisphere of Mars the summer lasts for no fewer than 381 days, and the
winter must be 306 days. In both hemispheres the white polar cap in the
course of the long winter season increases until it reaches a diameter of 45°
to 50°, while the long summer reduces it to a small area only 4° or 5° in
diameter. It is remarkable that one of these white regions—that at the south
pole—seems not to be concentric with the pole, but is placed so much to
one side that the south pole of Mars appears to be quite free from ice or
snow once a year.

Although many valuable observations of Mars were made in the course
of the nineteenth century, it is only since the very favourable opposition of

1877 that the study of the surface of Mars has made that immense progress
which is one of the most remarkable features of modern astronomy. Among
the observers who produced valuable drawings of the planet in 1877 we
may mention Mr. Green, whose exquisite pictures were published by the
Royal Astronomical Society, and Professor Schiaparelli, of Milan, who
almost revolutionised our knowledge of this planet. Schiaparelli had a
refractor of only eight inches aperture at his disposal, but he was doubtless
much favoured by the purity of the Italian sky, which enabled him to detect
in the bright portions of the surface of Mars a considerable number of long,
narrow lines. To these he gave the name of canals, inasmuch as they issued
from the so-called oceans, and could be traced across the reputed continents
for considerable distances, which sometimes reached thousands of miles.

The canals seemed to form a kind of network, which connected the
various seas with each other. A few of the more conspicuous of these so-
called canals appeared indeed on some of the drawings made by Dawes and
others before Schiaparelli's time. It was, however, the illustrious Italian
astronomer who detected that these narrow lines are present in such great
numbers as to form a notable feature of the planet. Some of these
remarkable features are shown in Figs. 51 and 52, which are copied from
drawings made by Professor William H. Pickering at the Lowell
Observatory in 1894.

Great as had been the surprise of astronomers when Schiaparelli first
proclaimed the discovery of these numerous canals, it was, perhaps,
surpassed by the astonishment with which his announcement was received
in 1882 that most of the canals had become double. Between December,
1881, and February, 1882, thirty of these duplications appear to have taken
place. Nineteen of these were cases of a well-traced parallel line being
formed near a previously existing canal. The remaining canals were less
certainly established, or were cases where the two lines did not seem to be
quite parallel. A copy of the map of Mars which Schiaparelli formed from
his observations of 1881–82 is given in Plate XVIII. It brings out clearly

these strange double canals, so unlike any features that we know on any
other globe.

PLATE XVIII.
SCHIAPARELLI'S MAP OF MARS IN 1881–82.

Subsequent observations by Schiaparelli and several other observers
seem to indicate that this phenomenon of the duplication of the canals is of
a periodic character. It is produced about the times when Mars passes
through its equinoxes. One of the two parallel lines is often superposed as
exactly as possible upon the track of the old canal. It does, however,
sometimes happen that both the lines occupy opposite sides of the former
canal and are situated on entirely new ground. The distance between the
two lines varies from about 360 miles as a maximum down to the smallest
limit distinguishable in our large telescopes, which is something less than
thirty miles. The breadth of each of these remarkable channels may range
from the limits of visibility, say, up to more than sixty miles.

The duplication of the canals is perhaps the most difficult problem which
Mars offers to us for solution. Even if we admit that the canals themselves
represent inlets or channels through which the melted polar snow makes its
way across the equatorial continents, it is not easy to see how the duplicate
canals can arise. This is especially true in those cases where the original

channel seems to vanish and to be replaced by two quite new canals, each
about the breadth of the English Channel, and lying one on each side of the
course of the old one. The very obvious explanation that the whole
duplication is an optical illusion has been brought forward more than once,
but never in a conclusive manner. We must, perhaps, be content to let the
solution of this matter rest for the present, in the hope that the extraordinary
attention which this planet is now receiving will in due time explain the
present enigma.

The markings on the surface of this planet are, generally speaking, of a
permanent character, so that when we compare drawings made one or two
hundred years ago with drawings made more recently we can recognise in
each the same features. This permanence is, however, not nearly so absolute
as it is in the case of the moon. In addition to the canals which we have
already considered, many other parts of the surface of Mars alter their
outlines from time to time. This is particularly the case with those dark
spots which we call oceans, the contours of which sometimes undergo
modifications in matters of detail which are quite unmistakable. Changes of
colour are often observed on parts of the planet, and though some of these
observations may perhaps be attributed to the influence of our own
atmosphere on the planet's appearance, they cannot be all thus accounted
for. Some of the phenomena must certainly be due to actual changes which
have taken place on the surface of Mars.

As an example of such changes, we may refer to the north-western part
of the notable feature, to which Schiaparelli has given the name of Syrtis
major.[18] This has at various times been recorded as grey, green, blue,
brown, and even violet. When this region (about the time of the autumnal
equinox of the northern hemisphere) is situated in the middle of the visible
disc, the eastern part is distinctly greener than the western. As the season
progresses this characteristic colour gets feebler, until the green tint is to be
perceived only on the shores of the Syrtis. The atmosphere of Mars is
usually very transparent, and fortunately allows us to scrutinise the surface
of the planet without putting obstacles in the way m the shape of Martian

clouds. Such clouds, however, are not invariably absent. Our view of the
surface is occasionally obstructed in such a manner as to make it certain
that clouds or mist in the atmosphere of Mars must be the cause of the
trouble.

Would we form an idea of the physical constitution of the surface of
Mars, then the question as to the character of the atmosphere of the planet is
among the first to be considered. Spectroscopic observations do not in this
case render us much assistance. Of course, we know that the planet has no
intrinsic light. It merely shines by reflected sunlight. The hemisphere which
is turned towards the sun is bright, and the hemisphere which is turned
away from the sun is dark. The spectrum ought, therefore, like that of the
moon, to be an exact though faint copy of the solar spectrum, unless the
sun's rays, by passing twice through the atmosphere of Mars, suffered some
absorption which could give rise to additional dark lines. Some of the
earlier observers thought that they could distinctly make out some such
lines due, as was supposed, to water vapour. The presence of such lines is,
however, denied by Mr. Campbell, of the Lick Observatory, and Professor
Keeler, at the Allegheny Observatory,[19] who, with their unrivalled
opportunities, both instrumental and climatic, could find no difference
between the spectra of Mars and the moon. If Mars had an atmosphere of
appreciable extent, its absorptive effect should be noticeable, especially at
the limb of the planet; but Mr. Campbell's observations do not show any
increased absorption at the limb. It would therefore seem that Mars cannot
have an extensive atmosphere, and this conclusion is confirmed in several
other ways.

The distinctness with which we see the surface of this planet tends to
show that the atmosphere must be very thin as compared with our own.
There can hardly be any doubt that an observer on Mars with a good
telescope would be unable to distinguish much of the features of the earth's
surface. This would be the case not only by reason of the strong absorption
of the light during the double passage through our atmosphere, but also on
account of the great diffusion of the light caused by this same atmosphere.

Also, it is needless to say, the great amount of cloud generally floating over
the earth would totally obscure many parts of our planet from a Martian
observer. But though, as already mentioned, we occasionally find parts of
Mars rendered indistinct, it must be acknowledged that the clouds on Mars
are very slight. We should expect that the polar caps, if composed of snow,
would, when melting, produce clouds which would more or less hide the
polar regions from our inspection; yet nothing of the kind has ever been
seen.

We have seen that there are very grave doubts as to the existence of
water on Mars. No doubt we have frequently spoken of the dark markings
as "oceans" and of the bright parts as "continents." That this language was
just has been the opinion of astronomers for a very long time. A few years
ago Mr. Schaeberle, of the Lick Observatory, came to the very opposite
conclusion. He contended that the dark parts were the continents and the
bright ones were the oceans of water, or some other fluid. He pointed to the
irregular shading of the dark parts, which does not suggest the idea of light
reflected from a spherical surface of water, especially as the contrasts
between light and shade are strongest about the middle of the disc.

It is also to be noticed that the dark regions are not infrequently
traversed by still darker streaks, which can be traced for hundreds of miles
almost in straight lines, while the so-called canals in the bright parts often
seem to be continuations of these same lines. Mr. Schaeberle therefore
suggests that the canals may be chains of mountains stretching over sea and
land! The late Professor Phillips and Mr. H.D. Taylor have pointed out that
if there were lakes or seas in the tropical regions of Mars we should
frequently see the sun directly reflected from them, thus producing a bright,
star-like point which could not escape observation. Even moderately
disturbed water would make its presence known in this manner, and yet
nothing of the kind has ever been recorded.

On the question as to the possibility of life on Mars a few words may be
added. If we could be certain of the existence of water on Mars, then one of

the fundamental conditions would be fulfilled; and even though the
atmosphere on Mars had but few points of resemblance either in
composition or in density to the atmosphere of the earth, life might still be
possible. Even if we could suppose that a man would find suitable
nutriment for his body and suitable air for his respiration, it seems very
doubtful whether he would be able to live. Owing to the small size of Mars
and the smallness of its mass in comparison with the earth, the intensity of
the gravitation on the neighbouring planet would be different from the
attraction on the surface of the earth. We have already alluded to the small
gravitation on the moon, and in a lesser degree the same remarks will apply
to Mars. A body which weighs on the earth two pounds would on the
surface of Mars weigh rather less than one pound. Nearly the same exertion
which will raise a 56-lb. weight on the earth would lift two similar weights
on Mars.

The earth is attended by one moon. Jupiter is attended by four
conspicuous moons. Mars is a planet revolving between the orbits of the
earth and of Jupiter. It is a body of the same general type as the earth and
Jupiter. It is ruled by the same sun, and all three planets form part of the
same system; but as the earth has one moon and Jupiter four moons, why
should not Mars also have a moon? No doubt Mars is a small body, less
even than the earth, and much less than Jupiter. We could not expect Mars
to have large moons, but why should it be unlike its two neighbours, and
not have any moon at all? So reasoned astronomers, but until modern times
no satellite of Mars could be found. For centuries the planet has been
diligently examined with this special object, and as failure after failure
came to be recorded, the conclusion seemed almost to be justified that the
chain of analogical reasoning had broken down. The moonless Mars was
thought to be an exception to the rule that all the great planets outside Venus
were dignified by an attendant retinue of satellites. It seemed almost
hopeless to begin again a research which had often been tried, and had
invariably led to disappointment; yet, fortunately, the present generation has
witnessed still one more attack, conducted with perfect equipment and with

consummate skill This attempt has obtained the success it so well merited,
and the result has been the memorable detection of two satellites of Mars.

This discovery was made by Professor Asaph Hall, the distinguished
astronomer at the observatory of Washington. Mr. Hall was provided with
an instrument of colossal proportions and of exquisite workmanship, known
as the great Washington refractor. It is the product of the celebrated
workshop of Messrs. Alvan Clark and Sons, from which so many large
telescopes have proceeded, and in its noble proportions far surpassed any
other telescope ever devoted to the same research. The object-glass
measures twenty-six inches in diameter, and is hardly less remarkable for
the perfection of its definition than for its size. But even the skill of Mr.
Hall, and the space-penetrating power of his telescope, would not have been
able on ordinary occasions to discover the satellites of Mars. Advantage
was accordingly taken of that memorable opposition of Mars in 1877,
when, as we have already described, the planet came unusually near the
earth.

Had Mars been attended by a moon one-hundredth part of the bulk of
our moon it must long ago have been discovered. Mr. Hall, therefore, knew
that if there were any satellites they must be extremely small bodies, and he
braced himself for a severe and diligent search. The circumstances were all
favourable. Not only was Mars as near as it well could be to the earth; not
only was the great telescope at Washington the most powerful refractor then
in existence; but the situation of Washington is such that Mars was seen
from the observatory at a high elevation. It was while the British
Association were meeting at Plymouth, in 1877, that a telegram flashed
across the Atlantic. Brilliant success had rewarded Mr. Hall's efforts. He
had hoped to discover one satellite. The discovery of even one would have
made the whole scientific world ring; but fortune smiled on Mr. Hall. He
discovered first one satellite, and then he discovered a second; and, in
connection with these satellites, he further discovered a unique fact in the
solar system.

Deimos, the outer of the satellites, revolves around the planet in the
period of 30 hours, 17 mins., 54 secs.; it is the inner satellite, Phobos, which
has commanded the more special attention of every astronomer in the
world. Mars turns round on his axis in a Martial day, which is very nearly
the same length as our day of twenty-four hours. The inner satellite of Mars
moves round in 7 hours, 39 mins., 14 secs. Phobos, in fact, revolves three
times round Mars in the same time that Mars can turn round once. This
circumstance is unparalleled in the solar system; indeed, as far as we know,
it is unparalleled in the universe. In the case of our own planet, the earth
rotates twenty-seven times for one revolution of the moon. To some extent
the same may be said of Jupiter and of Saturn; while in the great system of
the sun himself and the planets, the sun rotates on his axis several times for
each revolution of even the most rapidly moving of the planets. There is no
other known case where the satellite revolves around the primary more
quickly than the primary rotates on its axis. The anomalous movement of
the satellite of Mars has, however, been accounted for. In a subsequent
chapter we shall again allude to this, as it is connected with an important
department of modern astronomy.

The satellites are so small that we are unable to measure their diameters
directly, but from observations of their brightness it is evident that their
diameters cannot exceed twenty or thirty miles, and may be even smaller.
Owing to their rapid motion the two satellites must present some
remarkable peculiarities to an observer on Mars. Phobos rises in the west,
passes across the heavens, and sets in the east after about five and a half
hours, while Deimos rises in the east and remains more than two days
above the horizon.

As the satellites revolve in paths vertically above the equator of their
primary, the one less than 4,000 miles and the other only some 14,500 miles
above the surface, it follows that they can never be visible from the poles of
Mars; indeed, to see Phobos, the observer's planetary latitude must not be
above 68-3⁄4°. If it were so, the satellite would be hidden by the body of

Mars, just as we, in the British Islands, would be unable to see an object
revolving round the earth a few hundred miles above the equator.

Before passing from the attractive subject of the satellites, we may just
mention two points of a literary character. Mr. Hall consulted his classical
friends as to the designation to be conferred on the two satellites. Homer
was referred to, and a passage in the "Iliad" suggested the names of Deimos
and Phobos. These personages were the attendants of Mars, and the lines in
which they occur have been thus construed by my friend Professor Tyrrell:
—

"Mars spake, and called Dismay and Rout
To yoke his steeds, and he did on his harness
sheen."

A curious circumstance with respect to the satellites of Mars will be
familiar to those who are acquainted with "Gulliver's Travels." The
astronomers on board the flying Island of Laputa had, according to Gulliver,
keen vision and good telescopes. The traveller says that they had found two
satellites to Mars, one of which revolved around him in ten hours, and the
other in twenty-one and a half. The author has thus not only made a correct
guess about the number of the satellites, but he actually stated the periodic
time with considerable accuracy! We do not know what can have suggested
the latter guess. A few years ago any astronomer reading the voyage to
Laputa would have said this was absurd. There might be two satellites to
Mars, no doubt; but to say that one of them revolves in ten hours would be
to assert what no one could believe. Yet the truth has been even stranger
than the fiction.

And now we must bring to a close our account of this beautiful and
interesting planet. There are many additional features over which we are
tempted to linger, but so many other bodies claim our attention in the solar
system, so many other bodies which exceed Mars in size and intrinsic
importance, that we are obliged to desist. Our next step will not, however, at

once conduct us to the giant planets. We find outside Mars a host of objects,
small indeed, but of much interest; and with these we shall find abundant
occupation for the following chapter.

CHAPTER XI.
THE MINOR PLANETS.
The Lesser Members of our System—Bode's Law—The Vacant Region in the Planetary
System—The Research—The Discovery of Piazzi—Was the small Body a Planet?—
The Planet becomes Invisible—Gauss undertakes the Search by Mathematics—The
Planet Recovered—Further Discoveries—Number of Minor Planets now known—
The Region to be Searched—The Construction of the Chart for the Search for Small
Planets—How a Minor Planet is Discovered—Physical Nature of the Minor Planets—
Small Gravitation on the Minor Planets—The Berlin Computations—How the Minor
Planets tell us the Distance of the Sun—Accuracy of the Observations—How they
may be Multiplied—Victoria and Sappho—The most Perfect Method.

In our chapters on the Sun and Moon, on the Earth and Venus, and on
Mercury and Mars, we have been discussing the features and the
movements of globes of vast dimensions. The least of all these bodies is the
moon, but even that globe is 2,000 miles from one side to the other. In
approaching the subject of the minor planets we must be prepared to find
objects of dimensions quite inconsiderable in comparison with the great
spheres of our system. No doubt these minor planets are all of them some
few miles, and some of them a great many miles, in diameter. Were they
close to the earth they would be conspicuous, and even splendid, objects;
but as they are so distant they do not, even in our greatest telescopes,
become very remarkable, while to the unaided eye they are almost all
invisible.

In the diagram (p. 234) of the orbits of the various planets, it is shown
that a wide space exists between the orbit of Mars and that of Jupiter. It was
often surmised that this ample region must be tenanted by some other
planet. The presumption became much stronger when a remarkable law was
discovered which exhibited, with considerable accuracy, the relative
distances of the great planets of our system. Take the series of numbers, 0,
3, 6, 12, 24, 48, 96, whereof each number (except the second) is double of

the number which precedes it. If we now add four to each, we have the
series 4, 7, 10, 16, 28, 52, 100. With the exception of the fifth of these
numbers (28), they are all sensibly proportional to the distances of the
various planets from the sun. In fact, the distances are as follows:—
Mercury, 3·9; Venus, 7·2; Earth, 10; Mars, 15·2; Jupiter, 52·9; Saturn, 95·4.
Although we have no physical reason to offer why this law—generally
known as Bode's—should be true, yet the fact that it is so nearly true in the
case of all the known planets tempts us to ask whether there may not also be
a planet revolving around the sun at the distance represented by 28.

So strongly was this felt at the end of the eighteenth century that some
energetic astronomers decided to make a united effort to search for the
unknown planet. It seemed certain that the planet could not be a large one,
as otherwise it must have been found long ago. If it should exist, then
means were required for discriminating between the planet and the hosts of
stars strewn along its path.

The search for the small planet was soon rewarded by a success which
has rendered the evening of the first day in the nineteenth century
memorable in astronomy. It was in the pure skies of Palermo that the
observatory was situated where the memorable discovery of the first known
minor planet was made by Piazzi. This laborious and accomplished
astronomer had organised an ingenious system of exploring the heavens
which was eminently calculated to discriminate a planet among the starry
host. On a certain night he would select a series of stars to the number of
fifty, more or less, according to circumstances. With his meridian circle he
determined the places of the chosen objects. The following night, or, at all
events, as soon as convenient, he re-observed the whole fifty stars with the
same instrument and in the same manner, and the whole operation was
afterwards repeated on two, or perhaps more, nights. When the observations
were compared together he was in possession of some four or more places
of each one of the stars on different nights, and the whole series was
complete. He was persevering enough to carry on these observations for

very many groups, and at length he was rewarded by a success which amply
compensated him for all his toil.

It was on the 1st of January, 1801, that Piazzi commenced for the one
hundred and fifty-ninth time to observe a new series. Fifty stars this night
were viewed in his telescope, and their places were carefully recorded. Of
these objects the first twelve were undoubtedly stellar, and so to all
appearance was the thirteenth, a star of the eighth magnitude in the
constellation of Taurus. There was nothing to distinguish the telescopic
appearance of this object from all the others which preceded or followed it.
The following night Piazzi, according to his custom, re-observed the whole
fifty stars, and he did the same again on the 3rd of January, and once again
on the 4th. He then, as usual, brought together the four places he had found
for each of the several bodies. When this was done it was at once seen that
the thirteenth object on the list was quite a different body from the
remainder and from all the other stars which he had ever observed before.
The four places of this mysterious object were all different; in other words,
it was in movement, and was therefore a planet.

A few days' observation sufficed to show how this little body, afterwards
called Ceres, revolved around the sun, and how it circulated in that vacant
path intermediate between the path of Mars and the path of Jupiter. Great,
indeed, was the interest aroused by this discovery and the influence which it
has exercised on the progress of astronomy. The majestic planets of our
system had now to admit a much more humble object to a share of the
benefits dispensed by the sun.

After Piazzi had obtained a few further observations, the season for
observing this part of the heavens passed away, and the new planet of
course ceased to be visible. In a few months, no doubt, the same part of the
sky would again be above the horizon after dark, and the stars would of
course be seen as before. The planet, however, was moving, and would
continue to move, and by the time the next season had arrived it would have
passed off into some distant region, and would be again confounded with

the stars which it so closely resembled. How, then, was the planet to be
pursued through its period of invisibility and identified when it again came
within reach of observation?

This difficulty attracted the attention of astronomers, and they sought for
some method by which the place of the planet could be recovered so as to
prevent Piazzi's discovery from falling into oblivion. A young German
mathematician, whose name was Gauss, opened his distinguished career by
a successful attempt to solve this problem. A planet, as we have shown,
describes an ellipse around the sun, and the sun lies at a focus of that curve.
It can be demonstrated that when three positions of a planet are known, then
the ellipse in which the planet moves is completely determined. Piazzi had
on each occasion measured the place which it then occupied. This
information was available to Gauss, and the problem which he had to solve
may be thus stated. Knowing the place of the planet on three nights, it is
required, without any further observations, to tell what the place of the
planet will be on a special occasion some months in the future.
Mathematical calculations, based on the laws of Kepler, will enable this
problem to be solved, and Gauss succeeded in solving it. Gauss
demonstrated that though the telescope of the astronomer was unable to
detect the wanderer during its season of invisibility, yet the pen of the
mathematician could follow it with unfailing certainty. When, therefore, the
progress of the seasons permitted the observations to be renewed, the search
was recommenced. The telescope was directed to the point which Gauss's
calculations indicated, and there was the little Ceres. Ever since its re-
discovery, the planet has been so completely bound in the toils of
mathematical reasoning that its place every night of the year can be
indicated with a fidelity approaching to that attainable in observing the
moon or the great planets of our system.

The discovery of one minor planet was quickly followed by similar
successes, so that within seven years Pallas, Juno, and Vesta were added to
the solar system. The orbits of all these bodies lie in the region between the
orbit of Mars and of Jupiter, and for many years it seems to have been

thought that our planetary system was now complete. Forty years later
systematic research was again commenced. Planet after planet was added to
the list; gradually the discoveries became a stream of increasing volume,
until in 1897 the total number reached about 430. Their distribution in the
solar system is somewhat as represented in Fig. 55. By the improvement of
astronomical telescopes, and by the devotion with which certain
astronomers have applied themselves to this interesting research, a special
method of observing has been created for the distinct purpose of searching
out these little objects.

It is known that the paths in which all the great planets move through the
heavens coincide very nearly with the path which the sun appears to follow
among the stars, and which is known as the ecliptic. It is natural to assume
that the small planets also move in the same great highway, which leads
them through all the signs of the zodiac in succession. Some of the small
planets, no doubt, deviate rather widely from the track of the sun, but the
great majority are approximately near it. This consideration at once
simplifies the search for new planets. A certain zone extending around the
heavens is to be examined, but there is in general little advantage in pushing
the research into other parts of the sky.

The next step is to construct a map containing all the stars in this region.
This is a task of very great labour; the stars visible in the large telescopes
are so numerous that many tens of thousands, perhaps we should say
hundreds of thousands, are included in the region so narrowly limited. The
fact is that many of the minor planets now known are objects of extreme
minuteness; they can only be seen with very powerful telescopes, and for
their detection it is necessary to use charts on which even the faintest stars
have been depicted. Many astronomers have concurred in the labour of
producing these charts; among them may be mentioned Palisa, of Vienna,
who by means of his charts has found eighty-three minor planets, and the
late Professor Peters, of Clinton, New York, who in a similar way found
forty-nine of these bodies.

Fig. 55.—The Zone of Minor Planets between Mars and
Jupiter.

The astronomer about to seek for a new planet directs his telescope
towards that part of the sun's path which is on the meridian at midnight;
there, if anywhere, lies the chance of success, because that is the region in
which such a body is nearer to the earth than at any other part of its course.
He steadfastly compares his chart with the heavens, and usually finds the
stars in the heavens and the stars in the chart to correspond; but sometimes
it will happen that a point in the heavens is missing from the chart. His
attention is at once arrested; he follows the object with care, and if it moves

it is a planet. Still he cannot be sure that he has really made a discovery; he
has found a planet, no doubt, but it may be one of the large number already
known. To clear up this point he must undertake a further, and sometimes a
very laborious, enquiry; he must search the Berlin Year-Book and other
ephemerides of such planets and see whether it is possible for one of them
to have been in the position on the night in question. If he can ascertain that
no previously discovered body could have been there, he is then entitled to
announce to his brother astronomers the discovery of a new member of the
solar system. It seems certain that all the more important of the minor
planets have been long since discovered. The recent additions to the list are
generally extremely minute objects, beyond the powers of small telescopes.

Since 1891 the method of searching for minor planets which we have
just described has been almost abandoned in favour of a process greatly
superior. It has been found feasible to employ photography for making
charts of the heavens. A photographic plate is exposed in the telescope to a
certain region of the sky sufficiently long to enable very faint telescopic
stars to imprint their images. Care has to be taken that the clock which
moves the camera shall keep pace most accurately with the rotation of the
earth, so that fixed stars appear on the plate as sharp points. If, on
developing the plate, a star is found to have left a trail, it is evident that this
star must during the time of exposure (generally some hours) have had an
independent motion of its own; in other words, it must be a planet. For
greater security a second picture is generally taken of the same region after
a short interval. If the place occupied by the trail on the first plate is now
vacant, while on the second plate a new trail appears in a line with the first
one, there remains no possible doubt that we have genuine indications of a
planet, and that we have not been led astray by some impurity on the plate
or by a few minute stars which happened to lie very closely together. Wolf,
of Heidelberg, and following in his footsteps Charlois, of Nice, have in this
manner discovered a great number of new minor planets, while they have
also recovered a good many of those which had been lost sight of owing to
an insufficiency of observations.