Astronomy for everybody; a popular exposition of the wonders of the heavens

ALBERT R. MANN
LIBRARY

New York Stath Colleges

OF

Agriculture and Home Economics

Cornell University

Cornell University Library
QB 44.N6

Astronomy for everybody; a popular expos!

3 1924 003 160 540

The original of tliis book is in
tine Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

http://www.archive.org/details/cu31924003160540

Total Eclipse of the Sun of May 29, 1900.
Photographed by the party o£ the Smithsonian Institution.

Science for Everybody

ASTRONOMY FOR
EVERYBODY

A Popular Exposition of the Wonders
of the Heavens

BY
SIMON NEWCOMB, LL.D.

Professor, U. S. N., retired

FULLY ILLUSTRATED

NEW YORK
McCLURE, PHILLIPS & CO.

1907

Copyright, 19011, hy
McCLURE, PHILLIPS & CO.

t^c^7 id

Published, November, 1902, N
Fourth Impression

Preface

The present work grew out of articles contributed to
McClure's Magazine a few years since on the Unsolved
Problems of Astronomy, Total Eclipses of the Sun, and
other subjects. The interest shown in these articles
suggested an exposition of the main facts of astronomy
in the same style. The result of the attempt is now
submitted to the courteous consideration of the reader.

The writer who attempts to set forth the facts of as-
tronomy without any use of technical language finds him-
self in the dilemma of being obliged either to convey
only a very imperfect idea of the subject, or to enter
upon explanations of force and motion which his reader
may find tedious. In grappling with this difficulty the
author has followed a middle course, trying to present
the subject in such a way as to be intelligible and inter-
esting to every reader, and entering into technical ex-
planations only when necessary to the clear understand-
ing of such matters as the measure of time, the changes
of the seasons, the varying positions of the constella-
tions, and the aspects of the planets. It is hoped that
the reader who does not wish to master these subjects
will find enough to interest him in the descriptions and
illustrations of celestial scenery to which the bulk of the
work is devoted.

The author is indebted to Mr. Secretary Langley, of
the Smithsonian Institution, for the use of the picture
which forms the frontispiece.

Simon Newcomb.

Washington, October, 1902.

Contents

PART r. TEM CELESTIAL MOTIONS

PAGE

L A View of the Universe ; 3

What the universe is 6

A model of the xmiverse 7

n. Aspects op the Heavens 9

Apparent daily revolution of the stars 12

Changes ia the motions as we journey south 15

m. EEliATioN OP Time AND Longitude 19

Local time 21

Standard time 21

Where the day changes 23

rv. How THE Position op a Heavenly Body is Defined 25

Circles of the celestial sphere 27

V. The Annual Motion op the Eabth and its Eesttlts ... 31

The sun's apparent path in the sky 32

The ecliptic 33

The equinoxes and solstices 37

The seasons 39

Eelations between real and apparent motions summed

up 39

The year and the precession of the equinoxes 41

PART IL ASTRONOMICAL INSTRUMENTS

I. The Refracting Telescope 47

The lenses of a telescope 48

The image of a distant object 51

Power and defects of a telescope 53

Mounting of the telescope 55

The making of telescopes 59

Fraunhof er and Alvan Clark 61

viil CONTENTS

PAGE

n. The Eepmjcting Telescope 67

HE. The Photographio Telescope 71

IV. The Spectroscope 73

Nature and waTO length of light 74

The spectrum 75

How the stars are analysed 76

V. Other Astronomical Instrttments 79

The meridian circle and clock 80

How the position of stars are determined 81

PART III. TBE SUN, EARTH, AND MOON

I. An Introdtjctort Glance at the SoixAB System 87

n. The Sun 91

General description 91

Botation of the sun , 93

Density and gravity 94

Spots on the sun 95

The Faculae 98

The prominences and chromosphere 99

How the sun is made up. 101

The source of the sun's heat. 103

m. The Earth 107

Measuring the earth 107

The earth's interior 108

Its gravity and density. . Ill

Variations of latitude 114

The atmosphere 116

IV. The Moon 119

Distance of the moon 120

Eevolution and phases 130

The surface of the moon 123

Is there air or water on the moon S 127

Rotation of the moon 138

How the moon produces the tides 129

V, Eclipses op the Moon 133

The nodes of the moon's orbit. 134

Eclipse seasons 134

How an eclipse of the moon looks 136

CONTENTS ix

PAGE

TI. Eclipses op the Stm 139

Central, total, and annulaT eclipses. 140

Beanty of a total eclipse 141

Ancient eclipses 143

Prediction of eclipses 144

The sun's appendages 145

The corona 147

PART IV. THE PLANETS AND THEIR SATELLITES

I. Orbits and Aspects op the Planets 151

Distances of the planets • 154

Bode's law 154

Kepler's laws 155

n. The Planet Meecuet 157

Surface and rotation of Mercury 159

Observations of Schroter, Herschel, Schiaparelli, and

Lowell 160

Phases of Mercury 161

Transits of Mercury 162

m. The Planet Venus 167

The morning and evening star 167

Eotation of Venus 169

Atmosphere of Venus , 173

Has Venus a satellite ? 174

Transits of Venus 175

IV. The Plaitet Mars 177

Distance, dates of opposition, etc 178

Surface and rotation of Mars 179

The canals of Mars 180

Probable nature of the channels 184

The atmosphere of Mars 185

Supposed winter snowfall near the poles 187

The satellites of Mars 188

V. The Grottp op Minor Planets 191

Discovery of Ceres 191

Hunting asteroids 192

Orbits of the asteroids 194

Groupings of the orbits 195

The most curious of the asteroids 198

X CONTENTS

tAGB

TL JupiTEB AND rrs Satblutes 201

Aspect of Jupiter 201

Surface 202

Constitution 205

Rotation 206

Eesemblance of Jupiter to the sun 206

The sateUitea of Jupiter 208

Vn. Satubn and its System 213

Aspects of Saturn 313

Satellites of Saturn. 214

Varying aspects of Saturn's rings 215

What the rings are 219

System of Saturn's satellites 220

Physical constitution of Saturn 324

Vin. UBANtrS AND ITS SATELLITES 335

Discovery of Uranus 235

Old observations 236

Constitution of the planet 337

Its satellites 327

rX. Neptune and its Satelljtb 231

History of the discovery of Neptune 333

Satellite of Neptune 235

X. How the Heavens are Measured 237

Parallax 337

Measurement by the motion of light 239

Measurement by the sun's gravitation 340

Besults of measurements of the sun's distance 243

XI. Gravitation and the WEiOHiNa or the Planets 343

Accuracy of astronomical predictions based on the

theory of gravitation 244

How the planets are weighed 346

PART V. COMETS AND METEOBIG BODIES

I. Comets 355

Description of a comet 355

Orbits of comets 257

Halley's comet '. 360

Comets which have disappeared 362

CONTENTS xi

PAGE

Enoke's comet 264

Capture of comets by Jnpiter 265

Whence come comets ? 266

Brilliaut comets of OTir time 267

Nature of comets 274

n. Meteomc Bodies 277

Meteors 277

Canse of meteors 278

Meteoric showers 279

Comiection of comets and meteors 281

The zodiacal light 283

The impTUsion of light 286

PAET VI. THE FIXED 8TARS

I. General Review 291

Stars and nebulas 293

Spectra of the stars 293

Density and heat of the stars 296

n. Aspect of the Skt 299

The MUky Way 299

Brightness of the stars 300

Number of stars 301

Colours 303

Collection into constellatious 303

m. Description of the OoNSTBij:.ATioNS 305

To find the sidereal time 306

The northern constellations 307

The autumnal constellations 309

The winter constellations 313

The spring constellations 316

The summer constellations 317

rv. The Distances op the Stabs. 321

V. The Motions op the Stars 325

VL Variable andCompounb Stars. ,,, = ..o....... ........ 329

List of Illustrations

PAGE

Total Eclipse of the Sun of May 29, 1900. Bhotograplied by the

party of the Smithsonian Institution Frontispiece

The Celestial Sphere as it appears to ns 13

The Northern Sky and the Pole Star .-. 16

Circles of the Celestial Sphere 27

The Sun Crossing the Equator about -March Twentieth 32

The Orbit of the Earth and the Zodiac 33

How the Obliquity of the Ecliptic Produces the Changes of Seasons 35
Apparent Motion of the Sun along the Ecliptic in Spring and

Summer 36

Apparent Motion of the Sun from March till September 37

Precession of the Equinoxes 43

Section of the Object-glass of a Telescope 50

Axes on which a Telescope turns 57

Great Telescope of the Terkes Observatory 65

Section of a Newtonian Eeflecting Telescope 69

Wave Length of Light 74

Arrangement of the Colours of the Spectrum 75

A Meridian Instrument 80

Appearance of a Sun-spot 96

Frequency of Sun-spots in Different Latitudes on the Sun 97

Eevolution of the Moon Eound the Earth 131

Mountainous Surface of the Moon 134

Showing how the Moon would Move if it did not Eotate on its

Axis 139

How the Moon Produces Two Tides in a Day 131

The Moon in the Shadow of the Earth 133

xiv LIST OF ILLUSTRATIONS

PAGE

PaBsage of the Moon through the Earth's Shadow 136

The Shadow of the Moon Thrown on the Earth during a Total

Eclipse of the Sun 139

The Moon Passing Centrally over the Sun during an Annular

Eclipse 140

Orbits of the Four Inner Planets 153

Conjunctions of Mercury with the Sun 158

Elongations of Mercury 159

Phases of Venus in Different Points of its Orbit 168

Effect of the Atmosphere of Venus during the Transit of 1883. . 173
Map of Mars and its Canals as drawn at the Lowell Observatory 181
Drawings of Lacus Solis on Mars, by Messrs. Campbell and

Hussey 183

Separation of the Minor Planets into Groups 195

Distribution of the Orbits of the Minor Planets 196

Telescopic Views of Jupiter, one with the Shadow of a Satellite

Crossing the Planet 304

Perpendicular View of the Kings of Saturn 316

Showing how the Direction of the Plane of Saturn's Rings re-
mains Unchanged 317

Disappearance of the Bings of Saturn, according to Barnard,

when seen edgewise 318

Orbits of Titan and Hyperion, showing their relation 333

Measure of the Distance of an Inaccessible Object by Triangnla-

tion 337

Parabolic Orbit of a Comet 357

Donati's Comet, as drawn by G. P. Bond 368

Head of Donati's Comet, drawn by G. P. Bond 870

Great Comet of 1859, drawn by G. P. Bond 373

The Zodiacal Light in February and March 384

Ursa Major, or The Dipper 307

Ursa Minor 308

Cassiopeia 308

Lyra, the Harp ,,.,,.,,.,.,,, , . . . 311

LIST OF ILLUSTRATIONS xv

PAGE

The Hyades 313

The Pleiades, as seen with the naked eye 313

Telescopic View of the Pleiades, with Names of the Brighter Stais 314

Orion 316

The Northern Crown 317

Aqnila 318

Delphinns, the Dolphin 318

The Great Clnster of Hercules, photographed at the Lick Observ-
atory 319

Soorpius, the Scorpion 320

Measnrement of the Parallax of a Star. 323

Axctnros and the Snironnding Stars la Constellation Bootes 328

PART I
THE CELESTIAL MOTIONS

I

A VrEW or the Univeese

Let us enter upon our subject by taking a general
view of this universe in which we live, fancying ourselves
looking at it from a point without its limits. Far away,
indeed, is the point we must choose. To give a concep-
tion of the distance, let us measure it by the motion of
light. This agent, darting through 186,000 miles in
every second, would make the circuit of the earth several
times between two ticks of a watch. The standpoint
which we choose will probably be well situated if we take
it at a distance through which light would travel in
100,000 years. So far as we know, we should at this
point find ourselves in utter darkness, a black and star-
less sky surrounding us on all sides. But, in one direc-
tion, we should see a large patch of feeble hght spread-
ing over a considerable part of the heavens like a faint
cloud or the first glimmer of a dawn. Possibly there
might be other such patches in different directions, but
of these we know nothing. The one which we have men-
tioned, and which we call the universe, is that which we
are to inspect. We therefore fly toward it — ^how fast we
need not say. To reach it in a month we should have
to go a million times as fast as light. As we approach,
it continually spreads out over more of the black sky.

4 THE CELESTIAL MOTIONS

which it at length half covers, the region behind us being
still entirely black.

Before reaching this stage we begin to see points of
light glimmering here and there in the mass. Continu-
ing our course, these points become more numerous, and
seem to move past us and disappear behind us in the
distance, while new ones continually come into view in
front, as the passengers on a railway train see landscape
and houses flit by them. These are stars, which, when
we get well in among them, stud the whole heavens as
we see them do at night. We might pass through the
whole cloud at the enormous speed we have fancied, with-
out seeing anything but stars and, perhaps, a few great
nebulous masses of foggy light scattered here and there
among them.

But instead of doing this, let us select one particular
star and slacken our speed to make a closer inspection
of it. This one is rather a small star; but as we ap-
proach it, it seems to our eyes to grow brighter. In time
it shines like Venus. Then it casts a shadow; then we
can read by its light ; then it begins to dazzle our eyes.
It looks like a little sun. It is the Sun !

Let us get into a position which, compared with the dis-
tances we have been travelling, is right alongside of the
sun, though, expressed in our ordinary measure, it may
be a thousand million miles away. Now, looking down
and around us, we see eight star-like points scattered
around the sun at different distances. If we watch them
long enough we shall see them all in motion around the
sun, completing their circuit in times ranging from three

A VIEW OF THE UNIVERSE 5

months to more than 160 years. They move at very
different distances ; the most distant is seventy times as
far as the nearest.

These star-Kke bodies are the planets. By careful
examination we see that they differ from the stars in
being opaque bodies, shining only by light borrowed
from the sun.

Let us pay one of them a visit. We select the third
in order from the sun. Approaching it in a direction
which we may call from above, that is to say from a
direction at right angles to the line drawn from it to
the sun, we see it grow larger and brighter as we get
nearer. Wlien we get very near, we sec it looking like
a half -moon — one hemisphere being in darkness and the
other illuminated by the sun's rays. As we approach
yet nearer, the illuminated part, always growing larger
to our sight, assumes a mottled appearance. Still ex-
panding, this appearance gradually resolves itself into
oceans and continents, obscured o^'er perhaps half their
surface by clouds. The surface upon which we are look-
ing continually spreads out before us, filling more and
more of the sky, until we see it to be a world. We land
upon it, and here we are upon the earth.

Thus, a point which was absolutely invisible while we
were flying through the celestial spaces, which became a
star when we got near the sun, and an opaque globe when
yet nearer, now becomes the world on which we live.

This imaginary flight makes known to us a capital
fact of astronomy : The great mass of stars which stud
the heavens at night are suns. To express the idea in

6 THE CELESTIAL MOTIONS

another way, the sun is merely one of the stars. Com-
pared with its fellows it is rather a small one, for we
know of stars that emit thousands or even tens of thou-
sands of times the light and heat of the sun. Measur-
ing things simply by their intrinsic importance, there is
nothing special to distinguish our sun from the hundreds
of millions of its companions. Its importance to us and
its comparative greatness in our eyes arise simply from
the accident of our relation to it.

The great universe of stars which we have described
looks to us from the earth just as it looked to us during
our imaginary flight through it. The stars which stud
our sky are the same stars which we saw on our flight.
The great difference between our view of the heavens
and the view from a point in the starry distances is the
prominent position occupied by the sun and planets.
The former is so bright that during the daytime it com-
pletely obliterates the stars. If we could cut off the
sun's rays from any very wide region, we should see the
stars around the sun in the daj'time as well as by night.
These bodies surround us in all directions as if the earth
were placed in the centre of the universe, as was sup-
posed by the ancients.

What the Universe Is

We may connect what we have just learned about <tb»
the universe at large with what we see in the heavens.
What we call the heavenly bodies are of two classes.
One of these comprises the millions of stars the arrange-
ment and appearance of which we have just described.

WHAT THE UNIVERSE IS 1

The other comprises a single star, which is for us the
most important of a]l, and the bodies connected with it.
This collection of bodies, with the sun in its centre, forms
a little colony all by itself, which we call the solar system.
The feature of this system which I wish first to impress
on the reader's mind is its very small dimensions when
compared with the distances between the stars. All
around it are spaces which, so far as we yet know, are
quite void through enormous distances. If we could fly
across the whole breadth of the system, we should not be
able to see that we were any nearer the stars in front of
us, nor would the constellations look in any way different
from what they do from our earth. An astronomer armed
with the finest instruments would be able to detect a
change only by the most exact observations, and then
only in the case of the nearer stars.

A conception of the respective magnitudes and dis-
tances of the heavenly bodies, which wiU help the reader
in conceiving of the universe as it is, may be gained by
supposing us to look at a little model of it. Let us
imagine that, in this model of the universe, the earth on
which we dwell is represented by a grain of mustard seed.
The moon will then be a particle about one fourth the
diameter of the grain, placed at a distance of an inch
from the earth. The sun will be represented by a large
apple, placed at a distance of forty feet. Other planets,
ranging in size from an invisible particle to a pea, must
be imagined at distances from the sun varying from ten
feet to a quarter of a nule. We must then imagine all
these little objects to be slowly moving around the

8 THE CELESTIAL MOTIONS

sun at their respective distances, in times varying from
three months to 160 years. As the mustard seed per-
forms its revolution in the course of a year we must
imagine the moon to accompany it, making a revolution
around it every month.

On this scale a plan of the whole solar system can be
laid down in a field half a mUe square. Outside of this
field we should find a tract broader than the whole con-
tinent of America without a visible object in it unless
perhaps comets scattered around its border. Far beyond
the limits of the American continent we should find the
nearest star, which, hke our sun, might be represented
by a large apple. At still greater distances, in every
direction, would be other stars, but, in the general aver-
age, they would be separated from each other as widely
as the nearest star is from the sun. A region of the
little model as large as the whole earth might contain
only two or three stars.

We see from this how, in a flight through the universe,
like the one we have imagined, w« might overlook such
an insignificant little body as our earth, even if we made
a careful search for it. We should be like a person fly-
ing through the Mississippi Valley, looking for a grain
of mustard seed which he knew was hidden somewhere
on the American continent. Even the bright shining
apple representing the sun might be overlooked unless
we happened to pass quite near it.

n

Aspects of the Heavens

The immensity of the distances which separate us
from the heavenly bodies makes it impossible for us to
form a distinct conception of the true scale of the uni-
verse, and very difficult to conceive of the heavenly
bodies in their actual relations to us. If, on looking at a
body in the sky, there were any way of estimating its
distance, and if our eyes were so keen that we could see
the minutest features on the surface of the planets and
stars, the true structure of the universe would have been
obvious from the time that men began to study the heav-
ens. A little reflection will make it obvious that if we
could mount above the earth to a distance of, say, ten
thousand times its diameter, so that it would no longer
have any perceptible size, it would look to us, in the light
of the sun, like a star in the sky. The ancients had no
conception of distances like tliis, and so supposed that
the heavenly bodies were, as they appeared, of a con-
stitution totally different from that of the earth. We
ourselves, looking at the heavens, are unable to conceive
of the stars being millions of times farther than the
planets. All look as if spread out on one sky at the same
distance. We have to leam their actual arrangement
and distances by reason.

It is from the impossibility of conceiving these enor-

10 THE CELESTIAL MOTIONS

mous difFerences in the distances of objects on the earth
and the heavens, that the real difficulty of forming a
mental picture of them in their true relation arises. I
shall ask the reader's careful attention in an attempt
to present these relations in the simplest way, so as to
connect things as they are with things as we see them.

Let us suppose the earth taken away from under our
feet, leaving us hanging in mid space. We should then
see the heavenly bodies — sun, moon, planets, and stars —
surrounding us in every direction, up and down, east and
west, north and south. The eye would rest on nothing
else. As we have just explained, all these objects would
seem to us to be at the same distance.

A great collection of points 'scattered in every direc-
tion at an equal distance from one central point, must
all lie upon the inner surface of a hollow sphere. It fol-
lows that, in the case supposed, the heavenly bodies will
appear to us as if set in a sphere in the centre of which
we appear to be placed. Since one of the final objects
of astronomj' is to learn the directions of the heavenly
bodies from us, this apparent sphere is talked about in
astronomy as if it were a reality. It is called the celes-
tial sphere. In the case we have supposed, with the earth
out of the way, all the heavenly bodies on this sphere
would at any moment seem at rest. The stars would re-
main apparently at rest day after day and week after
week. It is true that, by watching the planets, we should
in a few days or weeks, as the case might be, see their
slow motion around the sun, but this would not be per-
ceptible at once. Our first impression would be that the

ASPECTS OP THE HEAVENS 11

sphere was made of some solid, crystalline substance,
and that the heavenly bodies were fastened to its inner
surface. The ancients had this notion, which they
brought yet nearer the truth by fancying a number of
these spheres fitting inside of each other to represent the
different distances of the heavenly bodies.

With this conception well in mind, let us bring the
earth back under our feet. Now we have to make a draft
upon the reader's power of conception. Considered in
its relation to the magnitude of the heavens, the earth
is a mere point ; yet, when we bring it into place, its sur-
face cuts off one half of the universe from our view, just
as an apple would cut off the view of one side of a room
from an insect crawling upon it. That half of the celes-
tial sphere which, being above the horizon, remains visi-
ble is called the visible hemisphere; the half below, the
view of which is cut off by the earth, is called the invisi-
ble hemisphere. Of course we could see the latter by
travelling around the earth.

Having this state of things well in mind, we must
make another draft on the reader's attention. We know
that the earth is not at rest, but revolves unceasingly
around an axis passing through its centre. The natural
result of this is an apparent rotation of the celestial
sphere in the opposite direction. The earth rotates from
west toward east ; hence the sphere seems to rotate from
east toward west. This real revolution of the earth, with
the apparent revolution of the stars which it causes, is
called the diurnal motion, because it is completed in a
day.

n THE CELESTIAL MOTIONS

Apparent Daily Revolution of the Stars
Our next problem is to show the connection between the
very simple conception of the rotation of the earth and
the more complicated appearance presented by the ap-
parent diurnal motion of the heavenly bodies which it
brings about. The latter varies with the latitude of the
observer upon the earth's surface. Let us begin with its
appearance in our middle northern latitudes.

For this purpose we may in imagination build a hollow
globe representing the celestial sphere. We may make
it as large as a Ferris wheel, but one of thirty or forty
feet in diameter would answer our purpose. Let Figure
1 be an inside view of this globe, mounted on two pivots,
P and Q, so that it can turn round on them diagonally.
In the middle, at O, we have a horizontal platform, NS,
on which we sit. The constellations are marked on the
inside of the globe, covering the whole surface, but
those on the lower half are hidden from view by the
platform. This platform, as is evident, represents the
horizon.

The globe is now made to turn on its pivots. What
will happen? We shall see the stars near the pivot P
revolving around the latter as the globe turns. The
stars on a certain circle KN will graze the edges of the
platform, as they pass below P. Those yet farther from
P will dip below the platform to a greater or less extent,
according to their distance from P. Stars near the circle
EF, halfway between P and Q, will perform half their
course above, and half below the platform. Finally,

REVOLUTION OF THE STARS

13

stars within the circle ST will never rise above the level
of the platform at all, and will remain invisible to us.

To our eyes the celestial sphere is such a globe as this,
of infinite dimensions. It seems to us to be continually

■y\VISfBLe\ HEMiSPHERe^

^r:!:^

INVISIBLE \HEMISP\ERe

Fig. 1.-

T
- 77te Celestial SpJie)'e as il appears to tis.

revolving round a certain point in the sky as a pivot,
making one revolution in nearly a day, and carrying the
sun, moon, and stars with it. The stars preserve their
relative positions as if fastened to the revolving celestial
sphere. That is to say, if we take a photograph of them

14 THE CELESTIAL MOTIONS

at any hour of the night, the same photograph will show
their appearance at any other hour, if we only hold it in
the right position.

The pivot corresponding to P is called^ the north celes-
tial pole. To dwellers in middle northern latitudes, where
most of us live, it is in the northern sky, nearly midway
between the zenith and the northern horizon. The
farther south we live, the nearer it is to the horizon, its
altitude above the latter being equal to the latitude of
the place where the observer stands. Quite near it is the
pole star, which we shall hereafter show how to locate.
To ordinary observation, the pole star seems never to
move from its position. In our time it is little more than
a degree from the pole, a quantity with which we need
not now concern ourselves.

Opposite, the north celestial pole, and therefore as far
below our horizon as the north one is above it, lies the
south celestial pole. ^

An obvious fact is that the diurnal motion as we see
it in our latitude is oblique. When the sun rises in the
east it does not seem to go straight up from the horizon,
but moves over toward the south at a more or less acute
angle with the horizon. So when it sets, its motion rela-
tive to the horizon is again oblique.

Now, imagine that we take a pair of compasses long
enough to reach the sky. We put one point on the sky
at the north celestial pole, and the other point far
enough from it to touch the horizon below the pole.
Keeping the first point at the pole we draw a complete
circle on the celestial sphere with the other point. This

REVOLUTION OF THE STAHS 15

circle just touches the north horizon at its lowest point
and, in our northern latitudes, extends to near the zenith
at its highest point. The stars within this circle never
set, but only seem to perform a daily course around the
pole. For this reason this circle is called the circle of
perpettuil apparition.

The stars farther south rise and set, but perform less
and less of their daily course above our horizon, tiU
we reach the south point of it, where they barely show
themselves.

Stars yet farther south never rise at all in our lati-
tudes. They are contained within the circle of perpetual
occultation, which surrounds and is centred on the south
celestial pole, as the circle of perpetual apparition is
centred on the north one.

Figure 2 shows the principal stars of the northern
heavens within the circle of perpetual apparition for the
Northern States. By holding it with the month on top
we shall have a view of the constellations as they are seen
about eight o'clock in the evening. It also shows how to
find the pole star in the centre by the direction of the
two outer stars or pointers in the Dipper, or Great Bear.

Now let us change our latitude and see what occurs.
If we journey toward the equator, the direction of ou.'
horizon changes, and during our voyage we see the pole
star constantly sinking lower and lower. As we ap-
proach the equator, it approaches the horizon, reaching
it when we reach the equator. It is plain enough that
the circle of perpetual apparition grows smaller until,
at the equator, it ceases to exist, each pole being in our

16 THE CELESTIAL MOTIONS

horizon. Now the diurnal motion seems to us quite dif-
ferent from what it is here. The sun, moon, and stars,
when they rise, commence their motion directly upwards.
If one of them rises exactly in the east, it will pass

Fig. 2. — The Northern Sky and the Pole Star.

through the zenith ; one rising south of the east will pass
south of the zenith; one rising north of the east, north
of the zenith.

Continuing our course into the southern hemisphere,
we find that the sun, while still rising in the east, gener-
ally passes the meridian to the north of the zenith. The

RESOLUTION OF THE STARS 17

main point of diiference between the two hemispheres is
that, as the sun now culminates in the north, its ap-
parent motion is not in the direction of the hands of a
watch, as with us, but in the opposite direction. In
middle southern latitudes, the northern constellations,
so familiar to us, are always below the horizon, but we
see new ones in the south. Some of these are noted for
their beauty, the Southern Cross, for example. Indeed,
it has often been thought that the southern heavens
were more brilliant and contained more stars than the
northern ones. But this view is now found to be incor-
rect. Careful study and counts of the stars show the
number to be about the same in one hemisphere as in the
other. Probably the impression we have mentioned
arose from the superior clearness of the sky in the
southern regions. For some reason, perhaps because of
the drier climate, the air is less filled with smoke and
haze in the southern portions of the African and Ameri-
can continents than it is in our northern regions.

What we have said of the diurnal motion of the
northern stars round and round the pole, applies to the
stars in the southern heavens. But there is no southern
pole star, and thetef ore nothing to distinguish the posi-
tion of the southern celestial pole. The latter has a
i number of small stars around it, but they are no thicker
tthan in any other region of the sky. Of course, the
southern hemisphere has its circle of perpetual appari-
jtion, which is larger the farther south we travel. That
eis to say, the stars in a certain circle around the south
'; celestial pole never set, but simply revolve around it.

18 THE CELESTIAL MOTIONS

apparently in an opposite direction from what they do
in the north. So, also, there is a circle of perpetual
occultation containing those stars around the north pole
which, in our latitudes, never set. After we go bej'ond
W south latitude we can no longer see any part of the
constellation Ursa Minor. StiU farther south the Great
Bear will only occasionally show itself to a greater or
less extent above the horizon.

Could we continue our journey to the south pole we
should no longer see any rising or setting of the stars.
The latter would move around the sky in horizontal
circles, the centre or pole being at the zenith. Of course,
the same thing would be true at the north pole.

Ill

Relation of Time and Longitude

We all know that a line running through any place
on the earth in a north and south direction, is called the
meridian of that particular place. More exactly, a me-
ridian of the earth's surface is a semicircle passing from
the north to the south pole. Such semicircles pass in
every direction from the north pole, and one may be
drawn so as to pass through any place. The meridian
of the Royal Observatory at Greenwich is now adopted
by most nations, our own included, as the one from which
longitudes are measured, and by which in the United
States and a considerable part of Europe the clocks
are set.

Corresponding to the terrestrial meridian of a place
is a celestial meridian which passes from the north celes-
tial pole through the zenith, intersects the horizon at its
south point, and continues to the south pole. As the
earth revolves on its axis it carries the celestial as well as
the terrestrial meridian with it, so that the former, in
the course of a day sweeps over the whole celestial
sphere. The appearance to us is that every point of the
celestial sphere crosses the meridian in the course of a
day.

Noon is the moment at which the sun passes the me-
ridian. Before the introduction of railways, people used

20 THE CELESTIAL MOTIONS

to set their clocks by the sun. But owing to the obliquity
of the ecliptic and the eccentricity of the earth's orbit
around the sun, the intervals between successive passages
of the sun are not exactly equal. The consequence is
that, if a clock keeps exact time, the sun will sometimes"
pass the meridian before and sometimes after twelve by
the clock. When this was understood, a distinction was
made between apparent and mean time. Apparent time
was the unequal time determined by the sun ; mean time
was that given by a clock keeping perfect time month
after month. The difference between these two is called
the equation of time. Its greatest amounts are reached
every year about the first of November and the middle
of February. At the former time, the sun passes the
meridian sixteen minutes before the clock shows twelve;
in February, fourteen or fifteen minutes after twelve.

To define mean time astronomers imagine a mean sun
which always moves along the celestial equator so as to
pass the meridian at exactly equal intervals of time, and
which is sometimes ahead of the real sun and sometimes j
behind it. This imaginary or mean sun determines the i
time of day. The subject will perhaps be a little easier i
if we describe things as they appear, imagining the earth
to be at rest while the mean sun revolves around it, cross-
ing the meridian of every place in succession. We thus
imagine noon to be constantly travelling around the
world. In our latitudes, its speed is not far from a
thousand feet per second ; that is to say, if it is noon at
a certain place where we stand, it will one second after-
ward be noon about one thousand feet farther west, in

STANDARD TIME 21

another second a thousand feet yet farther west, and so
on through the twenty-four hours, until noon will once
more get back where we are. The obvious result of this
is that it is never the same time of day at the same mo-
ment at two places east or west of each other. As we
travel west, we shall continually find our watches to be
too fast for the places which we reach, while in travelling
east, they will be too slow. This varying time is called
local or astronomical time. The latter term is used be-
cause it is the time determined by astronomical observa-
tions at any place.

Standard Time

Formerly the use of local time caused great inconven-
ience to travellers. Every railway had its own meridian
which it ran its trains by ; and the traveller was fre-
quently liable to miss his train by not knowing the rela-
tion between his watch or a clock and the railway time.
So in 1883, our present system of standard time was in-
troduced. Under this system, standard meridians are
adopted fifteen degrees apart, this being the space over
which the sun passes in one hour. The time at which
noon passes a standard meridian is then used throughout
a zone extending seven or eight degrees on each side.
This is called standard time. The longitudes which
mark the zones are reckoned from Greenwich. It hap-
pens that Philadelphia is about seventy-five degrees in
longitude, or five hours in time from Greenwich. More
exactly, it is about one minute of time more than this.
Thus the standard meridian which we use for the Middle

22 THE CELESTIAL MOTIONS

States passes a little east of Philadelphia. When mean
noon reaches this meridian, it is considered as twelve
o'clock throughout all our Eastern and Middle States
as far west as Ohio. An hour later, it is considered twelve
o'clock in the Mississippi Valley. An hour later, it is
twelve o'clock for the region of the Rocky Mountains.
In yet another hour, it is twelve o'clock on the Pacific
coast. Thus we use four different kinds of time. Eastern
time. Central time. Mountain time, and Pacific time, dif-
fering from each other by entire hours. Using this time,
the traveller only has to set his watch forward or back
one hour at a time, as he travels between the Pacific and
the Atlantic coast, and he will always find it correct for
the region in which he is at the time.

It is by this difference of time that the longitudes of
places are determined. Imagine that an observer in
New York makes a tap with a telegraph-key at the exact
moment when a certain star crosses his meridian, and
that this moment is recorded at Chicago as well as New
York. When the star reaches the meridian of Chicago,
the observer taps the time of its crossing over his meri-
dian in the same way. The interval between the two
taps shows the difference of longitude between the two
cities.

Another method of getting the same result is for each
observer to telegraph his local time to the other. The
difference of the two times gives the longitude.

In this connection, it must be remembered that the
heavenly bodies rise and set by local, not standard, time.
Hence the time of rising and setting of the sun, given m

WHERE THE DAY CHANGES 23

the almanacs, will not answer to set our watches by for
standard time, unless we are on one of the standard
meridians. One difference between these two kinds of
time is that local time varies continuously as we travel
east or west, while standard time varies only by jumps
of one hour when we cross the boundaries of any of the
four zones just described.

Where the Day Changes

Midnight, like noon, is continually travelling round
the earth, crossing all the meridians in succession. At
every crossing it inaugurates the beginning of another
day on that meridian. If it is Monday at any crossing,
it wiU be Tuesday when it gets back again. So there
must be some meridian where Monday changes to Tues-
day, and where every day changes into the day follow-
ing. This dividing meridian, called the "date line," is
determined only by custom and convenience. As colo-
nization extended toward the east and the west men
carried their count of days with them. Tlie result was
that whenever it extended so far that those going east
met those going west they found their time differing by
one day. What for the westward traveller was Monday
was Tuesday for the eastern one. This was the case
when we acquired Alaska. The Russians having reached
that region by travelling east, it was found that, when
we took possession by going west, our Saturday was their
Sunday. This gave rise to the question whether the
inhabitants, in celebrating the festivals of the Greek
Church, should follow the old or the new reckoning of

M THE CELESTIAL MOTIONS

days. The subject was referred to the head of the church
at St. Petersburg, and finally to Struve, the director of
the Pulkowa Observatory, the national astronomical in-
stitution of the empire. Struve made a report in favor
of the American reckoning, and the change to it was
duly carried out.

At the present time custom prescribes for the date line
the meridian opposite that of Greenwich. This passes
through the Pacific Ocean, and in its course crosses very
little land— ^only the northeastern corner of Asia and,
perhaps, some of the Fiji Islands. This fortunate cir-
cumstance prevents a serious inconvenience whjch might
arise if the date line passed through the interior of a
country. In this case the people of one city might have
their time a day different from those of a neighbouring
city across the line. It is even conceivable that residents
on two sides of the same street would have different days
for Sunday. But being in the ocean, no such incon-
venience follows. The date line is not necessarily a meri-
dian of the earth, but may deviate from one side to the
other in order to prevent the inconvenience we have
described. Thus the inhabitants of Chatham Island
have the same time as that of the neighbouring island of
New Zealand, although the meridian of 180° from
Greenwich runs between them.

IV

How THE Position or a Heavenly Bodt is Defined

In this chapter I have to use and explain some tech-
nical terms. The ideas conveyed by them are necessary
to a complete understanding of the celestial motions, and
of the positions of the stars at any hour when we may
wish to observe them. To the reader who only desires
a general idea of celestial phenomena, this chapter will
not be necessary. I must invite one who wants a knowl-
edge more thorough than this to make a close study of
the celestial sphere as it was described in our second
chapter. Turning back to our first figure, we see our-
selves concerned with the relation of two spheres. One
of these is the real globe of the earth, on the surface of
which we dwell, and which is continually carrying us
around by its daily rotation. The other is the apparent
sphere of the heavens, which surrounds our globe on all
sides at an enormous distance, and which, although it has
no reality, we are obliged to imagine in order to know
where to look for the heavenly bodies. Notice that we
see this sphere from its centre, so that everything we
see upon it appears upon its inside surface, while we see
the surface of the earth from the outside.

There is a correspondence between points and circles
on these two spheres. We have already shown how the
axis of the earth, which marks our north and south poles.

26 THE CELESTIAL MOTIONS

being continued in both directions through space, marks
the north and south poles of the celestial sphere.

We know that the earth's equator passes around it at
an equal distance from the two poles. In the same way
we have an equator on the celestial sphere which passes
around it at a distance of ninety degrees from either
celestial pole. If it could be painted on the sky we should
always see it, by day or night, in one fixed position. We
can imagine exactly how it would look. It intersects the
horizon in the east and west points, and is in fact the line
which the sun seems to mark out in the sky by its diurnal
course during the twelve hours that it is above the hori-
zon, in March or September. In our northernmost
States, it passes about halfway between the zenith and
the south horizon, but passes nearer the zenith the farther
south we are.

As we have circles of latitude parallel to the equator
passing around the earth both north and south of the
equator, so we have on the celestial sphere circles parallel
to the celestial equator, and therefore having one or the
other of the celestial poles as a centre. As the parallels
of latitude on the earth grow smaller and smaller toward
the pole, so do these celestial circles grow smaller toward
the celestial poles.

We know that longitude on the earth is measured by
the position of a meridian passing from the north to the
south pole through the place whose position is to be de-
fined. The angle which this meridian makes with that
through the Greenwich Observatory is the longitude of
the place.

CIRCLES OF CELESTIAL SPHERE

27

We have the same system in the heavens. Circles are
imagined to pass from one celestial pole to the other in
every direction, but aU intersecting the equator at right

Fig. 3. — Circles of the Celestial Sphere.

angles, as shown in Figure 3. These are called hour
circles. One of them is called the first hour circle, and
is so marked in the figure. It passes through the vernal

2S THE CELESTIAL MOTIONS

equinox, a point to be defined in the next chapter. This
takes a place in the sky corresponding to Greenwich on
the earth's surface.

The position of a star on the celestial sphere is defined
in the same way that the position of a city on the earth
is defined, by its latitude and longitude. But different
terms are used. In astronomy, the measure which corre-
sponds to longitude is called right ascension; that which
corresponds to latitude is called declination. We thus
have the following definitions, which I must ask the
reader to remember carefully.

The declination of a star is its apparent distance from
the celestial equator north or south. In the figure the
star is in declination twentj'-five degrees north.

The right ascension of a star is the angle which the
hour circle passing through it makes with the first hour
circle which passes through the vernal equinox. In the
figure the star is in three hours right ascension.

The right ascension of a star is, in astronomical usage,
generally expresssed as so many hours, minutes, and
seconds, in the way shown on Figure 3. But it may
equally well be expressed in degrees as we express the
longitude of places on the earth. The right ascension
expressed in hours may be changed into degrees by the
simple process of multiplication by 15. This is because
the earth revolves 15° in an hour. Figure 3 also shows
us that, while the degrees of latitude are nearly of
the same length all over the earth, those of longitude
continually diminish, slowly at first and more rapidly
afterwards, from the equator toward the poles. At the

POSITION OF A HEAVENLY BODY 29

equator the degree of longitude is about 69^- statute miles,
but at the latitude of 45° it is only about 42 miles. At
60" it is less than 35 miles, at the pole it comes down to
nothing, because there the meridians meet.

We may see that the speed of the rotation of the
earth follows the same law of diminution. At the equa-
tor, 15° is about 1,000 miles. We may therefore see
that, in that part of the earth, the latter revolves at
the rate of 1,000 miles an hour. This is about 1,500
feet per second. But in latitude 45° the speed is
diminished to little more than 1,000 feet per second.
At 60°, north, it is only half that at the equator ; at the
poles it goes down to nothing.

In applying this system the only trouble arises from
the earth's rotation. As long as we do not travel, we
remain on the same circle of longitude on the earth. But
by the rotation of the earth, the right ascension of any
point in the sky which seems to us fixed, is continually
changing. The only difference between the celestial
meridian and an hour circle is that the former travels
round with the earth, while the latter is fixed on the
celestial sphere.

There is a strict resemblance in almost every point
between the earth and the celestial sphere. As the former
revolves on its axis from west to east, the latter seems to
revolve from east to west. If we imagine the earth cen-
tred inside the celestial sphere with a common axis pass-
ing through them, as shown in the figure, we shall have a
clear idea of the relations we wish to set forth.

If the sun, like the stars, seemed fixed on the celestial

30 THE CELESTIAX MOTIONS

sphere from year to year, the problem of finding a star
when we knew its right ascension and declination would
be easier than it actually is. Owing to the annual revor
lution of the earth round 'the sun there is a continual
change in the apparent position of the sphere at a given
hour of the night. We must next point out the eiFect
of this revolution.

V
The Annual Motion of the Earth and its Results

It is well known that the earth not only turns on its
axis, but makes an annual revolution round the sun. The
result of this motion — in fact, the phenomenon by which
it is shown — is that the sun appears to make an an-
nual revolution around the celestial sphere among the
stars. We have only to imagine ourselves moving round
the sun and therefore seeing the latter in different direc-
tions, to see that it must appear to us to move among
the stars, which are farther than it is. It is true that
the motion is not at once evident because the stars are
invisible in the daytime. But the fact of the motion
will be made very clear if, day after day, we watch some
particular fixed star in the west. We shall find that it
sets earlier and earlier every day; in other words, it is
getting continually nearer and nearer the sun. More
exactly, since the real direction of the star is unchanged,
the sun appears to be approaching the star.

If we could see the stars in the daytime, all round the
sun, the case would be yet clearer. We should see that
if the sun and a star rose together in the morning the
sun would, during the day, gradually work past the star
in an easterly direction. Between the rising and setting
it would move nearly its own diameter relative to the star.
Next morning we should see that it had gotten quite

32

THE CELESTIAL MOTIONS

away from the star, being nearly two diameters distant
from it. The figure shows how this would go on at the
time of the spring equinox, after March twentieth. This
motion would continue month after month. At the end

^ARCH■^^;/^;;;;o^^:■■;•;;;v^;:■x;• >■;•;■■';.■/:

^^ ^i2AV.^ ■;/;;■■/:<;.;. ::':VV-;-^;;:

IARCH::;

Fig. 4. — 7%e Sun Crossing the Equator ahotd March TmadiA,

of the year the sun would have made a complete circuit
of the heavens relative to the star, and we should see the
two once more together.

The Sun's Apparent Path

How the above eflPect is produced will be seen by Fig-
ure 5, which represents the earth's orbit round the sun,
with the stars in the vast distance. When the earth is at
A, we see the sun in the line AM, as if it were among the
stars at M. As we are carried on the earth from A to B,
the sun seems to move from M to N, and so on through
the year. This apparent motion of the sun in one year
around the celestial sphere, was noticed by the ancients,
who seem to have taken much trouble to map it out. They

THE SUN'S APPARENT PATH

S3

imagined a line passing around the celestial sphere which
the sun always followed in its annual course, and which
was called the ecliptic. They noticed that the planets
followed nearly but not exactly the same general course
as the sun among the stars. A belt extending around on

Fig. 5. — T!i,e Orbit of the Earth and the Zodiac.

each side of the ecliptic, and broad enough to contain
all the known planets, as well as the sun, was called the
zodiac. It was divided into twelve signs, each marked
by a constellation. The sun went through each sign in
the course of a month and through all twelve signs in a
year. Thus arose the familiar signs of the zodiac, which

34. THE CELESTIAL MOTIONS

bore the same names as the constellations among which
they were situated. This is not the case at present,
owing to the slow motion of precession soon to be
described.

It wiU be seen that the two great circles we have de-
scribed spanning the entire celestial sphere are fixed in
entirely different ways. The equator is determined by
the direction in which the axis of the earth points, and
spans the sphere midway between the two celestial poles.
The ecliptic is determined by the earth's motion around
the sun.

These two circles do not coincide, but intersect each
other at two opposite points, at an angle of twenty-three
and a half degrees, or nearly one quarter of a right
angle. This angle is called the obliquity of the ecliptic,
To understand exactly how it arises we must mention
a fact about the celestial poles ; from what we have said
of them it will be seen that they are not determined by
anything in the heavens, but by the direction of the
earth's axis only; they are nothing but the two op-
posite points in the heavens which lie exactly in the
line of the earth's axis. The celestial equator, being
the great circle halfway between the poles, is also
fixed by the direction of the earth's axis and by nothing
else.

Let us now suppose that the earth's orbit around the
sun is horizontal. We may in imagination represent
it by the circumference of a round level platform with
the sun in its centre. We suppose the earth to move
around the circumference of the platform with its cen-

THE SUN'S APPARENT PATH 35

tre on the level of the platform ; then, if the earth's axis
were vertical, its equator would be horizontal and on a
level with the platform and therefore would always be
directed toward the sun in its centre, as the earth made its
annual course around the platform. Then, on the celes-
tial sphere, the ecliptic determined by the course of the
sun would be the same circle as the equator. The
obliquity of the ecliptic arises from the fact that the
earth's orbit is not vertical, as just supposed, Dut is in-

FiG. 6. — Hou) the Obliquity of the JEcliptic Produces the Changes of Seasons.

clined twenty-three and a half degrees. The ecliptic
has the same inclination to the plane of the platform;
thus the obliquity is the result of the inclination of the
earth's axis. An important fact connected with the sub-
ject is that, as the earth makes its revolutions around the
sun, the direction of its axis remains unchanged in space ;
hence its north pole is tipped away from the sun or
toward it, according to its position in the orbit. This
is shown in Figure 6, which represents the platform we
have supposed, with the axis tipped toward the right
hand. The north pole will always be tipped in this

36

THE CELESTIAL MOTIONS

direction, whether the earth is east, west, north, or south
from the sun.

To see the effect of the inclination upon the ecliptic
suppose that, at noon on some twenty-first day of March,
the earth should suddenly stop turning on its axis, but
continue its course around the sun. What we should then
see during the next three months is represented in Figure
7, in which we are supposed to be looking at the southern
sky. We see the sun on the meridian, where it will at
first seem to remain immovable. The figure shows the

Fig. 7. — Apparent Moiio7i of the Sun along the Ecliptic in Spring and
Summer.

celestial equator passing through the east and west
points of the horizon as already described and also the
ecliptic, intersecting it at the equinox. Watching the
result for a time equal to three of our months we should
see the sun slowly make its way along the ecliptic to
the point marked "summer solstice," its farthest north-
ern point, which it would reach about June twentieth.

Figure 8 enables us to follow its course for tlfiee
months longer. After passing the summer solstice, its

THE SUN'S APPARENT PATH

31

course gradually carries it once more to the equator,
which it again crosses about September twentieth. Its
course during the rest of the year is the counterpart of
that during the first six months. It is farthest south of
the equator on December twentieth, and again crosses it
on March twentieth.

We see that there are four cardinal points in this ap-
parent annual course of the sun. (1) Where we have
commenced our watch is the vernal equinox. (2) The
point where the sun, having reached its northern limit,
begins to again approach the equator. This is called the
summer solstice. (3) Opposite the vernal equinox is the

Fig. 8. — Apparent Motion oftlie Sun from March till September.

autumnal equinox, which the sun passes about September
twentieth. ( 4 ) Opposite the summer solstice is the point
where the sun is farthest south. This is called the winter
solstice.

The hour circles which pass from one celestial pole to
the other through these points at right angles to the
equator are called colures. That which passes through

38 THE CELESTIAL MOTIONS

the vernal equinox is the first meridian, from which right
ascensions are counted as already described. The two
at right angles to it are called the solstitial colures.

Let us now show the relation of the constellations to
the seasons and the time of day. Suppose that to-day
the sun and a star passed the meridian at the same mo-
ment; to-morrow the sun will be nearly a degree to the
east of the star, which shows that the star will pass the
meridian nearly four minutes sooner than the sun will.
This will continue day after day throughout the entire
year when the two will again pass the meridian at about
the same moment. Thus the star will have passed once
oftener than the sun. That is to say: In the course
of a year while the sun has passed the meridian three
hundred and sixty-five times, a star has passed it three
hundred and sixty-six times. Of course if we take a
star in the south it will have risen and set the same
number of times.

Astronomers keep the reckoning of this different ris-
ing and setting of the stars by using a sidereal day, or
star day, equal to the interval between two passages of
a star, or of the vernal equinox, across the meridian. They
divide this day into twenty-four sidereal hours, and these
into minutes and seconds according to the usual plan.
They also use sidereal clocks which gain about three
minutes and fifty-six seconds per day on the ordinary
clocks, and thus show sidereal time. Sidereal noon is the
moment at which the vernal equinox crosses the meridian
of the place. The clock is then set at hours, minutes,
nd seconds. Thus set and regulated, the sidereal

THE SEASONS 39

clock keeps time with the apparent rotation of the celes-
tial sphere, so that the astronomer has only to look at his
clock to see, by day or by night, what stars are on the
meridian and what the positions of the constellations are.

The Seasons

If the earth's axis were perpendicular to the plane of
the ecliptic, the latter would coincide with the equator,
and we should have no difference of seasons the year
round. The sun would always rise in the exact east and
set in the exact west. There would be only a very slight
change in the temperature arising from the fact that
the earth is a little nearer the sun in January than in
July. Owing to the obliquity of the ecliptic it follows
that, while the sun is north of the equator, which is the
case from March to September, the sun shines upon the
northern hemisphere during a greater time of each day
and at a greater angle, than on the southern hemisphere.
In the southern hemisphere the opposite is the case. The
sun shines longer from September till March than it does
on the northern hemisphere. Thus we have winter in
the northern hemisphere when it is summer in the
southern, and vice versa.

Relations between Real and Apparent Motions

Before going farther let us recapitulate the phenom-
ena we have described from the two points of view: one
that of the real motions of the earth ; the other that of
the apparent motions of the heavens, to which the real
motions give rise.

40 THE CELESTIAL MOTIONS

The real diurnal motion is the turning of the earth on
its axis.

The apparent diurnal motion is that which the stars
appear to have in consequence of the earth's rotation.

The real annual motion is that of the earth round the
sun.

The apparent annual motion is that of the sun around
the celestial sphere among the stars.

By the real diurnal motion the plane of our horizon is
carried past the sun or a star.

We then say that the sun or star rises or sets, as the
case may be.

About March twenty-first of every year the plane of
the earth's equator passes from the north to the south
of the sun, and about September twenty-first it repasses
toward the north.

We then say that the sun crosses to the north of the
equator in March, and to the south in September.

In June of every year the plane of the earth's equator
is at the greatest distance south df the sun, and in De-
cember at the greatest distance north.

We say in the first case that the sun is at the north-
ern solstice, and in the second that it is at the southern
solstice.

The earth's axis is tipped twenty-three and a half
degrees from the perpendicular to the earth's orbit.

The apparent result is that the ecliptic is inclined
twenty-three and a half degrees to the celestial equator.

During June and the other summer months the north-
ern hemisphere of the earth is tipped toward the sun.

PRECESSION OF THE EQUINOXES 41

Places in north latitude, as they are carried round by
the turning of the earth, are then in sunlight during
more than half their course ; those in south latitude less.

The result as it appears to us is that the sun is more
than half the time above the horizon, and that we have
the hot weather of summer, while in the southern hemi-
sphere the days are short, and the season is winter.

During our winter months the case is reversed. The
southern hemisphere is then tipped toward the sun, and
the northern hemisphere away from it. Consequently,
summer and long days are the order in the southern, and
the reverse in the northern hemisphere.

The Year and the Precession of the Equinoxes

We most naturally define the year as the interval of
time in which the earth revolves around the sun. From
what we have said, there are two ways of ascertaining its
length. One is to find the interval between two passages
of the sun past the same star. The other is to find the
interval between two passages of the sun past the same
equinox, that is, across the equator. If the latter were
fixed among the stars the two intervals would be equal.
But it was found by the ancient astronomers, from obser-
vations extending through several centuries, that these
two methods did not give the same length of year. It
took the sun about eleven minutes longer to make the
circuit of the stars than to make the circuit of the
equinoxes. This shows that the equinoxes steadily
shift their position among the stars from year to year.
This shift is called the precession of the equinoxes. It

42 THE CELESTIAL MOTIONS

does not arise from anything going on in the heavens, but
only from a slow change in the direction of the earth's
axis from year to year as it moves around the sun.

If we should suppose the platform in Figure 6 to last
for six or. seven thousand j^ears, and the earth to make its
six. or seven thousand revolutions around it, we should
find that, at the end of this time, the north end of the
axis of the earth, instead of being tipped toward our
right hand, as shown in the figure, would be tipped
directly toward us. At the end of another six or seven
thousand years it would be tipped toward our left ; at the
end of a third such period it would be tipped away from
us, and at the end of a fourth, or about twenty-six
thousand years in all, it would have gotten back to its
original direction. Since the celestial poles are deter-
mined by the direction of the earth's axis, this change in
the direction of the axis makes them slowly go around a
circle in the heavens, having a radius of about twenty-
three and a half degrees. At the present time the pole
star is a little more than a degree from the pole. But
the pole is gradually approaching it and will pass by it
in about two hundred years. In twelve thousand' years
from now the pole will be in the constellation Lyra, about
five degrees from the bright star Vega of that constella-
tion. In the time of the ancient Greeks their navigators
did not recognize any pole star at all, because what is
now such was then ten or twelve degrees from the pole,
the latter having been between it and the constellation of
the Great Bear. It was the latter which they steered by,
and which they called the Cynosure.

LENGTH OF THE YEAR 43

It follows from all this that, since the celestial equator
is the circle midway between the two poles, there must
be a corresponding shift in its position among the stars.
The effect of this shift during the past two thousand
years is shown in Figure 9- Since the equinoxes are the
points of crossing of the ecliptic and the equator, they
also change in consequence of this motion. It is thus
that the precession of the equinoxes arises.

_l^ Q,t 2000 YEARS ACQ

<>J:EI.E5TIAL equator 2000 YEARS AOO
CONSTELLATION PISCES

Fig. 9. — Precension of the Equinoxes.

The two kinds of year we have described are called
equinoctial and sidereal. The equinoctial year, also
called the solar year, is the interval between two returns
of the sun to the equinox. Its length is — ■

365 days 5 hours 48 minutes 46 seconds.

Since the seasons depend upon the sun's being north
or south of the equator, the solar or equinoctial year is
that used in the reckoning of time. The ancient astrono-
mers found that its length was about three hundred
and sixty-five and one quarter days. As far back as the
time of Ptolemj' the length of the year was known even
more exactly than this, and found to be a few minutes
less than three hundred and sixty-five and one quarter
days. The Gregorian Calendar, which nearly all civl-

U THE CELESTIAL MOTIONS

Used nations now use, is based upon a close approxima-
tion to this length of the year.

The sidereal year is the interval between two passages
of the sun past the same star. Its length is three hun-
dred and sixty-five days six hours and nine minutes.

According to the Julian calendar, which was in use
in Christendom until 1582, the year was considered to
be exactly 365J days. This, it will be seen, was 11
minutes 14 seconds more than the true length of the
solar year. Consequently, the seasons were slowly
changing in the course of centuries. In order to obviate
this, and have a year whose average length was as nearly
as possible correct, a decree was passed by Pope Gregory
XIII by which, in three centuries out of four, a day
was dropped from the Julian calendar. According to
the latter, the closing year of every century would be
a leap year. In the Gregorian calendar 1600 was still
to remain a leap year, but 1500, 1700, 1800, and 1900
were all common years.

The Gregorian calendar was adopted immediately by
all Catholic countries, and from time to time by Protes-
tant countries also, so that for the past 150 years it has
been universal in both. But Russia has held on to the
Julian calendar until this day. Consequently in that
country the reckoning of time is now 13 days behind
that in the other Christian countries. The Russian New
Year of 1900 occurred on what we call January 13. In
February of that year we only counted 28 days, but
Russia counted 29. Hence, in 1901, the Russian New
Year was carried still farther forward to our January 14.

PART II
ASTRONOMICAL INSTRUMENTS

I

The Refkacting Telescope

Theee is no branch of science more interesting to the
pubKc than that with which the telescope is concerned. I
assume that the reader wishes to have an inteUigent idea
as to what a telescope is and what can be seen with it.
In its most complete form, as used by the astronomer in
his observatory, the instrument is quite complex. But
there are a few main points about it which can be mas-
tered in a general way by a little close attention. After
mastering these points, the visitor to an observatory will
examine the instrument with much more satisfaction than
he can when he knows nothing about it.

The o ne great function of a telescope, as we all know,
i s to ma ke dist,a nt nb ^^pf^^p InnV i^op^i-ov t.r> 7^c ; to see an
object miles away as if it were, perhaps, only as many
3'ards. The optical appliances by which this is effected
are extremely simple. They are made with large well-
polished lenses, of the same kind as those used in a pair
of spectacles, differing from the latter only in their size
and general perfection. A telescope requires an ap-
phance for .rnllprtinp- the Hgbt pnTningr .frnm-tJio r-bjpr^^

SO as to form an Jmage of the -latter. There are two

ways in wTlirh +Tip li'^rTlt nfiay V.o fnllooforl^ npp hj pQggi'Tl ^.

the light throug h a set of lenses, and one bv rpflpcting it
from a concave mirror. Thus we have two different kinds

48 ASTRONOMICAL INSTRUMENTS

of telescope, one called ffrTfractji^ the other reflecting.
We begin with the former because it is tliomnrp ^1s^^a,^■

The Lenses of a Telescope

The lenses of a rsfiasiingielescope comprise two com-
binations or systems; the one an object-glass — or "ob;;^
jpgf-ivp," as it is sometimes called for shortness — ^which
faUU^jftJHja^jfliof a distant object in the focus of the
instrument ; and j^h°„"^^'"' PV pyppiecp- with which this
image is viewed.

The objective is the really difficult and delicate part
of the instrument. Its construction involves more refined
skill than that of all the other parts together. How great
is the natural aptitude required may be judged from the
fact that a generation ago there was but one man in the
world in whose ability to make a perfect object-glass of
the largest size astronomers everywhere would have felt
confidence. This man was Alvan Clark, of whom we
shall soon speak.

The obiect-glas s, as commonly made, consists of two ,
large lenses . The power of the telescope depends alto-
gether on thcs^iameter of these lenses, which is caUed
the tif""^^""^^ P^ the telescope. The aperture may vary
from Ihree or f o ur inche s, in the little telescope which
one has in his house, to more than three feet in the great
telescope of the Yerkes Observatory. One reason why
the power of the telescope depends on the diameter
of the object-glass is that, in order to see an object mag-
nified a certain number of times, in its natural bright-
ness, we need a quantity of light expressed by the square

THE LENSES OF A TELESCOPE 49

of the magnifying power. For example, if we have a
magnifying power of one hundred, we should need ten
thousand times the light. I do not mean that this quan-
tity of light is always necessary ; it is not so, because we
can commonly see an object with less than its natural
illumination. Still, we need a certain amount of light,
or it will be too dim.

In order that jji stinct vision of a distant object may
be secured in the telescope, the one grea t essential is that

the ohject-^laSS fih""1^ hn'ng- all the ravs fnmmprfyntn^

any one point of the object observed to the same focus.
If this is not brought about; if different rays come to
slightly different foci, then the object will look blurred,
as if it were seen through a pair of spectacles which did
not suit our eyes. Now, a single len s, no matter of what
sort of glass we make it, will not brina rays to the same
focus. Tlie reader is doubtless aware that ordinary light,
whether coming from the sun or a star, is of a countless
— multitude of diff p^'pint ^^nl""'''^ which can be separated by
passing the light through a triangular prism. These
colours range from red at one end of the scale, through
yellow, green, and blue, to violet at the other. A single
Ipp;; brinps fij^i^fif; di fferent rays to different f oci ; the red
farthest from the object-glass; the violet nearest to it.
^^"'°_°fpfiriiitinn nf th? ray; is called dispersion.

The astronomers of two centuries ago found it impos-
sible to avoid the dispersion of a lens. About 1750,
Dollond, of London, found that it was possible to cor-
rect this defect by using two different kinds of glass,
the one crown glass and the other flint glass. The prin-

50 ASTRONOMICAL INSTRUMENTS

ciple by which this is done is very simple. Crown glass
has nearly the same refracting power as flint, but it has
nearly twice the dispersive power. So DoUond made
an objective of two lenses, a section of which is shown in
the figure. First there was a convex lens of crown glass,
which is of the usual construction. Combined with this
is a concave lens of flint glass. These two lenses, being
of opposite curvatures, act on the light in opposite direc-
tions. The crown glass tends to bring the light to a
focus, while the flint, being concave, would make the rays
diverse. If it were used alone, we
^^^^^^^^^P should nnd that the rays passmg

CROWN OLAss through it, instead of coming to a

Fig. 10.— Section of tJie focus, diverge farther and farther

Object-glass of a Tele- f j.om a focus, in different direc-

scope.

tions. Now, the flint glass is made
with but little more than half the power of the crown.
This half power is sufficient to neutralize the dispersion
of the crown ; but it does not neutralize much more than
half the refraction. The combined result is that all the
rays passing through the combination are brought nearly
to one focus, which is about twice as far away as the
focus of the crown alone.

I say brought nearly to one focus. It happens, un-
fortunately, that the combined action of the two glasses
is such that it is impossible to bring all the rays of the
various colours absolutely to the same focus. The diver-
gence, in the case of the brighter rays, can be made very
small indeed, but it cannot be cured entirely. The larger
the telescope, the more serious the defect. If you look

THE IMAGE OF A DISTANT OBJECT 51

at a bright star through any large refracting telescope,
you will see it surrounded by a blue or purple radiance.
This is produced by the blue or violet light which the
two lenses will not bring to one focus.

The Image of a Distant Object

By the action of the nhjpctivp. in thus bringing rays
to a focus, the image of a distant object is formed in
the Jnrni r^'"ip This is a plane passing through th e
focus at right angles to the axi s or line of sight of the
telescope.

What is meant by the image formed by a telescope can
be seen by looking into the ground glass of a camera with
the photographer, as he sets his instrument for a picture.
You there see a face or a distant landscape pictured on
the ground glass. To all intents and purposes the
camera is a small telescope, and the ground glass, or the
point where the sensitive plate is to be fixed to take a
picture, is the focal plane. We may state the matter
in the reverse direction by saying that thp tplpprnp;^ is ^
l ^rge camera of lonp- focus, with which we can take
photographs of the heavens as the photographer takes
ordinary pictures with the camera.

Sometimes we can better comprehend what an object
is bj' understanding what it is not. In the celebrated
moon hoax of half a century ago or more, there was a
statement which illustrates what an image is not. The
writer said that Sir John Herschel and his friend finding
that, when they used enormous magnifying power, there
was not light enough for the image to be visible, the

52 ASTRONOMICAL INSTRUMENTS

friend suggested that the image should be illuminated
by artificial light. This was done with such brilliant
success that animals in the moon were made visible
through the telescope. If many people, even those of
the greatest intelligence, had not been deceived by this,
I should hardly deem it necessary to say that the image
of an object formed by a telescope is such that, in the
very nature of things, extraneous light cannot aid in its
formation. Its ^ffeclwejiess does not proceed from its
being a real image, but only from the fact that all the
ravs_J li.ini] .iim iiitt. p"iT]L."f " '^¥^ 4^!^'- obiect meet in a
co rm pwii i ilii ii j^ jirirt nf ■ th ^i illifJF j and there d ^yer j iyfi
again, j u s t as if a picture of the object were placed in
the focal plane. The fact is that the term picture is
perhaps a little better one than image to apply to this
representation of the object, only the picture is formed
by light and nothing else.

If an image or picture of the object is thus formed
so as to stand out before our eyes, one may ask why an
eyepiece is necessary to view it ; why the observer cannot
stand behind the picture, look toward the objective and
see the picture banging in the air, as it were. He can
really do so if he holds a ground glass in the focal plane,
as the photographer does with the camera. He can thus
see the image formed on the glass. If he looks into the
object-glass he can see it without any eyepiece. But
onh' a very small portion of it will be visible at any one
point, and the advantage over looking directly at the
object will be slight. To see it tn fldvawiaff c an pypgiece
mnst bp^^ii^pd This is nothing more than a little eye

POWER AND DEFECTS OF TELESCOPE 53

glass, essentially of the same kind that the watchmaker
uses to examine the works of a watch. The smaller th e
gyepiece, the .more closely the examination can be made,
and the greater the mafinifvii] |;7 pp--""'-

Power and Defects of a Telescope

The question is often asked, how great is the magnify-
ing power of some celebrated telescope. The answer is
that the magnifying power irpp"'^" """* — ^J " *^^~'
"hifrt"n'°°° ^'itjUl thr njipi-r- Th e smaller the latter
the greater the magni fyitip- pnwpr. Astronomical tele-
scopes are supplied with quite a large collection of eye-
pieces, varying from the lowest to the highest power,
according to the needs of the observer.

So far as the geometric principle goes, wc can get any
magnifying power we please on any telescope, however
small. By viewing the image with an ordinary micro-
scope, such as is used by physicians, we might give a
little four-inch telescope the magnification of Herschel's
great reflectors. But there are many practical difficul-
ties in carrying the magnification of any instrument
above a certain point. First there is the want of light
in seeing the surface of an object. If we looked at
Saturn with a three-inch telescope, using a magnifying
power of several hundred times, the planet would seem
dim and indistinct. But this is not the only difficulty
in using a high magnifying power with a small telescope.
The eff'ect of light having a wave length is such that
as a general rule we can get no advantage in carrying
the magnification above fifty, or one hundred at the

54 ASTRONOMICAL INSTRUMENTS

most, for each inch of aperture. That is to say, with a
three-inch telescope we should gain no advantage by
using a power much above one hundred and fifty, and
certainly none above three hundred.

But T IfiTgr tnhff-rrf ? also has its dpferti!- owing to
the ''^^"SfiiHIi^j "*- ViTig'"g "^' ^^^ light trr ibirrllitrlx^
thP 'ifliTT" ^"f't" There is a limit to the magnification
which can be used, rather difiicult to define exactly, but
of which the observer will be very sensible when he looks
into the instrument and sees the blue aureole already
mentioned.

But there is still another trouble, which annoys the
astronomer more than all others, but which the public
rarely understands.

We see a heavenly body through a thickness of atmos-
phere which, were it all compressed to the density that
it has around us, would be equal to about six miles. We
know that when we look at a body six miles away, we
see its outlines softened and blurred. This is mainly
because the atmosphere t hrough which the rays have to
jass is con stantly in Tmntinn. thus pro<^ucinpf an-irrpffnlar
refraction which mfl.kpi= i fh" Ir-irlj InnV ivavy anA ^rpm^-_

Idus. The softened and blurred effect thus produced is
magnified in a telescope as many times as the object
itself. The result is that "" lymi^rTgS'"' the, mg^nify-

inff pn ^^ jif we increase » rertai n indistincft^pss jri tllfi^

vi sion in the same proportion . The amount of this
indistinctness depends very much on the condition of
the air. The astronomer having this in mind tries to
find a perfectly clear air, or. rather, air which is very

MOUNTING OF THE TELESCOPE 55

steady, so that the heavenly bodies will look sharp when
seen through it.

We frequently see calculations showing how near the
moon can be brought to us by using some high magnify-
ing power. For example, with a power of one thou-
sand we see it as if it were two hundred and forty miles
away ; with about five thousand, as if it were forty-eight
miles away. This calculation is quite correct so far as
the apparent size of any object on the moon is concerned,
but it takes no account either of the imperfections of the
telescope or the bad effect produced by the atmosphere.
The result of both of these defects Is that such calcula-
tions do not give a correct idea of the truth. I doubt
whether any astronomer with any telescope now in exist-
ence could gain a great advantage, in the study of such
an object as the moon or a planet, by carrying his mag-
nification above a thousand, unless on very rare occa-
sions in an atmosphere of unusual stillness.

Mounting of the Telescope

Those who have never used a telescope are apt to think
that the work of observing with it is simply to point it at
a heavenly body and examine the latter through it.*

* The writer recalls that when Mr. James Lick was founding
the observatory which has since become so celebrated, the great
telescope was the only feature which seemed to interest him, and
his plan was to devote nearly all the funds to making the largest
lens possible. He did not see why such a complicated instrument
as that used by asti-onomers was necessary. The troublesome prob-
lem of seeing a heavenly body through a telescope had to be ex-
plained to him.

66 ASTRONOMICAL INSTRUMENTS

But let us try the experiment of pointing a great tele-
scope at a star. A result which perhaps we have not
thought of would be immediately presented to our sight.
The star, instead of remaining in the fi^l/l rff yipw* of
the telescope, very soon passes out of it by the diurnal
motion. This is because, as the earth revolves on its axis,
the star seems to move in the opposite direction. Thi s mo-
tioyi is mnltiplipd as manv t im"" iliM thr <-"1"'-'"p" "iipjTii
fpg With a high power, the star is out of the field
before we have time to examine it.

Then it must also be remembered that the field of view •
is .also magnified in the same way, so that it is smaller
than it appears, in proportion to the magnifying power.
For example, if a magnification of one thousand be used,
the field of view of an ordinary telescope would be about
two minutes in angular measure, a patch of the sky so
small that to the naked eye i^ would look like a mere
point. It would be as if we were looking at a star
through a hole one eighth of an inch in diameter in the
roof of a house eighteen feet high. If we imagine our-
selves looking through such a hole and trying to see a
star we shall readily realise how difficult will be the
problem of finding it and of following it in its motion.

This difficulty is overcome by a suitable mounting of
the telescope, so as to turn on two aj:es, at right angles to
each other. By the mounting Is meant the whole system
of machinery by the aid of which a telescope is pointed

*By this term is meant the s mall circul
which we see by looking into the telescope.

MOUNTING OF THE TELESCOPE 57

at a star and made to follow it in its diurnal motion. In
order not to distract the attention of the reader by be-
ginning a study of the instrument with a view of all the
details, we first give an outline, showing the relation of
the axes on which the telescope turns. The__gryicijial
axis, ca lled the polar axis^ is ad justed so as to ^p pa.i;a)lel
to the axis of the earth,, and ^^HH^fflTP tr p^'^i^" "* *^''
cfilestra.1 nnl e. Then, as the earth turns from west to-
ward east, a clockwork connected with this axis turns the

Si

Fig. 11. — Axes on which a Telescope turns.

^Tis1;riimpTit. from past towar d west, with an equal motion.
Thus the rotation of the earth is neutralized, as it were,
by the corresponding rotation of the telescope in the
opposite direction. When the instrument is pointed at
a star and the clockwork set going, the star when once
found will remain in the field of view.

In order that a telescope may be directed at any point
of the heavens at pleasure, there must be finother axis ,
at right angles to the p "^"r nr° This is called the

58 ASTRONOMICAL INSTRUMENTS

flpclinatinn axis. It passes through a sheath fixed to the
upper end of the polar axis so as to form a cross like
the letter T. By turning the telescope on the two axes,
it can be pointed wherever we choose.

Owing to the polar axis being parallel to that of the
earth, its inclination to the horizon is equal to the lati-
tude of the place. In our latitudes, especially in the
southern portions of the United States, it will be nearer
horizontal than vertical. But in the observatories of
northern Europe, it is more nearly vertical.

It will be seen that the contrivance we have described
does not solve the problem of bringing a star into the
field of view of the telescope, or as we commonly say,
of finding it. We might grope round for minutes or
even hours without succeeding in this. There are two
processes by which a star may be found :

Every telescope for astronomical purposes is supplied
with a smaller telescope fastened to the lower end of its
tube, and called the pnder. This finder is of low magni-
fying power, and therefore has a large field of view.
By sighting along the outside of it, the observer, if he
can see the star, can point the finder at it so nearly that
it will be in the field of view of the latter. Having found
it there, he moves the telescope so that the object shall
be seen in the centre of the field. Having brought it
there, it is in the field of view of the main telescope.

But most of the objects which the astronomer has to
observe are totally invisible to the naked eye. He must,
therefore, have a system by which a telescope can be
pointed at a star, without any attempt on his part to see

THE MAKING OF TELESCOPES 59

the latter. This is done by graduated circles, one of
which is attached to each axis. One of these circles has
degrees and fractions of a degree marked upon it, so as
to show the declination of that point in the heavens at
which the telescope is pointed. The other, attached to
the polar axis, and called the hour circle, is divided into
twenty-four hours, and these again into sixty minutes
each. When the astronomer wishes to find a star, he
simply looks at the sidereal clock, subtracts the right
ascension of the star from the sidereal time, and thus
gets its "hour angle" at the moment, or its distance east
or west of the meridian. He sets the declination circle
at the declination of the star, that is, he turns the tele-
scope until the degree on the circle seen through a mag-
nifying aparatus is equal to the declination of the star ;
and then he turns the instrument on the polar axis until
the hour circle reads its hour angle. Then, starting his
clockwork, he has only to look into the telescope and
there is the object.

If all this seems a compHcated operation to the reader,
he has only to visit an observatory and see how simply it
is all done. He may thus in a few minutes gain a practi-
cal idea of sidereal time, hour angle, declination, etc.,
which will make the whole subject much clearer than any
mere description.

The Making of Telescopes

Let us return to some interesting matters, mostly his-
torical, connected with the making of telescopes. The
great difficulty, which requires special native skill of the

60 ASTRONOMICAL INSTRUMENTS

rarest kind, is, as we have already intimated, that of con-
structing the object-glass. The^sjightfit .dPYJatifin frnia.
t^f r''"r°"' ff"""* — a defect consisting in some part of
' the object-glass being too thin by a hundred thousandth
part of an inch — ynil^'^ Tfil t'^°^°'°Cr'

The skill of the optician who figures the glass, that is
to say, who polishes it into the proper shape, is by no
means all that is required. The making of large disks
of glass of the necessary uniformity and purity is a
practical problem of equal difficulty. Any deviation from
perfect uniformity in the glass will be as injurious to its
performance as a defect in its figure.*

A century ago it was found especially difiicult to make
flint glass of the necessary uniformity. This substance
contains a considerable amount of lead, which, during
the process of melting the glass, would sink toward the
bottom of the pot, thus making the bottom portion of
greater refracting power than the upper portion. The
result was that, at that time, a telescope of four or five
inches aperture was considered of great size. Quite early
in the century, Guinand, a Swiss, found a process by
which larger disks of flint glass could be made. He pro-
fessed to have some secret process of doing this, but there
is some reason to believe that his secret consisted only in
the constant and vigorous stirring of the melted glass

* It is frequently proposed by persons not acquainted with the
delicate points of the problem to make a telescope of large size by
putting together different pieces of glass, each of the propet shape,
to form a lens. The idea, ingenious though it looks, is thor-
oughly impracticable, for the simple reason that it is impossible to
make two pieces of glass of exactly the same refracting power.

ALVAN CLARK AND HIS GENIUS 61

while it was being fused in the pot. However this may
have been, he succeeded in making disks of larger and
larger size.

To utilize these disks required an optician of corre-
sponding skill to grind and polish them into proper
shape. Such an artist was found in the person of Fraun-
hofer, of Munich, who, about 1820, made telescopes as
large as nine inches aperture. He did not stop here, but,
about 1840, succeeded in making two objectives, each of
fourteen German inches, or about fifteen English inches
in diameter. These, far exceeding any before made, were
at the time regarded as marvellous. One of these instru-
ments was acquired by the Pulkova Observatory in Rus-
sia ; the other was acquired by the Harvard Observatory
at Cambridge, Mass. The latter, after a lapse of more
than half a century, is still in efiicient use.

Alvan Clark and His Genius

After Eraunhofer's death it was doubtful whether his
skill had died with him, or had passed to a successor.
The latter appeared where none would have thought of
looking for him, in the person of an obscure portrait
painter of Cambridgeport, Mass., named Alvan Clark.
The fact that such a man, with scarcely the elements
of technical education and without training in the use
of optical instruments, should have done what he did,
illustrates in a striking way what an important element
native talent is in such a case. He seemed to have an
intuitive conception of the nature of the problem,
coupled with extraordinary acuteness of vision in solving

62 ASTRONOMICAL INSTRUMENTS

it. Moved by that irrepressible impulse which is a mark
of genius, he purchased in Europe the rough disks of
optical glass necessary to make small telescopes. Having
succeeded in making one of four inches aperture to his
satisfaction, the problem was to make his skill known to
astronomers. I regret to say that he found this a very
difficult part of his task. The director of the Harvard
Observatory would not believe that Mr. Clark could make
a reall}'' good telescope. When the optician took his
first instrument up to the observatory to be tested, the
astronomer called his attention to the fact that it showed
a little tail attached to the star, which, of course, had no
real existence, and was supposed to arise from a serious
defect in the figure of the glass. Mr. Clark saw it, but
was sure it had not been there before. He could not ex-
plain it at the time, but afterwards found that it was
caused by the unequal temperature of the air in the tube
of the telescope when it was exposed under the sky at
night.

Unable to secure any effective recognition at home,
he determined to try abroad. He made a larger instru-
ment, scanned the heavens with it and discovered several
close and difficult double stars. He wrote out descrip-
tions of these objects and sent them to Rev. W. R.
Dawes, an amateur astronomer in England, devoted to
this branch of the science. Mr. Dawes was a lovely char-
acter. He looked at the objects described by Clark and
found great difficulty in making them out. Yet the de-
scriptions were so accurate that it was evident to him
that Mr. Clark's instrument must be of the highest class.

CLARK'S GREAT TELESCOPES 63

He wrote asking him to look at some other objects and
describe them. When the description was received it was
found to be exact. No doubt could remain. The result
was a further correspondence, the purchase by Mr.
Dawes of the largest and best instrument that Mr. Clark
could then make, and a friendship which continued as
long as Mr. Dawes lived.

Mr. Clark now secured recognition in his own country
and became ambitious to make the largest refracting
telescope that had ever been known. This was one of
eighteen inches diameter, which was completed about
1860 for the University of Mississippi. While testing
it at his workshop, a discovery of a most interesting
character was made with it by Mr. George B. Clark, the
son. This was a companion of Sirius, which had been
known to exist by its attraction on Sirius, but had never
been seen by human eye. The breaking out of the Civil
War prevented the University of Mississippi from tak-
ing the telescope, and the latter was acquired by citizens
of Chicago. It is now mounted at the Northwestern
University in Evanston, 111.

The making of disks of glass of larger and larger
size was continued by the great glass works of Chance
& Company, in England. But they found the work
too delicate and too troublesome, and allowed it to pass
into the hands of Feil of Paris, son-in-law of Guinand.
With the glass supplied by these two parties, Mr. Clark
made larger and larger telescopes. First was the twenty-
six-inch telescope for the Naval Observatory at Wash-
ington and a similar one for the University of Virginia.

64 ASTRONOMICAL INSTRUMENTS

Then followed a still larger instrument, thirty inches in
diameter, for the Observatory of Pulkova, Russia. Next
was completed the thirty-six-inch instrument of the Lick
Observatory, which has done such splendid work.

After the death of Feil, the business was taken up by
Mantois, who made optical glass of a purity and uni-
formity that no one before him had ever approached.
He furnished the disks with which the Clarks figured
the objective for the Yerkes telescope of the University
of Chicago. This is about forty inches in diameter, and
is the largest refracting telescope now in actual use for
astronomical purposes.

Our readers have doubtless been interested in the great
telescope of the Paris Exposition of 1900, which is yet
larger than that of Chicago, being of forty-seven inches
aperture. This instrument is of such immense size that
it cannot be mounted and pointed at the heavens in the
usual way. It is therefore fixed in a horizontal, north
and south position, and the rays of the object to be ob-
served are reflected into it by an immense plane mirror.
The question whether this contrivance has been success-
ful with so large an instrument is one that is not yet
settled with astronomical precision. Nothing has yet
been done with this instrument, which, it is feared, is
so imperfect in make as to serve no better purpqse th^.!!
that of a toy.

The engineering problem of mounting a great tele-
scope is by no jneans a simple one. It was one in which
Mr. Clark was less successful than in the construction of
his object-glasses. In the case of the later telescopes the

GREAT TELESCOPES

65

Fig. 12. — Great Telescope of the Yerkes Observatory, moimUedhy Warner cGt

Smazey,

66 ASTRONOMICAL INSTRUMENTS

mountings of the great instruments were made by other
parties. That of the Pulkova telescope was made by the
Repsolds of Hamburg, the most noted makers of fine
astronomical instruments in Europe. The Lick and
Chicago telescopes were mounted by Warner & Swazey,
of Cleveland, Ohio, who are gaining the highest reputa-
tion in this class of work. In the case of the Chicago
telescope, arrangements were devised by them which sur-
pass all ever before thought of. The observer has only
to touch electric buttons to have all the work of pointing
and moving the telescope perfortned by electricity.

n

The Reflecting Telescope

Although the rRfract,^pg^^g^^f) p(;)p p. ,ii}|,|ti^a || ^n ^ost
.^eflfij^Qjge, there is another form of instrument of radi-
cally different construction. Tt s main fi^g ^tur^- is that
the functions of tjhfi fijljp'^^-gliiri'i 'Jirf pfirf^wr"'! hjr a
^ligll^^y ''OTI^^Yf tyirrnr^ That such a mirror reflects
parallel r ajs ^|j),]]^Tiy_LLpQ.Ti jj- tr. «■ fqpug is doubtless well
known to our readers. The focus is .c 'l'iatprl jj^qt^^' ^<''^-
way between the mirror and its centre of curvature .

This form of instrument has an enormous advantage
in its freedom from the "secon dary ab^ pTfjitii"^'^ which
we have already described as inherent in the refracting
telescope. Another advantage which it possesses is that
it can be made of larger dimensions than the other. The
extreme Hmit so far reached in the refractor, as we have
already stated, is four feet , but the forty-inch aperture
of the Yerkes telescope is, up to the present time, the
limit in actual use for astronomical research. But, more
than half a century ago, Lord Rosse constructed his
great reflector of six feet diameter. Judging by its
size alone, this instrument ought to give several times
more light, and therefore show far minuter stars, than
any refracting telescope yet made. But, for some rea-
son, its performance — and, indeed, that of reflectors
generally — ^has not corresponded to the size,

68 ASTRONOMICAL INSTRUMENTS

The practical difficulties in using a reflector are several
in number. The first and most obvious one is that the
rays are reflected back in the direction from which they
came. To see the image the observer must look into the
mirror as it were. If he does this directly, his head and
shoulders will cut off the light that falls on at least the
central regions of the mirror. Some contrivance for re-
flecting this light away is therefore necessary. Two
ways of doing this are in use. In what is known as the
^Vi^?f]'Ti\iU^"^ rpflpftnr. a smaller, slightly convex mirror

is JT^tprpnserl bp|,wppT^ +hp fpfys drirl thn prjp^i'paj miriTir

An opening is made in the centre of the latter, through
which the rays are reflected back by the smaller mirror.
The curvature and positions of the two are so adjusted
that the image of the distant object shall be formed in
this opening. The only telescope of this kind in actual
use is the great Melbourne reflector, of four feet diam-
eter, made by Sir Howard Grubb, of Dublin.

The contrivance most in use was designed by Sir Isaac
J^ewton. It consists of a diasrona] --^pfloptnT. which may
be a mere glass prism, p ifir r d junt-in n id a tha i fnnn Its
reflecting surface makes an angle of forty-five degrees
with the axis of the telescope, and therefore reflects the
rays laterally to the side of the tube. Here they are
observed with an ordinary eyepiece. This instrument is
known as the Newtonian reflector.

It is remarkable that, notwithstanding the immense
improvement in the mechanical processes necessary in
constructing and mounting a reflecting telescope, no at-
tempt has ever been made to even equal Lord Rosse's

Fig. lo.—Seciim of a Newtonian Reflecting Telescope.

70 ASTRONOMICAL INSTRUMENTS

great instrument in dimensions. The largest mirrors
so far successfully made and used have been about four
feet in diameter. About fifty years ago, Mr. Lassell
made one of this size, with which he discovered two
new satellites of Uranus. More recently, Mr. A. A.
Common, F.R.S., has constructed a mirror of the same
size. This has been used in taking photographs of
nebulas and other faint objects, for which this form of
telescope seems well designed.

The great difficulty in using a large mirror is that it
bends under the influence of its own weight. It would
seem that when the diameter exceeds four feet, no way
of completely avoiding this difficulty has yet been put
into successful use. A mirror of five feet diameter is,
however, being made at the Yerkes Observatory by Mr.
Ritchie, in which, it is hoped, all the difficulties will be
surmounted.

In the instruments of Lord Rosse and Mr. Lassell, the
mirror was made of an alloy, known as speculum metal.
Recently, however, the use of speculum metal has been
superseded by another arrangement. The concave mir-
ror is made of a large disk of glass, which is ground and
polished into nearly spherical form, or to speak more ac-
curately, a parabolic form,, because the latter is necessary
to bring all the rays to one focus. A thin coating of
silver is then deposited on the surface of the glass, which
is susceptible of a high polish, and reflects more light
than polished metal.

in

The Photographic Telescope

One of the greatest advances in practical astronomy
in our time has been brought about by jjhntngr^ph^^g
the heavenly bodies. This is so simple a process that the
slowness of its introduction may seem curious. Back in
the early '40's, Professor Draper, of New York, the well-
known chemist, succeeded in making a daguerreotype
of the moon. When the system of photography by our
present process on a glass negative was invented. Pro-
fessor Bond, of the Harvard Observatory, and Mr. L. M.
Ilutherfurd, an eminent astronomer of New York, both
began to apply the art to the moon and stars. Mr.
Rutherfurd brought his work to such perfection that his
photographs of the Pleiades and other clusters of stars
are still of great value in astronomy.

A photograph of the stars can be made by an ordinary
camera if we only mount it like an equatorial telescope
so that it shall f oUow the star in its diurnal motion. A
very few minutes exposure will suffice to take a picture
of more stars than can be seen by the naked eye ; in fact,
with a large camera, this will not require a minute. But
what is generally used by the astronomer is a photo-
graphic telescope. Any ordinary telescope wiU serve
the purpose, but in order to get the best results the
object-glass of the telescope must be especially made to

72 ASTRONOMICAL INSTRUMENTS

hrin p- to a fpfus tlinsp.ravs of }{fT^ t» wTniVl. Ap pVintn-
gjjpMn filwii iiiiiiiiiiiiiiiiiili nrnnilTi'nr So rapid has been the
progress during the past few years that the greater, part
of the astronomical work of the future seems Ukely to be
done by photography. The g^""<- nriTor^+j^rPf of the
method is that "hi " j ' ' '^"r nf 7P^° Vioa-Yfjily

Vin^Y r.T nf t>io j^^fj ps in the s] fY 1^1 t"l^"^ ^^- c^n be studied
and niriiiTwrflf' nt Iri'ill'"" ""'^^ all th e r^re t he astronomer
chooses to bestow upon it, while the nhsprv^,tio| i in the
heavens is nearly always more or legaJmrried; and made
difficult by the diurnal motion of the star.

Formerly the spots on the sun were investigated by
watching that luminary through the telescope, recording
the number of spots, and measuring their position on the
solar disk. Now, at the Greenwich Observatory and else-
where, a photograph of the sun is taken almost every
day, and the position of the spots is found by measuring
the photograph. Thus a study of the sun and the
changes going on on its surface is kept up from year to
year.

Formerly the astronomer studied the physical con-
stitution of a comet by making a drawing of it. This
was a rather uncertain process, and as a general rule no
two men would quite agree in the minute details. Now
the comet is photographed and a study is made upon the
negative. The same remark applies to nebulae. Draw-
ings of them are no longer made — only photographs
which show a great deal more than any drawing will.

IV

The Spectroscope

The spectroscope is an instrument for analysing
light. It is a much more recent instrument than the
telescope, having first been applied to astronomical ob-
servation about 1864. To convey an intelligent idea
of its use we must say something about the heat and light
radiated by the heavenly bodies.

We know that the sun, a gas light, or other bright
body gives us heat as well as light. A very simple obser-
vation will show that the rays of heat proceed in straight
lines like those of light, and that they can pass through
air and other transparent bodies without warming them,
just as light does. If we make a large fire on the hearth
in a perfectly cold room, we shall feel the heat on our
faces although the air may be frosty. A striking experi-
ment is that of making a lens out of ice and using it as
a burning glass. The rays of the sun passing through
the ice may be concentrated so as to burn the hand, and
that without the ice melting.

It was formerly supposed that heat and light were two
distinct agents ; now it is known that such is not the case.
As emitted by a hot body both may be called by the
general name of radiance. All radiance, when it falls
on a surface, produces heat, just as the blaze of the fire
produces heat on the walls of a room. But not all radi-

74 ASTRONOMICAL INSTRUMENTS

ance affects the optic nerve of the eye so as to produce a
sensation of light and enable us to see bodies.

It is now know that radiance consists of something in
the nature of waves in an ethereal medium which fills all
space, even to the most distant star. These waves are
exceedingly short. To form an idea of their length we
must measure by the micron, which is one thousandth
of a millimetre. Those which produce the sensation of
light on the optic nerve mostly range between four and
seven tenths of a micron. This allows between forty and

eighty thousand waves
to ,the inch. We rep-
resent these waves by
FiQ. 14.— Wave Length of Light. the little wave line in

the figure. The dis-
tance between the dotted lines is the wave lengths. The
peculiar feature of the radiance emitted by the sun, or
any other body that is not transparent, is that it is not
all of the same wave length, but of a very wide range
of wave lengths all mixed together. We must imagine
that between the rays which we represent in the figure
there are an infinity of others, all varying in their wave
lengths. In this respect radiance is like the waves of
the ocean, which range in length from several hundred
yards to a few inches, all piled upon each other.

When the radiance passes through a glass prism it is
refracted from its course. Difi'erent wave lengths are
refracted differently, but waves of the same length are
always refracted by the same amount. This is shown by
the familiar experiment of forming a spectrum of the

THE SPECTROSCOPE

75

sun with a triangular prism. Arranging the light to
be thrown on a screen, we see red light at the bottom,
then yellow above it, then in succession, green, blue, and
violet. This arrangement of colours
on a surface is called a spectrum. The
colour of the light in the spectrum
depends on the wave length. If the
wave length is greater than about
seventy-five one-hundredths of a mi-
cron, that is, one forty-four-thou-
sandth of an inch, the eye does not
see it, and, for us, it passes simply as
heat. From this length to one fifty-
thousandth it looks red, when a little
shorter it looks scarlet, then yellow,
and so on. Shorter than forty-three
one-hundredths of a micron it is diffi-
cult to see it at all. But the violet
light affects the photographic plate
even more strongly than the light
which looks brightest to the eye. The
light which is most easily photo-
graphed is the blue and violet, and as
we go toward the red the photo-
graphic efi'ect diminishes.

All bodies emit radiance, but, at
ordinary temperatures, the wave
lengths of this radiance are too long to be visible
to the eye. ■ Not until we heat a body red hot
does it emit radiance of wave length short enough to

FiO. 15. — Arrange-
ment of the Colours
of the Spectrum,
with the Dark lAnes
A,B, C,D,etc.,of
the Spectrum.

76 ASTRONOMICAL INSTRUMENTS

form light. As we make it hotter it still emits more and
more waves of long wave lengths, and also waves of
shorter and shorter wave lengths. Thus as we heat up
a piece of iron, it appears first as red hot, and afterward
as white hot.

The possibility of reaching conclusions about the con-
stitution of a hot body from the light which it emits arises
from the fact that different bodies emit light of different
wave lengths. If the body is solid, it emits light of all
wave lengths, and we cannot tell much about it. But if
it is a mass of transparent gas, it only emits light of cer-
tain wave lengths, depending on the nature of the gas.

The easiest way of making a gas emit its peculiar
light is by passing an electric spark or current through
it. Then, if we analyse the light produced by the spark
with a prism, we find that the spectrum is composed of
one or more bright lines, varying in position according
to the nature of the gas. Thus we have a spectrum of
hydrogen, another of oxygen, and others of almost all
the bodies which we know. Solid bodies, including all
the metals, can be made to give their spectrum by being
heated so intensely by the electric spark that a small
quantity of the body is changed into a gas. Thus we
may even form a spectrum of iron, which the practised
observer can immediately detect as iron by the position
and arrangement of the lines of the spectrum.

How the Stars are Analysed

The fundamental principle of spectrum analysis is
that if the light of an incandescent body passes through

HOW THE STARS ARE ANALYSED 77

a gas which is cooler than the body, the latter will cull
out and absorb from tlie light those wave lengths which
it would emit if it were itself incandescent. The result
is that the spectrum from the solid body will be seen
crossed by certain dark lines, depending on the nature
of the gas through which the light has passed. Thus,
if we observe an electric light through a prism in its
immediate neighbourhood, the spectrum will be unbroken
from one end to the other. But if the light is at a great
distance, we shall see it crossed by a great number of dark
lines. These lines are produced by the air through which
the light has passed culling out the light which has cer-
tain wave lengths. It is of interest that the aqueous va-
pour in the air is the most powerful agent in this, and
culls out great groups of lines, by which its presence in
the air can be immediately detected. The darkest of the
lines found in the spectrum of the sun are designated by
the letters A, B, C, etc., as shown in the preceding
figure.

We may describe the spectroscope in the most compre-
hensive way by saying that it is an instrument for
studying the spectra of bodies, whether in the heavens
or on the earth.

The studies of the heavenly bodies with the spectro-
scope have two objects. One is to determine the nature
of the bodies ; the other their motions to or from us.
The possibility of the latter is one of the most wonderful
achievements of modern science. If a star is coming
toward us, the wave length of the light which it emits is
slightly shorter in consequence of the motion ; if it is

78 ASTRONOMICAL INSTRUMENTS

going away from us, it is longer. Thus, by measuring
the positions of its lines in the spectrum, it is possible
to determine whether a star is approaching us or
moving away from us.

In recent years the studies of the spectra of stars have
been made almost entirely by photography. It is found
that, as in other cases, the sensitive plates now used in
that art will take impressions of objects which the eye
cannot see in the telescope. So the astronomer photo-
graphs the spectrum of a star, which will show all the
lines he can see with the naked eye, and perhaps a great
many more. The positions of these lines are measured
and studied, and the astronomer's conclusions are drawn
from these studies.

V

OtHEE AsTKONOMICAIi InSTEUMENTS

It is commonly supposed that the principal work of
an astronomer is to study the stars as he sees them in
his telescope. This is true only in the sense that a tele-
scope is a necessary part of almost every astronomical
instrument. But the mere studying of a star with a
telescope is a very small part of the astronomer's work.
The most important practical use of astronomy to our
race consists in the determination of the latitudes and
longitudes of points on the earth's surface, so that we
may know where towns and cities are situated and be
able to make a map of a state or country. This re-
quires a knowledge of the exact positions of the stars in
the heavens, that is to say, of their right ascension and
declination. We have shown in a former chapter how
these quantities correspond to longitude and latitude on
the earth's surface. Through that correspondence an
observer may determine his latitude by the star's dec-
lination and his longitude by its right ascension, com-
bined with a knowledge of the sidereal time at a place
of known longitude.

The figures and dimensions of the planets, the motions
of the satellites, the orbits of planets and comets, the
structure of nebulae and clusters of stars — all these offer
fields of astronomical investigation to which there is

80

ASTRONOMICAL INSTRUMENTS

no end, and in order to make these investigations other
instruments besides the telescope are necessary.

The Meridian Circle and Clock

The problem which demands most attention from the
vorking astronomer in an observatory is the determina-
tion of the positions of the heavenly bodies. The prin-

FiG. 16. — A Meridian hislrument.

cipal instrument for making these determinations is the
meridiem circle, called also a meridian instrument. This
consists of a telescope supported on a horizontal east and
west axis, at right angles to its length, so that its line
of sight can move only along the meridian. If it points

MERIDIAN CIRCLE AND CLOCK 81

exactly south you can turn it on the axis until the line
of sight passes through the zenith, and still farther untU
it passes through the pole on the north horizon; but
you cannot turn it east or west. This might seem to re-
strict its usefulness, but it is on this restriction of its
motion that its usefulness depends. The great value
of this instrument is that it enables us to determine the
right ascension of a star without taking any measure-
ment but one of time. In a former chapter we described
sidereal time, the units of which are slightly shorter
than those of our ordinary time, so that a sidereal clock
gains about two hours every month on an ordinary clock.
The sidereal time at which a star crosses the meridian is
the same as its right ascension ; the problem of determin-
ing the latter, therefore, is the simplest in the world.
We start our sidereal clock, set it on the exact sidereal
time, point the telescope of the meridian circle to various
stars as they are about to cross the meridian, and note
the exact moment at which each star passes. In the
instrument the meridian is shown by a very fine fibre or
spider's web fixed in the focus of the telescope. The
moment when the image of the star as seen in the tele-
scope crosses this spider line is that of passing the me-
ridian. The time by the sidereal clock then shows the
star's right ascension. If the clock could be set with
perfect exactness and the instrument revolved exactly
in the plane of the meridian, right ascensions would be
determined in the very simple way we have described.

It unfortunately happens, however, that no clock can
be set with such exactness as to satisfy the requirements

82 ASTRONOMICAL INSTRUMENTS

of the astronomer, who wants to know the time down to
the tenth or even to the hundredth of a second. More-
over, no meridian circle can have its axis set so exactly
east and west that the instrument shall not deviate a little
from the meridian. The astronomer must therefore make
allowances for the error of his clock and for the deviation
of his instrument; and these require much careful ob-
servation and calculation. Even when he does the best
he can, a single observation will always be hable to little
errors which he wishes to make as small as possible. He
does this by repeatedly determining the position of every
star which he puts upon his list. He generally has to be
satisfied with three or four observations on the great
mass of the stars, but on the more important stars he
makes them by scores or hundreds.

To determine the declination of a star, a graduated
circle is necessary. This consists of a brass or steel circle,
much like a carriage wheel, of which the axis is the same
as that on which the telescope of the meridian instrument
turns. The circle is firmly attached to the axis so
that it must turn with the telescope as the latter sweeps
along the celestial meridian. The graduations of the
circle consist of very fine marks or lines all round its
circumference. The latter being divided into three htin-
dred and sixty degrees, every degree is marked by such
a line. Between these it is common to mark thirty inter-
mediate lines, which are therefore two minutes apart.
Attached to one or both the stone piers which support
the instrument are four microscopes, so fixed that the
graduations on the circle are seen through them. When

MERIDIAN CIRCLE AND CLOCK 83

the instrument is turned on its axis, all these graduations
pass successively under each microscope, so that they
can be seen by the observer looking through the latter.
The position of the star is determined by measures with
the microscope on the graduation which happens to be
under it when the telescope is pointed at a star.

The equatorial telescope and the meridian circle are
the two principal instruments in the astronomical outfit
of an observatory. Many other instruments are more
or less in use for special purposes, but they are not of
great interest, save to one who is making a special study
of astronomy and who must therefore refer to books
specially written for the professional student of the
subject.

The precision with which a practised observer can
note the time of tranfsit of a star over the thread of his
instrument is remarkable. One method of doing this
consists in listening to and counting the beats of the
clock as the star approaches and crosses the thread.
He watches the exact position of the star at the beat
before the transit, and again at the beat following. By
comparing in his mind the opposite distances of the
star from the thread at the two clock beats, he estimates
the number of tenths of the second at which the transit
took place, and records the time in his notebook.

This method is now superseded in most observatories
by that of registration on a chronograph. This instru-
ment consists of a revolving cylinder, covered with
paper, having a pen-point resting upon it, so that, as
the cylinder revolves, the pen leaves a trace on the paper.

84 ASTRONOMICAL INSTRUMENTS

The pen is so connected with an electric current passing
through the clock, and through a key held by the ob-
server, that every beat of the clock and every pressure
of the key by the observer makes a notch in the trace
left by the pen. When the observer sees that a star has
reached the thread of his instrument he presses the key,
and the position of the notch thus made in the pen-trace
between two notches made by the clock gives the moment
at which the key was pressed.

The astronomer's clock must be of the highest at-
tainable perfection, running for a whole day or more
without a deviation of one tenth of a second. With a
common house clock, the change in the length of the
pendulum produced by changes of temperature between
the day and night would cause deviations of several
seconds. Hence in the astronomical clock these changes
must be neutralised. This is done by making the pen-
dulum of such a combination of different materials that
the unequal expansions of the latter shall neutralise each
other. The most common combination is that of a steel
rod bearing at its lower end a steel or glass jar of
quicksilver, which serves as the bob of the pendulum.
Then, when the temperature rises, the upward expansion
of the quicksilver compensates the downward expansion
of the steel.

PART III
THE SUN, EARTH, AND MOON

I

An Introductory Glance at the Solar System

We have shown how this comparatively small family
of bodies, on one of which we dwell, forms as it were a
little colony by itself. Small though it be when com-
pared with the whole universe as a standard, it is for us
the most important part of the universe. Before pro-
ceeding to a description of its various bodies in detail
we must take a general view to show of what kind of
bodies it is formed and how it is made up.

First of all we have the sun, the great shining central
body, shedding warmth and light on all the others and
keeping the whole system together by virtue of its
powerful attraction.

Next we have the planets, which revolve round the sun
in their regular orbits, and of which our earth is one.
The word planet means wanderer, a term applied in
ancient times because these bodies, instead of keeping
their places among the fixed stars, seemed to wander
about among them. The planets are divided into two
quite distinct classes, termed major and minor.

The major planets are eight in number and are, next
to the sun, the largest bodies of the system. For the
most part their distances from the sun are arranged in a
close approach to a certain regular order, ranging from
nearly forty millions of miles in the case of Mercury,

88 THE SUN, EARTH, AND MOON

the nearest one, to three thousand millions in the case of
Neptune. The latter is therefore seventy times as far
from the sun as Mercury. Still wider is the range of
their times of revolution. Mercury performs its circuit
round the sun in less than three of our months — Neptune
takes more than one hundred and sixty years for his loiig
journey. It has not yet made half a revolution since its
discovery in 1846.

The major planets are separated into two groups of
four planets each, with quite a broad gap between the
groups. The inner group is composed of much smaller
planets than the outer one ; all four together would not
make a body one quarter the size of the smallest of the
outer group.

In the gap between the two groups revolve the minor
planets, or asteroids as they are commonly called. They
are very small as compared with the major planets. So
far as we know they are all situated in a quite wide belt
ranging between a little more than the distance of the
earth out to four times that distance. For the most
part they are about three or four times as far from the
sun as the earth is. They are also distinguished from
the major planets by their indefinite number; some five
hundred are now known, and new discoveries are con-
tinually being made at such a rate that no one can set
any exact limit to them.

A third class of bodies in the solar system comprises
the satellites, or moons. Several of the major planets
have one or more of these small bodies revolving round
them, and therefore accompanying them in their revolu-

GLANCE AT THE SOLAR SYSTEM 89

tion around the sun. The two innermost planets, Mer-
cury and Venus, have no satellites, so far as we yet
know. In the case of the other planets their number
ranges from one (our moon) to eight, which form the
retinue of the planet Saturn. Each major planet. Mer-
cury and Venus excepted, is therefore the centre of a
system bearing a certain resemblance to the solar system.
These systems are sometimes designated by names de-
rived from those of their central bodies. Thus we have
the Martian System, composed of Mars and its satellites ;
the Jovian System, composed of Jupiter and its five
satellites; the Saturnian System, comprising the planet
Saturn, its rings, and satellites.

A fourth class of bodies consists of the comets. These
move round the sun in very eccentric orbits. We see
them only on their approach to the sun, which, in the
case of most of these bodies, occurs only at intervals of
centuries, or even thousands of years. Even then a
comet may fail to be seen unless under favourable
conditions.

Besides the preceding bodies we have a countless num-
ber of meteoric particles revolving round the sun in
regular orbits. These are probably related in some way
to the comets. They are completely invisible except as
they strike our atmosphere, when we see them as
shooting stars.

The following is the arrangement of the planets in
the order of their distance from the sun and with the
number of satellites of each :

90 THE SUN, EARTH, AND MOON

/• Inner Group of Major Planets:

Mercurj',

Venus.

Earth, with one satellite.

Mars, with two satellites.

//. Group of Minor Planets, or Asteroids.

III. Outer Group of Major Planets :

Jupiter, with five satellites.
Saturn, with eight satellites.
Uranus, with four satellites.
Neptune, with one satellite.

Instead of taking up these bodies in the order of their
distance from the sun, we shall, after describing the
latter, pass over Mercury and Venus to consider the earth
and moon. Then we shall return to the other planets
and describe them in order.

n

The Sun

In a description of the solar system its great central
body is naturally the first to claim our attention. We
see that the sun is a shining globe. The first questions
to present themselves to us are about the size and dis-
tance of this globe. It is easy to state its size when we
know its distance. We know by measurement, the angle
subtended by the sun's diameter. If we draw two lines
making this angle with each other, and continue them in-
definitely through the celestial spaces, the diameter of
the sun must be equal to the distance apart of the lines
at the distance of the sun. The exact determination is
a very simple problem of trigonometry. It will suffice
at present to say that the measure of the apparent
diameter of the sun, or the angle which it subtends to our
eye, is thirty-two minutes, making this angle such that
the distance of the sun is about 107.5 times its diameter
in miles. If, then, we know the distance of the sun, we
have only to divide it by 107.5 to get the sun's diameter.

The various methods of determining the distance of
the sun will be described in our chapter stating how dis-
tances in the heavens are measured. The result of all
the determinations is that the distance is very nearly
ninety-three million miles, perhaps one or two hundred
thousand miles more. Taking the round number, and

92 THE SUN, EARTH, AND MOON

dividing by 107.5, we find the diameter to be about 865,-
000 miles. This is about one hundred and ten times the
diameter of the earth. It follows that the volume or bulk
of the sun is more than one million three hundred thou-
sand times that of the earth.

The sun's importance to us arises from its being our
great source of heat and light. Were these withdrawn,
not onlj' would the world be enveloped in unending
night, but, in the course of a short time, in eternal frost.
We all know that during a clear night the surface of the
earth grows colder through the radiation into «pace of
the heat received from the sun during the day. With-
out our daily supply, the loss of heat would go on until
the cold around us would far exceed that which we now
experience in the polar regions. Vegetation would be
impossible. The oceans would freeze over, and all life
on the earth would soon be extinct.

The surface of the sun, which is all we can see of it,
is called the photosphere. This term is used to distin-
guish the visible surface from the vast invisible interior
of the sun. To the naked eye, the photosphere looks
entirely uniform. But through a telescope we see that
the whole surface has a mottled appearance, which has
been aptly compared te that of a plate of rice soup.
Examination under the best conditions shows that this
appearance is due to minute and very irregular grains
which are scattered all over the photosphere.

When we carefully compare the brightness of differ-
ent regions of the photosphere, we find that the apparent
centre of the disk is brighter than the edge. The differ-

ROTATION OF THE SUN 93

ence can be seen even without a telescope, if we look at
the sun through a dark glass, or when it is setting in a
dense haze. The falling off in the light is especially
rapid as we approach the extreme edge of the disk, where
it is little more than half as bright as at the centre.
There is also a difference of colour, the light of the edge
having a lurid appearance as compared with that of
the centre.

All this shows that the light of the sun is absorbed by
an atmosphere surrounding the sun. We readily see
that, the sun being a globe, the light which we receive
from the edge of its disk leaves it obliquely, while that
from the centre leaves it perpendicularly. The more
obliquely the light comes from the surface, the greater
the thickness of the sun's atmosphere through which it
must pass, and hence the greater the portion lost by the
absorption of that atmosphere. The sun's atmosphere,
like our own, absorbs the green and blue rays more than
the red. For this reason the light has a redder tint when
it comes from near the edge of the disk.

Rotation of the Sun

Careful observations show that the sun, like the
planets, rotates on an axis passing through its centre.
Using the same terms as in the case of the earth, we call
the points in which the axis intersects the surface the
poles of the sun, and the circle around it halfway be-
tween the poles the sun's equator. The period of rota-
tion is about twenty-six days. As the distance around
the sun is more than one hundred and ten times that

94 THE SUN, EARTH, AND MOON

round the earth, the speed of rotation must be more than
four times that of the earth's rotation to make it com-
plete the circuit in the time that it does. At the sun's
equator the speed is more than a mile a second.

The most curious feature of this rotation is that it
is completed in less time at the equator than at a distance
on each side of the equator. Were the sun a solid body,
like the earth, all its parts would have to rotate at the
same time. Hence the sun is not a solid body, but must
be either liquid or gaseous, at least at its surface.

The equator of the sun is inclined six degrees to the
plane of the earth's orbit. Its direction is such that in
our spring months the north pole is turned six degrees
away from us and the central point of the apparent disk
is about that amount south of the sun's equator. In our
summer and autumn months this is reversed.

The Sun's Density and Gravity

By the mean density of the sun we refer to the average
specific gravity of the matter composing it, or the ratio
of its weight to that of an equal volume of water. It is
known that the density is only about one fourth that of
the earth, and about four tenths greater than that of
water. Stated with more exactness, the figures are :

Density of sun : Density of earth = 0.2554.

Density of sun: Density of water = 1.4!ll5.

The mass or weight of the sun is about 334,000 times
that of the earth.

The force of gravity at the sun's surface is 27 times
that of the earth. If it were possible for a human being

SPOTS ON THE SUN 95

to be placed there, an ordinary man would weigh two
tons, and be crushed by his own weight.

Spots on the Sun

When the sun is carefully examined with a telescope,
one or more seemingly dark spots will generally, though
not always, be seen on its surface. These are, of course,
carried around by the rotation of the sun, and it is by
means of them that the time of rotation is most easily
determined. If a spot appears at the centre of the disk
it will, in six days, be carried to the western edge, and
there disappear. At the end of about two weeks it will
reappear at the eastern edge unless it has, in the mean-
time, died away, which is frequently the case.

The spots have a wide range in size. Some are very
minute points, barely visible in a good telescope, while on
rare occasions one is large enough to be seen with the
naked eye through a dark glass. They frequently ap-
pear in groups, and a group may sometimes be made out
with the naked eye as a minute patch when the individual
spots cannot be seen.

I^Tien the air is steady, and a good-sized spot is care-
fully examined with a telescope, it will be seen to be com-
posed of a dark central region or nucleus, surrounded
by a shaded border. If all the conditions are favourable,
this border will appear striated, like the edge of a
thatched roof. The appearance is represented in the cut,
which also shows the mottling of the photosphere.

The spots are of the most varied and irregular forms,
frequently broken up in many ways. The shaded border,

96

THE SUN, EARTH, AND MOON

or the thatched lines which form it, frequently encroaches
on the nucleus or may, in places, extend qxiite across it.
A most remarkable law connected with the spots, which
has been established by nearly three centuries of observa-
tion, is that their frequency varies in a regular period of
eleven years and about forty days. During a certain
year no spots will be visible for about half of the time.

Fig. 11.—.

of a
also the

with High Magnifying Power, shmi-
of the Photosphere.

This was the case in 1889 and again in 1900. The year
following a slightly greater number will show themselves ;
and they will increase year after year for about five
years. Then the frequency wiU begin to diminish, year
after year, until the cycle is completed, when it wiU again
begin to increase. These mutations have been traced back
to the time of Galileo, although it was not till about 1825
that they were found by Schwabe to take place in a
regular period.

SPOTS ON THE SUN

97

Years of greatest and least frequency, past and future
are as follows :

Greatest
1871
1882
1893
1904
1916
1927

Least

1878

1889

1900

1911

1922

1933

Another noteworthy law connected with the sun's spots
is that they are not found all over the sun ; but only in
certain regions of solar latitude. They are rather rare
on the sun's equa-
tor, but become
more frequent as we
go north or south
of the equator till
we get to fifteen de-
grees of latitude,
north or south. From
this region to twen-
ty degrees the fre-
quency is greatest;
then it falls off, so
that beyond thirty
degrees a spot is
rarely seen. These
regions are shown in
the accompanying figure, where the shading is darker

Fig. 18. — Frequency of Sun-spc i
ent Latitudes on the Sun.

Diffn:

98 THE SUN, EARTH, AND MOON

the more frequent the spots. If we made a white globe
to represent the sun, and made a black dot on it for every
spot during a number of years, the dotting would make
the globe look as represented in the figure.

The Faculce

Collections of numerous small spots brighter than the
photosphere in general are frequently seen on the sun.
These are often seen in the neighbourhood of a spot,
and occur most frequently in the regions of greater
spot frequency, but are not entirely confined to those
regions. They are, however, rare near the poles of
the sun.

That the spots and faculae proceed from some one
general cause has been brought out by the spectro-heho-
graph, an instrument devised by Professor George E.
Hale for taking photographs of the sun by the light of
a single ray of the spectrum, that emitted by calcium, for
example. The effect is the same as if we should look at
the sun through a glass which would allow the rays of
calcium vapour to pass, but would absorb all the others.
We should then see the calcium light of the sun and no
other.

When the sun is photographed by calcium light with
this instrument, the result is wonderful. The sun-spot
regions are now seen to be brighter than the others,
and faculae are found on every part of the sun. We thus
learn that eruptions of gas, of which calcium is the best
marked ingredient, are taking place all the time; but
they are more numerous in the sun-spot zones than else-

PROMINENCES AND CHROMOSPHERE 99

where. The sun-spots are therefore the effect of opera-
tions going on all the time, all over the sun, but giving
rise to a spot only in the exceptional cases when they are
very intense.

It was formerly supposed that the spots were openings
or depressions in the photosphere, showing a darker
region within. This view was based on the belief that,
when a spot was near the edge of the sun's disk, the
shaded border next the edge looked broader than the
other. But this view is now abandoned. We cannot cer-
tainly say that a spot is either above or below the photo-
sphere. We shall hereafter see that the latter is not a
mere surface as it seems to us, but a shell or covering
many miles, perhaps a hundred or more, in thickness.
The spots doubtless belong to this shell, being cooler por-
tions of it, but lying neither above nor below it.

The Prominences and Chromosphere

The next remarkable feature of the sun to be described
consists in the prominences. Our knowledge of these ob-
jects has an interesting history — which will be mentioned
in describing eclipses of the sun. The spectroscope
shows us that large masses of incandescent vapour burst
forth from every part of the sun. They are of such ex-
tent that the earth, if immersed in them, would be as a
grain of sand in the flame of a candle. They are thrown
up with enormous velocity, sometimes hundreds of miles
a second. Like the faculae, they are more numerous in
the sun-spot zones, but are not confined to those zones.
The glare around the sun caused by the reflection of .light

100 THE SUN, EARTH, AND MOON

by the air renders them entirely invisible to vision, even
with the telescope, except when, during total eclipses of
the sun, the glare is cut off by the intervention of the
moon. They may then be seen, even with the naked eye,
rising up as if from the black disk of the moon.

The prominences seem to be of two forms, the eruptive
and the cloud-hke. The first rise from the sun like im-
mense sheets of flame ; the latter seem to be at rest above
it, like clouds floating in the air. But there is no air
around the sun for these objects to float in, and we can-
not certainly say what supports them. Very likely, how-
ever, it is a repulsive force of the sun's rays, which will
be mentioned in a later chapter.

Spectrum analysis shows that these prominences are
composed mostly of hydrogen gas, mixed with the va-
pours of calcium and magnesium. It is to the hydrogen
that they owe their red colour. Continued study of the
prominences shows them to be connected with a thin layer
of gases which surrounds and rests upon the photosphere.
This layer is called the chromosphere, from its deep red
colour, similar to that of the prominences. As in the case
of the latter, most of its light seems to be that of hydro-
gen ; but it contains many other substances in seemingly
varying proportions.

The last appendage of the sun to be considered is the
corona. This is seen only during total eclipses as a soft
effulgence surrounding the sun, and extending from it in
long rays, sometimes exceeding the diameter of the sun
in length. Its exact nature is still in doubt. It will be
described in the chapter on eclipses,

HOW THE SUN IS MADE UP 101

How the Sun is Made Up

Let us now recapitulate what makes up the sun as we
see and know it.

We have first the vast interior of the globe which, of
course, we can never see.

What we see when we look at the sun is the shining
surface of this globe, the photosphere. It is not a real
surface, but more likely a gaseous layer several hundred
miles deep which we cannot distinguish from a surface.
This layer is variegated by spots, and in or over it rise
the faculaj.

On the top of the photosphere rests the layer of gases
called the chromosphere, which can be observed at any
time with a powerful spectroscope, but can be seen by
direct vision only during total eclipses.

Through or from the red chromosphere are thrown up
the equally red flames called the prominences.

Surrounding the whole is the corona.

Such is the sun as we see it. What can we say about
what it really is ? First, is it solid, liquid, or gaseous ?

That it is not solid we have already shown by the law
of rotation. It cannot be a liquid like molten metal, be-
cause it sends off from its surface such a flood of heat as
would cool off and solidify molten metal in a very short
time. For more than thirty years it has been understood
that the interior of the sun must be a mass of gas, com-
pressed to the density of a liquid by the enormous pres-
sure of its superincumbent portions. But it was still sup-
posed that the photosphere might be in the nature of a

102 THE SUN, EARTH, AND MOON

crust and the whole sun like an immense bubble. This
view, however, seems no longer tenable. It does not seem
likely that there is any solid matter on the sun.

Attempts have sometimes been made to learn the tem-
perature of the photosphere. It probably exceeds any
that we can produce on earth, even that of the electric
furnace, else how could calcium, the metallic base of lime,
one of the most refractory of substances, exist there in
a state of vapour.? We all know that the air around us
becomes cooler and rarer as we ascend above the surface
of the earth, owing to the action of gravity and the con-
sequent weight of the atmosphere, which gives rise to a
constantly increasing pressure as we descend.' Now,
gravity at the sun is twenty-seven times as powerful as
on the earth. Hence, going downward, temperature and
pressure increase at a far more rapid rate on the sun
than on the earth. Even in the photosphere the tempera-
ture is such that "the elements melt with fervent heat."
And, as we go below the surface, the heat must increase
by hundreds of degrees for every mile that we descend.
The result is that in the interior the gases of the sun are
subjected to two opposing forces which grow more and
more intense. These are the expansive force of the heat
and the compressing force of the gases above, produced
by the enormous force of gravity of the sun.

The forces thus set in play merely in the outer portions
of the sun's globe are simply inconceivable. Perhaps
the explosion of the powder when a thirteen-inch cannon
is fired is as striking an example of the force of ignited
gases as we are familiar with. Now suppose every foot

THE SUN'S HEAT 103

of space in a whole county covered with such cannon, all
pointed upward and all being discharged at once. The
result would compare with what is going on inside the
photosphere about as a boy's popgun compares with the

The Source of the Sun's Heat

Perhaps, from a practical point of view, the most com-
prehensive and important problem, of science is : How is
the sun's heat kept up? Before the laws of heat were
fully apprehended this question was not supposed to offer
any difficulties. Even to this day it is supposed by those
not acquainted with the subject, that the heat which we
receive from the sun may arise in some way from the pas-
sage of its rays through our atmosphere, and that, as a
matter of fact, the sun may not radiate any actual heat
at all — may not be an extremely hot body. But, modern
science shows that heat cannot be produced except by
the expenditure of some form of energy. The energy of
the sun is necessarily limited in quantity and is continu-
ally being lost through radiation.

It is very easy to imagine the sun as being something
like a white-hot cannon ball, which is cooling off by send-
ing its heat in all directions, as such a ball does. We
know by actual observation how much heat the sun sends
to us. It may be expressed in the following way :

Imagine a shallow basin with a flat bottom, and a
depth of one centimetre, that is, about four tenths of an
inch. Let the basin be filled with water, the latter then
being one centimetre deep. Expose such a basin to the

104 THE SUN, EARTH, AND MOON

rays of the vertical sun. The heat which the sun wUl
radiate to them will be sufficient to warm the water about
three and a half or four degrees Centigrade, or not very-
far from seven degrees Fahrenheit, in one minute. It
follows that if we suppose a thin spherical shell of water,
one centimetre thick, of the same radius as the earth's
orbit, and having the sun in its centre, that shell of water
will be heated with the rapidity just mentioned. The
heat which it receives will be the total amount radiated
by the sun. We can thus define how much heat the sun
loses every minute, day and year.

A very simple calculation will show that if the sun
were of the nature of a white-hot ball it would cool off so
rapidly that its heat could not last more than a few cen-
turies. But it has in all probability lasted milUons of
years. Whence, then, comes the supply? The answer
of modern science to this question is that the heat radi-
ated from the sun is supplied by the contraction of size
as heat is lost. We all know that in many cases when mo-
tion is destroyed heat is produced. When a cannon shot
is fired at the armour plate of a ship of war, the mere
stroke of the shot makes both plate and shot hot. The
blacksmith can make iron hot by hammering it.

These facts have been generalized into the statement
that whenever a body falls and is stopped in its fall by
friction, or by a stroke of any sort, heat is produced.
From the law governing the case, we know that the water
of Niagara, after it strikes the bottom of the falls, must
be about one quarter of a degree warmer than it was
during the fall. We also know that a hot body contracts

THE SUN'S HEAT 105

in volume when cooled. The contraction of a gaseous
body, such as we believe the sun to be, is greater than
that of a solid or liquid. The heat of the sun is radiated
from streams of matter constantly rising from the in-
terior, which radiate their heat when they reach the sur-
face. Being cooled they fall back again, and the heat
caused by this fall is what keeps the sun hot.

It may seem almost impossible that heat sufficient to
last for millions of years could be generated in this way ;
but the known force of gravity at the surface of the sun
enables us to make exact computations on the subject.
It is thus found that in order to keep up the supply of
heat it is only necessary that the diameter of the sun
should contract about a mile in twenty-five years — or
four miles in a century. This amount would not be per-
ceptible until after thousands of years. Yet the process
of contraction must come to an end some time. There-
fore, if this view is correct, the life of the sun must have
a limit. What its limit may be we cannot say with exact-
ness, we only know that it is several millions of years, but
not many millions.

The same theory implies that the sun was larger in
former times than it is now, and must have been larger
and larger every year that we go back into its history.
There was a time when it must have been as large as the
whole solar system. In this case it could have been
nothing but a nebula. We thus have the theory that the
sun and solar system have resulted from the contraction
of a nebula — through millions of years. This view is
familiarly known as the nebular hypothesis.

106 THE SUN, EARTH, AND MOON

The question whether the nebular hypothesis is to be
accepted as a proved result of science is one on which
opinions differ. There are many facts which support it
— such as the interior heat of the earth and the revolu-
tion and rotation of the planets all in the same direction.
But cautious and conservative minds will want some fur-
ther proof of the theory before they regard it as abso-
lutely estabhshed. Even if we accept it, we still have
open the question: How did the nebula itself originate,
and how did it begin to contract.'' This brings us to the
boundary where science can propound a question but
cannot answer it.

Ill

The Earth

The globe on which we live, being one of the planets,
would be entitled to a place among the heavenly bodies
even if it had no other claims on our attention. Insig-
nificant though it is in size when compared with the great
bodies of the universe, or even with the four giant planets
of our system, it is the largest of the group to which it
belongs. Of the rank which it might claim as the abode
of man we need not speak.

What is the earth.'' We may describe it in the most
comprehensive way as a globe of matter nearly eight
thousand miles in diameter, bound together by the mu-
tual gravitation of its parts. We all know that it is not
exactly spherical, but bulges out very slightly at the
equator. The problem of determining its exact shape
and size is an extremely diiBcult one, and we cannot say
that an entirely satisfactory result is yet reached. The
difficulty is obvious enough. There is no way of measur-
ing distances across the great oceans. The measurements
are necessarily limited to such islands as are visible from
the coasts of the continents or from each other. Of
course, the measures cannot be extended to either pole.
The size and shape must therefore be inferred from the
measures across or along the continents. Owing to the
importance of such work, the leading nations have from

108 THE SUN, EARTH, AND MOON

time to time entered into it. Quite recently our Coast
and Geodetic Survey has completed the measurement of a
line of triangles extending from the Atlantic to the
Pacific Oceans. North and south measurements both on
the Atlantic and Pacific coasts have been executed or are
in progress. The English have from time to time made
measures of the same sort in Africa, and the Russians
and Germans on their respective territories. Nearly all
these measures are now being combined in a work carried
on by the International Geodetic Association, of which
the geodetic authorities of the principal countries are
members.

The latest conclusions on the subject may be summed
up thus. We remark in the first place that by the
figure of the earth geodetists do not mean the figure
of the continents, but of the ocean level as it would
be if canals admitting the water of the oceans were
dug through the continents. The earth thus defined is
approximately an ellipsoid, of which the smaller diameter
/ is that through the poles, and which has about the
following dimensions:

Polar diameter, 7,899.6 miles, or 12,713.0 kilometres.

Equatorial " 7,926.6 miles, or 12,756.5 kilometres.

It will be seen that the equatorial diameter is twenty-
seven miles or forty-three kilometres greater than the
polar.

The Earth's Interior

What we know of the earth by direct observation is
confined almost entirely to its surface. The greatest
depth to which man has ever been able to penetrate com-

THE EARTH'S INTERIOR 109

pares with the size of the globe only as the skin of an
apple does to the body of the fruit itself.

I shall first invite the reader's attention to some facts
about weight, pressure, and gravity in the earth. Let
us consider a cubic foot of soil forming part of the outer
surface of the earth. This upper cubic foot presses upon
its bottom with its own weight, perhaps one hundred and
fifty pounds. The cubic foot below it weighs an equal
amount, and therefore presses on its bottom with a force
equal to its own weight with the weight of the other foot
added to it. This continual increase of pressure goes on
as we descend. Every square foot in the earth's interior
sustains a pressure equal to the weight of a column of
the earth a foot square extending to the surface. Not
many yards below the surface this pressure will be meas-
ured in tons ; at the depth of a mile it may be thirty or
forty tons; at the depth of one hundred miles, thou-
sands of tons ; continually increasing to the centre. Un-
der this enormous pressure the matter composing the
inner portion of the earth is compressed to the density of
a metal. By a process which we will hereafter describe,
the mean density of the earth is known to be five and one
half times that of water, while the superficial density, is
only two or three times that of water.

One of the most remarkable facts about the earth is
that the temperature continually increases as we pene-
trate below the surface in deep mines. The rate of in-
crease is diff^erent in different latitudes and regions. The
general average is one degree Fahrenheit in fifty or sixty
feet.

110 THE SUN, EARTH, AND MOON

The first question to suggest itself is, how far toward
the earth's centre does this increase of temperature ex-
tend? The most that we can say is that it cannot be
merely superficial, because, in that case, the exterior por-
tions would have cooled off long ago, so that we should
have no considerable increase of heat as we went down.
The fact that the heat has been kept up during the whole
of the earth's existence shows that it must still be very
intense toward the centre, and that the rate of increase
near the surface must go on for many miles into the
interior.

At this rate the material of the earth would be red hot
at a depth of ten or fifteen miles, while at one or two
hundred miles the heat would be sufficient to melt all the
substances which form the earth's crust. This fact sug-
gested to geologists the idea that our globe is really a
molten mass, like a mass of melted iron, covered by a cool
crust a few miles thick, on which we dwell. The exist-
ence of volcanoes and the occurrence of earthquakes
gave additional weight to this view, as did also other
geological evidence, showing changes in the earth's
surface which appeared to be the result of a liquid
interior.

But in recent years the astronomer and physicist have
collected evidence, which is as conclusive as such evidence
can be, that the earth is solid from centre to surface, and
even more rigid than a similar mass of steel. The sub-
ject was first developed most fully by Lord Kelvin, who
showed that, if the earth were a fluid, surrounded by a
crust, the action of the moon would not cause tides in the

EARTH'S GRAVITY AND DENSITY 111

ocean, but would merely tend to stretch out the entire
earth in the direction of the moon, leaving the relative
positions of the crust and the water unchanged.

Equally conclusive is the curious phenomenon which
we shall describe presently of the variation of latitudes
on the earth's surface. Not only a globe of which the
interior is soft, but even a globe no more rigid than steel
could not rotate as the earth does.

How, then, are we to reconcile the enormous tempera-
ture and the solidity? There seems to be only one solu-
tion possible. The matter of the interior of the earth
is kept solid by the enormous pressure. It is found ex-
perimentally that when masses of matter like the rocks
of the earth are raised to the melting point, and then
subjected to heavy pressure, the effect of the pressure is
to make them solid again. Thus, as we increase the tem-
perature we have only to increase the pressure also to
keep the material of the earth solid. And thus it is that,
as we descend into the earth, the increase of pressure
more than keeps pace with the rise of temperature, and
thus keeps the whole mass solid.

Gravity and Density of the Earth

Another interesting question connected with the earth
is that of its density, or specific gravity. We all know
that a lump of lead is heavier than an equal lump of iron,
and the latter heavier than an equal lump of wood. Is
there any way of determining what a cubic foot of earth
would weigh if taken out from a great depth of its vast
interior.'' If there is, then we can determine what the

112 THE SUN, EARTH, AND MOON

actual weight of the whole earth is. The solution de-
pends on the gravitation of matter.

Every child is familiar with gravitation from the time
it begins to walk, but the profoundest philosopher knows
nothing of its cause, and science has not discovered any-
thing respecting it except a few general facts. The
widest and most general of these facts, which may be said
to include the whole subject, is Sir Isaac Newton's theory
of gravitation. According to this theory, the mysterious
force by which all bodies on the surface of the earth tend
to fall toward its centre does not reside merely in the
centre of the earth, but is due to an attraction exerted
by every particle of matter composing our globe.
Whether this was the case was at first an open question.
Even so great a philosopher and physicist as Huyghens
believed that the power resided in the earth's centre, and
not in every particle, as Newton supposed. But the lat-
ter extended his theory yet farther by showing that every
particle of matter in the universe, so far as we have yet
ascertained, attracts every other particle with a force
that diminishes as the square of the distance increases.
This means that at twice the distance the attraction will
be divided by four ; at three times by nine ; at four times
by sixteen, and so on.

Granting this, it follows that all objects around us
have their own gravitating power, and the question
arises : Can we show this power by experiment, and meas-
ure its amount .'' The mathematical theory shows that
globes should attract small bodies at their surfaces with
B force proportJODed to their diameter. A globe two

ATTRACTION OF THE EARTH 113

feet in diameter, of the same specific gravity as the earth,
should attract with a force one twenty-miUionth of the
earth's gravity.

In recent times several physicists have succeeded in
measuring the attraction of globes of lead having a
diameter of a foot, more or less. This measurement is
the most delicate and difficult that has ever been made,
and the accuracy which seems to have been reached would
have been incredible a few j^ears ago. The apparatus
used is, in its principle, of the simplest kind. A very
light horizontal rod is suspended at its centre by a thread
of the finest and most flexible material that can be ob-
tained. This rod is balanced by having a small ball at-
tached to each end. What is measured is the attraction
of the globes of lead upon these two balls. The former
are placed in such a position as to unite their attraction
in giving the rod a slight twisting motion in the horizon-
tal plane. To appreciate the difficulties of the case, we
must call to mind that the attraction may not amount to
the ten-millionth part of the weight of the little balls.
It would be difficult to find any object so light that its
weight would not exceed this force. To compare the
weight of a fly with it would be like comparing the
weight of an ox with that of a dose of medicine. Not
only the weight of a mosquito but even of its finest limb
might exceed the quantity to be measured. If a mosquito
were placed under a microscope an expert operator could
cut off' from one antenna a piece small enough to express
the force measured. /

Yet the determination of this force has been made with

114. THE SUN, EARTH, AND MOON

such precision that the results of the two latest investiga-
tors do not differ by a thousandth part. These were
Professor Boys, F.R.S., of Oxford, England, and
Dr. Karl Braun, S.J., of Marienschein, in Bohemia.
They worked independently at the problem, meeting
and overcoming innumerable difficulties one after another,
getting greater and greater delicacy and precision in
their apparatus, and finally published their results al-
most at the same time, the one in England, the other in
Austria. The outcome of their experiments is that the
mean density of the earth is slightly more than five and
a half times that of water. This is a little less than the
density of iron, but much more than that of any or-
dinary stone. As the mean density of the materials
which compose the earth's crust is scarcely more than
one half of this amount, it follows that near the centre
the matter composing the earth must be compressed to
a density not only far exceeding that of iron, but prob-
ably that of lead.

The attraction of mountains has been known for more
Ihan a hundred years. It was first demonstrated by
Maskelyne about 1775 in the case of Mount Schehallion,
in Scotland. In all mountain regions where very accu-
rate surveys are made the attraction of mountains upon
the plumb line is very evident.

Variations of Latitude
We know that the earth rotates on an axis passing
through the centre and intersecting the earth's surface at
either pole. If we imagine ourselves standing exactly

VARIATIONS OF LATITUDE 115

on a pole of the earth, with a flagstaff fastened In the
ground, we should be carried round the flagstaff by the
earth's rotation once in twenty-four hours. We should
become aware of the motion by seeing the sun and stars
apparently moving in the opposite direction in horizontal
circles by virtue of the diurnal motion. Now, the great
discovery of the variation of latitude is this : The point
in which the axis of rotation intersects the surface is not
fixed, but moves around in a somewhat variable and ir-
regular curve, contained within a circle nearly sixty feet
in diameter. That is to say, if standing at the north
pole we should observe its position day by day, we should
find it moving one, two, or three inches every day, de-
scribing in the course of time a curve around one central
point, from which it would sometimes be farther away
and sometimes nearer. It would make a complete revolu-
tion in this irregular way in about fourteen months.

Since we have never been at the pole, the question
may arise : How is this known ? The answer is that by
astronomical observations we can, on any night, deter-
mine the exact angle between the plumb line at the place
where we stand and the axis on which the earth is rota-
ting on that particular day. Four or five stations for
making these observations were established around the
earth in 1900 by the International Geodetic Association.
One of these stations is near Gaithersburg, Md., another
is on the Pacific coast, a third is in Japan, and a fourth
in Italy. Before these were established observations
having the same object were made in various parts of
Europe and America. The two most important stations

116 THE SUN, EARTH, AND MOON

in the latter region were those of Professor Rees of Co-
lumbia University, New York, and of Professor Doolittle,
first at Lehigh, and later at the Flower Observatory,
near Philadelphia.

The variation which we have described was originally
demonstrated by S. C. Chandler, of Cambridge, in 1890
by means of a great mass of astronomical observations
not made for this special purpose. Since then investi-
gation has been going on with the view of determining
the exact curve described. What has been shown thus
far is that the variation is much wider some years than
others, being quite considerable in 1891, and very small
in 1894. It appears that in the course of seven years
there wiU be one in which the pole describes the greater
part of a comparatively wide circle, while three or four
years later it will for several months scarcely move from
its central position.

If the earth were composed of a fluid, or even of a
substance which would bend no more than the hardest
steel, such a motion of the axis as this would be impossi-
ble. Our globe must therefore, in the general average,
be more rigid than steel.

The Atmosphere

The atmosphere is astronomically, as well as physic-
alljs a most important appendage of the earth. Neces-
sary though it is to our life it constitutes one of the
greatest obstructions with which the astronomer has to
deal. It absorbs more or less of all the light that passes
through it, and thus slightly changes the colour of the

THE ATMOSPHERE 117

heavenly objects as we see them, and renders them some-
what dimmer, even in the clearest sky. It also refracts
the Kght passing through it, causing it to describe a
slightly curved line, concave toward the earth, instead of
passing straight to the astronomer's eye. The result of
this is that the stars appear slightly higher above the
horizon than they actually are. The light coming directly
down from a star in the zenith suffers no refraction. The
latter increases as the star is farther from the zenith,
but even forty-five degrees away it is only one minute
of arc, about the smallest amount that the unaided eye
can plainly perceive ; yet this is a very important quan-
tity to the astronomer. The nearer the object is to the
horizon the greater the rate at which the refraction in-
creases ; twenty-eight degrees above the horizon it is
about twice as great as at forty-five degrees ; at the hori-
zon it is more than one half a degree, that is more than
the whole diameter of the sun or moon. The result is that
when we see the sun just about to touch the horizon at
sunset or sunrise its whole body is in reality below the
horizon. We see it only in consequence of the refraction
of its light. Another result of the rapid increase near
the horizon is that, in this position, the sun looks decid-
edly flattened to the eye, its vertical diameter being
shorter than the horizontal one. Anyone may notice
this who has an opportunity to look at the sun as it is
setting in the ocean. It arises from the fact that the
lower edge of the sun is refracted more than the upper
edge.

When the sun sets in the ocean in the clear air of the

118 THE SUN, EARTH, AND MOON

tropics a beautiful effect may be noticed, which can
rarely or never be seen in the thicker air of our latitudes.
It arises from the unequal refraction of the rays.of light
by the atmosphere. Like a prism of glass the atmos-
phere refracts the red rays the least and the successive
spectral colours, yellow, green, blue, and violet, more
and more. The result is that, as the edge of the sun is
disappearing in the ocean, these successive rays are lost
sight of in the same order. Two or three seconds before
the sun has disappeared, the little spark of its Hmb
which still remains visible is seen to change colour and
rapidly grow paler. This tint changes to green and
blue, and finally the last glimpse which we see is that of
a disappearing flash of blue or violet light.

IV

The Moon

About one hundred years ago there was an unpopular
professor in the Government Polytechnique School of
Paris, still the great school of mathematics for the
French public service, who loved to get his students into
difficulties. One morning he addressed one of them the
question :

"Monsieur, have you ever seen the moon?"

"No, sir," replied the student, suspecting a trap.

The professor was nonplussed. "Gentlemen," said he,

"see Mr. , who professes never to have seen the

moon !"

The class all smiled.

"I admit that I have heard it spoken of," said the
student, "but I have never seen it."

I take it for granted that the reader has been more
observant than the French student professed to be, and
that he has not only seen the moon, but knows the phases
through which it goes and is familiar with the fact that
it describes a monthly course around the earth. I also
suppose that he knows the moon to be a globe, although,
to the naked eye, it seems like a flat disk. The globular
form is, however, very evident when we look at it with a
small telescope.

Various methods and systems of measurement all agree

120 THE SUN, EARTH, AND MOON

in placing the moon at an average distance of a little
less than two hundred and forty thousand miles. This
distance is obtained by direct measure of the paral-
lax, as will be explained hereafter, and also by calcula-
ting how far off the moon must be in order that, being
proj ectcd into space, it may describe an orbit around the
earth in the time that it actually does perform its
round. The orbit is elhptic, so that the actual distance
varies. Sometimes it is ten or fifteen thousand miles less,
at other times as much more, than the average.

The diameter of the moon's globe is a little more than
one fourth that of the earth ; more exactly, it is two
thousand one hundred and sixty miles. The most careful
measures show no deviation from the globular form
except that the surface is very irregular.

Revolution and Phases of the Moon

The moon accompanies the earth in its revolution
round the sun. To some the combination of the two
motions seems a Httle complex ; but it need not offer any
real difficulty. Imagine a chair standing in the centre
of a railway car in rapid motion, while a person is walk-
ing around it at a distance of three feet. He can go
round and round without varying his distance from the
chair and without any difficulty arising from the motion
of the car. Thus the earth moves forward in its orbit,
and the moon continually revolves around it without
greatly varying its distance from us.

The actual time of the moon's revolution around the
earth is twenty-seven days eight hours; but the time

MOON'S REVOLUTION AND PHASES 121

from one new moon to another is twenty-nine days thir-
teen hours. The difference arises from the earth's mo-
tion around the sun ; or, which amounts to the same thing,
the apparent motion of the sun along the echptic. To

V

/
/

/

I

I

I

i

SUN

\

\

T

\

\

\
\

I \

I »

/

\ I

V /

I \ I-

«?* laMOON

M

\-Jt

/ \ / W

kE / tat^--

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■-- ■-■■ ■■-- ./

Fig. 19. — Sevolution of the Moon Round the Earth.

show this, let AC be a small arc of the earth's orbit
around the sun. Suppose that at a certain time the earth
is at the point E, and the moon at the point M, between
the earth and the sun. At the .end of twenty-seven days
eight hours the earth will have moved from E to F.

122 THE SUN, EARTH, AND MOON

While the earth is making, this motion the moon will have
moved around the orbit in the direction of the arrows,
so as to have reached the point N. At the moment when
the hnes EM and FN are parallel to each other, the moon
will have completed her actual revolution, and will seem
to be in the same place among the stars as before. But
the sun is now in the direction FS. The moon therefore
has to continue its motion before it catches up to the sun.
This requires a little more than two days, and makes
the whole time between two new moons twenty-nine and
a half days.

The varying phases of the moon depend upon its
position with respect to the sun. Being an opaque globe,
without light of its own, we see it only as the light of
the sun illuminates it. When it is between us and the
sun its dark hemisphere is turned toward us, and it is
entirely invisible. The time of this position in the
almanacs is called "new moon," but we cannot commonly
see the moon for nearly two days after this time, because
it is lost in the bright twilight of evening. On the second
and third day, however, we see a small portion of the
illuminated globe, having the familiar form of a thin
crescent. This crescent we commonly call the new moon,
although the time given in the almanac is several days
earlier.

In this position, and for several days longer, we may,
if the sky is clear, see the entire face of the moon, the
dark parts shining with a faint gray light. This light
is that which is reflected from the earth to the moon.
An inhabitant of the moon, if there were such, would

SURFACE OF THE MOON 123

then see the earth in the sky like a full moon, looking
much larger than the moon looks to us. As the moon
advances in its orbit day after day, this light diminishes,
and about the time of first quarter disappears from our
sight owing to the brightness of the illuminated portion
of the moon.

Seven or eight days after the almanac time of new
moon, the moon reaches its first quarter. We then see
half of the illuminated disk. During the week following,
the moon has the form called gibbous. At the end of the
second week the moon is opposite the sun, and we see its
entire hemisphere hke a round disk. This we call full
moon. During the remainder of its course the phases
recur in reverse order, as we all know.

We might regard all these recurrences as too well
known to need description, yet, in the Ancient Mariner,
a star is described as seen between the two horns of the
moon as though there were no dark body there to inter-
cept our view of the star. Probably more than one poet
has described the new moon as seen in the eastern sky,
or the evening full moon as seen in the west.

The Surface of the Moon

We can see with the naked eye that the moon's surface
is variegated by bright and dark regions. The latter
are sometimes conceived to have a vague resemblance to
the human face, the nose and eyes being especially prom-
inent. Hence the "man in the moon." Through even
the smallest telescopes we see that the surface has an im-
mense variety of detail ; and the more powerful the tele-

124 THE SUN, EARTH, AND MOON

A ■'

w^':>:',i :Y ....

Fig. 20. — Mountainous Surface of the Moon.

SURFACE OF THE MOON 125

scope the more details we see. The first thing to strike
us on a telescopic examination will be the elevations, or
mountains as they are commonly called. These are best
seen about the time of the first quarter, because they then
cast shadows. At full moon they cannot be so well made
out, because we are looking straight down and see every-
thing illuminated. Although these elevations and de-
pressions are called mountains they are different in
form from the ordinary mountains of the earth.
There is, however, e.n almost exact resemblance be-
tween them and the craters of our great volcanoes.
A very common form is that of a circular fort, one
or more miles in diameter, with walls which may be
thousands of feet high. The inside of this fort may
be saucer shaped, a large portion of the surface being
flat. At first quarter we can see the shadow of the walls
cast upon the interior flat surface. In the centre a little
cone is frequently seen. The interior surface is by no
means perfectly flat and smooth. The higher power the
more details we shall see. Just what these consist of it is
impossible to say ; they may be solid rock or they may be
piles of loose stone. As we can see no object on the moon,
even with the most powerful telescope, unless it is more
than a hundred feet in diameter, we cannot say what the
exact nature of the surface is in its minutest portions.

The early observers with the telescope supposed that
the dark portions were seas and the brighter portions
continents. This notion was founded on the fact that the
darker portions looked smoother than the others. Names
were therefore given to these supposed oceans, such

126 THE SUN, EARTH, AND MOON

as Mare Procellarum, the Sea of Storms ; Mare Serenita-
tis, the Sea of Calms, etc. These names, fanciful though
they be, are still retained to designate the large dark
regions on the moon. A very shght improvement in the
telescope, however, showed that the idea of these dark
regions being oceans was an illusion. They are all cov-
ered with inequalities, proving that they must be com-
posed of sohd matter. The difference of aspect arises
from the lighter or darker shade of the materials which
compose the lunar surface. These are distributed over
the surface of the moon in a very curious way. One of
the most remarkable features is the long bright Unes
which radiate from certain points on the moon. A very
low telescopic power will show the most remarkable of
these ; a good eye might even perceive it without a tele-
scope. On the southern part of the moon's hemisphere,
as we see it, is a large spot or region known as Tycho,
and from this radiate a number of these bright streaks.
The appearance is as if the moon had been cracked and
the cracks filled up with melted white matter.

Whether we accept this view or not, it is impossible to
examine the surface of the moon without the conviction
that in some former age it was the seat of great volcanic
activity. In the centre of all the great circular moun-
tains we have described are craters which, it would seem,
must have been those of volcanoes. Indeed, a hundred
years ago it was supposed by Sir William Herschel that
there was an active volcano on the moon, but it is now
known that this appearance is due to the light of the
earth reflected from a very bright spot on the moon's

AIR OR WATER ON THE MOON? 127

surface. It can be easily seen about the time of the new
moon with a telescope of moderate size.

Is there Air or Water on the Moon?

One of the most important questions connected with
the moon is whether there is any air or water on its sur-
face. To these the answer of science up to the present
time is in the negative. Of course this does not mean
that there can absolutely not be a drop of moisture nor
the smallest trace of an atmosphere on our satellite ; all
we can say is that if any atmosphere surrounds the moon
it is so rare that we have never been able to get any evi-
dence of its existence. If the latter had such an append-
age of even one hundredth of the density of the earth's
atmosphere, its existence would be made known to us by
refraction of the Kght from a star seen alongside the
moon. But not the slightest trace of any such refrac-
tion can be discovered. If there is any such liquid as
water, it must be concealed in invisible crevices, or dif-
fused through the interior. Were there any large sheets
of water in the equatorial regions they would reflect the
light of the sun day by day, and would thus become
clearly visible. The water would also evaporate and form
more or less of an atmosphere of watery vapour.

All this seems to settle another important question;
namely, that of the habitability of the moon. Life, in
the form in which it exists on our earth, requires water
at least for its support, and in all its higher forms air
also. We can hardly conceive of a living thing made of
mere sand or other dry matter such as forms the lunar

128 THE SUN, EARTH, AND MOON

surface. If we supposed animals to walk about on the
moon, it is difficult to imagine what they could eat. Our
general conclusion must be that there is no life on the
moon subject to the laws which govern life on the surface
of this earth.

The total absence of air and water results in a state of
things on the moon such as we never experience on the
earth. So far as can be ascertained by the most careful
examination, not the slightest change ever takes place
on its surface. A stone lying on the surface of the earth
is continually attacked by the weather and in the course
of years is gradually disintegrated or washed away by
the wind and water. But there is no weather on the moon,
and a stone lying on its surface might rest there for un-
known ages undisturbed by any cause whatever. The
lunar surface is heated up when the sun shines on it and
it cools oif when the sun has set. Except for these
changes of temperature there is absolutely nothing going
on over the whole surface of the moon, so far as we can
see. A world which has no weather and on which nothing
ever happens — such is the moon.

Rotation of the Moon

The rotation of the moon on its axis is a subject on
which some are frequently so perplexed that we shall
explain it. Anyone who has carefully examined this
body knows that it always presents the same face to us.
This shows that it rotates on its axis in the same time
that it revolves around the earth. An idea frequently
entertained is that this shows tha,t it does not rotate at all,

HOW THE MOON PRODUCES TIDES 1£9

and many chapters have been written on this subject.
The whole difficulty arises from the different ideas which
people have of motion. In physics we say that a body
does not rotate when, if a rod were passed through it,
that rod always maintained the same direction when the
body moved about.
Now let us sup-
pose such a rod
passed through the
moon ; then, if the
latter did not ro-
tate on its axis the
rod would main-
tain its same direc-
tion while the
moon, revolving
around the earth,
would appear at
different points in
its orbit as we see it in Figure 21. A very little study
of this figure will show that as the moon went around
we should successively see every part of its surface
in succession if it did not rotate on its axis.

-A

How the Moon Produces the Tides

All of us who live on the seashore know that there is
a rise and fall of the ocean which in the general average
occurs about three quarters of an hour later every day,
and which keeps pace with the apparent diurnal motion
of the moQn. That is to say, if it is high tide to-day when

Fig. 21. — Showing how the Moon would Move if
it did 7iot Kotate on its Axis,

130 THE SUN, EARTH, AND MOON

the moon is in a certain position in the heavens, it will be
high tide when the moon is in or near that position day
after day, month after month, and year after year. We
have all heard that the moon produces these tides by its
attraction on the ocean. We readily understand that
when the moon is above any region its attraction tends
to raise the waters in that region; but the circumstar.ce
that most perplexes those who are not expert in the sub-
ject is that there are two tides a day, high tide occurring
not only under the moon, but on the side of the earth
opposite the moon. The explanation of this is that the
moon really attracts the earth itself as well as it does
the water. It continually draws the entire earth and
everything upon it toward itself. As it goes round the
earth in its monthly course, it thus keeps up a continual
motion of the latter. If it attracted every part of the
earth equally, the ocean included, there would then be
no tides, and everything would go on on the earth's sur-
face as if there were no attraction at all. But as the
attraction is as the inverse square of the distance, the
moon attracts the regions of the earth and oceans which
are nearest to it more than the average, and those that
are farthest from it less than the average.

To show the effect of these changes let A, C, and H be
the three points on the earth attracted by the moon.
Since the moon attracts C more than A, it tends to puU
C away from A and increase the distance between A and
C. At the same time pulling H more than C it tends to
increase the distance between H and C. If the whole earth
was a fluid, the attraction of the moon would be simply to

HOW THE MOON PRODUCES TIDES 191

draw this fluid out into the form of an ellipsoid, of which
the long diameter would be turned toward the moon. But
the earth itself, being solid, cannot be drawn out into this
shape, while the ocean, being fluid, is thus drawn out.
The result is that we have high tides at the two ends of
the ellipse into which the ocean is drawn, and low tides
in the mid-region.

The complete explanation of the subject requires a
statement of the laws of motion which cannot be made

MOON
.LIN.E-P-f:..PU-L.L„ „-../r\

Fig. 22. — Horn tlie Maoris Full on the Earth and Ocean Produces Two
Tides in a Day,

here. I will, however, remark that if the attraction of
the moon on the earth were always in the same direction,
the two bodies would be drawn together in a few days.
But owing to the revolution of the moon round the earth
the direction of the pull is always changing, so that the
earth is, in the course of a month, only drawn about
three thousand miles from its mean position by the
moon's pull.

It might be supposed that if the moon produces the
tides in this way we should always have high tide when
the moon is on the meridian and low tide when the moon
is in the horizon. But such is not the case, for two rea-
sons. In the first place it takes time for the moon to draw

132 THE SUN, EARTH, AND MOON

the waters out into the form of an eUipsoid, and when
it once gives them the motion necessary to keep this form,
that motion keeps up after the moon has passed the
meridian, just as a stone continues to rise after it has left
the hand or a wave goes forward by the momentum of
the water. The other cause is found in the interruption
of the motion by the great continents. The tidal wave,
as it is called, meeting a continent, spreads out in one
direction or the other, according to the lay of the land,
and may be a long time in passing from one point to
another. Thus arise all sorts of irregularities in the
tides when we compare those in different places.

The sun produces a tide as well as the moon, but a
smaller one. At the times of new and full moon the
two bodies unite their forces and cause the highest and
lowest tides. These are familiar to all dwellers on the
seacoast and are called spring tides. About the time
of the first and last quarters the attraction of the sun
opposes that of the moon and the tides do not rise so
high or fall so low, and these are called neap tides.

V
Eclipses op the Moon

The reader is doubtless aware that an eclipse of the
moon is caused by that body entering the shadow of the
earth, and that an ecKpse of the sun is caused by the
moon passing between us and the sun. Taking this
knowledge for granted, we shall explain the more inter-
esting features of these phenomena and the laws of their
recurrence.

The first question to be considered is: Why is there
not an eclipse of the moon at every full moon, since the
earth's shadow must always be in its place opposite the

" — -|- I

C2:-

Fig. 23. — Tlie Moon in the Shadow of the Earth.

sun? The answer i's that the moon commonly passes
either above or below the shadow of the earth, and so fails
to be eclipsed. This, again, arises from the fact that
the orbit of the moon has a small inclination, about five
degrees, to the plane of the ecKptic, in which the earth
moves, and in which the centre of the shadow always lies.
Returning to our former thought of the ecliptic being

134. THE SUN, EARTH, AND MOON

marked out on the celestial sphere, let us suppose that
we also mark out the orbit of the moon during the course
of its monthly period. We should then find the orbit of
the moon crossing that of the sun in two opposite points,
at the very small angle of five degrees. These points of
crossing are called nodes. At one node the moon passes
from below, or south of the ecliptic, to the north of it.
This is called the ascending node. At the other the
moon passes from north to south of the ecliptic. This is
called the descending node. The terms ascending and
descending are applied to the node, because to us in the
northern hemisphere, the north side of the ecliptic and
equator seem to be above the south side.

At the points halfway between the nodes the centre
of the moon is above the ecliptic by about one twelfth its
distance from us, that is, by about twenty thousand miles.
The sun being larger than the earth, the shadow of the
latter gradually grows smaller away from the earth. At
the distance of the moon its diameter is about three
fourths that of the earth, that is about six thousand
miles. Its centre being in the plane of the ecliptic, it
extends only about three thousand miles above and below
that plane. Hence it is that the moon will pass through
it only when near the nodes.

Eclipse Seasons

The line joining the sun and moon of course turns
round as the earth moves around the sun. It therefore
crosses the moon's nodes twice in the course of a year.
That is to say if we suppose the nodes to be marked in the

ECLIPSE SEASONS 135

sky, the ascending node at one point, and the descending
node at the opposite point, then the sun will appear to
us to pass each of these points in the course of a year.
While the sun is passing one node the shadow of the
earth will seem to be passing the other. It is only near
these two times of the year that an eclipse of the sun or
moon can occur. We may therefore call them eclipse
seasons. They commonly last about a month; that is
to say it is generally about a month from the time when
the sun gets near enough to a node to allow of an
eclipse until the time when it is too far past for an
eclipse to occur. In 1901 the seasons were May and
November.

If the moon's node stayed in the same place in the sky,
eclipses would occur only some time during these two
months. But, owing to the attraction of the sun on the
earth and moon, the position of the nodes is continually
changing in a direction opposite that of the motion of
the two bodies. Each node makes a complete revolution
around the celestial sphere in eighteen years and seven
months. Hence in this same period the eclipse seasons
win course all through the year. On an average they
occur about nineteen days earlier every year than they
did the year before. Thus it happens that in 1903 one
season occurs in March and April and the other season in
September and October. The change will keep going on
until, in the year 1910, the season which in 1901 was in
May will have gotten back to November, while the No-
vember one will have gotten back to May, each having
passed through all the intermediate months, and the two

136 THE SUN, EARTH, AND MOON

having changed places. By 1919 each will have made
an entire revolution through the year.

Let us imagine ourselves to be looking at the sun and
earth from the moon when the latter is about to enter the
earth's shadow. The earth, looking much larger than
the sun, will be seen to approach it, and at length will
begin to impinge on its disk and cut off a part of its
light. The region within which this wUl occur is called
the penumbra, and it is shown outside the shadow in the
figure. So long as the moon is only in this region, an

■■■: ■ 7 "' ^..-—<y — :

Fig. 24. — Passage of the Moon through llie EartKs Shadow.

ordinary observer would not notice any diminution in its
light, although such a diminution could be detected by
exact photometric measurements. The moon is not said
to be eclipsed until it begins to enter into the actual
shadow, where the whole direct light of the sun is cut off.

How an Eclipse of the Moon Looks

If we watch the moon when an eclipse is about to be-
gin, Tte shall see a small portion of her eastern edge grad-
ually grow dim and finally disappear. As the moon
advances in her orbit, more and more of her face thus
disappears from view by entering into the shadow. If,
however, we look very carefully, we shall see that the part

HOW AN ECLIPSE OP MOON LOOKS 1B7

immersed in the shadow has not entirely disappeared, but
shines with a very faint light. If the whole body of the
moon enters into the shadow, the eclipse is said to be
total ; if only a portion of her body dips into the shadow,
it is called partial. If the eclipse is total, the light which
illuminates the eclipsed moon wiU be very plainly seen,
because it is not drowned out by the dazzling light of the
uneclipsed portion. This light is of a dingy red colour,
and arises from the refraction of the earth's atmosphere,
which was described in a former chapter. In consequence
of this, those rays of the sun which just graze the earth,
or pass within a short distance of its surface,* are bent out
of their course and thrown into the shadow by refraction.
Thus they fill the shadow and fall on the moon. The red
colour is due to the same cause that makes the sun appear
red at sunset, namely, the absorption of the green and
blue rays by the atmosphere, which lets the red rays pass.

Two or three eclipses of the moon occur every year, of
which one, at least, is nearly always total. But, of
course, the eclipse will be visible only in that hemisphere ■
of the earth on which the moon is shining at the time.

When the moon is eclipsed an observer on that body
would see an eclipse of the sun by the earth. The cause
of the phenomenon we have described would then be plain
enough to him. The apparent size of the earth would
be much larger than that of the moon as we see it. Its
diameter would be between three and four times that of
the sun. At first this immense body would be invisible
when it approached the sun. What the observer would
see would be the cutting off of the light of the sun by the

138 THE SUN, EARTH, AND MOON

advancing but invisible earth. When the latter had
nearly covered the sun, its whole outline would be shown
to him by a red light surrounding it, caused by the re-
fraction of the earth's atmosphere. Finally, when the
last trace of true sunlight had disappeared, nothing
would be visible but this ring of bright red light having
inside of it the black but otherwise invisible body of the
earth.

The circumstances of an eclipse of the moon are quite
different from those of a solar eclipse, to be described in
the next chapter. It can aways be seen at the same in-
stant over the whole hemisphere of the earth on which
the moon is shining at the time. A curious phenomenon
occurs when the moon rises totally eclipsed. Then we
may see it on one horizon, say the eastern one, while the
sun is still visible on the western horizon. The explana-
tion of this seeming paradox is that both bodies are really
below the horizon, but are so elevated by refraction that
we can see them at the same time.

VI

Eclipses op the Sun

If the moon moved exactly in the plane of the ecliptic
she would pass over the face of the sun at every new
moon. But, owing to the inclination of her orbit, as de-
scribed in the preceding chapter, she will actually do so
only when the direction of the sun happens to be near one
of the moon's nodes. When this is the case we may see
an eclipse of the sun if we are only on the right part of
the earth.

Supposing the moon to pass over the sun, the first
question is whether it can wholly hide the sun from our
eyes. This depends not on the actual size of the two

Fig. 25. — The Shadow of the Moon Tlirown on i!te Earth during a Total
Eclipse oftlie Sun.

bodies but on their apparent size. We know that the sun
has about four hundred times the diameter of the moon.
But it is also four hundred times as far from us as the
moon. The curious result of this is that the two bodies
appear of nearly the same size to our eyes. Sometimes

140 THE SUN, EARTH, AND MOON

the moon appears a little the larger, and sometimes the
sun. In the former case the moon may entirely hide the
sun ; in the latter case she cannot do so.

One important difference between an eclipse of the
moon and of the sun is that the former is always the same
wherever it is visible, while an eclipse of the sun depends
upon the position of the observer. The most interesting
eclipses are those in which the centre of the moon passes
exactly over that of the sun. These are called central

Cr^" "'

Fig. 26. — 7%e Moon Passing Centrally over the Sun during an Annular

Eclipse.

eclipses. To see one, the observer must station himself
at a point through which the line joining the centres
shall pass. Then if the apparent size of the moon ex-
ceeds that of the sun, the former will completely hide the
sun from view. The eclipse is then said to be total.

If the sun appears the larger, a ring of its light wiU
surround the dark body of the moon at the moment of
central eclipse. The latter is then called annular (Latin
annulus, a ring).

The line of centres of the two bodies sweeps along the
surface of the earth, and its course may be shown by a
line marked on a map. Such maps, showing the regions

BEAUTY OF A TOTAL ECLIPSE 141

and lines of eclipses are published in the astronomical
ephemerides. An eclipse may be total or annular in a
region a few miles north or south of this central line, but
never for so far as one hundred miles. Outside this
limit an observer will see only a partial echpse, that is,
one in which the moon partly covers the sun. In yet
more distant regions of the earth there will be no ecHpse
at all.

Beauty of a Total Eclipse

A total eclipse is one of the most impressive sights that
nature offers to the eye of man. To see it to the best
advantage one should be in an elevated position com-
manding the widest possible view of the surrounding
country, especially in the direction from which the
shadow of the moon is to come. The first indication of
anything unusual is to be seen, not on the earth or in the
air, but on the disk of the sun. At the predicted moment
a little notch will be seen to form somewhere on the west-
ern edge of the sun's outline. It increases minute by
minute, gradually eating away, as it were, the visible
sun. No wonder that imperfectly civilised people, when
they saw the great luminary thus diminishing in size,
fancied that a dragon was devouring its substance.

For some time, perhaps an hour, nothing will be
noticed but the continued progress of the advancing
moon. It will be interesting if, during this time, the ob-
server is in the neighbourhood of a tree that will permit
the sun's rays to reach the ground through the small
openings in its foliage. The little images of the sun
which form here and there on the ground will then have

14.2 THE SUN, EARTH, AND MOON

the form of the partially eclipsed sun. Soon the latter
appears as the new moon, only instead of increasing, the
crescent form grows thinner minute by minute. Even
then, so well has the eye accommodated itself to the
diminishing light, there may be little noticeable darkness
until the crescent has grown very thin. If the observer
has a telescope with a dark glass for viewing the sun, he
will now have an excellent opportunity of seeing the
mountains on the moon. The unbroken limb of the sun
will keep its usual soft and uniform outline. But the
inside of the crescent, the edge of which is formed by
the surface of the moon, wiU be rough and jagged in
outline.

As the crescent is about to disappear the advancing
mountains on the rugged surface of the moon will reach
the sun's edge, leaving nothing of the latter but a row of
broken fragments or points of light, shining between
the hollows on the lunar surface. They last but a second
or two and then vanish.

Now is seen the glory of the spectacle. The sky is
clear and the sun in mid-heaven, and yet no sun is visible.
Where the latter ought to be the densely black globe of
the moon hangs, as it were, in mid-air. It is surrounded
by an effulgence radiating a saintly glory. This is the
sun's corona, already mentioned in our chapter on the
sun. Though bright enough to the unaided vision, it is
seen to the best advantage with a telescope of very low
magnifying power. Even a common opera glass may
suffice. With a telescope of high power only a portion
of the corona is visible, and thus the finest part of the

ANCIENT ECLIPSES 143

effect is lost. A common spy-glass, magnifying ten or
twelve times, is better, so far as effect is concerned, than
the largest telescope. Such an instrument will show not
only the corona itself but the so-called "prominences" — •
fantastic cloud-like forms of rosy colour rising here and
there, seemingly from the dark body of the moon.

Ancient Eclipses

It is remarkable that though the ancients were f amiHar
with the fact of eclipses, and the more enlightened of
them perfectly understood their causes, some even the
laws of their recurrence, there are very few actual ac-
counts of these phenomena in the writings of the ancient
historians. The old Chinese annals now and then record
the fact that an eclipse of the sun occurred at a certain
time in some province or near some city of the empire.
But no particulars are given. Quite recently the Assyri-
ologists have deciphered from ancient tablets a statement
that an eclipse of the sun was seen at Nineveh, B. C. 763,
June 15. Our astronomical tables show that there actu-
ally was a total eclipse of the sun on this day, during
which the shadow passed a hundred miles or so north of
Nineveh.

Perhaps the most celebrated of the ancient eclipses,
and the one that has given rise to most discussion, is that
known as the eclipse of Thales. Its principal historical
basis is a statement of Herodotus that in a battle between
the Lydians and the Medes the day was suddenly turned
into night. The armies thereupon ceased battle and were
more eager to come to terms of peace with each other. It

144i THE SUN, EARTH, AND MOON

is added that Thales, the Milesian, had predicted to the
lonians this change of day, even the very year in which
it should occur. Our astronomical tables show that there
actually was a total eclipse of the sun in the year B. C.
585, which was near enough to the time of the battle to
be the one alluded to, but it is now known that the path
of the shadow did not quite reach the seat of hostiHties
till after sunset. Some doubt therefore still rests on the
subject.

Prediction of Eclipses

There is a curious law of the recurrence of eclipses
which has been known from ancient times. It is based on
the fact that the sun and moon return to nearly the same
positions, relative to the node and perigee of the moon's
orbit, -after a period of six thousand five hundred and
eighty-five days eight hours, or eighteen years and
twelve days. This period is called the Saras. Eclipses
of every sort repeat themselves at the end of a Saros.
For example, the eclipse of May, 1900, may be regarded
as a repetition of those which occurred in the years 1846,
1864, and 1882. But when such an eclipse recurs it is
not visible in the same part of the earth, because of the
excess of eight hours in the period. During this eight
hours the earth performs one third of a rotation on its
axis, which brings a different region under the sun. Each
eclipse is visible in a region about one third of the way
round the world, or one hundred and twenty degrees of
longitude, west of where it occurred before. Only after
three periods will the recurrence be near the same region.
But in the meantime the moon's line of motion will have

THE SUN'S APPENDAGES 145

changed so that the path of its shadow will pass farther
north or south than before.

There are two series of eclipses remarkable for the
long duration of the total phase. To one of these the
eclipse of 1868, hereafter mentioned, belongs. This re-
curred in 1886, and will recur again in 1904. Unfortu-
nately, at the first recurrence, the shadow was cast almost
entirely on the Atlantic and Pacific Oceans, so that it
was not favourable for observation by astronomers. That
of 1904, September 9, will be yet more unfortunate for
us, because the shadow will pass only over the Pacific
Ocean. Possibly, however, it may touch some island
where observations may be made. The recurrence of
1922, September 1, wiU be visible in northern Australia,
where the duration of totality will be about four minutes.

To the other and yet more remarkable series belonged
the eclipse of May 7, 1883, and that of May 11, 1901.
At the successive recurrences of this eclipse the duration
of totahty -will be longer and longer through the twenti-
eth century. In 1937, 1955, and 1973 it will exceed
seven minutes, so that so far as duration is concerned, our
successors will see eclipses more remarkable than any
their ancestors have enjoyed for many centuries.

The Sun's Appendages

About 1863-64 the spectroscope began to be applied
to researches on the heavenly bodies. Mr. (now Sir
William) Huggins, of London, was a pioneer in observ-
ing the spectra of the stars and nebuls. For several
years it did not seem that much was to be learned in this

146 THE SUN, EARTH, AND MOON

way about the sun. The year 1868 at length arrived.
On August eighteenth there was to be a remarkable total
eclipse of the sun, visible in India. The shadow was one
hundred and forty miles broad ; the duration of the total
phase was more than six minutes. The French sent Mr.
Janssen, one of their leading spectroscopists, to observe
the eclipse in India and see what he could find out. Won-
derful was his report. The red prominences which had
perplexed scientists for two centuries were found to be
immense masses of glowing hydrogen, rising here and
there from various parts of the sun, of a size compared
with which our earth was a mere speck. This was not
all. After the sunlight reappeared, Janssen began to
watch these objects in his spectroscope. He followed
them as more and more of the sun came out, and con-
tinued to see them until after the eclipse was over. They
could be observed at any time when the air was sufficiently
clear and the sun high in the sky.

By a singular coincidence this same discovery was
made independently in London without any eclipse. Mr.
J. Norman Lockyer was then rising into prominence as
an enthusiastic worker with the spectroscope. It oc-
curred independently to him and to Mr. Huggins that
the heat in the neighbourhood of the sun was so intense
that any matter that existed there would probably take
the form of a gas shining by its own light. Both of
these investigators endeavoured to get a sight of the
prominences in this way; but it was not until October
twentieth, two months after the Indian eclipse, that Mr.
Lockyer succeeded in having an instrument of sufficient

THE SUN'S APPENDAGES 147

power completed. Then, at the first opportunity, he
found that he could see the prominences without an
eclipse !

At that time communication with India was by mail,
so that for the news of Mr. Janssen's discovery astrono-
mers had to wait until a ship arrived. By a singular
coincidence his report and Mr. Lockyer's communication
announcing his own discovery reached the French Acad-
emy of Sciences at the same meeting. This eminent body,
with pardonable enthusiasm, caused a medal to be struck
in commemoration of the new method of research, in
which the profiles of Lockyer and Janssen appeared to-
gether as co-discoverers. Since that time the promi-
nences are regularly mapped out from day to day by
spectroscopic observers in various parts of the world.

The greatest beauty of a total eclipse is due to the
sun's corona. The exact nature of this appendage is
still in doubt. Indeed, until photography was called to
the aid of the astronomer its structure was unknown. It
was described by observers simply as a soft light sur-
rounding the sun ; but when it is photographed and care-
fully examined it is found to be of a radial, hairy
structure which the reader can easily see from the fron-
tispiece of the book. It extends out farthest in the
direction of the sun's equator and least at the poles. The
rays which chance to be exactly at the poles go straight
out from the sun. But those on each side are found to
curve toward the equator, while farther from the equator
they are lost in the more powerful effulgence going out
from the region of the solar spots. Near the poles the

148 THE SUN, EARTH, AND MOON

forms are remarkably like those which iron filings assume
when scattered on paper above a magnet. It is therefore
a question whether there is not here something in the na-
ture of a magnetic force. But in the region called the
sun's equator this analogy ceases to hold. In describing
the sun we mentioned the much greater activity in the
regions of greater spottedness than elsewhere. "It now
seems as if the forces which throw out the corona are
also greatest where the sun's activity is greatest.

The probability now seems to be that the corona is
composed of matter thrown up from the sun, and kept
from falling back again by the repulsion of the solar
rays, and that it bears a certain resemblance to the tail
of a comet.

A very important question is whether the corona shines
mostly by reflected light, or by its own light, due to the
high temperature which it must have so near the sun.
No doubt its light arises from both sources, but it is not
yet known in what proportion. The fact is that its
spectrum shows some bright lines. These can be due
only to the light of the matter itself. Some observers
have supposed that they also saw dark lines in the spec-
trum. This, however, has not been proved. On the whole
the probability seems to be that the corona shines mostly
by its own light.

PART IV
THE PLANETS AND THEIR SATELLITES

1

Oebits and Aspects op the Planets

The orbits in which the planets revolve around their
central luminary are in strictness ellipses, or slightly
flattened circles. But the flattening is so slight that the
eye would not notice it without measurement. The sun
is not in the centre of the ellipse but in a focus, which
in some cases is displaced from the centre by an amount
that the eye can readily perceive. This displacement
measures the eccentricity of the ellipse, which is much
greater than the flattening. For example, in the case
of Mercury, which moves in a very eccentric orbit, the
flattening is only one fiftieth ; that is, if we represent
the greatest diameter of the orbit by fifty, the least
diameter will be forty-nine. But the distance of the
sun from the centre of the orbit is ten on the same
scale.

To show this we give a diagram of the orbits of the
inner group of planets showing quite nearly their forms
and respective locations. A simple glance will show that
the orbits are much nearer together at some points than
at others.

In explaining the various aspects and motions, real
and apparent, of the planets a number of technical ex-
pressions are used which we shall explain.

Inferior planets are those whose orbits lie within the

152 PLANETS AND THEIR SATELLITES

orbit of the earth. This class comprises only Mercury
and Venus.

Superior planets are those whose orbits lie without
that of the earth. These comprise Mars, the minor
planets or asteroids, and all four of the outer group of
major planets.

When a planet seems to us to pass by the sun, and so

Fio. 27. — Orbits o/i/ie l''our Inner Planets.

is seen as if alongside of it, it is said to be in conjunction
with the sun.

An inferior conjunction is one in which the planet is
between us and the sun.

A superior conjunction is one in which the planet is
beyond the sun.

ORBITS AND ASPECTS OF PLANETS 153

A little consideration will show that a superior planet
can never be in inferior conjuction, but an inferior planet
has both kinds of conjunction.

A planet is said to be m opposition when it is In the
opposite direction from the sun. It then rises at sunset,
and vice versa. Of course, an inferior planet can never
be in opposition.

The perihelion of an orbit is that point of it which is
nearest the sun ; the aphelion its most distant point from
the sun.

As the inferior planets. Mercury and Venus, perform
their revolutions they seem to us to swing from one side
of the sun to the other. Their apparent distance from
the sun at any time is called their elongation.

The greatest elongation of Mercury is generally about
twenty-five degrees, being sometimes more and sometimes
less, owing to the great eccentricity of the orbit of this
planet. The greatest elongation of Venus is almost
forty -five degrees.

When the elongation of one of these planets is east
from the sun we may see it in the west after sunset;
when west we may see it in the east in the morning sky.
As neither of them ever wanders from the sun farther
than the distances we have stated, it follows that a planet
seen in the east in the evening, or in the west in the morn-
ing, cannot be either Mercury or Venus.

No two orbits of the planets lie exactly in the same
plane. That is, if we regard any one orbit as horizontal,
all the others will be tipped by small amounts toward one
side or the other. Astronomers find it convenient to take

154 PLANETS AND THEIR SATELLITES

the orbit of the earth, or the ecliptic, as the horizontal or
standard one. As each orbit is centred on the sun it
wiU have two opposite points which lie on the same hori-
zontal plane as the earth's orbit. More exactly, these
are the points at which the orbit intersects the plane of
the ecliptic. They are called nodes.

The angle by which an orbit is tipped from the plane
of the ecliptic is called its inclination. The orbit of Mer-
cury has the greatest inclination, more than 6° The
orbit of Venus is inclined 3° 24' ; those of all the superior
planets less, ranging from 0° 46' in the case of Uranus
to 2° 30' in the case of Saturn.

Distances of the Planets

Leaving out Neptune, the distances of the planets
follow very closely a rule known as Bode's Law, after the
astronomer who first pointed it out. It is this : Take the
numbers 0, 3, 6, 12, etc., doubling each as we go along.
Then add 4 to each number, and we shall hit very nearly
on the scale of distances of all the planets except Nep-
tune, thus :

Mercury, + 4 = 4

actual distance 4

Venus, 3 +«4 = 7

7

Earth, 6 + 4 = 10

' 10

Mars, 13 + 4 = 16

' 15

Asteroids, 34 + 4 = 28

20 to 40

Jupiter, 48 + 4 = 53

• 52

Saturn, 96 + 4 = 100

• 95

Uranus, 193 + 4 = 196

' 192

Neptune, 384 + 4 = .888
On these actual distances we ]

■' 300

remark tha

; astronomers do

KEPLER'S LAWS 155

not use miles or other terrestrial measures to express
distances between the heavenly bodies, for two reasons.
In the first place, they are too short ; to use them would
be like stating the distance between two cities in centi-
metres. In the next place, distances in the heavens can-
not be fixed with the necessary exactness in our measures,
whereas, if we take the sun's distance from the earth as
the unit of measure, we c^n determine other distances
between the planets with great precision in terms of this
measure. So, to get the distances of the planets from
the sun in astronomical measure, we have to divide the
last numbers of the preceding table by ten, or insert a
decimal point before the last figure of each.

We have not in this table distracted the attention of
the reader by using unnecessary decimals. Actually, the
distance of Mercury is 0.387, etc. ; we have simply called
it 0.4 and multiplied it by 10 to get the proportion for
comparing with Bode's Law.

Kepler's Laws

The motions of the planets in their orbits take place
in accordance with certain laws laid down by Kepler,
and therefore known as Kepler's laws. The first of these
has already been mentioned ; the orbits of the planets are
ellipses, of which the sun is in one focus.

The second law is that the nearer the planet is to the
sun the faster it moves. With more mathematical exact-
ness, the areas swept over by the line joining the planet
and sun in equal times are all equal.

The third law is that the cubes of the mean distances

156 PLANETS AND THEIR SATELLITES

of the planets from the sun are proportional to the
squares of their times of revolution. This law requires
some illustration. Suppose one planet to be four times
as far from the sun as another. It will then be eight
times as long going around it. This number is reached
by taking the cube of four, which is sixty-four, and then
extracting the square root, which is eight.

The unit of measure which the astronomer uses to ex-
press distances in the solar system being the mean dis-
tance of the earth from the sun, it follows that the mean
distances of the inferior planets will be decimal fractions,
as we have just shown, while those of the outer ones will
A^ary from 1.5 in the case of Mars to 30 in the case of
Neptune. If we take the cubes of all these distances and
extract their square roots we shall have the times of the
revolution of the planets, expressed in years.

It will be seen that the outer planets are longer in
getting around their orbits, not only because they have
farther to go, but because they actually move more
slowly. If, as in the case first supposed, the outer planet
is four times as far from the sun, it will move only half
as fast. This is why it takes eight times as long to get
around. The speed of the earth in Its orbit is about
18.6 miles per second. But that of Neptune is only
about 3.5 miles per second, although it has thirty times
as far to go. This is why it takes more than one hun-
dred and sixty years to complete a revolution.

II

The Planet Mekcury

To set forth what is known of the major planets we
shall take them up in the order of their distance from the
sun. The first planet reached will then be Mercury. It
is not only the nearest planet to the sun, but much the
smallest of the eight; so small, indeed, that, but for its
situation, it would hardly be called a major planet. Its
diameter is about two fifths greater than that of the
moon, but, the volumes of bodies being proportional to
the cubes of their diameters, it has about three times the
volume of the moon.

It has far the most eccentric orbit of all the major
planets, though, in this respect, it is exceeded by some
of the minor planets to be hereafter described. In conse-
quence, its distance from the sun varies between wide
limits. At perihelion it is less than twenty-nine millions
of miles from the sun ; at aphelion it goes out to a distance
of more than forty-three millions of miles. It performs
its revolution around the sun in a little less than three
months ; to speak more exactly, in eighty-eight days. It
therefore makes more than four revolutions in a year.

Performing more than four revolutions around the
sun while the earth is performing one, we readily see
that it must pass conjunction with the sun at certain
regular though somewhat unequal intervals. To show

158 PLANETS AND THEIR SATELLITES

the exact nature of its apparent motion let the inner
circle of the diagram represent the orbit of Mercury and
the outer one that of the earth. When the earth is at E,
and Mercury at M, the latter is in inferior conjunction
with the sun. At the end of three months it will have re-
turned to the point M, but it will not yet "be in conjunc-

/ \

/ '^/^ A,

' / . \

k INFERIOR _ Jm ^j^ f

T CON Junction T ^^^ySi" i

I \

\ V^ ^y^^6t

"vr.t

''9 /■

\ %y^

\ y

Fig. 28. — Conjunctions of Mercury with the Sun,

tion, because, in the meantime, the earth has moved for-
ward in its orbit. When the earth reaches a certain point
F, Mercury will have reached the point N and will again
be in inferior conjunction. This revolution from one in-
ferior conjunction to another is called the synodic revolu-
tion of the planet. In the case of Mercury this is some-
what less than one third more than the time of actual

THE APPEARANCE OF MERCURY 159

revolution ; that is to say, the arc MN is a little less than,
one third of the circle.

Now suppose that when the earth is at E, Mercury,
instead of being at M is near the highest point A of the
orbit as represented in the figure. It will then be at
its greatest apparent distance from the sun as we see
it from the earth; or, in technical language, at its
greatest east elongation. Being east of the sun it will

,^eftl§^^";. / , %

EARTH .,HS:'- "' /■ < 'N ^\

G<^ ( :-0-r- C,

"^^5>-..^

"^^efe-

''*^->;^. \

/

'^io>

'^^^i^^.

/

^o:. -^

Fi3. 29. — Elongations of Mercury.

then set after the sun, by a time generally between
an hour and a quarter and an hour and a half. This
is the most convenient time for seeing it. If the sky
is clear, it will readily be seen in the twilight from half
an hour to an hour after sunset. At the opposite elonga-
tion, near C, it is west of the sun ; then it rises before the
sun and may be seen in the morning twilight.

' The Surface and Rotation of Mercury

The best time to make a telescopic study of Mercury
is late in the afternoon, when it is near east elongation.

160 PLANETS AND THEIR SATELLITES

or shortly after sunrise, if it rises before the sun. Sup-
posing it east of the sun, it will probably be visible in
the telescope at any time after noon, but the air is gen-
erally disturbed by the sun's rays so that it is hardly
possible to make a good observation at that time. Late
in the afternoon the air grows steadier, so that the planet
can be better observed. But, after sunset, the planet is
seen through a continually increasing extent of atmos-
phere, so that the seeming disturbance again begins to
increase. Owing to these circumstances it is the most
difficult of all the planets to study in a satisfactory way,
and observers differ very much as to what can be seen on
its surface.

The first observer who thought he could see any fea-
tures on the surface of this planet was Schroter, a Ger-
man. When Mercury presented the form of a crescent
he fancied that its south horn seemed blunted at inter-
vals. He attributed this to the shadow of a lofty moun-
tain ; and by observing the intervals between the blunted
appearance he concluded that the planet revolved on its
axis in twenty-four hours and five minutes. But Sir
William Herschel, who observed at the same time with
much more powerful instruments, could not see anything
of the kind.

Until quite recently nearly all observers agreed with
Herschel that no time of rotation could be certainly de-
termined. But a few years since, Schiaparelli, observing
with a fine telescope in the beautiful sky of northern
Italy, noticed that the aspect of the planet seemed un-
changed day after day. He was thus led to the conclu-

THE PHASES OF MERCURY 161

sion that it always presents the same face to the sun,
as the moon presents the same face to the earth. This
view was shared by Mr. Lowell, observing at the Flag-
staiF Observatory. But the observation is too difficult
to permit us to regard the fact as established. All that
a conservative astronomer would be willing to say is
that as yet we know nothing of the revolution of Mercury
on its axis.

Drawings showing the face of Mercury have been
made by several astronomers. As it is seen under all
ordinary conditions no special features are well marked.
Very different is the case at the Lowell Observatory in
Flagstaff, Ariz. The most singular feature of its sur-
face in the latter picture consists in the dark lines which
cross it. These have not been seen by other observers,
and, until they are established by independent evidence,
astronomers will be sceptical as to their reality. The
reason of this will be stated later in connection with the
planet Mars.

Owing to the various positions of Mercury relative to
the sun it presents phases like those of the moon. These
depend upon the relation of the dark and the illuminated
hemispheres relative to the direction in which we see the
planet. The hemisphere which is turned away from the
sun, being in darkness, is always invisible to us. At
superior conjunction the illuminated hemisphere is turned
toward us and the planet seems round, like a full moon.
As it moves from east elongation to inferior conjunction,
more and more of the dark hemisphere is turned toward
us, and less and less of the illuminated one. But this

162 PLANETS AND THEIR SATELLITES

disadvantage is counterbalanced by the fact that the
planet continually comes nearer during the interval, so
that we get a better view of whatever portion of the
illuminated hemisphere may be visible to us. Its appar-
ent form and size at different times during its synodic
revolution go through a series of changes similar to those
shown in the next chapter in the case of Venus.

The question whether Mercury has an atmosphere is
also one on which opinions differ, the prevailing opinion
being in the negative. It seems quite certain that, if it
has one, it is too rare to reflect the light of the sun.

Transits of Mercury

It wiU be readily seen that, if an inferior planet re-
volved around the sun in the same plane as the earth, we
should see it pass over the sun's disk at every inferior
conjunction. But no two planets revolve in the same
plane. Of all the major planets the orbit of Mercury
has the largest inclination to that of the earth. In con-
sequence, when in inferior conjunction, it commonly
passes a greater or less distance to the north or to the
south of the sun. If, however, it chances to be near one
of its nodes at the time in question, wc shall see it as
a black spot passing across the sun's disk. This phe-
nomenon is called a transit of Mercury. Such transits
occur at intervals ranging between three and thirteen
years. They are observed with much interest by as-
tronomers because it is possible to determine with great
precision the time at which the planet enters upon the
solar disk, and leaves it again. Knowing these times,

TRANSITS OF MERCURY 163

valuable information is afforded respecting the exact law
of motion of the planet.

The first observation of a transit of Mercury was made
by Gassendi on November 7, 1631. His observation is
not, however, of any scientific value at the present time,
owing to the imperfection of his instruments. A some-
what better but not good observation was made by Hal-
ley, of England, in 1677, during a visit to the island of
St. Helena. Since that time the transits have been ob-
served with a fair degree of regularity. The following
table shows the transits that will be visible during the
next fifty years, with the regions of the earth in which
each may be seen :

1907, November 14, visible in Europe and eastern United

States.
1914, November 7, visible in the same regions.
1924, May 7, the beginning will be visible on the Pacific

coast, but the whole transit only on the Pacific

Ocean and in eastern Asia.
1927, November 9, visible in Asia and eastern Europe.
1937, May 11, Mercury will graze the south limb of

the sun. The phenomenon will be visible in Europe,

but will occur before the sun rises in America.
1940, November 10, visible in the Western and Pacific

States.
1953, November 14, visible throughout the United

States.

Observations of transits of Mercury since 1677 have
brought out one of the most perplexing facts of astron-

164. PLANETS AND THEIR SATELLITES

omy. The orbit of this planet is found to be slowly
changing its position, its perihelion moving forward by
about forty-three seconds per century farther than it
ought to move in consequence of the attraction of all the
known planets. This deviation was discovered in 184i5
by Le Verrier, celebrated as having computed the posi-
tion of Neptune before it had ever been recognised in
the telescope. He attributed it to the attraction of a
planet, or group of planets, between Mercury and the
sun. His announcement set people to looking for the
supposed planet. About 1860, a Dr. Lescarbault, a
country physician of France, who possessed a small tele-
scope, thought he had seen this planet passing over the
disk of the sun. But it was soon proved that he must
have been mistaken. Another more experienced astron-
omer, who was looking at the sun on the same day, failed
to see anything except an ordinary spot. It was prob-
ably this which misled the physician-astronomer. Now,
for forty years, the sun has been carefully scrutinised
and photographed from day to day at several stations
without anything of the sort being seen.

Still, it is possible that little planets so minute as to
escape detection in passing over the sun's disk may re-
volve in the region in question. If so, their light would
be completely obscured by that of the sky, so that they
might not ordinarily be visible. But there is still a
chance that, during a total eclipse of the sun, when the
light is cut off from the sky, they could be seen. Ob-
servers have, from time to time, looked for them during
totftl eclipses. In one instance something of the sort was

INTRAMERCURIAL PLANETS 165

supposed to be found. During the eclipse of 1878,
Professor Watson, of Ann Arbor, and Professor Lewis
Swift, both able and experienced observers, thought that
they had detected some such bodies. But critical exam-
ination left no doubt that what Watson saw was a pair
of fixed stars which had always been in that place. How
it was with the observations of Professor Swift has never
been certainly ascertained, because he was not able to lay
down the position with such certainty that positive con-
clusions could be drawn.

Notwithstanding such failures, observers have repeated
tlie search during several of the principal total eclipses.
The writer did so during the eclipse of 1869, and again
during that of 1878, the search being made with a small
telescope. In recent times the powerful agency of
photography has been invoked by Professors Pickering
and Campbell during the eclipses of 1900 and 1901.
Campbell's results during the latter eclipse were the most
decisive yet reached. With his photographic telescope
some fifty stars were photographed, some as faint as the
eighth magnitude, but they were all found to be knowu
objects. It therefore seems certain that there can be no
intramercurial much brighter than the eighth magnitude.
It would take hundreds of thousands of such planets as
this to produce the observed motion of Mercury. So great
a number of these bodies would produce a far brighter
illumination of the sky than any that we see. The result
therefore seems to be conclusive against the view that the
motion of the perihelion of Mercurj' can be produced by
intramercurial planets. In addition to all these difBcul-

166 PLANETS AND THEIR SATELLITES

ties in supposing the planet to exist we have the difficulty
that, if it did exist, it would produce a similar though
smaller change in the position of the nodes of either Mer-
cury or Venus, or both.

Altogether, the evidence seems conclusive against the
reality of any bodies whose attraction could produce the
observed deviation, which still remains unexplained. The
most recent supposition on the subject is that the force
of gravitation deviates slightly from the law of the in-
verse square. But this requires farther investigation.

Ill

The Planet Venus

Of all the star-like objects in the heavens the planet
Venus is the most brilliant. The sun and moon are the
only heavenly bodies outshining it. In a clear and moon-
less evening it may be seen to cast a shadow. If an
observer knows exactly where to look for it, and has a
well-focused eye, it can be seen in the daytime when near
the meridian, provided that the sun is not in its immediate
neighbourhood. When it is east of the sun it may be seen
in the west, faintly before sunset and growing continually
brighter as the light diminishes. When west of the sun
it rises in the morning before the sun, and may then be
seen in the east. Under these circumstances it has been
called the evening and morning star respectively. The
ancients called it Hesperus when an evening star, and
Phosphorus when a morning star. It is said that, in the
early history of our race, Hesperus and Phosphorus were
not known to be the same body.

If Venus is examined with the telescope, even one of
low power, it will be seen to exhibit phases like those of
the moon. This fact was ascertained by Galileo when
he first directed his telescope toward the planet, and af-
forded him strong evidence of the truth of the Coperni-
can System. In accordance with a custom of the time he
published this discovery in the form of an anagram- — a

168 PLANETS AND THEIR SATELLITES

collection of letters which, when subsequently put to-
gether would state the discovery. Translated into Eng-
lish the anagram read, "The mother of the loves emulates
the phases of Cynthia."

What we have said of the synodic motion of Mercury
applies in principle to Venus, and need not therefore be
repeated. In the following cut the apparent size of the
planet is shown in various parts of its synodic orbit. As
the planet passes from superior to inferior conjunction
its globe continually grows larger in apparent size,

Fig. 30. — Phases of Venus in Different FoinU of Us Orbit.

though we cannot see its entire outline. But the fraction
of the disk illuniinated continually becomes smaller, first
having the shape of a half moon, and then the shape of
a crescent, which grows thinner and thinner up to the
time of inferior conjunction. In the latter position the
dark hemisphere is turned toward us and the planet is
invisible. Venus is at its greatest brightness about half-
way between inferior conjunction and greatest elonga-
tion. It then sets about two hours after the sun, if east
of it, and rises about two hours before the sun, if west
of it.

ROTATION OF VENUS 169

Rotation of Venus

The question of the rotation of Venus has interested
astronomers and the public ever since the time of Galileo.
But the difficulty of learning anything certain on the
subject is very great, owing to the peculiar glare of the
planet. When seen through a telescope no sharp and
well-defined markings are visible. Instead of this there
is a glare on the surface, varying by gentle gradations
from one region to another, as if we were looking upon
a globe of polished but slightly tarnished metal. Never-
theless, various observers have supposed that they could
distinguish bright or dark spots. As far back as 1667
Cassini concluded from these seeming spots that the
planet revolved on its axis in a little less than twenty-
four hours. During the next century Blanchini, an
Italian observer, published an extensive treatise on the
subject, illustrated with many drawings of the planet.
His conclusion was that Venus required more than twenty-
four days to revolve on it^ axis. Cassini, the son, de-
fended his father's conclusion by claiming that the planet
had always made one revolution and a little more between
the times of Blanchini's observations on successive even-
ings. Thus the Italian astronomer would naturally see
the spots on successive evenings a little farther advanced,
and estimated the motion by this advance, not being aware
that a whole revolution had been made during the interval.
At the end of twenty-four days the same hemisphere of
the planet would be presented to the earth as before, the
number of revolutions in the meantime being twenty-five.

170 PLANETS AND THEIR SATELLITES

Schroter tried to decide the question for Venus in the
same way that he supposed himself to have decided it for
Mercury. He directed his attention especially to the fine
sharp horns of the crescent, when the planet was nearly
between the earth and the sun. At certain intervals he
supposed one of them to be a little blunted. Ascribing
this appearance to the shadow of a high mountain, he
concluded that the time of rotation was twenty-three
hours twenty-one minutes.

From the time of Schroter no one professed to throw
any more light on the question until 1832. Then De
Vico, of Rome, announced that he had rediscovered the
markings found by Blanchini. He concluded that the
planet rotated in twenty-three hours twenty-one minutes,
in agreement with Schroter's result.

This close agreement between the results of observa-
tions by four distinguished observers led to the very gen-
eral acceptance of twenty-three hours twenty-one minutes
as the time of rotation of the planet. But there was much
to be said on the other side. The great Herschel, with
the most powerful telescopes that had ever been made, was
never able to make out any permanent markings on Venus,
If anything like a spot appeared, it varied and disap-
peared again so rapidly that no evidence of rotation could
be afforded by it. This negative result has always been
reached by the large majority of observers.

But a new and surprising theory has been recently put
forth by Schiaparelli, and maintained by Lowell. This is
that Venus rotates on its axis in the same period that it
revolves around the sun; in other words both Mercury

ROTATION OF VENUS 171

and Venus always present the same face to the sun, as the
moon presents the same face to the earth. Schiaparelli
reached this conclusion by noticing that a number of ex-
ceedingly faint spots could be seen on the southern hemi-
sphere of Venus for several days in succession in the
same position day after day. He could observe the planet
through several hours on each day, and the constancy
of the spots precluded the idea that the planet made one
rotation and a little more in the course of a day. Lowell
was led to the same conclusion by careful study of the
planet at his Arizona observatory.

The latest conclusion has been reached by the spectro-
scope. We have already explained how, with this instru-
ment, it can be determined whether a heavenly body is
moving toward us or from us. The principle applies
to a planet which we see by the reflected hght of the sun
as well as to a star. Hence, if Venus rotates, one part
of its disk will be moving toward us, and the other from
us. By comparing the dark lines of the spectrum shown
by the two edges of the disk of Venus it can then be de-
termined how various points of the disk are moving with
respect to the earth. It was thus found by Belopolsky
that the planet was affected by a quite rapid rotation.
The observation is so difficult, and the displacement of
the lines so small, that it was not possible to state a very
certain result, although the general fact was made very
probable. On the whole we must regard this conclusion
as the most likely that has yet been reached, although it
is at variance with the observations of Schiaparelli, as
well as those of the Lowell Observatory. But the spectre-

m PLANETS AND THEIR SATELLITES

scopic observations have not yet been made with sufficient
precision to teach us the exact time, of revolution. Re-
cent discoveries as to the nature of the atmosphere of
Venus make it almost certain that all the observers who
supposed that they saw markings on the planet were
mistaken.

Atmosphere of Venus

It is now .well established that Venus is surrounded
by an atmosphere which is probably denser than that of
the earth. This was shown in a remarkable and interest-

FiG. 31. — Effect of Hie Atmosphere of Veniis during tlie Trarmt of 188^.

ing way during the transit of Venus over the sun's disk
in 1882, which was observed by the writer at the Cape
of Good Hope. When the planet was a little more than
halfway on the disk, its outer edge appeared illuminated,
as shown on the figure. This illumination, however, did
not commence at the middle point of the arc, as it

ATMOSPHERE OF VENUS ITS

should have done had it been caused by regular refrac-
tion, but commenced at a point quite near one end of the
arc. This appearance was explained by Russell, of
Princeton, who showed that the atmosphere is so full
of vapour that we cannot see the light of the sun by
direct refraction through it. What we see is an illu-
minated stratum of cloud.s or vapour floating in an at-
mosphere. Such being the case, it is not at all likely that
astronomers on the earth can ever see the solid body of
the planet through these clouds. Hence the supposed
spots could only have been temporary clouds, continually
changing.

To illustrate the illusions to which the sight of even
good observers may be subject, we may mention the fact
that several such observers have supposed the whole hemi-
sphere of Venus to be visible when the planet was near
inferior conjunction. It then had the appearance fa-
miliarly known as "the new moon in the old moon's
arms," with which everyone who observes our satellite
when a narrow crescent is familiar. In the case of the
moon it is well known that we thus see the dark hemi-
sphere by the light reflected from the earth. But in
the case of Venus there is no possibility of a sufficient
reflection of light from the earth, or any other body.
The appearance has sometimes been explained by a possi-
ble phosphorescence covering the whole hemisphere of
Venus. But it is more likely due to an optical illusion.
It has generally been seen in the daytime, when the
sky is brightly illuminated, and when any faint light
like that of phosphorescence would be completely in-

174 PLANETS AND THEIR SATELLITES

visible. To whatever we might attribute the light, it
ought to be seen far better after the end of twilight in the
evening than during the daytime. The fact that it is not
seen then seems to be conclusive against its reality.

The appearance illustrates a well-known psychological
law, that the imagination is apt to put in what it is ac-
customed to see, even when the object is not there. We
are so accustomed to the appearance on the moon that
when we look at Venus the similarity of the general phe-
nomena leads us to make this supposed familiar addition
to it.

Has Venus a Satellite?

During the past two centuries several observers have
from time to time thought that they saw a satellite of
Venus. Countless observers, with good telescopes, have
seen nothing of the sort. We may safely say that Venus
Las no satellite visible in the most powerful telescopes
of our time. Quite likely these supposed satellites were
geeming objects quite familiar to astronomers under the
name of "ghosts." These are sometimes seen when a
telescope is pointed at a bright object, and are due to a
double reflection of light in the lenses either of the object-
glass or the eyepiece.

A few years ago the writer received a letter from the
owner of a very large telescope in England stating that,
by great care, he could see a very faint, round, and well-
defined aureole of light around the planet Mars. He
desired to know whether the object could be real, or how
the appearance was to be explained. In reply, he was
informed that such an appearance would be produced

TRANSITS OF VENUS 175

by the double reflection of light between the two inner
lenses of the obj ect- glass, provided their curvatures were
nearly, but not exactly the same. It was suggested that
he point the telescope at Sirius and see if a similar ap-
pearance did not surround the star. He probably found
that such was the case.

Transits of Venus

The transits of Venus across the sun's disk are among
the rarest phenomena of astronomy, as they occur, on
the average, only once in sixty years. For many cen-
turies past and to come there will be a regular cycle,
bringing about four transits in two hundred and forty-
three years. The intervals between the transits are one
hundred and five and a half years, eight years, one hun-
dred and twenty-one and a half years, eight years ; then
one hundred and five and a half years again, and so on.
The dates of the last six transits and the two next to
come are as follows :

1631, December 7, 1874, December 9,

1639, December 4, 1882, December 6,

1761, June 5, 2004, June 8,

1769, June 3, 2012, June 6.

It will be seen that no person now living is likely to see
this phenomenon, as the next transit does not occur until
2004. Yet, the time when Venus will appear upon the
disk on June 8 of that year can now be predicted for any
point on the earth's surface, within a minute or two.

176 PLANETS AND THEIR SATELLITES

The interest which has attached to these transits dur-
ing the past century arose from the fact that they were
supposed to afford the best method of determining the
distance of the sun from the earth. This fact and the
rarity of the phenomenon led to the last four transits
being observed on a large scale. In 1761, and again in
1769, the leading maritime nations sent observers to
various parts of the world to note the exact time at
which the planet entered upon and left the sun's dislc. In
1874 and 1882, expeditions were fitted up on a large
scale by the United States, Great B.ritain, France, and
Germany. On the first of these occasions American par-
ties occupied stations in China, Japan, and eastern
Siberia on the north, and in Australia, New Zealand,
Chatham Island, and Kerguelen Island in the south. In
1882 it was not necessary to send out so many expedi-
tions, because the transit was visible in this country. In
the southern hemisphere stations were occupied at the
Cape of Good Hope and other points. The observations
made by these expeditions proved of great value in de-
termining the future motions of Venus, but it was found
that other methods of determining the distance of the
sun would lead to a more certain result.

IV

The Planet Mars

More public interest has in recent years been con-
centrated on the planet Mars than on any other. Its
resemblance to our earth, its supposed canals, oceans,
climate, snowfall, etc., have all tended to interest us in
its possible inhabitants. At the risk of disappointing
those readers who would like to see certain proof that our
neighbouring world is peopled with rational beings, I
shall endeavour to set forth what is actually known on
the subject, distinguishing it from the great mass of illu-
sion and baseless speculation which has crept into popular
journals during the past twenty years.

We begin with some particulars which will be useful
in recognising the planet. Its period of revolution is
six hundred and eighty-seven days, or forty-three days
less than two years. If the period were exactly two years,
it would make one revolution while the earth made two,
and we should see the planet in opposition at regular in-
tervals of two years. But, as it moves a little faster than
this, it takes the earth from one to two months to catch
up with it, so that the oppositions occur at intervals of
two years and one or two months. This excess of one or
two months makes up a whole year after eight opposi-
tions ; consequently, at the end of about seventeen years.
Mars will again be in opposition at the same time of the

178 PLANETS AND THEIR SATELLITES

year, and near the same point of its orbit, as before. In
this period the earth will have made seventeen revolutions
and Mars nine.

The difference of a month or so in the interval be-
tween oppositions is due to the great eccentricity of the
orbit, which is larger than that of any other major
planet except Mercury. Its value is 0.093, or nearly
one tenth. Hence, when in perihelion, it is nearly one
tenth nearer the sun than its mean distance, and when
in aphelion nearly one tenth farther. Its distance from
the earth at opposition will be different by the same
amount, measured in miles, and hence in a much larger
proportion to the distance itself. If opposition occurs
when the planet is near perihelion, the distance from
earth is about forty-three milUon miles; but if near
the aphelion, about sixty million miles. The result of
this is that, at a perihelion opposition, which can occur
only in September, the planet will appear more than
three times as bright as at an aphelion opposition, occur-
ing in February or March. An opposition occurred
near the end of March, 1903; the next following early
in May, 1905. We shall then have oppositions near the
end of June, 1907, and in August, 1909, which will be
quite near to perihelion.

Mars, when near opposition, is easily recognised by its
brilliancy, and by the reddish colour of its light, which is
very different from that of most of the stars. It is
curious that a telescopic view of the planet does not give
so strong an impression of red light as does the naked eye
view.

SURFACE AND ROTATION OF MARS 179

The Surface and Rotation of Mars

The great Huygens, who flourished between 1650 and
1700, studying Mars with the telescope, was the first one
to recognise the variegated character of its surface, and
to make a drawing of the appearance which it presented.
The features delineated by Huygens can be recognised
and identified to this day. By watching them it was easy
to see that the planet rotated on its axis in a little more
than one of our days (24h. 37m.).

This time of rotation is the only definite and certain
one among all the planets besides the earth. For two
hundred years Mars has rotated at exactly this rate, and
there is no reason to suppose that the time will change
appreciably any more than the length of our day will.
The close approach to one of our days, the excess being
only thirty-seven minutes, leads to the result that, on
successive nights, Mars will, at the same hour, present
nearly the same face to the earth. But, owing to the ex-
cess in question, it will always be a little farther behind
on any one night than on the night before, so that, at the
end of forty days, we shall have seen every part of the
planet that is presented to the earth.

All that was known of Mars up to a quite recent
period could be embodied in a map of the planet, showing
the bright and dark regions of its surface, and in the
fact that a white cap would be generally seen to surround
each of its poles. When a pole was inclined toward us,
and therefore toward the sun, this cap gradually grew
smaller, enlarging again when the pole was turned from

180 PLANETS AND THEIR SATELLITES

the sun. In the latter case it would be invisible from the
earth, so that the growth would be recognised only by
its larger size when it again came into sight. These caps
were naturally supposed to be snow and ice which formed
around the poles during the Martian winter, and partly.
or wholly melted away during the summer.

The Canals of Mars

In 1877 commenced Schiaparelli's celebrated observa-
tions on the surface of Mars, and his announcement of
the so-called canals. The latter consisted of streaks
passing from point to point on the planet, and slightly
darker than the general surface. Seldom has more mis-
apprehension been caused by a mistranslation than in
the present case. Schiaparelli called these streaks canale,
an Italian word meaning channels. He called them so
because it was then supposed that the darker regions
of the surface were oceans, and the streams connecting
the oceans were therefore supposed to be water, and so
were called channels. But the translation "canals" led
to a widespread notion that these streaks were the works
of inhabitants, as canals on the earth are the works of
men.

Up to the present time there is some disagreement be-
tween observers and astronomical authorities on the sub-
ject of these channels. This arises from the fact that
they are not well-defined features on an otherwise uni-
form surface. Everywhere on the planet are found
variations of shade — ^light and dark patches, so faint
and ill defined that it is generally difficult to assign exact

*o o '^ b o '^ ''"' S

S

"k.

182 PLANETS AND THEIR SATELLITES

form and outline to them, running into each other by
insensible gradations. The extreme difficulty of making
them out at aU, and the variety of aspects they present
under different illuminations and in different states of
our atmosphere, has resulted in a great variety of in-
consistent delineations of these objects. At one extreme
we have the drawings made by the observers at the Lowell
Observatory at Flagstaff, Ariz. These show the chan-
nels as fine dark lines, so numerous as to form a network
covering the greater part of the surface of the planet.
In Schiaparelli's map they are rather broad faint bands,
not nearly so well defined as in Lowell's drawings. Low-
ell's channels are much more numerous than those seen
by Schiaparelli. We might therefore suppose that all
marked by the latter could be identified on Lowell's map.
But such is far from being the case ; there is only a gen-
eral resemblance between the features seen at the two
stations. One of the most curious features of Lowell's
drawings is that the points where the channels cross each
other are marked by dark round spots like circular
lakes. No such spots as these are shown on Schiaparelli's
map.

One of the best marked features of Mars is a large,
dark, nearly circular spot, surrounded by white, which
is called Lacus Solis, or the Lake of the Sun. All ob-
servers agree on this. They also agree in a considerable
part as to certain faint streaks or channels extending
from this lake. But when we go farther we find that
they do not agree as to the number of these channels,
nor is there an exact agreement as to the surrounding

THE CANALS OF MARS

183

features. It will be interesting to study two drawings
of this region made at the Lick Observatory, probably
under the best possible conditions, by Campbell and
Ilussey, respectively.

It is not likely that any observatory is more favoured
by its atmosphere for observations on this planet than
the Lick on Mount Hamilton. Its telescope is the largest
and finest in the world that has ever been especially

Figs. 33-34.— X>»'

of Laeus Solis on Mars, by Messrs. Campbell and
Hvssey,

directed to Mars, and Barnard is one of the most cautious
observers. It is therefore very noteworthy that on the
face of Mars, as presented to Barnard in the Lick tele-
scope, the features do not quite correspond to the chan-
nels of Schiaparelli and Lowell. When the air was ex-
ceptionally steady he could see a vast number of minute
and very faint markings, which were not visible in the
smaller telescopes used by the other observers. These

184 PLANETS AND THEIR SATELLITES

were sa intricate that it was impossible to represent them
on a drawing. They were not confined to the brighter
regions of the planet, or the supposed continents, but
were found to be more numerous on the so-called seas.
They showed no such regularity that they could be con-
sidered as channels running from one region to another.
The eye could indeed trace darker streaks here and there,
and some of these corresponded to the supposed channels,
but they were far more irregular than the features on
SchiapareUi's and Lowell's maps.

The matter was explained by CeruUi, a careful and in-
dustrious Italian observer, in a way which seems very
plausible. He found that after he had been studying
Mars for two years he was able, by looking at the moon
through an opera glass, to see, or fancy he saw, lines
and markings upon its surface similar to those of Mars.
This phenomenon is not to be regarded as a pure illusion
on the one hand, or an exact representation of objects
on the other. It grows out of the spontaneous action of
the eye in shaping slight and irregular combinations of
light and shade, too minute to be separately made out,
into regular forms.

Probable Nature of the Channels

The probable facts of the case may be summed up as
follows :

1. The surface of. Mars is extremely variegated by
regions diifering in shade, and having no very distinct
outlines.

2. There are numerous dark streaks, generally some-

THE ATMOSPHERE OF MARS 185

what indefinite in outline, extending through consider-
able distances across the planet.

3. In many cases the dark portions appear as if
chained together to a greater or less extent, and thus
give rise to the appearance of long dark channels.

The appearance on which this third phenomenon,
which we may regard as identical with that observed by
Cerulli, is based, may be well illustrated by looking, with
a magnifying glass, at a stippled portrait engraved on
steel. Nothing will then be seen but dots, arranged in
various hnes and curves. But take away the magnifying
glass and the eye connects these dots into a well-defined
collection of features representing the outlines of the
human face. As the eye makes an assemblage of dots into
a face, so may it make the minute markings on the planet
Mars into the form of long, unbroken channels.

The features which we hatVe hitherto described do not
belong to the two polar regions of the planet. Even when
the snowcaps have melted away, these regions are seen
so obUquely that it would be difficult to trace any well-
defined features upon them. The interesting question is
whether the caps which cover them are really snow which
falls during the Martian winter and melts again when
the sun once more shines on the polar regions. To throw
light on this question we have to consider some recent
results as to the atmosphere of the planet.

The Atmosphere of Mars

All recent observers are agreed that, if Mars has any
atmosphere at all, it is much rarer than our own, and

186 PLANETS AND THEIR SATELLITES

contains little or no aqueous vapour. This conclusion is
reached from observations both with the telescope and
the spectroscope. The most careful eye observations of
the planet show that the features are rarely, if ever, ob-
scured by anything which can be considered as clouds in
the IMartian atmosphere. It is true that the features are
not always seen with the same distinctness ; but the varia-
tions in the appearance are no greater than would be due
to the changes in the steadiness and purity of our own
atmosphere, through which the astronomer necessarily
makes his observations. Although, near the edge of the
apparent disk of the planet, the features appear to be
softened, as if seen through a greater thickness of the at-
mosphere, this appearance is, at least in part, due to the
obliquity of the line of sight, which prevents our getting
so good a view of the edge of the disk as of its centre.
Something of the same sort may be noticed when the
moon is viewed with the naked eye or an opera glass. Yet
it is quite possible that a certain amount of the softening
may be due to a rare atmosphere on Mars.

The most careful spectroscopic examination of the
planet was made by Campbell, who compared its spec-
trum with that of the moon. He could not detect the
slightest difference between the two spectra. Now, if
Mars had an atmosphere capable of exerting a strong
selective absorption on light, we should see lines in the
spectrum due to this absorption or, at least, some of the
lines would be strengthened. Our general conclusion
therefore must be that, while it is quite probable that
Mars has an atmosphere, it is one of considerable rarity.

IS THERE SNOW ON MARS? 187

and does not bear much aqueous vapour. Now snow can
fall only through the condensation of aqueous vapour in
the atmosphere. It does not therefore seem likely that
much snow can fall on the polar regions of Mars.

Another consideration is that the power of the sun's
rays to melt snow is necessarily limited by the amount of
heat that they convey. In the polar regions of Mars the
rays fall with a great obliquity, and even if all the heat
conveyed by them were absorbed, only a few feet of snow
could be melted in the course of the summer. But
far the larger proportion of this heat must be reflected
from the white snow, which is also kept cool by the
intense' radiation into perfectly cold space. We there-
fore conclude that the amount of snow that can fall
and melt around the polar regions of Mars must be
very small, being probably measured by inches at the
outside.

As the thinnest fall of snow would suffice to produce a
white surface, this does not prove that the caps are not
snow. But it seems more likely that the appearance is
produced by the simple condensation of aqueous vapour
upon the intensely cold surface, producing an appear-
ance similar to that of hoarfrost, which is only frozen
dew. This seems to me the most plausible explanation
of the polar caps. It has also been suggested that the
caps may be due to the condensation of carbonic acid.
We can only say of this, that the theory, while not impos-
sible, seems to lack probability.

The reader will excuse me from saying anything in
this chapter about the possible inhabitants of Mars. He

188 PLANETS AND THEIR SATELLITES

knows just as much of the subject as I do, and that is
nothing at all.

The Satellites of Mars

No discovery more surprised the whole world than that
of two satellites of Mars by Professor Asaph Hall, at
the Naval Observatory, in 1877. They had failed of
previous detection owing to their extreme minuteness. It
was not considered likely that a satellite could be so small
as these were found to be, and so no one had taken the
trouble to make a careful search with any great telescope.
But, when once discovered, they were found to be by no
means difficult objects. Of course the ease with which
they can be seen depends on the position of Mars both in
its orbit and with respect to the earth. They are never
visible except when the planet is near its opposition. At
each opposition they may be observed for a period of
three, four, or even six months, according to circum-
stances. At an opposition near perihelion they may be
seen with a telescope of less than twelve inches diameter ;
how small a one will show them depends on the skill of the
observer, and the pains he takes to cut off the light of
the planet from his eye. Generally a telescope ranging
from twelve to eighteen inches in diameter is necessary.
The difficulty in seeing them arises entirely from the
glare of the planet. Could this be eliminated they could
doubtless be seen with much smaller instruments. Owing
to the glare, the outer one is much easier to see than tha
inner one, although the inner one is probably the brighter
of the two.

THE SATELLITES OF MARS 189

Professor Hall assigned the name Deimos to the outer
and Phobos to the Inner, these being the attendants of
Mars in ancient mythology. Phobos has the remark-
able peculiarity that it revolves around the planet in
less than nine hours, making its period the shortest of
any yet known in the solar system. This is little more
than one third the time of the planet's rotation on its
axis. The consequence of this is that, to the inhabitants
of the planet, its nearest moon rises in the west and
sets in the east.

Deimos performs its revolution in 30 hours 18 minutes.
The result of this rapid motion is that some two days
must elapse between its rising and setting.

Phobos is only 3,700 miles from the surface of the
planet. It must therefore be an interesting object to the
inhabitants of Mars, if they have telescopes.

In size these bodies are the smallest visible to us in the
solar system, with the possible exception of Eros and
possibly some others of the fainter asteroids. From Pro-
fessor Pickering's photometric estimates their diameter
was estimated to be not very different from seven miles.
Their apparent size as we view them is therefore not very
different from that of a small apple hanging over the
city of Boston, and seen with a telescope from the city
of New York. In this respect they form a singular con-
trast to nearly or quite all of the other satellites, which
are generally a thousand miles or more in diameter. The
one exception to this is the fifth satellite of Jupiter, to be
described in the chapter on Jupiter and its satellites.
Although this is much less than a thousand miles in diam-

190 PLANETS AND THEIR SATELLITES

eter, it must considerably exceed the satellites of Mars
in size.

The satellites have been most useful to the astronomer
in enabling him to learn the exact mass of Mars. How
this is done will be explained in a subsequent chapter,
where the methods of weighing the planets are set forth.

The satellites also offer many curious and difficult
problems in gravitation. Their orbits seem to have a
slight eccentricity, and the position of the planes in which
they revolve changes in consequence of the bulging of
the planet at its equator, produced by its rotation. The
calculation of these changes and their comparison with
observations have opened up a field of research in which
Professor Hermann Struve, now of the University of
Koenigsberg, Germany, has taken a leading part.

V

The Gkoup of Minor Planets

The seeming gap in the solar system between the
orbits of Mars and Jupiter naturally attracted the at-
tention of astronomers as soon as the distances of the
planets had been accurately laid down. It became very
striking when Bode announced his law. There was a
row of eight numbers in regular progression, and every
number but one represented the distance of a planet.
That one place was vacant. Was the vacancy real, or
was it only because the planet which filled it was so small
that it had escaped notice?

This question was settled by Piazzi, an Italian as-
tronomer who had a little observatory in Palermo in
Sicily. He was an ardent observer of the heavens, and
was engaged in making a catalogue of all the stars
whose positions he could lay down with his instrument.
On January 1, 1801, he inaugurated the new century
by finding a star where none had existed before; and
this star soon proved to be the long-looked-f or planet. It
received the name of Ceres, the goddess of the wheat
field.

It was a matter of surprise that the planet should be
so small; and when its orbit became known it proved to
be very eccentric. But new revelations were soon to
come. Before the new planet had completed a revolu-

192 PLANETS AND THEIR SATELLITES

tion after its discovery, Dr. Olbers, a physician of Bre-
men, who employed his leisure in astronomical observa-
tions and researches, found another planet revolving in
the sama region. Instead of one large planet there were
two small ones. He suggested that these might be
fragments of a shattered planet, and that, if so, more
would probably be found. The latter part of the con-
jecture proved true. Within the next three years two
more of these little bodies were discovered, making four
in all.

Thus the matter remained for some forty years.
Then, in 1845, Hencke, a German observer, found a
fifth planet. The year following a sixth was added,
and then commenced the curious series of discoveries
which, proceeding year by year, are now carrying the
number known rapidly past five hundred.

Hunting Asteroids

Up to 1890 these bodies had been found by a few
observers who devoted especial attention to the search,
and caught the tiny stars as the hunter does game. They
would lay traps, so to speak, by mapping the many small
stars in some small region of the sky near the ecliptic,
familiarise themselves with their arrangement, and then
watch for an intruder. Whenever one appeared, it was
found to be one of the group of minor planets, and the
hunter put it into his bag.

Quite a succession of planet-hunters appeared, some
of them little known for any other astronomical work.
The most successful of these in the fifties was Gold-

HUNTING ASTEROIDS 193

Schmidt, of Paris, a jeweller if I mistake not. Three
were discovered by Professor James Ferguson at the
Washington Observatory. Palisa, of Vienna, made a
record for himself in this work. In this country Pro-
fessors C. H. F. Peters, of Clinton, and James C. Wat-
son, of Ann Arbor, were very successful. The last three
observers carried the number above the two hundred
mark.

About 1890 the photographic art was found to offer
a much easier and more effective means of finding these
objects. The astronomer would point his telescope at
the sky and photograph the stars with a pretty long
exposure, perhaps half an hour, more or less. The stars
proper would be taken on the negative as small round
dots. But if a planet happened to be among them it
would be in motion, and thus its picture would be taken
as a short line, and not as a dot. Instead of scanning
the heavens the observer had only to scan his photo-
graphic plate, a much easier task, because the planet
could be recognised at once by its trail.

Recently a dozen or more of these bodies have been
found nearly every year. Of course the unknown ones
are smaller and more difficult to find as the years elapse.
But there is as yet no sign of a limit to the number.
Most of those newly discovered are very minute, yet the
number seems to increase with their smallness. Even
the larger of these bodies are so small that they appear
only as star-like points in ordinary telescopes, and
their disks are hard to make out even with the most
powerful instruments. So far as can be determined,

194 PLANETS AND THEIR SATELLITES

the diameters of the largest ones, naturally the earliest
discovered, are only three or four hundred miles. The
size of the smallest can be inferred only in a rough
way from their brightness. They may be twenty or
thirty miles in diameter.

Orbits of the Asteroids

The orbits of these bodies are for the most part very
eccentric. In the case of Polyhymnia, the eccentricity
is about 0.33, which means that at perihelion it is one
third nearer the sun than its mean distance, and at aphe-
lion one third more. It happens that its mean distance
is just about three astronomical units; its least distance
from the sun is therefore two, its greatest four, or twice
as great as the least.

The large Inclination of most of the orbits is also
noteworthy. In several cases It exceeds twenty degrees.
In that of Pallas It Is twenty-eight degrees.

Olbers' idea that these bodies might be fragments of
a planet which had been shattered by some explosion is
now a;bandoned. The orbits range through too wide a
space ever to have joined, as they would have done if
the asteroids had once formed a single body. In the
philosophy of our time t;hese bodies have been as we see
them since the beginning. On the theory of the nebular
hypothesis the matter of all the planets once formed
rings of nebulous substance moving round the sun. In
the case of all the other planets the material of these
rings gradually gathered around . the densest point of
the ring, thus agglomerating into a single body. But

GROUPING OF THE ASTEROIDS 195

it is supposed that the ring forming the minor plan-
ets did not collect in this way, but separated into in-
numerable fragments.

Groupings of the Orbits

There is a curious feature of the orbits of these bodies
which may throw some light on the question of their
origin. I have explained that the planetary orbits are
nearly exact circles, but that these circles are not cen-
tred on the sun. Now imagine ourselves to look down
upon the solar system from an immense height, and sup-
pose that the orbits of the minor planets were visible
as finely drawn circles. These circles would appear to
interlace and cross
each other like an
intricate network,
filling a broad ring
of which the outer
diameter would be
nearly or quite
double the inner
one.

But suppose we
could pick all these
circles up, as if they
were made of wire,
and centre them all on the sun, without changing their
size. The diameters of the larger ones would be double
those of the smaller, so that the circles would fill a broad
space, as shown in the figure. Now, the curious fact is

Fig. 36. — Separaiiou of the Minor Planets
into Groups.

196 PLANETS AND THEIR SATELLITES

•■^JUPtTCK

that they would not fill the whole space uniformly, but
would be collected into distinct groups. These groups
are shown on the figures of their orbits, given above, and,
on a different plan, and more com-
pletely, in the second figure, which is
arranged on a plan explained as fol-
lows: Every planet performs its revo-
lution in a certain number of days,
which is greater the farther the planet
from the sun. Since the complete cir-
cumference of the orbit measures
1,296,000", it follows that if we
divide this number by the time of revo-
lution, the quotient will show through
what angle, on the average, the planet
moves along its orbit in one day. This
angle is called the mean motion of the
planet. In the case of the minor
planets it ranges from 400" to more
than 1 ,000 ', being greater the shorter
the time of revolution and the nearer
the planet is to the sun.

Now we draw a vertical line and
mark off on it values of the mean mo-
tion, from four hundred to one thou-
sand seconds, differing by ten sec-
onds. Between each pair of marks
we make as many points as there are planets having
mean motions between the limits. For example, be-
tween 550" and 560" there are three dots. This means

■ |JUPITER

Fig. S&.—Bistrihi-
iion of the Or-
bits of the Minor
Planets.

GROUPING OF THE ASTEROIDS 197

that there are three planets having mean motions between
550" and 560". There are also four planets between
560" and 570", and one between 570" and 580". Then
there are no more till we pass 610", when we find six
planets between 610" and 620", followed by a multitude
of others.

Examining the diagram we are able to distinguish
five or six groups. The outermost one is between 400
and 460", and is nearest to Jupiter. The times of revo-
lution are not far from eight years. Then there is a wide
gap extending to 560", when we have a group of ten
planets between 540" and 580". From this point down-
wards the planets are more numerous, but we find very
sparse or empty points at 700", 750", and 900". Now
the most singular feature of the case is that these empty
spaces are those in which the motion of a planet would
have a simple relation to that of Jupiter. A planet with
a mean motion of 900" would make its circuit round the
sun in one third the time that Jupiter does ; one of 600
in half the time ; one of 750" in two fifths of the time.
It is a law of celestial mechanics that the orbits of planets
having these simple relations to another undergo great
changes in the course of time from their action on each
other. It was therefore supposed by Kirkwood, who first
pointed out these gaps in the series, that they arose be-
cause a planet within them could not keep its orbit per-
manently. But it Is curious that there is no gap, but on
the contrary, a group of planets whose mean motion is
nearly two thirds of that of Jupiter. Hence the view Is
doubtful.

198 PLANETS AND THEIR SATELLITES

The Most Curious of the Asteroids

One of these bodies is so exceptional as to attract our
special attention. All the hundreds of minor planets
known up to 1898 moved between the orbits of Mars
and Jupiter. But in the summer of that year Witt, of
Berlin, found a planet which, at perihelion, came far
within the orbit of Mars — in fact within fourteen million
miles of the orbit of the earth. He named it Eros.
The eccentricity of its orbit is so great that at aphelion
the planet is considerably outside the orbit of Mars.
Moreover the two orbits, that of the planet and of Mars,
pass through each other like two links of a chain, so
that if the orbits were represented of wire they would
hang together.

Owing to the inclination of its orbit, this planet
seems to wander far outside the limits of the zodiac.
When nearest the earth, as it was in 1900, it was for a
time so far north that it never set in our middle lati-
tudes, and passed the meridian north of the zenith. This
peculiarity of its motion was doubtless one reason why
it was not found sooner. During its near approach in
the winter of 1900-'01 it was closely scrutinised and
found to vary in brightness from hour to hour. Care-
ful observation showed that these changes went through
a regular period of about two and a half hours. At
this interval it would fade away a little with great uni-
formity. Some observers maintained that It was fainter
at every alternate minimum of light, so that the real
period was five hours. It was supposed that this indi-

MOST CURIOUS OF THE ASTEROIDS 199

cated that the object was really made up of two bodies
revolving round each other — perhaps actually joined
into one. But it seems more likely that the variations
of light were due to there being light and dark regions
on the surface of the little planet, which therefore
changed in brightness according as bright or dark
regions predominated on the surface of the hemisphere
turned toward us. The case was made perplexing by
the gradual disappearance of the variations after they
had been well established by months of observation.
There seems to be some mystery in the constitution of
this body.

From a scientific point of view Eros is most interesting
because, coming so near the earth from time to time,
its distance may be measured with great precision, and
the distance of the sun as well as the dimensions of the
whole solar system thus fixed with greater exactness
than by any other method. Unfortunately, the nearest
approaches occur only at very long intervals. What
is most tantalising is that there was such an approach in
1892 before the object was recognised. At that time it
was photographed a number of times at the Harvard
Observatory, but was lost in the mass of stars by which
it was surrounded. Its distance was, astronomically, only
sixteen hundredths, or some fifteen millions of miles,
while the nearest approaches of Mars are nearly forty
millions. There will not be another approach so near for
more than sixty, perhaps not for more than a hundred
years.
' In 1900 it approached the earth within aLout thirty

200 PLANETS AND THEIR SATELLITES

millions of miles, and a combined effort was made at
various observatories to lay down its exact position from
night to night among the stars by photography, with a
view to determining its parallax. But the planet was
faint, the observations were difficult, and it is not yet
known what measure of success was reached.

Variations of light which might be due to a rotation
on their axes have been suspected in the case of other
asteroids besides Eros, but nothing has yet been settled.

VI

Jupiter and it's Satellites

JrpiTEE, the "giant planet," is, next the sun, the
largest body of the solar system. It is, in fact, more than
three times as large, and about three times as massive as
all the other planets put together. Yet, such is the pre-
ponderating mass of our central luminary that the mass
of Jupiter is less than one thousandth part that of the
sun.

This planet is in opposition in September, 1903, Octo-
ber, 1904, November, 1905, and so on for several years
afterward, about a month later every year. Near the
time of opposition it may easily be recognised in the even-
ing sky, both by its brightness and its colour. It is then,
next to Venus, the brightest star-like object in the
heavens. It can easily be distinguished from Mars by its
whiter colour. If we look at it with a telescope of the
smallest size, even with a good ordinary spy-glass, we
shall readily see that instead of being a bright point, like
a star, it is a globe of very appreciable dimensions. We
shall also see what look Uke two shadowy belts crossing
the disk. These were noticed and pictured two hundred
years ago by Huygens. As greater telescopic power was
used it was found that these seeming belts resolved them-
selves into very variegated cloud-like forms, and that
they vary, not only from month to month, but even from

202 PLANETS AND THEIR SATELLITES

night to night. By careful observation on the aspect
which they present from hour to hour, and from night to
night, it was found that the planet rotates on its axis
in about 9 hours 55 minutes. The astronomer may there-
fore in the course of a single night see every part of the
surface of the planet presented to his view in succession.

Two features presented by the planet will at once
strike the careful observer with the telescope. One of
these is that the disk does not seem uniformly bright, but
gradually shades off near the limb. The latter, instead
of being bright and hard is somewhat soft and diffuse.
In this respect the appearance forms quite a contrast to
that presented by the moon or Mars. The shading off
toward the edge is sometimes attributed to a dense atmos-
phere surrounding the planet. While this is possible,
it is by no means certain.

The other feature to which we allude is an ellipticity of
the disk. Instead of being perfectly round, the planet
is flattened at the poles, like our earth, but in a much
greater degree. The most careful observer, viewing the
earth from another planet, would see no deviation from
the spherical form, but, viewing Jupiter, the deviation is
very perceptible. This is owing to its rapid rotation on
its axis, which causes its equatorial regions to bulge out,
as, to a smaller degree, in the case of the earth.

Surface of Jupiter

The features of Jupiter, as we see them with a tele-
scope, are almost as varied as those of the clouds which
we see in our atmosphere. There are commonly elon-

SURFACE OF JUPITER 203

gated strata of clouds, apparently due to the same cause
that produces stratified clouds on the earth, namely, cur-
rents of air. Among these clouds round white spots are
frequently seen. The clouds are sometimes of a rosy
tinge, especially those near the equator. They are
darkest and most strongly marked in middle latitudes,
both north and south of the equatorial regions. It is
this that produces the appearance of dark belts in a small
telescope.

The appearance of Jupiter is, in almost every point,
very different from that of Mars or Venus. Comparing
it with Mars, the most strongly marked difference con-
sists in the entire absence of permanent features. Maps
of Mars may be constructed and their correctness tested
by observations generation after generation, but owing
to the absence of permanence, no such thing as a map of
Jupiter is possible.

Notwithstanding this lack of permanence, features
have been known to endure through a number of years,
and some of them may be permanent. The most remark-
able of these was the great red spot, which appeared in
middle latitudes, on the southern hemisphere of the planet,
about the year 1878. For several years it was a very
distinct object, readily distinguished by its colour. After
ten years it began to fade awaj', but not at a uniform
rate. Sometimes it would seem to disappear entirely,
then would brighten up once more. These changes con-
tinued but, since 1892, faintness or invisibility has been
the rule. If the spot finally disappeared, it was in so un-
certain a way that no exact date for the last observation

s

1^

s

I

CONSTITUTION OF JUPITER 205

of it can be given. Some observers with good eyes still
report it to be visible from time to time.

Constitution of Jupiter

The question of the constitution of this curious planet
is still an unsettled one. There is no one hypothesis that
readily explains all the facts, which suggest many points,
but prove few, unless negatively.

Perhaps the most remarkable feature of the planet is
its small density. Its diameter is about eleven times that
of the earth. It follows that, in volume, it must exceed
the earth more than thirteen hundred times. But its
mass is only a little more than three hundred times that
of the earth. It follows from this that its density is much
less than that of the earth ; as a matter of fact, it is only
about one third greater than the density of water. A
simple computation shows that the force of gravity at its
surface is between two and three times that at the surface
of the earth. Under this enormous gravitation we might
suppose its interior to be enormously compressed, and its
density to be great in comparison. Such would certainly
be the case were it made up of solid or fluid matter of the
same kind that composes the surface of the earth. From
this fact alone the conclusion would be that its outer por-
tions at least were composed of aeriform matter. But
how reconcile this form with the endurance of the red
spot through twenty-five years? This is the real diffi-
culty of the case.

Nevertheless, the hypothesis is one which we are forced
to accept without great modification. Besides the evi-

206 PLANETS AND THEIR SATELLITES

dence of vapour as shown by the constantly changing
aspect of the planet, we have another almost conclusive
piece of evidence in the law of rotation. It is found that
Jupiter resembles the sun in that its equatorial region
rotates in less time than the regions north of middle lati-
tude, although the circuit they have to make is longer.
This is probably a law of rotation of gaseous bodies in
general. It seems, therefore, that Jupiter has a greater
or less resemblance to the sun in its physical constitution,
a view which quite corresponds with its aspect in the tele-
scope. The difference in the time of rotation at the equa-
tor and in middle latitudes is, so far as we yet know, about
five minutes. That is to say, the equatorial region rotates
in nine hours fifty minutes and those in middle latitudes in
nine hours fifty-five minutes. This corresponds to a dif-
ference of velocity of the motion between the two amount-
ing to about two hundred miles an hour; a seemingly
impossible difference were the surface liquid.

It is a singular fact that no well-defined law of rotation
in different latitudes has yet been made out, as has been
done in the case of the sun. Were we to accept the re-
cults of the meagre observations at our disposal we might
be led to the conclusion that the difference of time is
not a gradually varying quantity, as we go from the
equator toward the poles, but that the change of five
minutes occurs very near a certain latitude and almost
suddenly. But we cannot assume this to be the case
without more observations than are yet on record. The
subject is one of which an accurate investigation is
greatly to be desired.

CONSTITUTION OF JUPITER 207

Yet another resemblance between Jupiter and the sun
is that they are both brighter in the centre of their disk
than toward the circumference. In the case of Jupiter,
the shading off is very well marked. The extreme cir-
cumference especially is more softened than that of any
of the other planets.

The apparent resemblance between the surfaces of
these bodies, taken in connection with the brightness of
the planet, has led to the question whether Jupiter may
not be, in whole or in part, self-luminous. This again is
a question which needs investigation. The idea that the
planet can emit much light of its own seems to be nega-
tived by the fact that the satellites completely disappear
when they pass into its shadow. We may therefore say
with entire certainty that Jupiter does not give enough
light to enable us to see a satellite by that light alone.
We can hardly suppose that this would be the case if the
satellite received one per cent as much light from the
planet as it does from the sun. It is also found that the
light which Jupiter sends out is somewhat less than
that which it receives from the sun. That is to say, all
the light which it gives out, when estimated in quantity,
may be reflected light, without supposing the planet
brighter than white bodies on the surface of the earth.
But this still leaves open the question whether the white
spots, sometimes so much brighter than the rest of the
planet, may not give us more light than can fall upon
them. This also is a question not yet investigated.

The hypothesis which best lends itself to all the facts
seems- to be that the planet has a solid nucleus, of which

208 PLANETS AND THEIR SATELLITES

the density may be as great as that of the earth or any
other sohd planet, and that the small average density
of the entire mass is due to the vapourous character of the
matter which surrounds this nucleus. In all probability
the nucleus is at a very high temperature, even ap-
proximating that at the surface of the sun, but this
temperature gradually diminishes as we ascend through
the gaseous atmosphere, as we suppose to be the case
with the sun ; hence it may happen that, at the surface,
none of the material that we see is at a high enough
temperature to radiate a sensible amount of heat.

On the whole we may describe Jupiter as a small sun
of which the surface has cooled off till it no longer emits
light.

The Satellites of Jupiter

When Galileo first turned his little telescope on the
planet Jupiter he was delighted and surprised to find it
accompanied by four minute companions. Watching
them from night to night, he found them to be in rev-
olution around their central body as, upon the theory
not fully accepted in his time, the planets revolve
around the sun. This remarkable resemblance to the
solar system was a strong point in favor of the Coper-
nican Theory.

These bodies can be seen with a common spy-glass, or
even a good opera- glass. It has even been supposed that
good eyes sometimes see them without optical assistance.
They are certainly as bright as the smallest stars visible
to the naked eye, yet the glare of the planet would seem
to be an insuperable obstacle to their visibility, even to

THE SATELLITES OF JUPITER 209

the keenest vision. A story has been told, by Arago, I
think, of a woman who professed to be able to see them at
any time and even pointed out their positions. It was
found, however, that she described them as on the op-
posite side of the planet to that on which they were really
situated. It was then found, or inferred, that she took
the positions from an astronomical ephemeris, in which
diagrams of them were given, but in which the pictures
were made upside down in order that the satellites might
be seen as in an ordinary inverting telescope. But it
seems quite likely that, when the two outer satellites
chance to be nearly in the same straight line, they may
be visible by their combined light.

From the measures of Barnard it may be inferred that
these bodies range somewhere between two and three thou-
sand miles in diameter. Hence, they do not differ greatly
from our moon in size.

Only four satellites were known until 1892 ; then Bar-
nard, with the great Lick telescope, discovered a fifth,
much nearer the planet than the four others. It makes
a revolution in a little less than twelve hours, the short-
est periodic time known except that of the inner satelhte
of Mars. Still, however, it is a little longer than the
rotation time of the planet. The next outer one, or the
innermost of the four previously known, still called the
first satellite, revolves in about one day- eighteen and a
half hours, while the outer one requires nearly seventy
days to perform its circuit.

In its visibility the fifth satellite is the most difficult
known object -in the solar system. Through only a few

210 PLANETS AND THEIR SATELLITES

of the most powerful telescopes of the world has it ever
certainly been seen by the human eye. Its orbit is de-
cidedly eccentric. Owing to the eUipticity of the planet,
it possesses the remarkable peculiarity that its major axis,
and, therefore, the perihelion point of its orbit, performs
a complete revolution in about a year.

It has sometimes been questioned whether these satel-
Htes are round bodies, Hke the planets and most other
satellites. Some observers, especially Barnard and W. H.
Pickering, noticed curious changes in the form of the
first satellite as it was crossing the surface of the planet.
At one time it looked like a double body. But Barnard,
by careful and repeated study, "showed that this appear-
ance was partly due to the varying shade of the back-
ground on which the satellite was seen projected upon
the planet, and partly to the differences in the shade of
various parts of the satellite itself.

During their course around the planet these bodies
present many interesting phenomena, which can be ob-
served with a moderate sized telescope. These are their
eclipses and transits. Of course Jupiter, like any other
opaque body, casts a shadow. As the satellites make
their round they nearly always pass through the shadow
during that part of their course which is beyond the
planet. Exceptions sometimes occur in the case of the
fourth and most distant satellite, which may pass above
or below the shadow, as our moon passes above or below
that of the earth. When a satellite enters the shadow, it
is seen to fade away gradually, and finally to disappear
from sight altogether.

THE SATELLITES OF JUPJTER 211

For the same reason the satellites generally pass across
the disk of the planet in that part of their course which
lies on this side of it. The general rule is that, when a
satellite has impinged on the planet, it looks brighter
than the latter, owing to the darkness of the planet's limb.
But, as it approaches the central regions, it may look
darker than the background of the planet. Of course
this does not arise from any change in the brightness of
the satellite, but only from the fact, already mentioned,
that the planet is brighter in its central regions than at
its limb.

Yet more interesting and beautiful is the shadow of a
satellite which, under such circumstances, may often be
seen upon the planet, looking like a black body crossing
alongside the satellite itself. Such a shadow is shown in
the picture of Jupiter on page 204.

The phenomena of Jupiter's satellites, including their
transits and those of their shadows, are all predicted in
the astronomical ephemerides, so that an observer can
always know when to look for an eclipse or transit.

The eclipses of the inner of the four older satellites
occur at intervals of less than two days. By noting their
times, an observer in unknown regions of the earth can
determine his longitude more easily than by any other
method. He has first to determine the error of his watch
on local time by certain simple astronomical observations,
quite familiar to astronomers and navigators. He thus
finds the local time at which an eclipse of the satellite
takes place. He compares this with the time predicted
in the ephemeris. The difference gives his longitude

212 PLANETS AND THEIR SATELLITES

according to the system set forth in our chapter on Time
and Longitude.

The principal drawback of this method is that it is
not very accurate. Observations of the time of such an
eclipse are doubtful to a large fraction of a minute. This
corresponds to 16 minutes of longitude, or 15 nautical
miles at the equator. In the polar regions the effect of
the error is much smaller, owing to the convergence of
the meridians. The method is, therefore, most valuable
to polar explorers.

VII

Saturn and its System

Among the planets, Saturn is next to Jupiter in size
and mass. It performs its revolution round the sun in
twenty-nine and a half years. When the planet is visible
the casual observer will generally be able to recognise
it without difficulty by the slightly reddish tint of its
light, and by its position in the heavens. During the
next few years it will be in opposition first in summer
and then in autumn, about twelve or thirteen days later
each year. Starting from August, 1903, opposition will
occur in August of 1904-'05, September of 1906-'08, Oc-
tober of 1909-'10, and so on. At these times Saturn will
be seen each evening after dark in the eastern or south-
eastern sky, moving toward the south as the evening ad-
vances. It looks a good deal like Arcturus, which, for a
few years to come, will be visible at the same seasons,
only high up in the south or southwest, or lower down in
the west.

Although Saturn is far from being as bright as Jupi-
ter, its rings make it the most magnificent object in the
solar sj'stem. There is nothing else like them in the
heavens, and it is not surprising that they were an
enigma to the early observers with the telescope. To
Galileo they first appeared as two handles to the planet.
After a year or two they disappeared from his view. We

214. PLANETS AND THEIR SATELLITES

now know that this occurred because, owing to the motion
of the planet in its orbit, they were seen edge-on, and
are then so thin as to be invisible in a telescope as imper-
fect as Galileo's. But the disappearance was a source
of great embarrassment to the Tuscan philosopher, who
is said to have feared that he had been the victim of some
illusion on the subject, and ceased to observe Saturn.
He was then growing old, and left to others the task of
continuing his observations. Of course the handles soon
reappeared, but there was no way of learning what they
were. After more than forty years the riddle was solved
by Huyghens, the great Dutch astronomer and physicist,
who announced that the planet was surrounded by a thin
plane ring, nowhere touching it, and inclined to the
ecliptic.

Satellites of Saturn

Besides his rings, Saturn is surrounded by a retinue of
eight satellites^ — a greater number than any other planet.
The existence of a ninth has been suspected, but awaits
confirmation. They are very unequal in size and dis-
tance from the planet. One, Titan, may be seen with a
small telescope ; the faintest, only in very powerful ones.

Titan was discovered by Huyghens just as he had
made out the true nature of the rings. And hereby
hangs a little tale which has only recently come out
through the publication of Huyghens's correspondence.
Following a practice of the time, the astronomer sought
to secure priority for his discovery without making it
known, by concealing it in an anagram, a collection of
letters which, when properly arranged, would inform the

ASPECTS OP SATURN'S RINGS ^15

reader that the companion of Saturn made its revolu-
tion in fifteen days. A copy of this was sent to Wallis,
the celebrated English mathematician. In his reply the
latter thanked Huyghens for his attention, and said he
also had something to say, and gave a collection of letters
longer than that of Huyghens. When the latter inter-
preted his anagram to WalHs, he was surprised to receive
in reply a solution of the Wallis anagram announcing
the very same discovery, but, of course, in different lan.-
guage and at greater length. It turned out that Wallis,
who was expert in ciphers, wanted to demonstrate the
futility of the system, and had managed to arrange his
own letters so as to express the discovery, after he knew
what it was. Huyghens did not appreciate the joke.

Varying Aspects of Saturn's Rings

The Paris Observatory was founded iri 1666 a-s one of
the great scientific institutions of France which adorned
the reign of Louis XIV. Here Cassini discovered the
division in the ring, showing that the latter was really
composed of two, one outside the other, but in the same
plane. The outer of these rings has somewhat the ap-
pearance of being again divided, by a line called the
Encke division, after the astronomer who first noticed it,
but the exact nature of this division is still in doubt. It
certainly is not sharp and well defined like the Cassini
division, but only a slight shade.

To understand the varying appearance of the rings
we give a figure showing how they and the planet would
look if we could see them perpendicularly (which we

216 PLANETS AND THEIR SATELLITES

never can). We notice first the dark Cassini division,
separating the rings into two, an inner and an outer one,
the latter being the narrower. Then, on the outer ring,
we see the faint and grey Encke division, which is much

less marked and much
harder to see than
the other. Passing
to the inner ring,
the latter shades off
gradually on the in-
ner edge, where there
is a grey border
called the "crape
ring." This was first
described by Bond,
of the Harvard Ob-
servatory, and was
long supposed to be
a separate and dis-
tinct ring. But careful observation shows that such is
not the case. The crape ring joins on to the ring out-
side of it, and the latter merely fades away into the
other.

The rings of Saturn are inclined about twenty-seven
degrees to the plane of its orbit, and they keep the same
direction in space as the planet revolves round the sun.
The efi'ect of this will be seen by the figure, which shows
the orbit of the planet round the sun in perspective.
-When the planet is at A the sun shines on the north
(upper) side of the ring. Seven years later, when the

Fig. 39.-

-Perpendicular View of the Rings
of Saturn.

ASPECTS OF SATURN'S RINGS 217

planet is at B, the ring is presented to the sun edgewise.
After passing B the sun shines on the south (lower) side
at an inclination which continually increases till the
planet makes C, when the inclination is at its greatest,

..^B.

\A

C"\

Fig. 40. — Slmiiing how ilie Direction of the Plane of Saturn's Riitgs re-
mains Unchanged as the Planet moves round the Sun.

twenty-seven degrees. Then it diminishes as the planet
passes to D, at which point the edge of the ring is again
presented to the sun. From this point to A and B the
sun again shines on the north side.

The earth is so near the sun in comparison with Saturn
that the rings appear to us nearly as they would to an
observer on the sun. There is a period of fifteen years,
during which we see the north side of the rings, and at
the middle of which we see them at the widest angle. As
the years advance, the angle grows narrower and the
rings are seen more and more edgewise till they close up
into a mere line crossing the planet, or perhaps disappear
entirely. Then they open out again, to close up in
another fifteen years. A disappearance occurred in 1892
and another will take place in 1907.

218 PLANETS AND THEIR SATELLITES

With this view of what the shape of the rings really is,
we may understand their appearance to us. The rings
are always seen very obliquely, never at a greater angle
than twenty-seven degrees. The general outline pre-

FiOS. 41-42

ranee of the Rings of Saturn, according to Bar-
nard, when seen edgewise.

sented by the planet and rings is that seen in Figure 40.
The best views are obtained when the rings are seen at a
considerable angle. The divisions and the crape ring
are then seen. The shadow of the globe of the planet on
the ring will be seen as a dark notch. A dark line cross-

WHAT THE RINGS ARE 219

ing the planet like a border to the inner ring is the
shadow of the ring on the planet.

Very interesting are rather rare occasions when the
plane of the ring passes between the earth and the sun.
Then the sun shines on one side of the ring while the
other side is presented to us, though, of course, at a very
small angle. The chances for observing Saturn at such
times are rather few, especially in recent times. At both
the last occasions, 1877 and 1892, this only happened
for a few days, when the planet was not well situated for
these observations. Nevertheless, in October, 1892, Bar-
nard got a look at it from the Lick Observatory, and
found that the rings were totally invisible, though their
shadow could be seen on the planet. This shows that the
rings are so thin that their edges are invisible in a
powerful telescope.

What the Rings are

When it became accepted that the laws of mechanics,
as we learn them on the earth, govern the motions of the
heavenly bodies, another riddle was presented by the
rings of Saturn. What keeps the rings in place.? What
keeps the planet from running against the inner ring
and producing, to modify Addison's verse, a "wreck of
matter and crash of worlds" that would lay the whole
beautiful structure in ruins.? It was for a time supposed
that a liquid ring might be proof against such a catas-
trophe, and then it was shown that such was not the case.
Finally it was made clear that the rings could not be co-
hering bodies of any kind, but were merely clouds ot

£20 PLANETS AND THEIR SATELLITES

minute bodies, perhaps little satellites, perhaps only par-
ticles like pebbles or dust, or perhaps like a cloud of
smoke. This view had to be accepted, but was long with-
out direct proof. The latter was finally brought out by
Keeler with his spectroscope. He found that when the
light of the rings was spread out into a spectrum, the
dark spectral lines did not go straight across it, but were
bent and broken in such a way as to show that the matter
of the rings was revolving round the planet at unequal
speeds. At the outer edge it revolved most slowly; the
speed continually increased toward the inner edge, and
was everywhere the sanus*that a satellite would have if it
revolved round the planet at that distance. This most
beautiful discovery was made at the Allegheny Observa-
tory near Pittsburg, Pa.

System of Saturn's Satellites •

In making known his discovery of the satellite Titan,
Huyghens congratulated himself that the solar system
was now complete. There were now seven great bodies
and seven small ones, the magic number of each. But
within the next thirty years Cassini exploded all this
mysticism by discovering four more satellites. Then,
after the lapse of a century, the great Herschel found
yet two more. Finally, the eighth was found by Bond
at the Harvard Observatory in 1848.

In 1898 photographs of the sky taken at the South
American branch of the Harvard Observatory showed a
star near Saturn, but farther than the outermost known
■satellite, which seemed to be in a different position each

SATELLITES OF SATURN

221

night. It has not yet been decided whether this was a
satellite, because Satuftl lia'^ been among the countless
faint stars of the Milky Way, among which the satellite
might be lost.

The following is a list of the eight satellites, with
their distances from the planet in radii of the latter,
their times of revolution, and the discoverer of each :

No.

Discoverer.

Date
of Dis-
covery.

Distance

from

Planet.

Herschel

HerscheU

Cassini. . .".

Cassini

Cassini

Huyghens

Bond

*-1789
1789
1684
1684
1673
1655
1848
1671

3.3

4.3

5.8

6.8

9.5

31.7

26.8

64.4

Cassini

Time
of Revo-
lution.

Mimas . . .
Enceledas
Tethys...

Dione

Rhea

Titan

Hyperion.
Japetus . .

d. h.
23

9
21

18
12

15 28
21 7
70 22

The most noteworthy features of this list are the wide
range of distances among the satellites, and the relation
between the times of revolution of the four inner ones.
The five inner ones seem to form a group by themselves.
Then there is a gap exceeding in breadth the distance of
the innermost of the five, when we have another group of
two. Titan and Hyperion. Then there is a gap wider
than the distance of Hyperion, outside of which comes
Japetus, the outermost yet known.

A curious relation anaong the periods is that the period
of the third satellite is almost exactly twice that of the
first; and -that of the fourth almost twice that of the

222 PLANETS AND THEIR SATELLITES

second. Also, four periods of Titan are almost exactly
equal to three of Hyperion.

The result of the latter relation is a certain very curi-
ous action of these two satellites on each other, through
their mutual gravitation. To show this we give a dia-
gram of the orbits. That of Hyperion, the outer of the

Fig. 43. — Orbits of Titan and Hyperion, showing their relation.

two, is very eccentric, as will be seen by the figure. Sup-
pose the satellites to be in conjunction at a certain mo-
ment; Titan, the inner and larger of the two at a point
A, Hyperion at the point a just outside. At the end of
sixty-five days Titan will have made three revolutions
and Hyperion four, which will bring them again into'

SATELLITES OF SATURN

conjunction at very nearly, but not exactly, the same
point. Titan will have reached the point B, and Hy-
perion b. At a third conjunction the two will be a little
above the line Bb, and so on. Really the conjunctions
occur closer together than we have been able to draw
them in the figure. In the course of nineteen years the
point of conjunction will have slowly moved all round
the circle, and the satellites will again be in conjunction
at A.

Now the effect of this slow motion of the conjunction-
point round the circle is that the orbit of Hyperion, or,
more exactly, its longer axis, is carried round with the
conjunction-point, so that the conjunctions always occur
where the distance of the two orbits is greatest. The
dotted line shows how the orbit of Hyperion is thus car-
ried halfway round in nine years.

An interesting feature of this action is that it Us, so
far as we know, unique, there being no case Uke it else-
where in the solar system. But there may be something
quite similar in the mutual action of the first and third,
and of the second and fourth satellites of Saturn on each
other.

A yet more striking effect of the mutual attraction of
the matter composing the rings and satellites is that,
excepting the outer satellite of all, these bodies all keep
exactly in the same plane. The effect of the sun's at-
traction, if there were nothing to counteract it, would
be that in a few thousand years the orbits of these bodies
would be drawn around into different planes, all having,
however, the same inclination to the plane of the orbit

224. PLANETS AND THEIR SATELLITES

of Saturn. But, by their mutual attraction, the planes
of the orbits are all kept together as if they were solidly
attsiched to the planet.

Physical Constitution of Saturn

There is a remarkable resemblance between the phys-
ical make-up of this planet and that of its neighbour
■ Jupiter. They are alike remarkable for their small den-
sity, that of Saturn being even less than that of water.
Another point of likeness is the rapid rotation, Saturn
turning on its axis in 10 hours 14 minutes, a little more
than the period of Jupiter. The surface of the planet
also seems to be variegated with cloud-like forms, similar
to those of Jupiter, but far fainter, so that they cannot
be seen with any distinctness.

What has been said of the probable cause of the small
density of Jupiter applies equally to Saturn. The prob-
ability is that the planet has a comparatively small but
massive nucleus, surrounded by an immense atmosphere,
and that what we see is only the outer surface of the
atmosphere.

A curious fact which bears on this view is that the
satellite Titan is far denser than the planet. Its cubical
contents are about one ten-thousandth those of the planet.
But its mass, as found from the motion of Hyperion, is
one forty-three-hundredth that of the planet.

VIII

XJiANUS AND ITS SATELLITES

Ukanus is the seventh of the major planets In the order
of distance from the sun. It is commonly considered a
telescopic planet; but one having good eyesight can
easily see Uranus without artificial help, if he only knows
exactly where to look for it, so as to distinguish it from
the numerous small stars having the same appearance.
Had any of the ancient astronomers made so thorough an
examination of the sky from night to night as Dr. Gould
did of the^outhern heavens after he founded the Cordoba
Observatory, they would have upset the notion that there
were only seven planets.

Uranus was discovered in 1782 by Sir William Her-
schel, who at first supposed it to be the nucleus of a
comet. But its motion soon showed that this could not be
the case, and before long the discoverer found that it
was a new addition to the solar system. In gratitude to
his royal benefactor, George III, he proposed to call the
planet Georgium Sidus, a name which was continued in
England for some seventy years. Some continental as-
tronomers proposed that it should be called after its
discoverer, and the name Herschel was often assigned to
it. But by 1850 the name Uranus, originally proposed
by Bode (author of the "Law"), and always used in
Germany, became universal.

PLANETS AND THEIR SATELLITES

When the orbit of the planet was determined, so that
its course in former years could be mapped out, the curi-
ous fact was brought to light that it had been seen and
recorded nearly a century before, as well as a few years
previously. Flamsteed, Astronomer Royal of England,
while engaged in cataloguing the stars, bad marked it
down as a star on five occasions between 1690 and 1715.
What was yet more singular, Lemonnier, at the Paris Ob-
servatory, had recorded it eight times in the course of
two months, December, 1768, and January, 1769. But
he had never reduced and compared his observations, and
not till Herschel announced the planet did Lemonnier
know how great a prize had lain for ten years within
his grasp.

The period of revolution of Uranus is eighty-four
years, so that its position in the sky changes but slowly
from year to year. During the first ten years of our cen-
tury it will be in or near the region of the Milky Way,
which we see in summer and autumn, low down in the
southern sky. This will make it difficult of detection by
the naked eye.

The distance of Uranus is about twice that of Saturn.
In astronomical units it is 19.2 ; in our familiar measures
1,790,000,000 miles, or 2,870,000,000 kHometres.

Owing to this great distance, it is hard to see with cer-
tainty any features on its surface. In a good telescope
it appears as a pale disk with a greenish hue. Some ob-
servers have fancied that they saw faintly marked fea-
tures on its surface, but this is probably an illusion. We
may regard it as certain that it rotates on its axis ; but

THE SATELLITES OF URANUS 227

no ocular evidence of this has ever been obtained, and of
course the period is unknown. But the measures of Bar-
nard showed a shght eUipticity of the disk which, if real,
would prove a rapid rotation.

The spectroscope shows that the constitution of
Uranus is materially different from that of any of the
six planets which revolve between it and the sun. None
of the latter gives a spectrum which is strikingly different
from that of ordinary sunlight. But when the light of
Uranus is spread out into a spectrum, a number of more
or less shaded bands are seen, totally unlike the lines of
an ordinary spectrum. Whether these bands are really
what they appear, or whether they are composed of a
multitude of fine dark Hues invisible singly, owing to
the f aintness of the light, has not yet been ascertained ;
but the probabilities are that such is the case. Whether
it is or not, the spectrum indicates that the light reflected
from the planet has passed through a dense medium of a
constitution quite different from that of our atmosphere.
But it is as yet impossible to determine the nature of this
medium.

The Satellites of Uranus

There are four of these bodies moving round Uranus
as he travels in his orbit. The two outer ones can be
seen in a telescope of twelve inches aperture or more ; the
inner ones only in the most powerful telescopes of the
world. The difficulty of seeing them does not arise. from
their small size, for they are probably nearly or quite as
large as the others, but from their being blotted out by
the glare of the planet.

228 PLANETS AND THEIR SATELLITES

The history of these bodies is somewhat pecuhar. Be-
sides the two brighter ones, Herschel, before 1800,
thought he caught glimpses from time to time of four
others, and thus it happened that for more than half a
century Uranus was credited with six satellites. This
was because during all that time no telescope was made
which could claim superiority over Herschel's.

Then about 1845, Lassell, of England, undertook the
making of reflecting telescopes, and produced his two
great instruments, one of two, the other of four feet
aperture. The latter he afterwards took to the Island
of Malta, in order to make observations under the fine
sky of the Mediterranean. Here he and his assistant
entered upon a careful examination of Uranus, and
reached the conclusion that none of the additional satel-
lites supposed by Herschel had any existence. But, on
the other hand, two new ones were found so near the
planet that they could not have been seen by any pre-
vious observer. During the next twenty years these
newly found bodies were looked for in vain with the best
telescopes then In use in Europe, and some astronomers
professed to doubt their existence. But in the winter of
1873 they were found with the twenty-six-inch Wash-
ington telescope, which had just been completed, and
were shown to move in exact accordance with the observa-
tions of Lassell.

The most remarkable feature of these bodies is that
their orbits are nearly perpendicular to the orbit of the
planet. The result is that there are two opposite points
of the latter orbit where that of the satellite is seen edge-

THE SATELLITES OF URANUS 229

wise. When Uranus is near either of these points, we,
from the earth, see the sateUites moving as if swinging
up and down in a north and south direction on each
sidfe of the planet, Hke the bob of a pendulum. Then, as
the planet moves on, the apparent orbits slowly open out.
At the end of twenty years we see them perpendicularly.
They then seem to us almost circular, but appear to close
up again year after year as the planet moves on its
course. The orbits were last seen edgewise in 1882, and
will be again so seen about 1924. For several years to
come the orbits are seen from a nearly perpendicular
standpoint, which is the most favourable condition for
observing the satellites.

It is quite possible that continued observations of these
bodies will yet enable the astronomer to reach some con-
clusion to the hitherto unsolved problem of the rotation
of Uranus on its axis. In the cases of Mars, Jupiter, and
Saturn, the satellites revolve very nearly in the plane of
the equators of the several planets to which they belong.
If this is true of Uranus, it would follow that the equator
of the planet was nearly perpendicular to its orbit, and
that its north pole, at two opposite points in its orbit,
would point almost exactly to the sun. Such being the
case, the seasons would be vastly more marked than they
are on our earth. Only on or near the equator of Uranus
would a denizen of the planet see the sun every day. If
he lived in middle latitudes there would be a period equal
in length to five or ten of our years during which the sun
would never reach his horizon. Then, moving rapidly
upwards, it would rise and set, giving him day and night,

PLANETS AND THEIR SATELLITES

but in time it would get so far up toward the north pole
that it would never set during a period equal to that at
which it never rose.

The fact that all the satellites revolve in almost ex-
actly the same plane gives some colour to this view, but
does not quite prove it, because it is not impossible that
their planes are kept together by their mutual action.
If, however, this is the case, and if the equator of Uranus
does not coincide with the orbits, the latter will, in the
course of centuries, undergo a change which our succes-
sors will be able to determine. In this way they will be
enabled to learn something of the equator and poles of
Uranus, even if their telescopes are noi powerful enough
to afford any visual evidence on the subject.

IX

Neptune and its Satellite

So far as yet known, Neptune is the outermost planet
of our solar system. In size and mass it is not very
different from Uranus, but its greater distance, 30 astro-
nomical units, instead of 19.2, makes it fainter and
harder to see. It is far below the limit of visibility by
the naked eye, but quite a moderate-sized telescope would
show it if one could only distinguish it from the numer-
ous stars of similar brightness that stud the heavens.
This needs astronomical appliances of a more refined
and complex sort.

The disk of Neptune is to be made out only with a
telescope of considerable power. It is then seen to be of a
bluish or leaden tint, perceptibly different from the sea-
green of Uranus. Of course nothing can be known by
direct observation about its rotation on its axis. Its spec-
trum shows bands like those of Uranus, and it seems likely
that the two bodies are much alike in their constitution.

The discovery of Neptune in 1846 is regarded as one
of the most remarkable triumphs of mathematical as-
tronomy. Its existence was made known by its attraction
on the planet Uranus before any other evidence had been
brought out. The history of the circumstances leading
to the discovery is so interesting that we shall briefly
mention its main points.

232 PLANETS AND THEIR SATELLITES

History of the Discovery of Neptune

During the first twenty years of the nineteenth cen-
tury Bouvard, of Paris, an eminent mathematical astron-
omer, prepared new tables of the motions of Jupiter,
Saturn, and Uranus, then supposed to be the three outer-
most planets. He took the deviations of these planets,
produced by their attraction on each other, from the
calculations of Laplace. He succeeded fairly well in
fitting his tables to the observed motions of Jupiter and
Saturn, but found that all his efforts to make tables that
would agree with the observed positions of Uranus were
fruitless. If he considered only the observations made
since the discovery by Herschel, he could get along ; but
no agreement could be obtained with those made previ-
ously by Flamsteed and Lemonnier, when the planet wag
supposed to be a fixed star. So he rejected these old
observations, fitted his orbit into the modern ones, and
published his tables. But it was soon found that the
planet began to move away from its calculated position,
and astronomers began to wonder what was the matter.
It was true that the deviation, measured by a naked eye
standard, was very small ; in fact, if there had been two
planets, one in the real and one in the calculated position,
the naked eye could not have distinguished them from
a single star. But the telescope would have shown them
well separated.

Thus the case stood until 1845. At that time there
lived in Paris a young mathematical astronomer, Lever-
rier, who had already made a name in his science, having

DISCOVERY OF NEPTUNE 233

communicated to the Academy of Sciences some re-
searches which gave Arago a very high opinion of his
abihties. Arago called his attention to the case of
Uranus and suggested that he should investigate the sub-
ject. The idea occurred to Leverrier that the deviations
were probably caused by the attraction of an unknown
planet outside of Uranus. He proceeded to calculate in
what orbit a planet should move to produce them, and
laid his result before the Academy of Sciences in the
summer of 1846.

It happened that, before Leverrier commenced his
work, an English student at the University of Cam-
bridge, Mr. John C. Adams, had the same idea and set
about the same work. He got the result even before
Leverrier did, and communicated it to the Astronomer
Royal. Both computers calculated the present position
of the unknown planet, so that, were it possible to dis-
tinguish it from a fixed star, it would only have been
necessary to search in the region indicated in order to
find the planet. Unfortunately, however, Airy was in-
credulous as to the matter, and did not think the chance
of finding the planet sufiicient to go through the labori-
ous operation of a search until his attention was attract-
ed by the prediction of Leverrier, and the close agree-
ment between the two computers was remarked.

The problem of finding the planet was now taken up.
Very thorough observations were made upon the stars in
the region by Professor Challis at the Cambridge Obser-
vatory. I must explain that, as it was not easy with the
imperfect instruments of that time to distinguish so

234. PLANETS AND THEIR SATELLITES

small a planet from the great number of fixed stars
which studded the heavens around it, it was necessary
to proceed by determining the position of as many stars
as possible several times, in order that, by a comparison
of the observations, it could be determined whether any
of them had moved out of its place.

While Mr. Challis was engaged in this work it oc-
curred to Leverrier that the astronomers of Berlin were
mapping the heavens. He therefore wrote to Encke, the
director of the Berlin Observatory, suggesting that they
should look for the planet. Now it happened that the
Berlin astronomers had just completed a map of that
part of the sky in which the planet was located. So, on
the very evening after the letter was received, they took
the map to the telescope and proceeded to search about
to see if any object was seen in the telescope which was
not on the map. Such an object was very soon found,
and, by comparing its position with that of the stars
around it, it seemed to have a slight motion. But Encke
was very cautious and waited for the discovery to be con-
firmed on the night following. Then it was found to
have moved so much that no doubt could remain, and he
wrote Leverrier that the planet actually existed.

When this news reached England, Professor Challis
proceeded to examine his own observations, and found
that he had actually observed the planet on two occa-
sions. Unfortunately, however, he had not reduced and
compared his observations, and so failed to recognise the
object until after it had been seen at Berlin.

The question of the credit due to Adams gave rise to

THE SATELLITE OF NEPTUNE 235

much controversy, Arago in France claiming that, in the
history of the affair, the name of Adams should not even
be mentioned — the whole credit should go to Leverrier.
This he did on the principle that it was not the person
who first did a thing, but he who first published it, who
should receive the credit. But the English claimed that,
as Adams had actually preceded Leverrier and, if he had
not printed his paper, had at least communicated it to
public authorities, and had enabled Challis to see, al-
though not to recognise, the planet, he should get his due
share of credit. The whole question thus raised was one
of honour, and subsequent astronomers have taken the
very proper course of honouring both men all they could
for so wonderful a work.

The Satellite of Neptune

Of course the newly found planet was observed by
astronomers the world over. The result was that Mr.
Lassell soon found that Neptune was accompanied by
a satellite. This obj ect was observed at the few observa-
tories then possessing telescopes of sufficient power to
make it out. Its time of revolution was found to be
nearly six days.

The most curious feature of this satelHte is that, con-
trary to the rule in the case of all the bodies of the solar
system except Uranus, it moves from east toward west.
In the case of Uranus we cannot consider the motion as
being east or west, we should rather call it a north and
south motion.

It would be very interesting to know whether the

236 PLANETS AND THEIR SATELLITES

planet Neptune revolves on its axis in the same direction
as the satellite moves. But this cannot be determined,
because it is so distant and its disk so faint and diffuse
that no markings can be detected upon it. Indeed, if we
reflect that the rotation of a planet so near us as Venus
has never been certainly determined, we may easily see
how hopeless is the prospect of determining that of
Neptune.

But, in spite of this, there is remarkable evidence that
the planet has a rapid rotation. It is found that the
orbit of the satellite is very slowly changing its position
from year to year. During the half century since obser-
vations commenced, this change amounts to several de-
grees. The only way in which it can be accounted for
is by supposing that Neptune, like the earth and the other
rapidly rotating planets, is an oblate ellipsoid, and that
the plane of the planet's equator does not coincide with
that of the orbit of the satellite. In time the astronomer
will be able to learn from this motion the position of the
poles and equator of the planet Neptune, but this may
require a century of observation, or even several centuries.

X

How THE Heavens are Measured

Distances in the heavens may be determined by a
method similar to that employed by an engineer in de-
termining the distance of an inaccessible object — say a
mountain peak. Two points, A and B, are taken as a
base line from which to measure the distance of a third
point, C. Setting up his instrument at A, the engineer
measures the angle between B and C. Setting it up at
B he ineasures the angle between A and C. Since the
sum of the three angles of a triangle is always one hun-
dred and eighty degrees, the angle at C is found by

ei-^ —

Fig. 44. — Measure of ihe Diaimice of an Inaccessible Object hij IVianrjulaiion,

subtracting the sum of the angles at A and B from that
quantity. It will readily be seen that the angle at C
is that subtended by the base line as it would appear if
viewed by an observer at C. Such an angle is, in a
general way, called a parallax. It is the difference of
direction of the point C as seen from the points A and B.
It will readily be seen that, with a given base line.

238 PLANETS AND THEIR SATELLITES

the greater the distance of the object the less will be its
parallax. At a sufficiently great distance the latter wiU
be so small that the observer cannot get any evidence of
it. To all appearance the lines B C and A C will then
have the same direction. The distance at which the
parallax cannot be made out depends, of course, on the
accuracy of the measurement, and the length of the
base line.

The moon being the nearest of all the heavenly bodies
has the largest parallax. Its distance can therefore be
determined with the greatest precision by measurement.
Even Ptolemy, who lived only one or two centuries after
Christ, was able to make an approximate measure of the
distance of the moon. But the parallax of a planet is so
small that it can be determined only with the most refined
instruments.

The ends of the base line used in the determination
may be any two points on the earth's surface — say the
observatories of Greenwich and the Cape of Good Hope.
In the case of the transits of Venus, which we have al-
ready described, there were a number of different sta-
tions at various points on the earth's surface, from which
the direction of Venus at the beginning and end of its
transit could be inferred. This method of determining
distances is called triangnlation.

The idea of a triangulation, as thus set forth, gives an
understanding only of the general principle involved in
the problem. One can readily see that it would be out
of the question for two observers in distant parts of the
earth to get the exact direction of a planet at the same

THE MOTION OF LIGHT 239

moment of time. The actual determination of the paral-
lax requires a combination of observations too complex
to be set forth in the present book, but the fundamental
principle is that just explained.

In order to get the dimensions of the whole solar sys-
tem, it is only necessary to know the distance of any one
planet from us at any given moment. The orbits and
motions of all the planets are mapped down with the
greatest possible exactness, but with the map before us
we are in the position that one would be who had a very
exact map of a country, only there was no scale of miles
upon it. So he would be unable to measure the distance
from one point to another on his map until he knew the
scale. It is the scale of our map of the solar system
which the astronomer stands in need of and which he has
riot, even with the most refined instruments, yet been able
to determine as accurately as he could wish.

The fundamental unit aimed at is that already de-
scribed — the mean distance of the earth from the sun.
Measures of parallax are by no means the only method of
determining this distance. Within the last fifty years
other methods have been developed, some of which are
fully as accurate as the best measures of parallax, per-
haps even more so.

Measurement hy the Motion of Light

One of the most simple and striking of these methods
makes use of the velocity of light. By observations of
Jupiter's satellites, made when the earth was at different
points of its orbit, it has been found that light passes

240 PLANETS AND THEIR SATELLITES

over a distance equal to that of the earth from the sun
in about eight minutes twenty seconds, or five hundred
seconds. This determination has been more accurately
made in another way by the aberration of the stars.
This is a slight change in their position due to the com-
bined motion of the earth and the ray of light by which
we see the star. By accurate observations on the aberra-
tion, it is found that light travels from the earth to the
sun in almost exactly 499-6 seconds. It f oUows that if we
can find how far light will travel in one second, we can
determine the distance of the sun by multiplying the re-
sult by 499.6. The measurement of the velocity of light
is one of the most difficult problems in physics, as it re-
quires the measurement of intervals of time only a few
millionths of a second in duration. Those who are inter-
ested in the subject will see the method of doing this
explained in special treatises ; at present it is sufficient to
say that light is found to travel 299,860 kilometres, or
186,300 miles in a second. Multiply this by 499.6 and
we have 93,075,480 miles for the distance of the sun from
the earth.

Measurement by the Sun's Gravitation

A third method rests on the measures of the sun's
gravitation upon the moon. One eff^ect of this is that,
as the moon performs its monthly revolution round the
earth, it is at its first quarter a little more than two
minutes behind its average position, to which it catches
up at full moon, and passes ; so that at last quarter it is
two minutes ahead of the mean position. Toward new

THE SUN'S GRAVITATION Ml

moon it falls behind again to the average place. Thus
a slight swing goes on in unison with the moon's mo-
tion around the earth. The amount of this swing is
inversely proportional to the distance of the sun. Hence,
by measuring this amount, its distance may be deter-
mined. As in other astronomical measurements, the
difficulty of the determination is very great. A swing
hke this is very hard to measure without error; more-
over, the problem of determining just how much swing
the sun would produce at a given distance is one of the
difficult problems of celestial mechanics, which has not
yet been solved so satisfactorily as to leave no doubt
whatever on the result.

The fourth method also rests on gravitation. If we
only knew the exact relation between the mass of the
earth and that of the sun; that is to say, if we could
determine precisely how many times heavier the sun is
than the earth, we could compute at what distance the
earth must be placed from the sun in order to revolve
around it in one year. The only difficulty, therefore, is
to weigh the earth against the sun. This is most exactly
done by finding the change in the position of the orbit
of Venus produced by the earth's attraction. By com-
paring the positions of the orbit of Venus by its transits
in 1761, 1769, 1874, and 1882, it is found that the orbit
has a progressive motion, indicating that the mass of
the sun is 332,600 times that of the earth and moon com-
bined. Thus we are enabled to compute the distance of
the sun by still another method.

242 PLANETS AND THEIR SATELLITES

Results of Measurements of the Sun's Distance

We have described four methods of making this fun-
damental determination in astronomy, and in order that
the reader may see just what degree -of certainty and
precision astronomical theory and measurements have
reached, we give the separate results of these methods.
The first column shows the parallax of the sun, which
is the quantity actually used by astronomers. It is the
same thing as, the angle under which the equatorial
radius of the earth would be seen by an observer at the
distance of the sun from us. This is followed by the
accompanying distance in miles.

Measures of parallax 8.800

Velocity of light 8.778

Motion of moon 8.784

Mass of the earth 8.763

Dist. 93,908,000 miles
" 93,075,480 "
" 93,958,000 "
" 93,113,000 "

The difference between these results is no greater than
the liability of error .wherever mathematical demonstra-
tions and instrumental measurements of such extreme
minuteness and complexity as these are required. From
the close agreement between results reached by methods
so widely different in their principles, we have a striking
proof of the correctness of the astronomical views of the
universe. Yet discrepancies exceeding a hundred thou-
sand miles will not be tolerated by astronomers longer
than is absolutely necessary.

XI

Gravitation and the Weighing of the Planets

No work of the human intellect farther transcends
what would seem possible to the ordinary thinker than the
mathematical demonstrations of the motions of the heav-
enly bodies under the influence of their mutual gravita-
tion. We have learned something of the orbits of the
planets round the sun ; but the following of the orbit is
not the fundamental law of the planet's motion ; the lat-
ter IS determined by gravitation alone. This law, as
stated by Newton, is so comprehensive that nothing can
be added. The law that every particle of matter in the
universe attracts every other particle, with a force which
varies inversely as the square of the distance between
them, is the only law of nature which, so far as we know,
is absolutely universal and invariable in its action. All
the other processes of nature are in some way varied or
modified by heat and cold, by time or place, by the pres-
ence or absence of other bodies. But no operation that
man has ever been able to perform on matter changes its
gravitation in the slightest. Two bodies gravitate by
exactly the same ambunti no matter what we do with
them, no matter what obstacles we interpose between
them, no matter how fast they move. All other natural
forces admit of being investigated, but gravitation does
not. Philosophers have attempted to explain it, or to

244 PLANETS AND THEIR SATELLITES

find some caiise for it, but nothing has yet been added
to our knowledge by these attempts.

The motions of the planets are governed by their
gravitation. Were there only a single planet moving
round the sun it would be acted on by no force but the
sun's attraction. By purely mathematical calculation it
is shown that such a planet would describe an ellipse,
having the sun in one focus. It would keep going round
and round in this ellipse forever. But in accordance with
the law, the planets must gravitate towards each other.
This mutual gravitation is far less than that of the sun,
because in our solar system the planets are of much
smaller mass than the central body. In consequence of
this mutual attraction the planets deviate from the
ellipse. Their orbits are very nearly, but not exactly,
eUipses. Still, the problem of their motion is one of pure
mathematical demonstration. It has occupied the ablest
mathematicians of the world since the time of Newton.
Every generation has studied and added to the work of
the preceding one. One hundred years after Newton,
Laplace and Lagrange showed that the ellipses near
which the planets move gradually change their form and
position. These changes can be calculated thousands,
tens of thousands, or even hundreds of thousands of
years in advance. Thus it is known that the eccentricity
of the earth's orbit round the sun is now slightly
diminishing, and that it will continue to diminish for
about forty thousand years. Then it will increase so
that in the course of many thousands more of years it
will be greater than it now is. The same is true of all

GRAVITATION AND WEIGHING ^45

the planets. Their orbits gradually change their form
back and forth through tens of thousands of years, like
"great clocks of eternity which count off ages as ours
count off seconds." The ordinary reader would be justi-
fied in some incredulity as to the correctness of these
predictions for thousands of years to come, were it not
for the striking precision with which the motions of the
planets are actually predicted by the mathematical
astronomer. This precision is reached by solving the
very difficult problem of determining the effect of each
planet on the motions of all the other planets. We might
predict the motions of these bodies by assuming that each
of them moves round the sun in a fixed ellipse, which,
as I have just said, would be the case if it were not
attracted by any other body. Our predictions would
then, from time to time, be in error by amounts which
might amount to large fractions of a degree; perhaps,
in the course of a long time, to even more. To form an
idea of this error we may say that one degree is the angle
at which we see the breadth of an ordinary window frame
at the distance of a hundred yards. The planet might
then be predicted as in a line with one side of such a
frame when in reality it would be at the other side or in
the middle of the window.

But, taking account of the attraction of all the other
planets, the prediction is so exact that the refined obser-
vations of astronomy hardly show any appreciable de-
viation. If we should mark on the side of a distant house
a row of a hundred points, each apparently as far from
the other as the average error of these predictions, the

MG PLANETS AND THEIR SATELLITES

whole row would seem to the naked eye as a single point.
The history of the discovery of Neptune, which was
mentioned in the preceding chapter, affords the most
striking example that we possess of the certainty of these
predictions.

How the Planets are Weighed

I shall now endeavour to give the reader some idea of
the manner in which the mathematical astronomer reaches
these wonderful results. To make them, he must, of
course, know the pull each planet exerts upon the others.
This is proportional to what the physicist and astrono-
mer call the mass of the attracting planet. This word
means quantity of matter, and around us on the surface
of the earth, it has nearly the same meaning as the word
weight. We may therefore say that, when the astrono-
mer determines the mass of a planet, he is weighing it.
He does this on the same principle by which the butcher
weighs a ham in the spring balance. When the butcher
picks the ham up he feels a pull of the ham toward the
earth. When he hangs it on the hook, this pull is trans-
ferred from his hand to the spring of the balance. The
stronger the pull the farther the spring is pulled down.
What he reads on the scale is the strength of the pull.
You know that this pull is simply the attraction of the
earth on the ham. But, by a universal law of force, the
ham attracts the earth exactly as much as the earth does
the ham. So what the butcher really does is to find how
much or how strongly the ham attracts the earth, and
he calls that pull the weight of the ham, On the same

HOW THE PLANETS ARE WEIGHED 247

principle, the astronomer finds the weight of a body by
finding how stronw is its attractive pull on some other
body.

In applying this principle to the heavenly bodies, you
meet at once a difiiculty that looks insurmountable. You
cannot get up to the heavenly bodies to do your weigh-
ing ; how then will you measure their pull ? I must begin
the answer to this question by explaining more exactly
the diff^erence between the weight of a body and its mass.
The weight of obj ects is not the same all over the world ;
a thing which weighs thirty pounds in New York would
weigh an ounce more than thirty pounds in a spring
balance in Greenland, and nearly an ounce less at the
equator. This is because the earth is not a perfect sphere,
but a little flattened. Thus weight varies with the place.
If a ham weighing thirty pounds were taken up to the
moon and weighed there, the pull would only be five
pounds, because the moon is so much smaller and lighter
than the earth. But there would be just as much ham
on the moon as on the earth. There would be another
weight of the ham on the planet Mars, and yet another
on the sun, where it would weigh some eight hundred
pounds. Hence, the astronomer does not speak of the
weight of a planet, because that would depend on the
place where it was weighed ; but he speaks of the mass of
the planet, which means how much planet there is, no
matter where you might weigh it.

At the same time we might, without any inexactness,
agree that the mass of a heavenly body should be fixed by
the weight it would have at some place agreed upon, say

24.8 PLANETS AND THEIR SATELLITES

New York. As we could not even imagine a planet at
New York, because it may be larger than the earth itself,
what we are to imagine is this : Suppose the planet could
be divided into a million million million equal parts, and
one of these parts brought to New York and weighed.
We could easily find its weight in pounds or tons. Then
multiply this weight by a million million million and we
shall have a weight of the planet. This would be what
the astronomers might take as the mass of the planet.

With these explanations, let us see how the weight of
the earth is found. The principle we apply is that round
bodies of the same specific gravity attract small objects
on their surface with a force proportional to the diameter
of the attracting body. For example, a body two feet
in diameter attracts twice as strongly as one of a foot,
one of three feet three times as strongly, and so on. Now,
our earth is about forty million feet in diameter ; that is,
ten million times four feet. It follows that if we made
a little model of the earth four feet in diameter, having
the average specific gravity of the earth, it would attract
a particle with one ten-millionth part of the attraction
of the earth. We have shown in our chapter on the earth
how the attraction of such a model has actually been
measured, with the result of showing that the total mass
of the earth is five and one half times that of an
equal bulk of water. Thus this mass becomes a known
quantity.

We come now to the planets. I have said that the
mass or weight of a heavenly body is determined by its
attraction on some other body. There are two ways in

HOW THE PLANETS ARE WEIGHED M9

which the attraction of a planet may be measured. One
is by its attraction on the planets next to it, causing
them to deviate from the orbits in which they would move
if left to themselves. By measuring the deviations, we
can determine the amount of the pull, and hence the mass
of the planet.

The reader will readily understand that the mathe-
matical processes necessary to get a result in this way
must be very delicate and complicated. A much simpler
method can be used in the case of those planets which
have satellites revolving round them, because the attrac-
tion of the planet can be determined by the motions of
the satellite. The first law of motion teaches us that a
body in motion, if acted on by no force, will move in a
straight line. Hence, if we see a body moving in a curve,
we know that it is acted on by a force in the direction
toward which the motion curves. A familiar example is
that of a stone thrown from the hand. If the stone were
not attracted by the earth it would go on forever in the
line of throw, and leave the earth entirely. But under
the attraction of the earth it is drawn down and down,
as it travels onward, until finally it reaches the ground.
The faster the stone is thrown, of course, the farther it
will go, and the greater will be the sweep of the curve of
its path. If it were a cannon ball, the first part of the
curve would be nearly a right line. If we could fire a
cannon ball horizontally from the top of a high moun-
tain with a velocity of five miles a second, and if it were
not resisted by the air, the curvature of the path would
be equal to that of the surface of our earth, and so the

250 PLANETS AND THEIR SATELLITES

ball would never reach the earth, but would revolve round
it like a little satellite in an orbit of its own. Could this
be done the astronomer would be able, knowing the
velocity of the ball, to calculate the attraction of the
earth. The moon is a satellite, moving like such a ball,
and an observer on Mars would be able, by measuring
the orbit of the moon, to determine the attraction of the
earth as well as we determine it by actually observing
the motion of falling bodies around us.

Thus it is that when a planet like Mars or Jupiter has
satellites revolving around it, astronomers on the earth
can observe the attraction of the planet on its satellites
and thus determine its mass. The rule for doing this is
very simple. The cube of the distance between the planet
and satellite is divided by the square of the time of revo-
lution. The quotient is a number which is propor-
tional to the mass of the planet. The rule applies
tO' the motion of the moon round the earth and of
the planets round the sun. If we divide the cube
of the earth's distance from the sun, say ninety-three
millions of miles, by the square of three hundred and
sixty-five and a quarter, the days in a year, we shall got
a certain quotient. Let us call this number the sun-
quotient. Then, if we divide the cube of the moon's dis-
tance from the earth by the square of its time of revolu-
tion, we shall get another quotient, which we may call
the earth-quotient. The sun-quotient will come out
about three hundred and thirty thousand times as large
as the earth-quotient. Hence it is concluded that the
mass of the sun is three hundred and thirty thousand

HOW THE PLANETS ARE WEIGHED 251

times that of the earth; that it would take this number
of earths to make a body as heavy as the sun.

I give this calculation to illustrate the principle; it
must not be supposed that the astronomer proceeds ex-
actly in this way and has only this simple calculation to
make. In the case of the moon and earth, the motion and
distance of the former vary in consequence of the attrac-
tion of the sun, so that their actual distance apart is a
changing quantity. So what the astronomer actually
does is to find the attraction of the earth by observing
the length of a pendulum which beats seconds in various
latitudes. Then by very delicate mathematical processes
he can find with great exactness what would be the time
of revolution of a small satellite at any given distance
from the earth, and thus can get the earth-quotient.

But, as I have already pointed out, we must, in the
case of the planets, find the quotient in question by means
of the satellites; and it happens, fortunately, that the
motions of these bodies are much less changed by the at-
traction of the sun than is the motion of the moon. Thus,
when we make the computation for the outer satellite of
Mars, we find the quotient to be 3709-375T0 that of the
sun-quotient. Hence we conclude that the mass of Mars
1^ 3,093,600 tli^t of the sun. By the corresponding quo-
tient, the mass of Jupiter is found to be about -j^^ij
that of the sun ; Saturn, ~^^ ; Uranus, 22J100 '■> Neptune,

I have set forth only the great principle on which the
astronomer has proceeded for the purpose in question.
The law of gravitation is at the bottom of all his work.

252 PLANETS AND THEIR SATELLITES

The eflFects of this law require mathematical processes
which it has taken two hundred years to bring to their
present state, and which are still far from perfect. The
measurement of the distance of a satellite is not a job
to be done in an evening; it requires patient labor ex-
tending through months and years, and then is not as
exact as the astronomer would wish. He does the best
he can and must be satisfied with the result until he can
devise an improvement on his work, which he is always
trying to do with varying success.

PART V
COMETS AND METEORIC BODIES

I

Comets

Comets differ from the heavenly bodies which we have
hitherto studied in their pecuUar aspects, their eccentric
orbits, and the rarity of their appearance. Some mystery
still surrounds the question of their constitution, but this
does not detract from the interest of the phenomena
which they present. When one of these objects is care-
fully examined we find it to embody three features which,
however, are not separate and distinct, but merge into
each other.

First we have what, to the naked eye, appears to be
a star of greater or less brilliancy. This is called the
nucleus of the comet.

Surrounding the nucleus is a cloudy nebulous mass,
like a little bunch of fog, shading off very gradually to-
ward the edge, so that we cannot exactly define its bound-
ary. This is called the coma (Latin for hair). Nucleus
and coma together are called the head of the comet,
which looks like a star shining through a patch of mist
or fog.

Stretching away from the comet is the tail, which may
be of almost any length. In small comets the tail may be
ever so short, while in the greatest it stretches over a long
arc of the heavens. It is narrow and bright near the head
of the comet and grows wider and more diffuse as it

256 COMETS AND METEORIC BODIES

recedes from the head. It is therefore always more or
less fan-shaped. Toward the end it fades away so gradu-
ally that it is impossible to say how far the eye can
trace it.

Comets differ enormously in brightness, and, notwith-
standing the splendid aspect which the brighter ones as-
sume, the great majority of these objects are quite invis-
ible to the naked eye. Such are called telescopic comets.
There is, however, no broad distinction to be drawn be-
tween a telescopic comet and a bright one, there being
a regular range of brightness from the faintest of these
objects to the most brilliant. Sometimes a telescopic
comet has no visible tail ; this, however, is the case only
when the object is extremely faint. Sometimes, also, the
nucleus is almost wholly wanting. In such a case all
that can be seen is a small hairy mass, like a very thin
cloud, which may be a little brighter in the centre.

From the historical records it would appear that from
twenty to thirty comets visible to the naked eye gener-
ally appear in the course of a century. But when the
telescope was employed in sweeping the heavens it was
found that these objects were more numerous than had
been supposed. Quite a number are now found every
year by diligent observers. Doubtless the number de-
pends very largely on accident, as well as on the skill
applied in the search. Sometimes the same comet will be
found independently by several observers. The credit is
then given to the one who first accurately fixes the posi-
tion of the comet at a given time, and telegraphs the fact
to an observatory.

ORBITS OF COMETS

257

Orbits of Comets

Soon after the invention of the telescope it was found
that comets resembled the planets in moving in orbits
around the sun. Sir Isaac Newton showed that their
motions were ruled by the sun's gravitation in the same
way as the motions of the planets. The great difference
was that, instead of the orbits being nearly circular, like
those of the planets, they were so elongated that, in most
cases, it could not be determined where the aphelion, or
farther end, was. As many of our readers may desire
an exact statement of the nature of cometary orbits, and
the laws governing them, we shall enter into some
explanations of the subj ect.

It was shown by Newton
that a body moving under
the influence of the sun's at-
traction would always de-
scribe a conic section. This
curve is of three kinds, an
ellipse, a parabola, and a
hyperbola. The first, as we
all know, is a closed curve
returning into itself. But
the parabola and the hyper-
bola are not such ; each of them extends out without end
in two branches. In the case of the parabola these two
branches approach more nearly to having the same direc-
tion as we get out farther, but in the case of the hyper-
bola they always diverge from each other.

Fig. 4o. — Parabolic OfOU cf a
Comet.

258 COMETS AND METEORIC BODIES

Having these curves in mind, let us imagine the earth
to leave us hanging in space at some point of its orbit,
our planet pursuing its course without us, until, at the
end of a year, it returns to pick us up again. During
the interval of its absence we, hanging in mid-space,
amuse ourselves by firing off balls to perform their revo-
lutions around the sun like little planets. The result will
be that all the balls we send off with a velocity less than
that of the earth, that is to say, less than eighteen and
six tenths miles per second, will move around the sun in
closed orbits, smaller than the orbit of the earth, no mat-
ter what direction we send them in. A very simple and
curious law is that these orbits will always have the same
period if the velocity is the same. All the balls sent with
the velocity of the earth will be one year in making their
revolution and will, therefore, come together, at the point
from which they started, at the same moment. If the
velocity exceeds eighteen and six tenths miles a second, the
orbit will be larger than that of the earth and the period
of revolution will be longer the greater the velocity.
With a speed exceeding about twenty-six miles a second,
the attraction of the sun could never hold in the ball,
which would fly away for good in one of the branches of
a hyperbola. This would happen no matter in what
direction we threw the object. There is, therefore, at
every distance from the sun, a certain limiting velocity
which, if a comet exceeds, it will fly off from ^ the sun
never to return ; while, if it falls short, it will be sure to
get back at some time.

The nearer we are to the gun, the greater is this linijt-

ORBITS OF COMETS 259

ing velocity. It varies inversely as the square root of
the distance from the sun, hence, four times away from
the sun, it is only half as great. The rule for finding
the limiting velocity at any point in space is very simple.
It is to take the speed of a planet passing through that
point in a circular orbit, and multiply it by the square
root of 2. This is 1.414. . . .

It follows that if the astronomer, by means of his ob-
servations, can find the velocity with which a comet is
passing a known point of its orbit, he can determine the
distance to which it will fly from the sun and the period
of its return. By a careful comparison of observation
made during the whole period of visibility of the comet
he can generally reach a definite conclusion on the
subject.

It is a curious fact that no comet nas yet been seen of
which the speed certainly exceeds the limit which we have
described. It is true that, in many cases, a slight excess
has been calculated from the observations, but this excess
was no greater than might result from the necessary
errors of observations on bodies of this kind. Commonly
the speed is so near the limit that it is impossible to say
whether it exceeds it or not. It is then certain that the
comet will fly out to an immense distance, not returning
for hundreds, thousands, or tens of thousands of years.
There are also cases in which the speed of the comet Is
found to be less than the limit by a considerable amount.
Such comets complete their revolutions in shorter periods
and are called periodic comets.

So far as we know, the history gf the motion of the

260 COMETS AND METEORIC BODIES

large majority of the comets is this. They appear to
us as if falling toward the sun from some great distance,
we know not what. If a comet fell exactly toward the
sun, it would fall into it, but this is a case which has not
been known to occur and which, for reasons to be ex-
plained later, cannot be expected ever to occur. As it
approaches the sun, it acquires greater and greater
velocity, speeds around the central body in a great curve,
and, by the centrifugal force thus generated, flies off
again, returning to the abyss of space nearly in the
direction from which it came.

Owing to the faintness of these objects they are visible,
even in powerful telescopes, only in that part of their
orbit which is comparatively near the sun. This is what
makes it so difficult in many cases to determine the exact
period of a comet which has only been seen once.

H alley's Comet

The first of these objects which was found to return
in a regular period is celebrated in the history of astron-
omy under the name of Halley's comet. It appeared in
August, 1682, and was observed for about a month, when
it disappeared from view. Halley was able, from the ob-
servations made upon it, to compute the position of the
orbit. He found that the latter was in the same position
as that of a bright comet observed by Kepler in 1607.

It did not seem at all likely that two comets should
move precisely in the same orbit. Halley therefore
judged that the real orbit was an ellipse, and that the
comet had a period of about seventy-five years. If this

HALLEY'S COMET 261

were the case, it should have been visible at intervals of
about seventy-five years in the past.

So he subtracted this period from the several dates in
order to determine whether any comets were recorded.
Subtracting seventy-five from 1607 we have 1632. He
found that a comet had actually appeared in 1531, which
he had reason to believe was moving in the same orbit.
Again subtracting seventy-five from this year we have
the year 1456. A comet really did appear in 14i56, which
spread such horror throughout Christendom that Pope
Calixtus III ordered prayers to be offered for protection
against the comet as well as against the Turks, who were
at war against Europe. It is probable that the myth
of "the Pope's Bull against the comet" refers to this
circumstance.

Other possible appearances of the comet were found in
past history, but Halley was not able to identify the
comet with exactness, owing to the absence of any pre-
cise description of the body. But the four well-observed
dates, 1456, 1531, 1607, and 1682, afforded ample
ground for predicting that the comet would again return
to the sun about 1758. Clairaut, one of the most eminent
mathematicians then in France, was able to calculate wV"\t
effect would be produced by the action of Jupiter di.d
Saturn on the period of the comet. He found that this
action would so delay its return that it would not reach
perihelion until the spring of 1759. It appeared accord-
ing to the prediction, and actually passed perihelion on
March twelfth of that year.

The next predicted return was in 1835. Several

262 COMETS AND METEORIC BODIES

mathematicians now made computations of the effect of
the planets in changing its period. So exact was their
work that two of them hit the time within five days : Pro-
fessor Rosenberger assigned November eleventh as the
date of return, and Pontecoulant predicted it for Novem-
ber thirteenth. It actually passed perihelion on November
sixteenth. After being observed for several months it
disappeared from view and has not since been seen. But
so exact is astronomical science that an astronomer could,
at any time during the intervening interval, have pointed
his telesco^? exactly at the object, after making the
necessary calculations to determine its position.

Its next return is now approaching, but the exact date
has not yet been computed. It will probably be some
time between 1910 and 1912.

Comets which have Disappeared

The most striking discovery of a comet after Halley
announced the one which bears his name, was made
by the French astronomer LexeU, in June, 1770. The
object soon became visible to the naked eye. On laying
down the orbit in which it moved, it was found, to the
surprise of astronomers, that the orbit was an ellipse,
with a period of only about six years. Its return was,
therefore, confidently predicted, but it never reappeared.
The cause was, however, speedily discovered. When it
returned at the end of six years, it was on the opposite
side of the sun, and therefore could not be seen. Passing
out to complete its revolution, it was found by calculation
that it must have gone into the immediate neighbourhood

COMETS WHICH HAVE DISAPPEARED 263

of the planet Jupiter, which, by Its powerful attraction,
started the comet off into some new orbit, so that it never
again came within reach of the telescope. This, also,
explained why the comet had not been seen before. Three
years before Lexell found it, it had come from the neigh-
bourhood of the planet Jupiter, which had thrown it into
an orbit different from its former one. Thus the giant
planet of our system had, so to speak, given the comet a
pull about 1767 so that it should pass into the immediate
neighbourhood of the sun, and having allowed it to make
two revolutions around the sun, again encountered it in
1779, and gave it a new swing off, no one knows where.
Since that time twenty or thirty comets, found to be
periodic, have been observed, most, but not all of them,
at two or more returns.

The most remarkable fact brought out by the study
of these objects has been that they da not appear to be
of seemingly infinite duration, like the planets, but are,
as a general rule, subject to dissolution and decay, like
living beings. The most curious case of a comet being
completely disintegrated is that of Biela's comet. This
was first observed in 1772, but was not known to be peri-
odic. It was again seen in 1805, and again the astrono-
mer did not notice the identity of the orbit in which it was
moving with that of the comet of 1772. In 1826 it was
discovered a third time, and now, on computing the orbit
by the improved methods which had been invented, its
identitji- with the former comets was brought out. The
time of revolution was fixed at six and two thirds years.
It should, therefore, appear in 1832 and 1839. But on

264 COMETS AND METEORIC BODIES

these returns the earth was not in a position to admit of
its being seen. Toward the end of 1845 it again ap-
peared and was observed in November and December.
In January, 1846, as it came nearer to the earth and sun,
it was found to have separated into two distinct bodies.
At first the smaller of these was quite faint, but it seemed
to increase in brightness until it became equal to the other.

The next return was in 1852. The two bodies were
then found to be far more widely separated than before.
In 1846 their distance apart was about two hundred
thousand miles ; in 1852 more than a million miles. The
last observations were made in September, 1852. Al-
though since that time the comet should have completed
seven revolutions, it has never again been seen. From the
former returns it was possible to compute the position
where it should appear with a good deal of precision, and
from its non-appearance we conclude that it has been
completely disintegrated. We shall, in the next chapter,
learn a little more about the matter which composed it.

Two or three comets have disappeared in the same way.
They were observed for one or more revolutions, growing
fainter and more attenuated on each occasion, and finally
became completely invisible.

Encke's Comet

Of the periodic comets the one that is most frequently
and regularly observed bears the name of Encke, the
German astronomer who first accurately determined its
motion. Its first discovery was made in 1786, but, as
was often the case then, its orbit could not at first be

ENCKE'S COMET ^66

determined. It was again seen in 1795 by Miss Caroline
Herschel. It was found again in 1805 and 1818. Not
until the latter date was the accurate orbit determined,
and then the periodic character of the comet and its iden-
tity with the comet observed in previous years was
established.

Encke now found the period to be about three years
and one hundred and ten days, varying a little according
to the attraction of the planets, especially of Jupiter.
In recent times it has been observed somewhere at almost
every return. Its last return was in September, 1901.

What has given this comet its celebrity is the theory of
Encke that its orbit was continually becoming smaller,
probably through its motion being resisted by some
medium surrounding the sun. A number of able mathe-
maticians have investigated this subject on the various
returns of the comet. Sometimes there appears to be
evidence of a retardation, like that found by Encke, and
sometimes not. The question is, therefore, still in an un-
settled condition. The computations are so intricate and
difficult, and, indeed, the whole problem of the motion
of a comet under the influence of the planets is so compli-
cated, that it is almost impossible to secure a solution
which can be guaranteed as absolutely correct.

Capture of Comets by Jupiter

A remarkable case, in which a new comet was made
a member of the solar system, occurred in the years 1886-
1889. In the latter year a comet was observed by Brooks
of Geneva, New York, which proved to be revolving in

266 COMETS AND METEORIC BODIES

an orbit with a period of only seven years. As it was
quite bright, the question arose why it had never been
observed before. This question was soon answered by
the discovery that in the year 1886 the comet had passed
close to Jupiter. The attraction of the planet had so
changed its course as to throw the comet into the orbit
which it now describes. Several other periodic comets
pass so near to Jupiter that there is little doubt that they
were brought into the system in this way.

The question therefore arises whether this may not be
true of all periodic comets. This question must be an-
swered in the negative, because Halley's comet does not
pass near any planet. The same is true of Enclce's
comet, which does not come near enough to the orbit of
Jupiter to have been drawn into its present orbit. With-
out the action of that planet, so far as we know, these
comets always have been members of the system.

Whence Come Comets?

It was supposed, until a recent time, that comets might
come into the solar system from the vast spaces between
the stars. This view, however, seems to be set aside
by the fact that no comet has been proved to move with
a much higher speed than it would get by falling to the
sun from a distance, which, though far outside the solar
system, is much less than the distance of the stars. We
shall see hereafter that the sun itself is in motion through
space. Even if we grant that comets come from space
far outside the solar system, the fact that we have just
cited still shows that they partook of the motion of the

WHENCE COME COMETS? 267

sun and solar system through space while they were still
outside that system.

The view which now seems established by a study of
the whole subject is that these objects have their regular
orbits, differing from those of the planets in their great
eccentricities. Their periods of revolution are generally
thousands, and sometimes tens of thousands, and even
hundreds of thousands of years. During this long inter-
val they fly out to an enormous distance beyond the con-
fines of the system. If, as they return to the sun, they
chance to pass very near a planet, two things may hap-
pen : Either the comet may be given an additional swing
that will accelerate its speed, throw it out to a greater
distance than it ever had before or possibly to a distance
from which it can never return, or the speed may be re-
tarded and the comet made to move in a smaller orbit.
Thus it is that we have comets of so many different
periods. If comets come from the regions of the fixed
stars, there is no reason why the motion of one might
not be directly toward the sun, so that it would fall into
our central luminary. But such an occurrence is hardly
possible when the comet belongs to our system, because
one of these bodies nearing an orbit passing through the
sun would have fallen into the sun on its first round, long
ages ago, and never could have a chance to fall in again.

Brilliant Comets of Our Time

The very bright comets which appear from time to
time are of the greatest interest to every beholder. It is
purely a matter of chance, so far as our knowledge ex-

S68 COMETS AND METEORIC BODIES

Fig. 46. — SonaiVs Cornel, as drawn hy O. P. Bond.

BRILLIANT COMETS OF OUR TIME 269

tends, when one shall appear. Of what are called great
comets, there were five or six during the nineteenth cen-
tury. The most remarkable and brilliant of all appeared
in 1858, and bears -the name of Donati, its discoverer, an
astronomer of Florence, Italy. Its history will show the
changes through which such a body goes. It was first
seen on June second, but was then only a faint nebulosity,
visible in the telescope like a minute white cloud in the
heavens. No tail was then visible, nor was there the
slightest indication of what the little cloud would grow
into until the middle of August. Then a small tail
gradually began to form. Early in September the ob-
ject became visible to the naked eye. From that time it
increased at an extraordinary rate, growing larger and
more conspicuous night after night. Its motions were
such that it seemed to move but little for the period of a
whole month, floating in the western sky night after
night. It attained its greatest brilliancy about October
tenth. Careful drawings of it were made from time to
time by George P. Bond, of the Harvard Ob:ervatory.
We give two of these, one a naked eye view, the other a
telescopic one showing what the head of the comet looked
like. After October tenth it rapidly faded away. It soon
travelled toward the south, and passed below our horizon,
but was followed by observers in the southern hemisphere
until March, 1859.

Before the comet had passed out of sight, computers
began to calculate its orbit. It was soon found not to
move in an exact parabola, but in a very elongated ellipse.
The period was not far from nineteen hundred years, but

270 COMETS AND METEORIC BODIES

Fig. 47, — Head of DonctWs Cornet, drawn by 0. P. Bond,

BRILLIANT COMETS OF OUR TIME 271

may have been a hundred years more or less than this. It
must therefore have been visible at its preceding return
sometime in the first century before Christ, but there is
no record by which it could be identified. It may be
expected again in the thirty-eighth or thirty-ninth
century.

A very remarkable case of several comets moving in
very nearly the same orbit is afforded by the comets of
1843, 1880, and 1882. The first of these was one of the
most memorable comets on record, as it passed so near
the sun as almost to graze the surface. In fact, it must
have passed quite through the outer 'portions of the
solar corona. It came into view with remarkable sudden-
ness in the neighbourhood of the sun, about the end cf
February. It was visible in full daylight. By a singular
coincidence it appeared shortly after the well-known pre-
diction of Miller that the end of the world was to come
in the year 1843. Those who had been alarmed by this
prediction saw in the comet an omen of the approaching
catastrophe.

The comet disappeared from view in April, so that the
time of observation was rather short. The period of
revolution now became a subject of interest. It was
found, however, that its orbit did not differ sensibly from
the parabola. But the time of observation was so brief
that any estimate of the period would be somewhat un-
certain. All that could be said was that the comet would
not return for several centuries.

Great, therefore, was the surprise when, thirty-seven
years later, a comet was seen by observers in the southern

S.n COMETS AND METEORIC BODIES

Fig. 48.~Great Comet of 1859, drawn by G. P. Bond.

BRILLIANT COMETS OF OUR TIME 273

hemisphere and found to be moving in almost the same
orbit. The first sign which it gave of its approach was
its long tail rising above the horizon. This was seen in
the Argentine Republic, at the Cape of Good Hope, and.
in Australia. Not until the fourth of February did the
head become visible. It swept around the sun, again
passed to the south, and disappeared without observers
in the northern hemisphere seeing it.

The question now arose whether this could possibly be
the same comet that had appeared in 1843. Previously
it had been supposed that when two such bodies moved in
the same orbit with a long interval between they must be
the same. In the present case, however, the hypothesis of
identity seemed to be incompatible with the observations.
The question was set at rest by the appearance in 1882
of a third comet moving in about the same orbit. ' This
certainly could not be a return of the comet which had
appeared a little more than two years before. The re-
markable spectacle was therefore offered of three bright
comets all moving in the same orbit at varying intervals
of time. Possibly there were more even than these three,
for, in 1680, a comet had passed very near the sun. Its
orbit, however, was somewhat different from those of the
three comets" already mentioned.

The most probable explanation of the case seems to be
that these comets were parts of some nebulous mass which
gradually broke up, its different meinbers pursuing their
courses independently. The result would be that, for
many ages, the objects would all continue in nearly the
same orbit.

274) COMETS AND METEORIC BODIES

Besides these, brilliant comets appeared in 1859, 1860,
and 1881. How long we may have to wait for another
no one can say. It is probable that Halley's comet, when
it appears eight or ten years hence, will at least be visible
to the naked eye, but no one can predict even its apparent
brightness. At its return in 1835 it was so small an
affair that it was difficult to explain the excitement it
caused in 1456 and later, except by supposing a great
diminution in the dimensions, at least of its tail.

Nature of Comets

The question of the exact nature of a comet is still in
doubt. In the case of large and bright comets, it is possi-
ble that the nucleus may be a solid body, though probably
much smaller than it looks. Some light on the question
is thrown by an observation, which is unique, made at the
Cape of Good Hope when the great comet of 1882
made a transit across the sun's disk, as Mercury and
Venus are sometimes known to do. Unfortunately, as-
tronomers generally were not prepared for such a phe-
nomenon, as the comet had been visible only in the
southern hemisphere, and the transit occurred only a
week or two after its first discovery. Hence it happened
that the Cape Observatory was the only one at which an
observation of the greatest interest in astronomy could
be made ; and here the circumstances were extremely un-
favourable. The sun was about to set behind Table Moun-
tain as the comet approached it. By careful watching,
two of the astronomers, Messrs. Finlay and Elkin, were
enabled to keep sight of the comet until it actually disap-

NATURE OF COMETS 275

peared at the limb of the sun. This happened fifteen
minutes before the sun disappeared from view: During
this time, if the nucleus were a solid body, it ought to
have been seen as a black spot projected against the sun.
Nothing of the sort could be made out. The conclusion
is either that the substance of the comet was transparent
to the sun's rays, or that the solid nucleus was too small to
be distinguished under the circumstances. Unfortunate-
ly, owing to the low altitude of the sun and the bad condi-
tion of the air, it was impossible to be quite sure how
small the nucleus must have been to be invisible. It
seemed certain, however, that the solid portion, if any
such the comet had, was much smaller than the apparent
nucleus as seen in the telescope.

There seems also to be some reason for suspecting that
a comet is nothing but a collection of meteoric matter,
consisting perhaps of separate objects, of sizes ranging
anywhere from that of grains of sand to masses as large
as the meteorites which sometimes fall from the sky. The
question then is to explain how the parts are kept to-
gether through so many revolutions of the comet. The
changes of shape which the nucleus often undergoes as it
is passing near to the sun seem to show that this hypothe-
sis may be near the truth.

The spectra of those comets whose light has been
analysed by the spectroscope are remarkable in showing
that this hght is not merely reflected sunlight. The
principal feature is three bright bands, which bear a
striking resemblance to those given by the compounds
of carbon and hydrogen. Taking this fact by itself, the

276 COMETS AND METEORIC BODIES

conclusion would be that the comet is a glowing gas,
shining as incandescent gases do in our chemical labora-
tories. That such should be the case and the whole case
seems impossible for two reasons. The comet cannot be
hot enough to glow; and its light fades out to nothing
as it recedes from the sun. The most likely conclusion
seems to be that the action of the sun's rays causes a glow
through some process which has not yet been made clear
to us.

What seems certain is that the matter of which a bright
comet is composed is volatile. When a bright comet is
carefully scrutinised with a telescope, masses of vapour
can be seen from time to time slowly rising from its head
in the direction of the sun, then spreading out and mov-
ing away from the sun so as to form the tail. The latter
is not an appendage which the comet carries as a^nimals
carry their tails, but is like a stream of smoke issuing
from a chimney.

It frequently happens that when a comet is first dis-
covered it has no tail at all. The latter begins to form
when the sun is approached. The nearer the comet ap-
proaches the sun, and the greater the heat to which it is
exposed, the more rapidly the tail develops. All this
shows that the matter which composes a great comet is
in part volatile. When warmed by the heat of the sun it
begins to evaporate, just as water would under the same
circumstances. The steam or vapour thus arising is re-
pelled by the sun, so as to form a stream of matter issu-
ing from the comet.

II

Meteoric Bodies

EvEEY reader of this book must frequently have seen
what is familiarly called a "shooting star" — an object
like a star, which darts through the heavens a greater or
less distance, and then disappears. These objects are, in
astronomy, called by the generic name of meteors. They
are of every degree of brightness, but the brighter they
may be, the more rarely they appear. One who is out
much at night will seldom pass a year without seeing such
a meteor of striking brilliancy. Once or twice in a life-
time he will see one that illuminates the whole sky with
its light.

On almost any clear night in the year a watcher may
see three or four or even more meteors in the course of an
hour. Sometimes, however, they are vastly more numer-
ous, for example, between the tenth and fifteenth of
August, more and brighter ones than usual will be seen.
On a number of occasions in history they have coursed the
heavens in such numbers as to fill the beholders with sur-
prise and terror. There were remarkable cases of this
kind in 1799 and 1833. In the latter year, especially,
the negroes of the South were so terrified that the recol-
lection of the phenomenon is brought down by tradition
to the present day.

278 COMETS AND METEORIC BODIES

Cause of Meteors

The cause of meteors was unknown until after the he-
ginning of the nineteenth century. It is now, however,
well made out. Besides the known objects of the solar
system — planets, satellites, and comets — ^there are, cours-
ing through space, and revolving around the sun, count-
less millions of particles, or minute collections of matter,
too small to be seen with the most powerful telescope.
Quite likely the greater number of these objects are
scarcely larger than pebbles, or even grains of sand. The
earth, in its course around the sun, is continually encoun-
tering them. One in the line of motion of the earth
may have a velocity amounting to many miles a sec-
ond ; perhaps ten, twenty;, thirty, or even forty. Meet-
ing the atmosphere with this immense velocity causes the
body to be immediately heated to so high a temperature
that its substance dissolves away with a brilliant effusion
of light no matter how solid it may be. What we see
is the course of a particle thus burning away as it darts
through the rare regions of the upper atmosphere.

Of course, a meteor will appear brighter and last
longer the larger and solider it is. Sometimes it is so
large and solid that it comes within a few miles of the
earth before being finally melted and dissolved away.
Then, the people in the region over which it is passing,
see a remarkably bright meteor. In such a case it fre-
quently happens that in a few minutes after the meteor
has passed a loud explosion, like the firing of a cannon,
is heard coming from the region through which it passed.

METEORIC SHOWERS 279

This arises from the concussion of the air compressed by
the rapid flight.

In rare cases the mass is so large that it reaches the
earth without heing melted or evaporated. Then we have
the fall of a meteoric stone, as it is called, which com-
monly occurs several times a year in some part or another
of the world. There is at least one case on record in
which a man was killed by the fall of such a body. When
these stones are dug up they are found to be composed
mostly of iron. Specimens of them are kept in our
museums, where they may be examined by anyone who
wishes to see them. Some remarkable ones are found at
the Smithsonian Institution, Washington, D. C.

How, these objects originated we cannot say, and even
a guess on the subject would be hazardous. When found
they bear marks on their surface of having been melted ;
this, however, is a natural result of their passage through
the air, by which the surface is always heated far above
the melting point.

Meteoric Showers

The greatest discovery of our times on the subject of
meteors is connected with the meteoric showers already
referred to, which occur at certain seasons of the year.
The most remarkable of these occur in November, and the
meteors of the shower are called Leonides, because their
lines of apparent motion all diverge from the constella-
tion Leo. It was found by historical research on the
subject that this shower had recurred at intervals of
about one third of a century for at least thirteen hundred

280 COMETS AND METEORIC BODIES

years. The earliest account is the following from an
Arabian writer :

In the year 599, on the last day of Moharren, stars shot hither
and thither, and flew against each other like a swarm of locusts ;
people were thrown into consternation and made supplication to
the Most High ; there was never the like seen except on the
coming of the messenger of God ; on whom be benediction and
peace.

The first well-described shower of this class occurred on
November 12, 1799. It was seen by Humboldt, then on
the Andes. He seems to have considered it as a very re-
markable display, but made no exact investigation as to
its cause.

The next recurrence was in 1833, which seems to have
been the most remarkable one ever observed. The as-
tronomer Olbers suggested from this that the shower had
a period of thirty-four years, and predicted a possible
return in 1867, which actually appeared in 1866. In
1866 and 1867 the observations were more carefully
made than ever before, and led to the remarkable astro-
nomical discovery, just alluded to, that of the relation
between meteors and comets. To explain this we must
define the radiant point of meteors.

It is found that if, during a meteoric shower, we mark
the course of each meteor by a line on the celestial sphere,
and continue these lines backward, we shall find them aU
to meet at a certain point in the heavens. In the case
of the November meteors this point is in the constellation
Leo ; in the August meteors it is in Perseus. It is called
the radiant point of the shower. The lines in which the

COMETS AND METEORS 281

meteors move are the same as if they were all shot out
from this one point, but it must not be supposed that the
meteors are actually seen at this point; they may begin
to show themselves at any distance from it less than
ninety degrees ; but when they are seen they are moving
from the point. This shows that the meteors are all mov-
ing in parallel lines when they encounter our atmosphere.
The radiant point is what, in perspective, is called the
vanishing point.

Connection of Comets and Meteors

The period of the November meteors, thirty-three
years, being known, and the exact position of the radiant
point determined, it became possible to calculate the orbit
of these objects. This was done by Leverrier soon after
the shower of 1866. Now it happened that, in December,
1865, a comet appeared which passed its perihelion in
January, 1866. Careful study of its motion showed that
its period was about thirty-three years. This orbit was
computed by Oppolzer, who published it without noticing
its resemblance to that of the meteors. Then it was no-
ticed by Schiaparelli that there was an almost perfect re-
semblance between the orbit of Oppolzer's comet and the
Leverrier orbit of the November meteors. So near to-
gether were they that no doubt could be felt that the two
orbits were identical. The evident fact was that the
bodies which produced these November meteors were fol-
lowing the comet in its orbit. It was therefore concluded
that these objects had originally formed part of the
comet and had gradually separated from it. When a

282 COMETS AND METEORIC BODIES

comet is disintegrated in the manner described in the last
chapter, those portions of its mass which are not com-
pletely dissipated continue to revolve around the sun as
minute particles, which get gradually separated from
each other in consequence of there being no sufficient bond
of attraction, but they still follow each other in line in
nearly the same orbit.

The same thing was found to be true of the August
meteors. They are found to move in an orbit very near
to that of a comet observed in 1862. The period of this
comet could not be exactly determined, but it is supposed
to be between one and two hundred years.

The third remarkable case of this kind occurred in
1872. We have already spoken of the disappearance of
Biela's comet. It happens that the orbit of this body
nearly intersected that of the earth at the point which
the latter passes toward the end of November. From the
observed period of this comet it should have passed this
point about the first of September, 1872, between two
and three months before the passage of the earth
through the same point. From the analogy of the other
cases it was therefore judged that there would be a
meteoric shower on the evening of November 27, 1872,
and that the radiant point would be in the constellation
Andromeda. This prediction was fulfilled in every re-
spect. The Andromedes, as these meteors are called, now
recur with great regularity.

There are now some disappointing circumstances to
narrate. The comet of 1866 should have reappeared
sometime during the years 1898-1900, but it was not

COMETS AND METEORS 283

seen. Probably it was missed, not because of its com-
plete disintegration, but because it happened to pass its
perihelion at a time when the earth was too far away to
admit of the comet being visible. But, what is still more
curious is that the meteors themselves, a shower of which
was expected in 1899-1900, did not reappear in great
numbers at either date. The probable reason for this is
that the swarm was deflected from its course by the at-
traction of the planets, which continually changes the
orbit of every object of this kind.

The general conclusion is that the countless thousands
of comets which in time past have coursed around the
sun, leave behind minute fragments of their mass, which
follow in their orbits like stragglers from an army, and
that, when the earth encounters a swarm of these frag-
ments a meteoric shower is produced. But it is still an
open question whether all these meteoric particles can
be fragments of comets, with the probabilities in favor
of a negative answer. If we are to accept the conclu-
sions drawn by Professor Elkin from recent photographs
of meteors, the velocities of these bodies sometimes exceed
the parabolic limit described in the last chapter. If this
be so, they must be wanderers through the infinite stellar
spaces, having no connection with our system.

The Zodiacal Light

This is a very soft, faint light, surrounding the sun,
extending out to about the orbit of the earth, and lying
nearly in the plane of the ecliptic. In tropical latitudes
it may be seen on any clear evening about an hour or

284 COMETS AND METEORIC BODIES

less after sunset. In our latitudes it is best seen in
the springs when, about an hour and a half after sunset,
it may always be seen in the west and southwest, extend-
ing upward toward the Pleiades. It is best seen at this

Fig. 49. — Tlie Zodiacal lAght in February and March.

season because, lying in the plane of the ecliptic, it makes
a greater angle with the horizon then than at other sea-
sons. In autumn it may be seen in the morning before
daybreak, rising from the east and extending toward
the south.

It is said that in regions where the atmosphere is
clearer than with us, it may be seen all night, spanning
the heavens like a complete circle. If so, the light is so

THE ZODIACAL LIGHT 285

faint as to elude ordinary vision, and this continuity does
not seem to be well established.

But there' is associated with it a phenomena which is
still one of the mysteries of astronomy. In the heavens,
immediately opposite the sun, there is always a faint
light, to which the term Gegenschein is applied. This is
a German word, of which the best English equivalent is
counter-glow. The light is so faint that it can be seen
only under the most favourable conditions. When it falls
in the Milky Way the light of that body is sufficient to
drown it out, as is that of the moon, if the latter is above
the horizon.

It passes through the Milky Way in June and Decem-
ber of each year, and can therefore not be seen during
these months. Nor is it likely to be seen during the first
part of January or July. At other times it must be
looked for when the sun is considerably below the horizon,
the sky perfectly clear and the moon not in sight. It
may then be seen as an extremely faint impression of
light, to which no exact outline can be assigned, The
observer will find it by sweeping his eye over the region
of the spot exactly opposite the sun.

There can be little doubt that the zodiacal light is
caused by the reflection of the light of ti.e sun from a
swarm of very minute bodies, perhaps in the nature of
meteors, continually revolving around it. We might
naturally attribute the Gegenschein to the same cause,
but the question would then arise why it is only seen
opposite the sun. It has been suggested that possibly
the earth has a tail, like a comet, and that the Gegen-

Z86 COMETS AND METEORIC BODIES

schein is simply this tail seen endwise. This is not an
impossibility, but there is no proof that it is true.

The Impulsion of Light

Facts are now being discovered, and physical theories
developed, the ultimate outcome of which may be an ex-
planation of a number of mysterious phenomena asso-
ciated with the earth and the universe. Tliese phenomena
are presented by the corona of the sun, the tails of comets,
the aurora, terrestrial magnetism and its variations,
nebulsE, the Gegenschein, and the zodiacal light. The
theories in question belong rather to the physicist than
the astronomer, and the writer does not feel competent to
explain them fully in their latest form, nor to define
where estabUshed facts end and speculation begins. He
must therefore limit himself to a few points.

First in order we have a pressure exerted by light,
which was pointed out by Maxwell thirty years ago, but
which seems to have been very generally overlooked, by
astronomers at least. This principle was deduced by
Maxwell from the electro-magnetic theory of light, and
may be stated as follows:

When a pencil of light impinges perpendicularly on
an opaque object, it produces a pressure upon the surface
of that object, determined by the condition that if the
object were set in motion with the velocity of light, and
the force against it were kept up, the power required to
keep up the pressure would be equal to that carried by
the ray of light.

Another way of expressing the principle is this : Sup-

THE IMPUI.SION OF LIGHT 287

posing the rays of light to be parallel, the work done by
the pressure upon a surface moving through any length
of the pencil is equal to the energy of the light con-
tained in that length.

By the aid of this principle and a knowledge of the
heat or energy contained jn the rays of the sun, it is
possible to calculate the pressure in question. It is found
to be too slight to be detected by any ordinary mode of
measurement. The great difficulty arises from the fact
that, if the experiment is not tried in a vacuum, the pres-
sure will be confused with that exerted by the surround-
ing air. A vacuum so nearly perfect that the slight
residuum of air still contained within it shall not exert a
force comparable with the light has not yet been at-
tained. Our conclusion must therefore depend on obser-
vations made on minute particles contained in the celes-
tial spaces ; and we cannot ascend into these spaces to
make the experiments, nor can we send matter up there
to be experimented upon. All we can do is to observe
matter already at hand. Here, then, is a wide gap which
we cannot bridge over in practice.

The other element in the case is the discovery that par-
ticles smaller than atoms, called corpuscles or ions, are
thrown off with high velocity from intensely heated
bodies. The sun being such a body, it follows that such
ions must be shot out from it.

On Maxwell's theory, the explanation of a comet's tail
is simple in the extreme. Being in the vacuum of celes-
tial space, the matter of the comet evaporates on the
side next to the sun, and, there being no pressure to hin-

288 COMETS AND METEORIC BODIES

der its expansion, it begins by flying otF in all directions,
especially toward the sun. It "condenses into very minute
particles, which are acted upon by the sun's rays and
thus thrown in the direction away from the sun. That
the tail of the comet was produced by a repulsion like this
has been evident ever since observations were made, but
not until Maxwell's law was understood could any ex-
planation be given of the seeming repulsion of the matter
of the tail by the sun.

The explanations of the other phenomena we have men-
tioned are not yet so simple and satisfactory that they
may be clearly stated in a short space. The reader who
is interested in the subject must therefore be referred to
special papers and treatises.*

♦The papeis to which the present writer is principally Indehted for the
views in question are by Prof. J. J. Thompson, in Ihe Popular Science Monthly
for August, 1901, and to the article by Prof. John Cox In the number for Jan-
uary, 1902. These papers again set forth the investigations of Arrhenius, the
Swedish physicist, who seeni= to have made the most successful endeavour
to explain the phenomena in question on the principles which we have
mentioned.

PART VI
THE FIXED STARS

I

.General Review

Having completed our survey of that small section of
the universe in which we have our dwelling, our next task
is to fly iii imagination to those distant parts of space
occupied by the thousands of stars which stud our sky.
This is the field of astronomy in which the most wonder-
ful discoveries have been made in recent times. We now
know things about many stars which, even to such an
observer as Sir William Herschel, would have seemed far
beyond the possibilities of human ken. But the very
vastness of the field and the minuteness of the details
into which recent research has gone render it impossible
to undertake anything like a comprehensive survey within
the limits of the present little book. All we can do is to
point out the more salient features of the universe of stars
as they have been brought to light by observers and in-
vestigators of the past and present. The reader who
desires further details and a wider idea of the methods
and results of recent research relating to the stars may
find them in a volume which the present author has re-
cently devoted to the subject.*

From the childhood of the race men have inquired:
"What is a star.f"' To this question no answer was pos-

• The Stars, a Study of the Universe. G. P. Putnam's Sons, New York,

W2 THE FIXED STARS

sible until recent times. Even within the last century
little more could be said than that they were shining
bodies whose nature was to us a mystery. At the present
time we may define the stars as immense globes of matter,
generally millions of times the size of the earth, so in-
tensely hot that they shine by their own light, and so
massive that they may continue to give light and heat for
unknown millions of years without cooling off. What
we have said of the sun probably applies in a greater or
less degree to the great majority of the stars. It is true
that we cannot study their surfaces because, even in the
most powerful telescopes, they appear as mere points of
light. But the analogy with our sun and with other
heavenly bodies leads us to believe that each of them re-
volves on its axis as the sun does, and that, could we see
it at the proper distance, it would present much the same
appearance as our sun. We have abundant evidence that
rotation is the order of nature in the case of all the heav-
enly bodies. In the few cases where it is possible to
decide whether a star does or does not rotate, the question
has been answered in the affirmative.

There are innumerable differences of detail among the
star:;. Indeed it would seem that no two are exactly alike
in their physical constitution, any more than two men
are alike in their personal appearance and make-up. In
the chapter on the sun we tried to give an idea of the
enormous temperature of that body, which far exceeds
any degree of heat we can produce on the earth. We
have good reason to believe that, while the stars differ
widely in temperature, the great majority of them are

STARS AND NEBULA 293

far hotter even than the sun. This is true of their sur-
faces and must be still more true of their vast interiors.

Stars and Nebulae

Stars are not the only bodies which fill these distant
regions of space. Scattered over the sky are immense
masses of exceedingly rare matter which, from their
cloud-like appearance, are called nehuloe. In size these
bodies far exceed the sun or stars. A nebula only as
large as our solar system would probably be invisible in
the most powerful telescope, and could never be impressed
even on the most delicate photograph of the sky unless
above the ordinary brightness. Those that we know have
probably hundreds or thousands of times the extent of
our whole solar system. We may therefore classify those
bodies of the universe which shine by their own light as
stars and nebulse.

Spectra of the Stars

When we read of astronomical discoveries, we common-
ly think of them as being made by looking through a
telescope. But this is no longer the case. The greatest
astronomical development of recent times consists in
proving the existence of dark bodies of the nature of
planets, revolving around many stars. These objects
are absolutely invisible in any telescope which it would
be possible to construct. Such an instrument could tell
us nothing about the constitution of a star. The great
engine of progress has been the spectroscope, which is
described in a previous chapter. From what hac there

a91 THE FIXED STARS

been said the reader will see that, using words in their
ordinary sense, we do not see anything by the aid of a
spectroscope. What we do with it is to analyse the rays
of light into their component parts, just as a chemist
analyses a compound body into its simple elements. A
spectroscopic analysis is more complicated from the fact
that the number of elements which compose a ray of light
is generally indefinite. The great advantage of spectro-
scopic analysis arises from the fact that it is independent
of distance. The farther a star is away, the more diffi-
cult it is to see, whether we look at it with the naked eye
or through a telescope. Its light diminishes as the square
of the distance increases; twice as far away it gives us
only one fourth the light ; three times as far away, only
one ninth the Hght, and so on. But if enough light comes
from the star to enable its spectrum to be analysed, the
result can be reached equally well no matter how great
the distance. As the chemist could analyse a mineral
brought from the planet Mars, were such a thing pos-
sible, as easily as he could if he found it on the earth,
so, when a ray of light reaches the spectroscope, the fact
that it may have been hundreds of years on its way, does
not interfere with the drawing of conclusions from it.

When the spectrum of a star is formed it is always
found to be crossed by numerous dark lines. This shows
that all the stars, like our sun, are surrounded by atmos-
pheres which are not as hot as the central body. But this
does not imply that the atmosphere is cold. On the con-
trary, it is probably hotter than the flame of any furnace
we have on earth, even in the case of the cooler stars.

SPECTRA OF THE STARS 295

When the spectra of stars are carefully compared, it is
always found that hardly any two are exactly alike.
This shows that their atmospheres all differ in their phys-
ical constitution, or in the temperature of the substancss
which compose them. A great number of the dark lines
of their spectra are found to be identical with those pro-
duced by known substances on earth. This shows that
the substances of which the stars are made up are iden-
tical, in at least a great part, with those on the earth.

One of the most abundant of these substances is hydro-
gen. Several lines of hydrogen are found in nearly all
the stars. Another substance which seems to be almost
universal throughout the universe is iron. Yet another is
calcium, the metallic base of lime. We all know that this
substance abounds on the earth, and we have, in its diffu-
sion among the stars, an example of the unity of nature
in its widest extent.

Yet, variety is also the rule. Besides lines due to known
substances, many stars show lines which have not yet been
identified with those of any element that we know of.
This is especially the case in the class known as Orion
stars, because many of them are found in the constella-
tion Orion. These stars are mostly very white or even
blue in colour, and show a number of fine dark lines which
are to a greater or less extent the same in all Orion stars,
but are not those produced by any known chemical
element. We therefore have reason to believe that there
are in the stars other chemical elements than those with
which we are acquainted.

There is a very curious case in which an element first

296 THE FIXED STARS

excited Interest through its being found in the sun and
stars. For some time after the study of the sun's spec-
trum had been commenced, it was known that certain well-
marked lines in it were not produced by any substance
then known. But continued research led to the discovery
that this substance existed in a Norwegian mineral,
cleveite, and- perhaps elsewhere on the earth. From its
existence on the sun it was called helium. Its spectrum
was no sooner made known than it was found that helium
existed in many stars which are, for that reason, called
"helium stars."

Density and Heat of the Stars

In many cases some idea can be obtained of the
density of a star, or, in ordinary language, of its
specific gravity. It is very remarkable that, in near-
ly all such cases the density is found to be far less
than that of our ordinary solid or liquid substances;
frequently no greater than that of air, sometimes
even less. In this respect our sun, although its den-
sity is so small, seems to be an exception, and it is
likely that only a very small proportion of the stars are
as dense as the sun. This affords one proof of the high
temperature of these bodies, which must be such that all
liquid or solid substances exposed to it would boil away
as water boils when put on a fire, thus changing
its substance into a vapour. We have reason to be-
lieve that the stars are for the most part masses of
this intensely hot vapour, surrounded perhaps by a
somewhat colder surface. Possibly many of the stars

DENSITY AND HEAT OF THE STARS 297

may be of the nature of bubbles, but this is far from
being established.

A star, like the sun, must be hotter in the interior than
at its surface. From the latter alone can heat be radiated ;
hence the surface is continually cooling off, and if the
matter composing the body were at rest, the cooling
would soon go so far that a crust would form, as it does
on a mass of molten iron. The only way in which this
can be prevented is that, as the superficial portions cool,
the greater density which they thus acquire causes them
to sink down into the seething mass below, portions of
which arise to take their place, cool off, and sink in their
turn. Thus there is a continual interchange of matter
between the inside and the surface, much as in a boiling
pot the water at the bottoin is continually being forced
up to the top, while that on the top continually sinks
down.

It follows from this that there must be a limit to the
smallness of a star. If such a body were no larger than
the moon, it would, in a few thousand years, so far cool
off that a crust would form over its surface. This would
cut off the currents by which the hot matter is brought
to the surface and the star would soon cease to shine. As
there can be little doubt that the age of most of the stars
is to be reckoned by millions of years, it follows that they
must be so large that they can lose heat for millions of
years and yet a cool crust not form on their surface.

We have said that our sun is among the colder of the
stars and also that it is among the smaller. These two
facts fit well together. The smaller a star is the more

298 THE FIXED STARS

rapidly it cools off, just as a cup of water cools off faster
than a pot full.

The revelations of the spectroscope makes it very prob-
able that every star has a life history. It began as a
nebula, which, in the course of ages, slowly condensed
into an intensely hot, blue-coloured star. The condensa-
tion going on, the star becomes yet hotter, until it reaches
its highest temperature. Then, cooling off, its colour
changes to white, yellow, and red, and the lines in its
spectrum become darker and more numerous. Finally, its
light dies away, as a fire flickers out when the supply of
fuel is exhausted, and the star becomes a dark opaque
body, — its life has ended. The greater the mass of the
star the longer its life. Thus it is that the stars we
observe seem to be of all ages, from the infantile nebula
to the star dying of old age.

II

Aspect op the Sky

Not only to the ordinary beholder, but to the learned
student of the heavens, the most wonderful feature of
the sky is the Milky Way. This is a girdle apparently
spanning the sky and perhaps, in reality, spanning the
entire universe of stars, uniting them, as it were, into a
single system — one "stupendous whole." It may be seen
at some time of the night every day of the year, and at
some convenient hour in the evening of every month ex-
cept May. During this month it extends round the
horizon in the early evening, and is invisible through
the denser strata of the air. Of course it will even
then become visible in the east and northeast later
at night.

The smallest telescope will show the Milky Way to be
formed of immense congeries of stars, too faint in their
light to be separately visible at their great distance from
us. Careful observation, even with the naked eye, will
show that these stars are not equally scattered along the
whole extent of their course, but are frequently collected
m great masses or clusters, with comparatively empty
spaces around or between them. These are especially
marked in the portions of the belt visible in the south in
the evenings of summer and autumn.

A remarkable fact connected with the universe Is that

300 THE FIXED STARS

the stars are not equally thick in all directions, there be-
ing more in a given space around the belt of the Milky
Way, and the number growing smaller as we pass away
from that belt. This is true even of the brightest stars,
and yet more true of the fainter ones. The poles of the
Milky Way are those two points in the heavens which
are ninety degrees from every point of the Milky Way.
If we imagine one to hold a rod in his hand, so that the
Milky Way shall be at right angles to it, the two ends of
the rod will point to the two poles in question. To give
an idea of the thickness of the stars we may say that,
near the poles of the Milky Way, a round circle of the
sky one degree in diameter will commonly contain two
or three stars visible in quite a small sized telescope. In
the region of the Milky Way, such a circle may contain
eight, ten, perhaps even fifteen or twenty such stars.

Brightness of the Stars

No one can look at the sky without seeing that the
stars difi^er enormously in their brightness, or, in the
language of astronomy, in their magnitude. They re-
semble men in that a very few far outshine all their fel-
lows, a greater number are less bright, and, as we come
down to smaller and smaller stars, we find the number
to continually increase. Those visible to the naked eye
were classified by the ancient astronomers as of six orders
of magnitude. About twenty of the brightest in the sky
were designated as of the first magnitude. The forty
next in order of brightness were called of the second mag-
nitude ; a larger number were of the third, and so on to

BRIGHTNESS OF THE STARS 301

the sixth magnitude, which included the faintest stars
that the best eye could see under a clear sky.

Modern astronomers carry this system down to the
telescopic stars. Those which are one degree fainter than
the smallest visible to the naked eye are called of the
seventh magnitude; the next in brightness are of the
eighth, and so on. The faiiitest that can be seen or
photographed with the largest telescopes are probably
of the fifteenth, sixteenth, or seventeenth magnitude.

The reader will of course understand that the magni-
tude of a star does not express its real brightness, because
a shining body looks brighter the nearer it is to us. No
matter how bright a star may be, if it were removed far
enough away it would grow so faint as to be invisible.
The smallest star in the heavens if brought near enough
to us would shine as of the first magnitude.

It was formerly believed that the actual brightness of
the different stars was nearly the same, and that some
looked brighter than others only because they were
nearer to us. But the case is now known to be different.
Estimates of the distance of the stars show that among
the nearest to us are many quite invisible to the naked
eye, while some of the first magnitude are so far away
that their distance is immeasurable. The brightest ones
probably emit hundreds of thousands of times as much
light as the smallest ones.

Number of Stars

The whole number of stars in the heavens which can
be seen by the ordinary eye is between five and six thou-

302 THE FIXED STARS

sand. Possibly a very keen eye might see more than six
thousand, but most eyes will see even less than five thou-
sand. Of these only one half can be above the horizon at
the same time, and of this half a great number will be
so near the horizon as to be obscured by the great thick-
ness of the atmosphere in that direction. The number
which can readily be seen on a clear evening by an
ordinarily good eye will probably range between fifteen
hundred and two thousand. Stars visible to the naked
eye are called lucid stars, to distinguish them from tele-
scopic stars, which can be seen only by the aid of a
telescope.

It is impossible to make even an estimate of the total
number of telescopic stars. It is commonly supposed
that between fifty and one hundred million can be seen
with large telescopes, and it is now possible, with spe-
cially arranged telescopes, to photograph stars which
are fainter than the smallest the eye can see in any tele-
scope. There is no sign of any limit to the number. As
we pass to fainter and fainter degrees of brightness the
stars are found to be more and more numerous. All that
we can say of the total number is that it must be counted
by hundreds of millions.

We have, in fact, some reason for inferring that the
great majority of the stars are invisible in the most
powerful telescope we can make, owing to their distance.
The distance of the great majority is such that only the
brightest of them can become known to us.

Minute stars are here and there collected Into clusters
in various parts of the sky. Some of these clusters are

NUMBER OF STARS 303

visible to the naked eye. Those in and near the Milky
Way frequently contain hundreds oi even thousands of
stars too small to be seen separately vdthout a telescope.
The stars differ from each other in colour, although
not in so marked a degree as terrestrial objects. The
most casual observer cannot fail to note the difference
between the bluish white of Alpha Lyras and the reddish
light of Arcturus. There seems to be a regular grada-
tion in the colour of the stars from blue, through yellow,
to red. These differences of colour are connected with
differences in the spectra of the stars. As a general rule,
the redder a star is, the greater the number and intensity
of the dark lines that can be seen in the green and blue
parts of its spectrum.

Constellations

A slight examination of the heavens shows that the
stars are not scattered equally over the sky, but that there
is more or less of a tendency to collect into constellations.
This is especially the case with the brighter stars. But
no well-marked dividing line between the constellations is
possible ; that is, we cannot draw a line showing exactly
where one constellation ends and another begins. Never-
theless a division into constellations was made in ancient
times and has been followed by astronomers down to the
present time.

How and by whom the constellations were first mapped
out and named no one knows. The Chinese had their
asterisms — collections of stars smaller than what we call
constellations — in the earliest years of their history.

304- THE FIXED STARS

What we know of the constellations dates from Ptolemy,
who lived in the second century after Christ. His names
are still in use. As many of them are those of the gods,
goddesses, and heroes of Grecian mythology — Perseus,
Andromeda, Cepheus, Hercules, etc. — it seems likely that
they were assigned during or after the heroic age.

In modern times quite a number of new constellations
have been carved out of or drawn between the older ones.
This is especially the case in the southern hemisphere,
which was imperfectly known to the ancient Greeks.

Ill

Description of the Consteli-ations

The present chapter is intended for those who wish to
be able to recognise the principal constellations, and to
know where to look for the several planets. The problem
of pointing out the constellations is complicated by the
effect of the twofold motion of the earth ; on its axis and
around the sun. In consequence of the former the con-
stellations change their apparent position in the course of
the night, and the result of the latter is that different
constellations are seen at different seasons.

We explained in a former chapter how, in consequence
of the motion of the earth in its orbit round the sun, the
latter seems to us to perform an annual circuit among
the constellations. Hence, if a star is east of the sun, we
shall see it approach nearer to the sun every day. If we
look out night after night at the same hour we shall find
it farther and farther advanced toward the west. In
consequence of this change it must rise and set earlier
every day than it did the day before. More exactly, the
time between two risings and settings of the same star is
twenty-three hours fifty-six minutes four and a half sec-
onds. While in the course of a year the sun rises three
hundred and sixty-five times, a star rises three hundred
and sixty-six times. The latter will therefore during the
year have risen at every hour of the day and night.

306 TH^ FIXED STARS

Astronomers avoid all confusion from this cause by
the use of sidereal time, that is star-time, or time meas-
ured by the stars. As already explained, a sidereal day
is the interval between two successive pasages of a star
over the meridian, and is three minutes fifty-sis seconds
less than our ordinary day. It is divided into twenty-
four sidereal hours, and each hour into sidereal minutes
and seconds. A sidereal clock gains three minutes fifty-
six seconds daily on an ordinary clock and thus shows
the same time at the same position of the stars the year
around.

One who wishes to keep the run of the stars will find it
very convenient to have some idea of sidereal time. This
may be had by the following rule : Double the number of
the month; the product will be the sidereal time at six
o'clock in the evening. At seven o'clock it will be one
hour later, and at eight it will be two hours later, and so on.

Suppose, for example, that one looks at the sky in
November at nine o'clock in the evening. This is the
eleventh month; multiplying by two gives twenty-two,
adding three gives twenty-five, from which we drop
twenty-four, giving one hour as the sidereal time. The
time thus obtained will not often be more than an hour
in error, except during the first week or ten days of the
month, when it may be an hour or more too great. It
may then be diminished by one hour.

Applying the same rule in January we have five hours
as the sidereal time at nine in the evening. But early in
the month the sidereal time at nine in the evening will be
four hours instead of five.

THE NORTHERN CONSTELLATIONS 307

At hours sidereal time the equinoctial colure is on
the meridian ; at six hours, the solstitial colure, and so on.

The Northern Constellations

With this preliminary explanation let us proceed to the
study of the constellations. I assume the reader to be
somewhere in the latitude of the United States. Then
the principal northern constellations will never set, and
will be visible in whole or in part every evening in the
year. With them, therefore, we begin.

A figure, showing these constellations, is found in the
first part of the present book (Fig. 2). To see how they
will appear hold the cut with the month at top ; we then
have thfe position at eight o'clock in the evening. For a
later hoilr turn it a little in the direction of the arrows.
For example, in July, at ten o'clock, we hold it so as to
have August at the top. The Roman numerals on top

give, the sidereal time
without the trouble of
calculating it.

First find Ursa Ma-
jor, the Great Bear,
generally called the Dip-
per, an implement which
the constellation resem-
FiG. ZO.-Ursa Major, or Tlie Dipper. bles much more than it

does a bear. This you
can always do except perhaps in autumn when, if you
are far south, it may be more or less below the northern
horizon. Notice the pair of stars forming the outside

808

THE FIXED STARS

Fig. 51. — Ursa Minor.

of the bowl of the dipper. They are called the Pointers,
because they point toward the pole star, as shown by the

dotted line. This is the cen-
tral star of the map. It is
called Polaris.

The pole star belongs to

the constellation Ursa Minor,

the Lesser Bear; the rest of

the constellation you will

see by following a curved

line of stars from the pole toward XVI hours. You will

thus fall on another star as bright as Polaris but a little

redder in colour. This is Beta Ursae Minoris.

If you cannot see the pointers you will still easily find
Polaris if you know the exact north, because it is nearly
midway between the zenith and the northern horizon- —
nearer the latter, however,
the farther south we are. It
can be easily distinguished
from its neighbour. Beta,
by its whiter colour, Beta
being slightly red or dingy
in comparison.

On the opposite side of
the pole, at the same dis-
tance as Ursa Major, is
Cassiopeia, the Lady in the Chair. The chair has a very
crooked back but could be made comfortable by a cushion
in the hollow.

There are several other constellations in the region

Fig. 82. — Cassiopeia.

THE AUTUMNAL CONSTELLATIONS 309

around the pole, but they have few bright stars and are
of less interest than those we have mentioned. Among
them is Draco, the Dragon, whose form coils itself up be-
tween the Bears, and whose head is represented by a tri-
angle of stars in XVIII hours, near the August zenith.

The Autumnal Constellations

The zenithal and southern constellations to be looked
for will vary with the season. We begin with the posi-
tion of the sphere at hours sidereal time, which occurs
at ten o'clock in October, eight in November, and six in
December.

The equinoctial colure is first to be imagined. It
passes from the pole upward near the westernmost bright
star of Cassiopeia and can be traced south through the
eastern side of the square of Pegasus. The latter easily
recognised landmark of the sky is formed by four stars
of the second or third magnitude. The square is fifteen
degrees on a side.

Northeast from the northeast corner of the square is
the Great Nebula of Andromeda. It is plainly visible to
the naked eye as a whitish, ill-defined patch of light, and
is a fine object when seen in a telescope.

The Milky Way now spans the heavens like a slightly
inclined arch, resting on the east and west regions of the
horizon, and having its keystone a little north of the
zenith, in Cassiopeia. Tracing it from this constellation
toward the east, we first have Perseus, which stands in
the Milky Way itself. The brightest star in this con-
stellation is Alpha Persei, of the second magnitude.

310 THE FIXED STARS

East of Alpha is a white mass like a httle cloud. With
a small telescope, even with a good field gla:ss, we see this
mass to be a collection or cluster of small stars. It is the
Great Cluster of Perseus and, in the figure of the con-
stellation, forms the hilt of the hero's sword.

In a sort of offshoot toward the south (or southeast
as the constellation is now situated) lies a row of three
stars. The middle and brightest of these is the wonder-
ful variable star, Algol, whose changes will be described
in a later chapter. It is also called Beta Persei.

Below Perseus, the first large constellation is Auriga,
the Charioteer. It is marked by Capella, the Goat, a star
of the first magnitude and one of the brightest now above
the horizon — indeed, one of the four or five brightest in
the sky. But it has no other striking stars.

In the southeast are Aldebaran and the Pleiades,
which will be described later. Meanwhile let us follow
the course of the Milky Way from the zenith toward
the west.

The first collection of bright stars west of Cassiopeia
is now Cygnus, the Swan, lying centrally in the Milky
Way. Five stars are arranged somewhat in the form of
a cross and mark the body, neck, and extended wings of
the bird. The brightest of the group is Alpha Cygni,
or Deneb, nearly, but not quite, of the first magnitude.

Low and to the right of Cygnus, and a little outside of
the Milky Way, is the constellation Lyra, the Harp,
marked by the beautiful and very bright bluish star,
Vega. It has no other star of greater magnitude than
the third, but what it has will re^ay careful study.

THE AUTUMNAL CONSTELLATIONS 311

In the figure given here, notice the star to the left of
Vega ; Epsilon Lyrae it is called. A keen eye will, on care-
ful examination, see that this star is really composed of
two, lying so close together that it is not easy to dis-
tinguish them. With an opera glass this will more easily
be accomplished. But the
most curious fact is that if
a telescope be pointed at
the pair, each of the stars
will be found to be double,
so that Epsilon Lyrae is
really composed of four
stars.

Another star, about as
near to Vega as Epsilon is,
lies at one corner of a par-
allelogram or elongated
diamond, which stretches

south of Vega. At the farther blunt corner of the dia-
mond lies Beta Lyrae, marked B in the figure, a remark-
able variable star. To the left of it is Gamma. The law
of variation will be described in a later chapter.

To the right of Lyra, and in the Milky Way, lies
Aquila, the Eagle. It will be described later.

The other constellations low in the west will be de-
scribed later. At present we shall pass rapidly over the
constellations of the Zodiac.

If the ecliptic were painted on the sky we should now
see it rising to the north of the east point of the horizon,
passing in the south to mid-sky, where it would cross the

Fig. 53. — Lyra, the Harp.

31-2 THE FIXED STARS

equator at a small angle, and then, passing to the west,
reach the western horizon twenty-three degrees south
of west. At the time we suppose, Sagittarius, the Archer,
is mostly below the western horizon. Capricornus, the
Goat; Aquarius, the Water Bearer, and Pisces, the
Fishes, fill up the space to the meridian. The stars of
these constellations are mostly faint, few or none exceed-
ing the third magnitude.

Reaching the meridian, we see the square of Pegasus
above the Zodiac, not far south of the zenith. East of
it is the constellation Aries, the Ram. Three of its prin-
cipal stars, of the second, third, and fourth magnitudes,
form an obtuse triangle. The brightest is Alpha Arietis.

Two thousand years ago this constellation marked the
first sign of the zodiac, and the equinox was just below
Alpha Arietis, as explained in speaking of the precession
of the equinoxes.

Southeast from the square of Pegasus is a widely
extended constellation, Cetus, the Whale. Its two bright-
est stars, Alpha and Beta, are of the second magnitude.
The latter lies nearly below the southeast star of the
square of Pegasus and is quite by itself. Alpha is some
distance farther east. West of Alpha, and a little south,
is a remarkable star, Mira Ceti, the wonderful star of
Cetus, which is invisible to the naked eye except for a
month or two in each year, when it attains the fourth,
third, and often the second magnitude.

A little west of south, quite low down, is Fomalhaut,
nearly of the first magnitude, in the constellation Pisces
Australis, the Southern Fish.

THE WINTER CONSTELLATIONS 313

Fig. 54.— ?7i« Hyades.

The Winter Constellations

The next position of the stars we shall describe comes
six hours after the preceding one ; that is at two o'clock
A. M. in November and at
eight o'clock P. M. in Feb-
ruary. During this six-
hour interval another sec-
tion of the Milky Way has
risen in the east and passed
over toward the south. The
Milky Way now passes
nearly through the zenith,
resting on the horizon near
the north and south points.

Near its course and east of the meridian we see the
constellation Taurus, the Bull, of which the brightest star

is Aldebaran, form-
ing the eye of the
bull in the mytho-
logical figure. Alde-
baran is easily rec-
ognised by its red
colour. It lies on the
end of one branch of
a V-shaped cluster
called Hyades. No-
tice the pretty pair
of stars in the middle
of one leg.

Fig. 55. — The Pleiades, as seen with the
naked. eye.

314! THE FIXED STARS

Near by is the best known cluster in the sky, the
Pleiades, or "seven stars." Only six stars are made out
by ordinary unaided vision, but to a good eye five others

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Fig. 86. — Telescopic View of ihe Pleiades,wit/i, Names of the BrighXer Stars.

are visible, making eleven in all. The term "seven stars"
is therefore a misnomer ; as a reason for it, it was said in
ancient times that the number was originally seven but
that one faded away. This "lost Pleiad" is probably a
myth, as we do not find stars fading away perj^nently.

THE WINTER CONSTELLATIONS 315

With a telescope we find the cluster to contain quite a
number of yet smaller stars, as can be seen by the tele-
scopic view which we give.

The central and brightest star of the group is
called Alcyone, and was supposed by Maedler to be the
central star of the universe. But this notion is quite
baseless.

East of Taurus and near the zenith is Gemini, the
Twins, marked by two stars nearly of the first magnitude,
Castor and Pollux. The latter is the northernmost and a
little the brighter of the two.

The next zodiacal constellation is Cancer, the Crab,
but it contains no conspicuous stars. Its most noticeable
feature is ProBsepe, a cluster of stars, which are singly
invisible to the naked eye, and look collectively like a
small patch of light. The smallest telescope will show a'
dozen stars in the patch.

Leo, the Lion, is also well up in the east. It may be
recognised by Regulus, a star nearly of the first magni-
tude, and a curved row of stars in the form of a sickle,
of which Regulus is the handle.

In the south we now have the most brilliant constella-
tion in the heavens, the beautiful Orion. The three stars
of the second magnitude in a row forming the belt of the
warrior are familiar from childhood to all who watch the
sky. Below them hangs another row of three stars, the
upper one quite faint. The middle one of these has a
hazy aspect, and is really not a star at all, but one of the
most splendid objects in the sky, the Great Nebula of
Orion. A mere spy-glass will show its character, but a

316

THE FIXED STARS

large telescope is required to bring out the magnificence
of its form.

The comers of the constellation are marked by four
stars. The brighter of the two uppermost, Alpha

Orionis, or Betel-
guese, is reddish in
colour. At the oppo-
site comer is Rigel,
blue in colour and
also of the first mag-
nitude. The two up-
per stars are in the
shoulders of the fig-
ure. Midway and
above them a triangle
of small stars forms
the head.

East of Orion is
Canis Minor, the Lit-
tle Dog, containing
Procyon, of the first magnitude. Below it and south-
east of Orion is another collection of bright stars forming
the constellation Canis Major, the Great Dog, containing
Sirius, the Dog Star, the brightest fixed star in the
heavens.

The Spring Constellations

The third position of the sphere, sidereal time twelve
hours, occurs in February at two A. M. ; in May at
eight P. M. Lyra has now risen in the northeast and
Capella is going downward in the northwest. The Milky

* ♦ *

*

Fig. 57. — Orion.

THE SPRING CONSTELLATIONS 317

Way may not be visible at all unless the air is very clear.
It will then be seen skirting the northern and western
horizon. Regulus has passed the meridian, and Orion
and Canis Major have set, or are low down in the
southwest.

In mid-heaven, southeast of the zenith, is Arcturus, cf
a dingy yellow col-
our, but one of the I "
brightest first magni-
tude stars.

East of Arcturus
(now below it) is
Corona Borealis, the
Northern Crown, a
beautiful semicircle of
stars, of which the
brightest is of the
second magnitude.

Near the zenith is
Coma Berenices, the Hair of Berenice, a collection of
faint stars mostly of the fifth magnitude. East of south
across the meridian from Leo is Virgo, the Virgin, con-
spicuous only by Spica, a white star of nearly the first
magnitude. Libra, the Balance, east and southeast of
Virgo, has no conspicuous stars.

The Summer Constellations

The fourth position of the sphere, eighteen hours
sidereal time, occurs in May at two A. M. ; in August at
eight P. M. Capella has now set, Lyra is near the zenith.

Fig 58. — The Northern Crown.

318

THE FIXED STARS

Fig. 69. — Aqnila.

Cassiopeia is in the northeast, and the most splendid por-
tion of the Milky Way is near the meridian. We have
described all the constellations
that lie near its course north of
Lyra ; let us now trace it to the
south.

One of the noticeable fea-
tures of the Milky Way now to
be seen is the great bifurcation,
or separation into two branches.
The split can be traced from
Cygnus, where it begins, past
Lyra and halfway to the south-
ern horizon. Here we see AquUa,
the Eagle, in the cleft, marked by Altair, of the first
magnitude. It is in a line between two other stars of the
third and fourth magnitudes.

At this point the westernmost branch of the Milky
Way diverges yet farther and
seems to terminate, but if the air
is clear we shall see that it recom-
mences near the horizon.

East of Aquila is a small but
very pretty constellation of which
the scientific name is DelpJiinus,
the Dolphin, but which is popu-
larly known as Job's CoflSn.

Between Lyra and the beautiful
Corona, now some distance west of
the zenith, lies the widely extended

Fig. 60. — Delphinus, the
Dolphin.

THE SUMMER CONSTELLATIONS 319

Fia. 61. — 2%« Oreat Cluster of Hercules, photographed at the Lick Observatorv.

820

THE FIXED STARS

constellation, Hercules. Alpha, its brightest star, is below
the second magnitude and may be known by its reddish
colour and by a white star. Alpha Ophiuchi, a little
farther east. The most remarkable object in this con-
stellation is the Great Cluster of Hercules which, to the
naked eye, is a very faint patch, but which a great tele-
scope resolves into a universe of stars.

Near the horizon, west of south, is the zodiacal con-
stellation Scorpius, the Scorpion. Its western boundary

is a curved row of stars
forming the claws of the
animal; east of them is
Antares, or Alpha Scor-
pii, reddish in colour,
and nearly of the first
magnitude.

In the Milky Way,
due south, and therefore
east of Scorpius, is Sag-
ittarius, the Archer, with
quite a collection of
stars of the second and
third magnitudes. The bow and arrow of the archer are
easily imagined.

Next toward the east are Capricornus and Aquarius,
already mentioned. The brightest star in the former
has a companion so close to it that it is a sign of not bad
eyesight to be able to distinguish it.

Fio. 62. — Scorpiun, the Scorpion.

IV

The Distances of the Staus

"The principles on which distances in the heavens are
determined was set forth in our chapter explaining how
the heavens are measured. For distances of the moon and
nearer planets, we use, as a base line for measurement, the
radius of the earth, or the line joining two points of ob-
servation on its surface. But this is entirely too short to
serve for measuring a distance so great as that even of the
nearest star. For this purpose we take as a base line the
whole diameter of the earth's orbit. As the earth moves
* from one side of the orbit to the other, the stars must seem
to have a slight motion in the opposite direction. But this
motion is found to be almost immeasurably small. It can
be made out with sufficient precision only by comparing
the stars among themselves in the following way :

Let the little circle on the left of the following figure
represent the orbit of the earth. Let S be the star, sup-
posed to be near us, of which we wish to measure the dis-
tance. Let the dotted lines almost parallel to each other
show the direction of a star T many times farther away.
When the earth is at one side of its orbit, say at P, we
measure the small angle SPT, which seems to us to sepa-
rate these two stars. When the earth goes to the opposite
side, it is readily seen that the corresponding angle SQT
will be greater. We again measure it. The difference

322 THE FIXED STARS

between these two angles will furnish a basis for com-
puting, by trigonometric methods, the distance of the
nearest star when that of the farthest is known. Practi-
cally we have to assume that the star T is at an infinite dis-
tance, so that the dotted lines are parallel. Then the
measured difference between the angles will enable us to
calculate the angle subtended by the radius of the earth's
orbit, as seen from the star S. This angle is what astrono-

FiG. 63. — MeOsSureineiit of the Parallax of a Star.

mers habitually use in their computations, not the dis-
tance of the star. It is called the Parallax of the star.
If we wish to obtain the distance of the star, we have to
divide the number 206,265 by the parallax of the star
expressed as a fraction of a second. This will give its
distance in terms of the radius of the earth's orbit as a
unit of measure. One second is the angle subtended by
an object one inch in diameter at a distance of 206,265
inches, or more than three mUes. It is, of course, com-
pletely invisible to the naked eye.

It will be seen that this method of measurement implies
t})at we know which of the 1 .vo stars is the nearer ; in fact,
that we know the farther star to be at an almost infinite
distance. The question may be asked how this knowl-
edge is obtained, and how a star is selected as being near
to us. The most careful measures that can be made with
the finest instruments show that the great mass of small

THE DISTANCES OF THE STARS 323

telescopic stars do not have the slightest change in their
relative positions, but remain as if fixed on the celestial
sphere from year to year. Now and then, however, an
exception is found. A very bright star is probably nearer
to us than the fainter ones, and if a star shows any
change in its position, the astronomer may proceed to
measure and determine its parallax.

So far as has yet been determined, the nearest star to
us is Alpha Centauri, a star of nearly the first magnitude,
in the southern hemisphere. The parallax of this star
is 0.75". By the rule we have given, its distance will be
nearly 275,000 times that of the sun. Such a distance
transcends all our power of conception over and over
again. A crude idea of it may be obtained by reflecting
that light itself, the speed of which we have already
described, would require more than four years to reach
us from this star. We see the latter, not as it is now, but
as it was more than four years ago. At such a distance
not only does the earth's orbit itself vanish away to a
point, but a ball as large as the whole body of Neptune
would be barely visible to the naked eye as the minutest
possible point.

The next star in the order of distance is supposed to be
about one half as far again as Alpha Centauri, and there
are some half dozen others, within three or four times its
distance. In all, the parallaxes of about one hundred
stars have been determined with more or less exactness;
but even in these cases the parallax is sometimes so small
that we cannot be sure it is real. It seems likely that only
about fifty stars are within seven times the distance of

324 THE FIXED STARS

Alpha Centauri. The distance of the stars whose paral-
laxes are too small to be measured is a matter of judg-
ment rather than calculation. The probability seems to
be that at least the brighter stars are scattered through
space with some approach to uniformity. If this is the
case, many of the fainter telescopic stars, perhaps the
large majority of the smallest ones found on photo-
graphs of the heavens, must be more than one thousand
times the distance of Alpha Centauri. The light by
which their presence is made known to us must have been
on its way to our system during the whole period of
human history.

V

The Motions of the Stars

If I were asked what is the greatest fact that the intel-
lect of man has ever brought to light I should say it was
this:

Through all human history, nay, so far as we can dis-
cover, from the infancy of time, our solar system — sun,
planets, and moons — has been flying through space
toward the constellation Lyra with a speed of which we
have no example on earth. To form a conception of this
fact the reader has only to look at the beautiful Lyra and
reflect that for every second that the clock tells off, we
are ten miles nearer to that constellation. Every day that
we live we are nearer to it by almost, perhaps quite, a
million of miles. For every sentence that we utter, for
every step that we take in the streets we are miles nearer
to this star. We approached it by tens of thousands of
miles while the writer has been penning these lines, and
the reader has been carried nearer by a thousand miles
while perusing them. This has been going on through
all human history, and we have reason to believe that it
will remain true for our remotest posterity. One of the
greatest problems of astronomy is, when and how did
this journey begin and when and how will it end? Before
this question our science stands dumb. The astronomer
can tell no more about the beginning or the end of the

326 THE FIXED STARS

journey than can the untutored child. He can only im-
press upon the mind of his followers the magnitude of
the problem.

Notliing can give us a better conception of the enor-
mous distance of the stars than the reflection that not-
withstanding the rapid motion, carrying us unceasingly
forward through all the ages that the human race has
existed on earth, ordinary observation would fail to show
any change in the appearance of the constellation toward
which we are travelling. From what we know of the dis-
tance of Vega we have reason to suppose that our solar
system will not reach the region in which that star is now
situated until the end of a period ranging somewhere
between half a million and a million of years from the
present time.

It does not follow, however, that our posterity, if any
such shall then live on the earth, will find Vega when they
arrive at its present place. It also is going on its own
journey and is passing away from its present location
almost as rapidly as we are approaching it.

What is true of our sun and of Vega is true, so far as
we know, of every star in the heavens. Each of these
bodies is flying straight ahead through space like a ball
shot out from a cannon, with a speed which in most cases
is almost inconceivable. It would be a very slow moving
star of which the velocity did not exceed that of a cannon
shot. In the great majority of cases it ranges from five
to thirty miles per second — frequently more than fifty
miles. Indeed there are two stars, of which Arcturus is
one, whose speed we have reason to believe approaches

THE MOTIONS OF THE STARS 3^7

two hundred miles a second. These motions of the stars
are called their proper motions.

We have described the proper motions as so many miles
per second. But owing to the enormous distance of the
stars, raj)id as the proper motions are in reality, they
seem slow indeed when we observe them. So slow are they
that if Ptolemy should come to life after his sleep of near-
ly eighteen hundred years, and be asked to compare the
heavens as they are now with those of his time, he would
not be able to see the slightest difference in the configura-
tion of a single constellation. Even to the oldest Assyrian
priests, the constellation Lyra and the star Vega looked
exactly as they do to us to-day, notwithstanding the im-
measurable distance by which we have approached them.

To resuscitate an inhabitant of the ancient world who
would be able to perceive any change, we should have to
go back four thousand years perhaps, to the time of Job,
and we should have to take one of the swiftest moving
stars in the heavens, Arcturus. Bringing Job to life and
showing him the constellation Bootes, of which Arcturus
is the brightest star, he would perceive the latter to have
moved through about half of the distance in the accom-
panying diagram between the stars marked "1" and "2."

In considering these motions, the most natural thought
to present itself is that the stars are describing vastly ex-
tended orbits around some centre, as the planets are
moving round the sun, and that the motions we see are
simply the motions in these orbits. But the facts do not
support this view. The most refined observations yet
made do not show the slightest curvature in the path of

THE FIXED STARS

any star. Every one seems to be going straight ahead on
its own account, never swerving to the right or left. It
does not seem possible to admit the existence of bodies
large and massive enough to control such rapid motions.
A body massive enough to attract Arcturus from its head-

PiQ. 6i.~Arclunis and the Surrounding Stars in Constellation Bootes.

long course would throw all that part of the universe in
which we live into disorder. The problem where the
rapidly moving stars came from and whither they are
going is therefore for us insoluble. What makes the case
yet more difficult is that different stars move in different
directions, without any seeming order, so that one motion
seems to have no connection with another, unless in a few
very rare cases.

VI

Variable and Compound Stars

As a general rule the starry heavens may be taken as
a symbol of eternal unchangeability. The proverb-
makers have told us in all time how everything on the
earth is subject to alternation and decay, while the stars
of heaven remain as we see them, age after age. But it
is now known that, although this is true of the great
majority of the stars, there are some exceptions. These
are so little striking that they were never noticed by the
ancient astronomers.

The first person in history to observe a change in a star
was one Daniel Fabritius, a diligent watcher of the
heavens, who lived three centuries ago.

In August, 1696, he noticed a star of the third magni-
tude before unknown in the constellation Cetus, which
soon faded away again, and disappeared from view in
October. In subsequent years it was found to show
itself at regular intervals of about eleven months.

Two centuries elapsed before another case of the kind
was known. Then it was found that the star Algol, in
Perseus, faded away from the second to the fourth magni-
tude for a few hours at intervals of a little less than three
days.

Early in the nineteenth century other stars were found
to be subject to a more or less regular variation of their

330 THE FIXED STARS

light. As observers studied the heavens with greater care,
more and more of such stars were found, until at the pres-
ent time the list of them numbers four or five hundred,
and is constantly increasing. Of these some vary in an
irregular way, but a large majority go through a regu-
lar period.

The easiest of these objects to notice is Beta Lyra,
which is marked B on the figure of that constellation
already given. It can be seen at some hour of any clear
evening, spring, summer, or autumn. If the reader as he
takes his evening walk will, night after night, compare
this star with the one nearest to it and nearly of the same
magnitude, he will see that while on some evenings the two
appear perfectly equal, on others Beta will be of a mag-
nitude fainter than the other. Careful and continued
watching will show that the change takes place in a
period of about six days and a half. That is to say, if
the two stars are equal on a certain evening, they will
again appear equal at the end of six or seven days, and
so on indefinitely. Midway between the two times of
equality the variable one will be at its faintest. If the
observer notes the magnitudes at this time with the
greatest precision, a curious fact will be brought out.
Every alternate minimum, as the phase of least light is
called, is slightly fainter than that preceding or follow-
ing. The actual period is therefore nearly thirteen days,
during which time there are two maxima of equal bright-
ness and two slightly diff^erent minima.

It is now known that the variation of light in this case
is not really inherent in the star itself, but arises from

VARIABLE AND COMPOUND STARS 331

the fact that the star is a double one, composed of two
stars revolving around each other, and so near together
as almost to touch. As they revolve, each one in succes-
sion whoUy or partially hides the other. This fact is not
brought out by the telescope, because the most powerful
telescope that could be made would not show the two
stars separately. It is the result of long and careful
study of the spectrum of the star, which is found to be
a double one, the lines in one of which alternately cover
and recede from the lines of the other.

In the extent of variation of its light the most remark-
able of the more conspicuous variable stars is Omicron
Ceti, already mentioned as seen by Fabritius. It is now
found to go through a regular period in three hundred
and thirty days. During about two weeks of this time it
is at its brightest, and is then sometimes of the second
magnitude and sometimes much fainter — occasionally
only of the fifth. After each maximum it gradually
fades away for a few weeks and disappears from view to
the naked eye. But with a telescope it can be seen all the
year round.

The period of eleven months makes the maximum occur
about a month earlier every year. During some years it
will occur when the star is so near the sun that it cannot
be easily observed. This will be the case during the years
1903-'O5.

Algol, also called Beta Persei, being in northern dec-
lination, can be seen in our latitudes at some time on
almost every night of the year. In autumn and winter
it is visible in the early evening. The peculiarity of its

332 THE FIXED STARS

variation is that it remains of the same brightness nearly
all the time, but fades away for a few hours at intervals
of about two days and twenty-one hours. It is now
known that this is due to the partial eclipse of the ctar
by a dark body nearly as large as itself, revolving round
it. It is true that this body has never been seen by human^
eye and never will be. Its existence is made known by its
causing the star to revolve in a small orbit. It is true
that this motion of the bright star is too small to be
observed with the telescope, but it is made certain by
means of the spectroscope, which shows a change in the
wave length of the light coming from the star.

Different variable stars differ very widely in the extent
of their variation. In most cases the latter is so slight
that only an expert observer would notice it. Frequently
it cannot be determined until after a long study by
various observers whether a "suspected variable" is really
such.

These objects form a very interesting subject of ob-
servation for those who have at command little or no
instrumental facilities. No telescope is needed unless the
star is, at some of its phases, invisible to the naked eye.
The points to be noticed and recorded are the exact
magnitude of the star from minute to minute or hour to
hour, as it is going through its most rapid change, in
order to learn at what moment its brightness is greatest
or least.

What adds to the interest of the astronomer in these
objects is the evidence now being gathered that, many,
perhaps most of the stars, are not single bodies, but more

VARIABLE AND COMPOUND STARS 333

or less complex systems of bodies having the widest di-
versity in their construction. Double stars have been
familiar to every observer of the heavens since the time
of the great Herschel. But it is only in the time of our
generation that the spectroscope has begun to make
known to us pairs of stars revolving round each other,
of which the components are so close together that the
most powerful telescope can never separate them. The
history of science offers no greater marvel than the dis-
coveries of invisible planets moving round many of the
stars which are now being made, and in which the Lick
observatory has recently taken the lead.

It now seems more or less probable that the changes of
light in all stars having a regular and constant period is
due to the revolution of large planets or other stars
around them. Sometimes the variation is slight and is
caused in the way we have described, by one body par-
tially eclipsing the other as it passes across it. In this
case there may be no real variation In the light ; the star
eclipsed shines just as bright behind the eclipsing body
as when it Is not eclipsed. But It now seems that, if the
darker body revolves in a very eccentric orbit, so as to be
much nearer the bright body at some times than at others,
its attraction produces such a change in the other as to
greatly Increase Its light. Just how this effect is pro-
duced it is as yet Impossible to say.

THE END.