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NEW YORK

^/Ki CJ.C.L.Tf

JUL 1 3 '33

UNIVERSITY OF CALIFORNIA
AT LOS ANGELES

L-.

" JUL 1 3 'W

7/f^ STORY OF THE STARS.

NEW

DESCRIPTIVE

ASTRONOMY,

JOEL DORM AN STEELE, Ph.D.,

AUTHOR OF THE FOURTEEN-VVEEKS SERIES IN NATURAL SCIENCE.

' ' The heavens cteolare the glory of God; and the fi'nmament showezn
is handiwork. " — PsAlM XIX, 1.

A. S. BARNES & COMPANY,
NEW YORK AND CHICAGO.

A POPULAR SERIES

NATURAL SCIENCE,

J. DoRNiAN Steele, PH.n., K.G.S.,

A uihor o/ the Fourteen Weeks Series in Natural Science, etc., etc.

New Popular Chemistry. New Descriptive Astronomy.

New Popular Physics. New Hygienic Physiology.

New Popular Zoology. Popular Geology.

An Introduction to Botany.

The Publishers can supply (to Teachers only) a Key containing Answers to the
Questions and Problems in Steele's entire Series.

BARNES' HISTORICAL SERIES,

ON THE FLAN OK

STEELE'S FOURTEEN-WEEKS IN THE SCIENCES.

A Brief History of the United States.
A Brief History of France.

A Brief History of Ancient Peoples.

A Brief History of Mediaeval and Modern Peoples^
A Brief General History.

A Brief History or Greece.

A Brief History of Rome.

A Popular History of the United States.

Library S '^Z S

PREFACE TO THE FIRST EDITION.

~r~\URING the past few years great advances
-■-^ have been effected in astronomical science.
Physics has come to the help of Mathematics, and,
not content with the old question, where the heav-
enly bodies are, has sought to find out what they are.
Valuable discoveries have been made concerning
Meteors, Shooting Stars, the Constitution of the Sun,
the Motion of the Heavenly Bodies, &c. The investi-
gations connected with Spectrum Analysis have been
especially suggestive. On every hand the facts of
the New Astronomy have been accumulating. Until
recently, however, they were scattered through many
expensive books, and were consequently beyond the
reach of the most of our schools. It has been the
aim to collect in this little volume the most interest-
ing features of the larger works.

Believing that Natural Science is full of fascina-
tion, the author has sought to weave the story of
those far-distant worlds into a form that may attract
the attention and kindle the enthusiasm of the pupil.

This work is not written for the information of
scientific men, but for the inspiration of youth.

20482'?

VI PREFACE TO THE FIRST EDITION.

Therefore the pages are not burdened with a multi-
tude of figures which no memory could retain.

Mathematical tables and data, Questions for Re-
view, a very valuable Guide to the Constellations,
and an Apparatus for Illustrating Precession, are
given in the Appendix, where they may be useful
for reference.

Those persons having a small telescope will find
valuable assistance in the '' List of interesting Ob-
jects for a common Telescope." The Index contains
the pronunciation of many difficult names.

Particular attention is called to the method of
classifying the measurements of Space, and the
practical treatment of the subjects of Parallax, Har-
vest Moon, Eclipses, the Seasons, Phases of the Moon,
Time, Nebular Hypothesis, Spectrum Analysis, and
Precession.

To teachers hitherto compelled to use a cumber-
some set of charts, it is hoped that the star maps
here offered will present a welcome substitute. The
geometrical figures, showing the actual appearance of
the constellations, will relieve the mind confused with
the idea of numberless rivers, serpents, and classical
heroes. Only the brightest stars are given, since in
practice it is found that pupils remember the general
outlines alone, while the details are soon forgotten.

Many of the cuts are copied from the French edi-
tion of Guillemin's '^ Heavens." Acknowledgment

ASTRONOMY. vil

for much valuable material is hereby made to this
excellent work, and also to "Chambers's Astron-
omy," "Newcomb's Astronomy," and Young's "The
Sun."

Finally, the author commits this little work to the
hands of the young, to whose instruction he has con-
secrated the energies of his life, in the earnest hope
that, loving Nature in all her varied phases, they
may come to understand somewhat of the wisdom,
power, beneficence, and grandeur displayed in the
Divine harmony of the Universe.

"One God, one law, one element,

And one far-off Divine event
To which the whole creation moves. "

READING REFERENCES.

Chambers's Astronomy. — Young's The Sun.— Ball's Elements of Astronomy.—
Newcomb's Popular Astronomy. — Lockyer's Spectrum Analysis. — Proctor's Other
Wbrlds than Ours, Saturn, The Moon, Poetry of Astronomy, &c. — Delaunay's Cours
D'Astronomie. — Haughton's Manual of Astronomy. — Newcomb and Holden's As-
tronomy. — Lockyer's Elements of Astronomy. — Norton's Spherical and Physical
Astronomy. — Herschel's Outlines of Astronomy. — Robinson's Astronomy, — Mitch-
ell's Popular Astronomy. — Arago's Popular Astronomy. — Airy's Lectures on
Astronomy. — Hind's Solar System, and Introduction to Astronomy. — Lockyer's Ele-
mentary Lessons in Astronomy.— Proctor's Star Atlas. — Heis's Star Atlas. — Peck's
Popular Astronom}-. — Gillet and Rolfe's Astronomy.- Sharpless and Phillips's As-
tronomy. — Peabody's Elements of Astronomy. — Schellen's Spectrum Analysis. —
Winchell's World-Life (excellent reading in connection with the Nebular Hypothe-
sis).— Flammarion's Wonders of the Heavens.— Guillemin's The Heavens, revised by
Proctor. — Loomis's Elements of Astronomy. — Proctor's Easy Star Lessons. — Olm-
stead's Letters on Astronomy. — Routledge's Historj' of Science. — Buckley's History
of Natural Science. — Williamson's Problems on the Globes. — The Popular Science
Monthly (1872-1884;. — Rambosson's Histoire Des .Astres.

SUGGESTIONS TO TEACHERS.

^l"^HIS work is designed to be recited in the topical method. On hear-
ing the title of a paragraph, the pupil should be able to draw upon
the blackboard the diagram, and to state the substance of what is con-
tained in the book. It will be noticed that the order of topics, in treat-
ing of the planets and also of the constellations, is uniform. If, each
day, a portion of the class write their topics in full upon the black-
board, it will be found a valuable exercise in spelling, punctuation,
and composition. Every point which can be illustrated in the heavens
should be shown to the class. No description or apparatus can equal
the reality in the sky. After a constellation has been traced, the pupil
should be practised in star-map drawing.

The article on "Celestial Measurements," near the close of the
work, should be constantly referred to during the term. In the figures,
and especially in the star-maps, it should be remembered that the
right-hand side represents the west ; and the left-hand, the east. To
obtain this idea correctly, the book should, in general, be held up
toward the southern sky.

For the purpose of more easily finding the heavenly bodies at any
time, Whitall's Movable Planisphere is of great service. It may be
procured of the publishers of this work. A tellurian is invaluable in
explaining Precession of the Equinoxes, Eclipses, Phases of the Moon,
etc. Messrs. A. S. Barnes & Co. , New York City, furnish a good in-
strument at a low price. A small telescope, or even ah opera-glass,
will be useful. A good star-map, and as many advanced works upon
Astronomy as can be secured, should be included in the teacher's
outfit.

X SUGGESTIONS TO TEACHERS.

The pupil should, at the outset, get a distinct idea of the circles
and planes of the celestial sphere. The subject of angular measure-
ments can easily be made clear in this relation. A circle contains
360° ; 90" reach from horizon to zenith ; 180" produce opposition ;
while smaller distances can be shown in the sky (see pp. 216, 228).

Never let a pupU recite a lesson, nor answer a question, except it be
a mere definition, in the language of the book. The text is designed to
interest and instruct the pupil ; the recitation should afford him an
opportunity of expressing what he has learned, in his own style and
words.

Teachers desiring additional information are advised to read " New-
comb's Astronomy," Young's "The Sun," Proctor's Works, "Chambers's
Astronomy," and Ball's " Elements of Astronomy."

TABLE OF CONTENTS

PAGE

INTRODUCTORY REMARKS 1

I. INTRODUCTION.

HISTORY OF ASTRONOMY 5

SPACE 24

The Thkee Systems op Circles 36

The Zodiac 31

II. THE SOLAR SYSTEM 35

THE SUN 36

THE PLANETS 55

Vulcan 71

Mercury 71

Venus 77

The Earth 82

The Seasons 95

Precession and Nutation 104

Refraction, Aberration, and Parai,lax 112

The Moon 122

Eclipses 138

The Tides 147

Mars 150

The Minor Planets 154

Jupiter 157

Saturn 164

Uranus 170

Neptune 172

Xll TABLE OF CONTENTS.

PAGE

METEORS AND SHOOTING STARS 175

COMETS 185

ZODIACAL LIGHT 196

III. THE SIDEREAL SYSTEM 201

THE STARS 203

THE CONSTELLATIONS 214

Northern Circumpolar Constellations 214

Equatorial Constellations 220

Southern Constellations 238

DOUBLE STARS, COLORED STARS, VARIABLE STARS,

CLUSTERS, MAGELLANIC CLOUDS, &C 239

Nebula 246

The INIiLKY Way 253

The Nebular Hypothesis 255

CELESTIAL CHEMISTRY.— Spectrum Analysis 258

TIME 263

CELESTIAL MEASUREMENTS 271

IV. APPENDIX 289

•

Tables 291

Questions 293

Guide to the Constellations 313

Apparatus 317

List of Interesting Objects Visible with an Ordinary

Telescope 319

Index 323

INTRODUCTORY REMARKS*

A STROif OM Y (astron, a star ; nomos, a law) treats of the
/-\ Heavenly Bodies — the sun, moon, planets, stars, etc.,
and, as our globe is a planet, of the earth also. It
is, above all others, a science that cultivates the imagination.
Yet its theories and distances are based upon rigorous matliemat-
ical demonstrations. Thus the study has at once the beauty of
poetry and the exactness of Geometry.

The great dome of the sky, lilled with glittering stars, is one
of the most sublime spectacles in nature. To enjoy this fully, a
night must be chosen when the air is clear, and the moon is ab-
sent. We then gaze upon a deep blue, an immense expanse
studded with stars of varied color and brilliancy. Some shine
with a vivid light, perpetually changing and twinkling ; others,
more constant, beam tranquilly and softly upon us ; while many
just tremble into our sight, like a wave that, struggling to reach
some far-off land, dies as it touches the shore.

In the presence of such weird and wondrous beauty, the ten-
derest sentiments of the heart are aroused. A feeling of awe and
reverence, of softened melancholy mingled with a thought of
God, comes over us, and awakens the better nature within us.
Those far-off lights seem full of meaning to us, could Ave but
read their message ; they become real and sentient, and, like the
soft Cj es in pictures, look lovingly and inquiringlj'^ upon us. "We
come into communion with another life, and the soul asserts its
immortality more strongly than ever before. We are humbled
as we gaze upon the infinity of suns, and strive to comprehend

* This Introduction is designed merely to furnish su{;gestive material for conversa-
tion at the first lesson, preparatory to beginning the study. It is not intended for com-
mittal. Other topics may he found in the Questions given in the Appendix.

2 INTRODUCTORY REMARKS.

their enormous distances, and their magnificent retinue of worlds.
The powers of the mind are aroused, and eager questionings
crowd upon us. What are those glittering fires 1 WTiat is their
distance 1 Are they worlds like our own ? Do living, thinking
beings dwell upon them ? Are they promiscuously scattered
tlirough space, or is there a system in the universe ? Can we
ever hope to fathom those mysterious depths, or are they closed
to us forever 1

Some of these problems have been solved ; others yet await
the astronomer whose eye shall be keen enough to read the mys-
terious scroll of the heavens. Two hundred generations of study
have revealed to us such startling facts, that we Avonder how man
in liis feebleness can grasp so much, see so far, and penetrate so
deeply into the mysteries of the universe. Astronomy has meas-
ured the distance of a few stars, and of all the planets ; com-
puted the mass, size, days, years, seasons, and many physical
features of the planets ; made a map of the moon ; tracked
many of the comets in their immense sidereal journeys ; and, at
last, analyzed the structure of the sun and stars, and announced
the very elements of wliich they are composed.

Observing for several evenings those stars which shine with a
clear, steady light, we notice that they change their position with
respect to the others. They are therefore called planets (literally
wanderers). Others remain immovable, and shine with a shift-
ing, twinkling light. They are termed the fixed stars, although
it is now knoAvn that they also are in motion — the most rapid of
any known even to Astronomy — but through such immense
orbits that they seem to us to be stationary. Then, too, diag-
onally girdling the heavens, is a whitish, vapory belt — the Milhy
Wmj. This is composed of multitudes of millions of suns — of
which our OAvn sun itself is one — so far removed from us that
their light mingles, and makes only a fleecy whiteness.

Tliis magnificent panorama of the heavens is before us, inviting
our study, and waiting to make known to us the grandest revela-
tions of science.

I.
INTRODUCTION

( 1. Among the Chinese.

2. Among the Chaldeans.

(\. Thales.

1 2. Anaximander.

3. Among the Grecians. ■{ 3. Pythagoras.

I 4. Auaxagoras <fe Eudoxus.
V 5. Hipparchus.

„ ^ (1. The School at Alexandria.

4. The Egyptians -^ j, ptokiny and his Theory.

1. History

l-H

2. Space.

5. The Saracens.

6. Astrology.

7. The C.opernican System.
S. Tycho Braheu

9. Kepler's Laws.

( 1. His Telescope,

10. Galileo \ 2. His Discoveries.

( 3. Their Reception.

Newton, and the Law
OF Gravitation

{a.
6.
c.

a. Laws of Motion.

Their Application to Moon's

Pha.ses.
Tlie Result

1. Celestial Sphere.

The Three Sys-
tems OF Cir-
cles.

'^ 3. The Zodiac.

a. The Principal Circle.

1. The Hori- ) 6. Tlie Subord. Circle,
zon. l c. Points.

d. Measurements.

( a. The Principal Circle.

2. The Equi- 1 6. The Subord. Circle.

noctial.

3. The

Ecliptic.

Points.
[ d. Measurements.

a. The Principal Circle.
6. The Subord. Circle.
r. Points.
d. Measurements.

Fig. S.

Sir Isaac Newtcm.

L-THE HISTORY.

Astronomy is the most ancient of the sciences.
The study of the stars is doubtless as old as man
himself, and hence many of its discoveries date back
of authentic records, amid the mysteries of tradition.
In tracing its history, we shall speak only of those

6 THE HISTORY.

prominent facts that will enable us to understand its
progress and glorious achievements.

The Chinese boast much of their astronomical dis-
coveries. Indeed, their emperor claims a celestial
ancestry, and styles himself the Son of the Sun.
They possess an account of a conjunction of four
planets and the moon, which occurred in the 25th
century before Christ. They have also the first record
of an eclipse of the sun (b.c. 2128) ; and one of their
emperors put to death the chief astronomers Ho and
Hi for failing to announce the solar eclipse of
2169 B.C.

The Chaldeans. — The Chaldean shepherds, watch-
ing their flocks by night under a sky famed for its
clearness and brilliancy, could not fail to become
familiar with many of the movements of the heav-
enly bodies. Their priests were astronomers ; and
their temples, observatories. When Alexander took
Babylon (b.c. 331), he found a record of their obser-
vations reaching back nineteen centuries.* The
Chaldeans divided the day into hours, invented the
sun-dial, and discovered the Saros, or Chaldean Pe-
riod — the length of time in which eclipses of the sun
and the moon repeat themselves in the same order.

The Grecians. — Though the Asiatics were patient
observers, they did not classify their knowledge, and
lay the basis of a science. This became the work of
the western mind.

Thales (B.C. 640-548), one of the seven sages of

* Many astronomical inscriptions have been found in the ruins of Nineveh. In the
public library of that city there was a series of about se^'enty-two A'olunies, called the
Observations of Bel. One book treated of the polar star (then Alpha of the Dragon),
another of Venus, and a third of Mars. Tlie earliest of these records are thought tO
date back as fiw as 2540 b.c. (See Kecords of the Past, Vol. I.)

THE GRECIANS. 7

Greece, has been styled the Father of Astronomy.
He taught that the earth is round, and that the moon
receives her light from the sun. He determined
when the equinoxes and the solstices occur, and also
predicted an eclipse of the sun that is famous for
having terminated a war between the Medes and the
Lydians. These nations were engaged in a fierce
battle, but the awe produced by the darkening of the
sun was so great, that both sides threw down their
arms and made peace.

Anaximander (B.C. 610-546) invented the sun-dial,
and explained the cause of the moon's phases.

Pythagoras (b.c. 570-500) founded a celebrated
astronomical school at Crotona, Italy, where were
educated hundreds of enthusiastic pupils. * He was
emphatically a dreamer. He conceived a system of
the universe, in many respects correct ; yet he ad-
vanced no proof, made few converts to his views,
and they were soon well-nigh forgotten.

He held that the sun is the center of the solar sys-
tem, the planets revolving about it in circular orbits ;
that the earth rotates daily on its axis, and revolves
yearly round the sun ; that Venus is both morning
and evening star ; that the planets are placed at
intervals corresponding to the scale in music, and
that they move in harmony, making the '* music of
the spheres," but that this celestial concert is heard
only by the gods, — the ears of man being too gross
for such divine melody. He also believed that the
planets are inhabited, and he even attempted to cal-
culate the size of the animals in the moon.

* §ee Bjimes's History of the Ancient Peoples, p. 17*.

8 THE HISTORY.

Anaxagoras (B.C. 500-428) taught that there is but
one God, and that the sun is only a fiery globe, and
should not be worshipped. He attempted to explain
eclipses and other celestial phenomena by natural
causes, saying that there is no such thing as chance
or accident, these being only names for unknown
laws. For his audacity and impiety, as his country-
men considered it, he and his family were doomed to
perpetual banishment.

EuDOXUS, who lived in the fourth century B.C., in-
vented the theory of the Crystalline Spheres. He
held that the heavenly bodies are set, like gems, in
hollow, transparent, crystal globes, which are so
pure that they do not obstruct our view, while they
all revolve around the earth ; and that the planets
are placed in one globe, but have a power of moving
themselves, under the guidance — as Aristotle taught
— of a tutelary genius, who resides in each, and rules
over it as the mind rules over the body.

HiPPARCHUS, who flourished in the second century
B.C., has been called the Newton of Antiquity. He
was the most celebrated of the Greek astronomers.
He calculated the length of the year to within six min-
utes, discovered the precession of the equinoxes, and
made the first catalogue of the stars — 1080 in number.

The Egyptians. — Egypt, as well as Chaldea, was
noted for its knowledge of the sciences long before
they were cultivated in Greece. It was the practice
of the Greek philosophers, before aspiring to the
rank of teacher, to travel for years through these
countries, and gather wisdom at its fountain-head.
Pythagoras spent thirty years in this kind of study.

THE EGYPTIANS. 9

Two hundred years after Pythagoras, the cele-
brated school of Alexandria was established.* Here
were concentrated in vast libraries and princely
halls nearly all the wisdom and learning of the
world. Here flourished the sciences and arts, under
the patronage of munificent kings.

At this school, Ptolemy (a.d. 70), a Grecian, wrote
his great work, the Almagest, which for fourteen
centuries was the text-book of astronomers. In this
work was given what is known as the Ptolemaic
System. It was founded largely upon the materials
gathered by previous astronomers, such as Hippar-
chus, whom we have already mentioned, and Era-
tosthenes, who computed the size of the earth by the
means even now considered the best — the measure-
ment of an arc of the meridian.

Ptolemaic System. — To the early astronomers, the
movements of the planets seemed extremely com-
plex. Venus, for instance, was sometimes seen as
evening star in the west, and then again as morning
star in the east. Sometimes she appeared to be
moving in the same direction as the sun, then, going
apparently behind the sun, she seemed to pass on
again in a course directly opposite. At one time,
she would recede from the sun more and more slowly
and coyly, until she would appear to be entirely sta-
tionary ; then she would retrace her steps, and seem
to meet the sun.

An attempt was made to account for all "these
facts by an incongruous system of '' Cycles and

* See Barnes's General History, p. 154.

THE HISTORY.

epicycles,*' as it is called.* The advocates of this
theory assumed that every planet revolves in a
circle, and that the earth is the fixed center around
which the sun and the heavenly bodies move. They
then conceived that a bar, or something equivalent,
is connected at one end with the earth ; that at some
part of this bar the sun is attached ; while between
that and the earth, Venus is fastened — not to the bar
directly, but to a sort of crank : and further on, Mer
cury is hitched on in the same way.

In Fig. 3, let A be the earth : S, the sun : A B D F,
the bar (real or imaginary) ; B C, the short bar or
crank to which Venus is tied ; D E, another bar for
Mercury ; F G, a fourth bar. with still another short
crank, at the end of which, H, Mars is attached.

Fig. 3.

The Ptolemaic System,

Thus they had a complete system. They did not
exactly understand the nature of these bars —
whether they were real or only imaginary — but they
did comprehend their action, as they thought ; and

• Hilton refers to this when he speaks of the heavens as —

" With centric and eccentric scribbled o'er,
Cycle and epicycle, orb in orb."

THE SARACENS. ll

SO they supposed the bar revolved, carrying the sun
and planets along in a large circle about the earth ;
while all the short cranks kept flying around, thus
sweeping each planet through a smaller circle.

By this theory, we can see that the planets would
sometimes go in front of the sun and sometimes
behind ; and their places were so accurately pre-
dicted, that the error could not be detected by the
rude instruments then in use. As soon as a new
motion of one of the heavenly bodies was discovered,
a new crank, and of course a new circle, was added
to account for the fact. Thus the system became
more and more complicated, until, at last, a coip.-
bination of five cranks and circles was necessary to
make the planet Mars keep pace with the Ptolemaic
theory. No wonder that Alfonso, of Castile, a
celebrated patron of Astronomy, revolted at the
cumbersome machinery, and cried out, " If I had
been consulted at the Creation, I could have done
the thing better than that."

The Saracens. — After the destruction of the
library at Alexandria, learning found a home among
the Mohammedans. Bagdad on the Tigris, and Cor-
dova on the Guadalquiver became centers of
science, literature, and art. The treasures of Grecian
knowledge were eagerly gathered by the Caliphs,
and we are told that it was not uncommon to see,
entering the gates of Bagdad, a whole train of
camels loaded with Greek manuscripts. Gerbert,
afterward Pope Sylvester II., learned the elements
of astronomy at the University of Cordova, going,
after the custom of the time, to Spain for instruc-

THE HISTORY.

Fig. U.

tion, as, formerlY, philosophers had gone to Egypt
In the Moorish schools, geography was already
taught by the use of the globe. The first observ-
atory in Europe was erected at
Seville (1196). The fragments
of Saracenic learning that have
come down to us show that
the Arabs had constructed astro-
nomical tables, and endeavored
to perfect them by means of sys-
tematic observation of the
heavens. With the down-
fall of the Moors, and
the Revival of Learning,
Spain ceased to take the
lead in scientific study.

iVi* {Jiraiila, Muorish OO^

ASTROLOGY. 13

Astrology. — During all these centuries, astronomy
owed its development quite as much to a desire of
foretelling the future, as to a love for science. It
was the prevalent belief that the stars rule the des-
tinies of men. The Chaldeans scanned the heavens
for purposes of divination, so that Chaldean and
astrologer became synonymous. Tiberius, Emperor
of Rome, practised astrology. Hippocrates himself,
the Father of Medicine (b.c. 470), ranked this among
the most important branches of knowledge for the
physician. The mysterious study possessed a pecu-
liar fascination for the Arabians, and they culti-
vated it assiduously. The Moorish astronomers were
astrologers as well, and popularized the art in west-
ern Europe. This superstition reached the height of
its influence during the Middle Ages.

The issue of any important undertaking and the
fortunes of an individual were foretold by the as-
trologer, who drew up a Horoscope representing the
position of the sun, moon, and planets at the begin-
ning of the enterprise, or at the birth of the person.
It was a complete and complicated system, and con-
tained regular rules, which guided the interpretation,
and which were so abstruse as to require years for
their mastery. Venus foretold love ; Mars, war ;
the Pleiades (Ple'-ya-dez), storms at sea.

The ignorant were not the only dupes of this
visionary system. Lord Bacon believed in it most
firmly. Kepler, by casting nativities, eked out his
miserable pittance as royal astronomer. So late even
as the reign of Charles II., Lilly, a famous astrolo-
ger, was called before a committee of the House of

14 THE HISTORY.

Commons, to give his opinion on the probable issue
of some enterprise then under consideration.

However foolish the system of Astrology may
have been, it preserved the science of Astronomy
during the Dark Ages, and prompted to accurate
observation and diligent study of the heavens.

The Copemican System.^About the commence-
ment of the sixteenth century. Copernicus, breaking
away from the theory of Ptolemy, that was still
taught in the institutions of learning in Europe,
revived the theory of Pythagoras. He saw how beau-
tifully simple is the idea of considering the sun the
grand center about which revolve the earth and the
planets. He noticed how constantly, when we are
riding swiftly, we forget our own motion, and think
that the trees and fences are gliding by us in the
contrary direction. He applied this thought to the
movements of the heavenly bodies, and maintained
that, instead of all the starry host revolving about
the earth once in twenty-four hours, the earth simply
turns on its own axis, and thus produces the ap-
parent daily revolution of the sun and stars ; while
the yearly motion of the earth about the sun, trans-
ferred in the same manner, would account for the
solar movements.

Though Copernicus thus simplified the Ptolemaic
theory, he yet found that the idea of circular orbits
for the planets would not explain all the phenomena,
and therefore retained the "cycles and epicycles"
Alfonso had so heartily condemned. For forty years,
this illustrious astronomer carried on his observa-
tions in the upper part of a humble, dilapidated

Kepler's laws. 15

farm-house, through the roof of which he had an
unobstructed view of the sky. The work containing
his theory was published just in time to be laid upon
his death-bed.

Tycho Brahe, a celebrated Danish astronomer,
next propounded a modification of the Copernican
system. He rejected the idea of cycles and epicycles,
but, influenced by certain passages of Scripture,
maintained, with Ptolemy, that the earth is the
center, and that all the heavenly bodies daily re-
volve about it in circular orbits. Brahe was a noble-
man of wealth, and, in addition, received large sums
of money from the government. He erected a mag-
nificent observatory, and made many beautiful and
rare instruments. Clad in his robes of state, he
watched the heavens with the intelligence of a
philosopher and the splendor of a king. His inde-
fatigable industry and zeal resulted in the accumu-
lation of a vast fund of astronomical knowledge,
which, however, he lacked the ability to apply to any
further advance in science.

His pupil, Kepler, saw these facts, and in his fruit-
ful mind they germinated into three great truths,
called Kepler's laws. These form one of the most
precious conquests of the human mind. They are
the three arches of the bridge over which Astronomy
crossed the gulf between the Ptolemaic and Coper-
nican systems.

Kepler's Laws. — Kepler, taking the investigations
of his master, Tycho Brahe, determined to find what
is the exact shape of the orbits of the planets. He
adopted the Copernican theory — that the sun is the

10 THE HISTORY.

center of the system. At that time, all believed the
orbits to be circular. They reasoned thus : the circle
is perfect ; it is the most beautiful figure in nature ;
it has neither beginning nor ending ; therefore, it is
the only form worthy of God, and He must have
used it for the orbits of the worlds He has made.

Imbued with this romantic view, Kepler com-
menced with a rigorous comparison of the places of
the planet Mars as observed by Brahe, with the
places as stated by the best tables that could be com-
puted on the circular theory. For a time, they
agreed, but in certain portions of the orbit the obser-
vations of Brahe would not fit the computed place
by eight minutes of a degree. Believing that so
good an astronomer could not be mistaken as to the
facts, Kepler exclaimed. " Out of these eight minutes
we will construct a new theory that will explain the
movements of all planets."

He resumed his work, and for eight years con-
tinued to imagine every conceivable hypothesis, and
then patiently to test it — "hunt it down," as he
called it. Each in turn proved false, until nineteen
had been tried. He then determined to abandon the
circle and to adopt another form. The ellipse sug-
gested itself to his mind. Let us see how this figure
is made.

Attach a thread to two pins, as at FF in the figure ;
next, move a pencil along with the thread, the latter
being kept tightly stretched, and the point will mark
a curve, flattened in proportion to the length of the
string, — the longer the string, the nearer a circle
will the figure become. This figure is the ellipse.

KEPLER S LAWS.

The two points F F are called the foci (singular,
focus). We can now understand Kepler's attempt,
and the triumph which crowned his seventeen years
of unflagging toil.

Fifl. a.

First Law. — With this figure he constructed an
orbit having the sun at the center, and again fol-
lowed the planet Mars in its course. But very soon
there was as great a discrepancy between the ob-
served and computed places as before. Undismayed
by this failure, Kepler assumed another hypothesis,
and determined to place the sun at one of the foci
of the ellipse. Once more he "hunted down" the
theory. For a whole year he traced the planet along
the imaginary orbit, and it did not diverge. The
truth was disciovered at last, and Kepler (1609) an-
nounced his first great law —

Planets revolve in ellipses, with the sun at one focus.

18 THE HISTORY.

Second Law. — Kepler knew that the planets do
not move with equal velocity in the different parts
of their orbits. He next set about establishing some
law by which this speed could be determined, and
the place of the planet computed. He drew an
ellipse, and once more marked the various positions
of the planet Mars. He soon found that when at
its perihelion (point nearest the sun) its motion
is fastest, but when at its aphelion (point furthest
from the sun) its motion is slowest. Again he
"hunted down" various hypotheses, until, at last.

he discovered that though, in going from B to A, the
planet moves more slowly, and from D to C more
rapidly, yet the space inclosed between the lines SB
and SA is equal to that inclosed between SD and SC.
Hence the second law —

A line connecting the center of the earth with the
center of the sun passes over equal spaces in equal
times.

Third Law. — Kepler, not satisfied with the dis-
covery of these laws, now determined to ascertain if
there were not some relation existing between the

GALILEO. 19

times of the revolutions of the planets about the sun
and their distances from that body. With the same
wonderful patience, he took the figures of Tycho
Brahe, and began to compare them. He tried them
in every imaginable relation. Next he took their
squares, then he attempted their cubes. Here was
the secret ; but he toiled around it, made a blunder,
and waited for months, until, once more, his patience
triumphed, and he reached (1618) the third law —

The squares of the times of revolution of the planets
about the sun are proportional to the cubes of their
mean distances from the sun.*

In rapture over the discovery of these three laws,
so marked by that Divine simplicity which pervades
all the laws of nature, Kepler exclaimed, ''Nothing
holds me. The die is cast. The book is written, to
be read now or by posterity, I care not which. It
may well wait a century for a reader, since God has
waited six thousand years for an observer, f

Galileo. — Contemporary with Kepler was the great
Florentine philosopher, Galileo. He discovered the
laws of the pendulum and of falling bodies, as we
have already learned in Physics. He was, however,
educated in and believed the Ptolemaic system. A
disciple of the Copernican theory happening to come
to Pisa, where Galileo was teaching as professor in

* For example : Tlie square of Jupiter's period is to the square of Mars's period, as
the cube of Jupiter's distance is to the cube of Mars's distance ; or, representing the
earth's time of revolution by P, and her distance from the sun by p, then letting D and
d represent the same in another planet, we have the proportion P» : D' : : p' : d'.

t Kepler, strangely enough, believed in the " Music of the Spheres." He made
Saturn and Jupiter take the bass, Mars the tenor, Earth and Venus the counter, and
Mercury the treble. This shows what a streak of folly or superstition may run through
the cliaracter of the noblest man. However, as Johnson says, a mass of metal may be
gold, though there be in it & littlo vein of tiu.

20 THE HISTORY.

the University, drew his attention to its simplicity
and beauty. His clear, discriminating mind per-
ceived its perfection, and he henceforth advocated it
with all the ardor of his unconquerable zeal. Soon
after, he learned that one Jansen, a Dutch watch-
maker, had invented a contrivance for making dis-
tant objects appear near. With his profound knowl-
edge of optics and philosophical instruments, Galileo
caught the idea, and soon had a telescope completed.
It was a very simple affair — only a piece of lead pipe
with a lens set at each end ; but it was destined to
overthrow the old Ptolemaic theory, and revolu-
tionize the science of Astronomy.

Discoveries made with the Telescope. — Galileo
now examined the moon. He saw her mountains and
valleys, and watched the dense shadows upon her
plains. On January 8, 1610, he turned the telescope
toward Jupiter. Near it he saw three bright stars,
as he considered them, which were invisible to the
naked eye. The next night he noticed that they had
changed their relative positions. Astonished and
perplexed, he waited three days for a fair night in
which to resume his observations. The fourth night
was favorable, and he found the three stars had
again shifted. Night after night he watched them,
discovered a fourth star, and finally found that they
were rapidly revolving around Jupiter, each in its
elliptical orbit, with its own rate of motion, and all
accompanying the planet in its journey around the
sun. Here was a miniature Copernican system, hung
up in the sky for every one to see and examine for
liimself.

NEWTON. 21

Reception of the Discoveries. — Galileo met with
the most bitter opposition. Many refused to look
through the telescope lest they might become victims
of the philosopher's magic. Some prated of the
wickedness of digging out valleys in the fair face of
the moon. Others doggedly clung to the theory they
had held from their youth.* But the truth of the
Copernican system was now fully established. Phil-
osophers gradually adopted this view, and the Ptole-
maic theory became a relic of the past.

Newton, a young man of twenty-four years, was
spending the summer of 1666 in the country, on
account of the plague which prevailed at Cambridge,
his place of residence. One day, while sitting in a
garden, an apple chanced to fall to the ground near
him. Reflecting upon the strange power that causes
all bodies thus to descend to the earth, and remem-
bering that this force continues, even when we as-
cend to the tops of high mountains, the thought oc-
curred to his mind, "May not this same force extend
to a great distance out in space ? Does it not reach
the moon ? "

Laws of Motion. — To understand the reasoning
that now occupied the mind of Newton, let us apply
the laws of motion as we have learned them in

* As a specimen of the arguments adduced against the new system, the following l>y
Sizzi is a fair instance. " There are seven windows in the head, through which the air
is admitted to the body, to enlighten, to warm, and to nourish it,— two nostrils, two
eyes, two ears, and one mouth. So in the heavens there are two favorable stars, Jupiter
and Venus ; two unpropitious. Mars and Saturn ; two luminaries, the Sun and Moon ;
and Mercury alone, undecided and indifferent. From which, and from many other phe-
nomena in Nature, such as the seven metals, etc., we gather that the number of planets
is necessarily seven. Moreover, the satellites are invisible to the naked eye, can exercise
no influence over the earth, and would be useless, and therefore do not exist, liesides,
the week is divided into seven days, which are named from the seven planets. Now, if
we increase the number of planets, this whole system falls to the ground."

22 THE HISTORY.

Physics. "When a body is set in motion, it will con-
tinue to move forever in a straight line, unless
another force is applied. As there is no friction in
space, the planets do not lose any of their original
velocity, but move now with the same speed which
they received at the beginning. But this would
make them all pass along straight lines, and not cir-
cular orbits. What causes the curve ? Obviously,
another force. For example : I throw a stone into
the air. It does not move in a straight line, but in
a curve, because the earth constantly bends it down-
ward.

Application. — Just so the moon is moving around
the earth, not in a straight line, but in a curve. Can
it not be that the earth bends it downward, just as
it does the stone ? Newton knew that a stone falls
toward the earth sixteen feet the first second. He
conceived, after a careful study of Kepler's laws,
that the attraction of the earth diminishes according
to the square of the distance. He supposed (accord-
ing to the measurement then received) that a body
on the surface of the earth is exactly four thousand
miles from the center. He now applied this imag-
inary law. Suppose the body is removed four thou-
sand miles from the surface of the earth, or eight
thousand miles from the center. Then, as it is twice
as far from the center, its weight will be diminished
2^, or 4 times. If it were placed 3, 4, 5, 10 times fur-
ther away, its weight would then decrease 9, 16, 25,
100 times. If, then, the stone at the surface of the
earth (four thousand miles from the center) falls
sixteen feet the first second, at eight thousand miles

NEWTON. 23

it would fall only four feet ; at 240,000 miles, or the
distance of the moon, it would fall only about one.-
twentieth of an inch (exactly .053).

Next the question arose, "How far does the moon
fall toward the earth, i. e., bend from a straight line,
every second ? " For sixteen years, with a patience
rivaling Kepler's, this philosopher sought to solve the
problem. He toiled over interminable columns of
figures, to find how much the moon's path curves
each second. At last, he reached a result, which was
nearly, but not quite, exact. Disappointed, he laid
aside his calculations. Repeatedly he reviewed
them, but could not find a mistake. At length,
while in London, he learned of a new and more
accurate measurement of the distance from the cir-
cumference to the center of the earth. He hastened
home, inserted this new value in his calculations,
and soon found that the result would be correct.
Overpowered by the thought of the grand truth just
before him, his hand faltered, and he called upon a
friend to complete the computation.

From the moon, Newton passed on to the other
heavenly bodies, calculating and testing their orbits.
Finally, he turned his attention to the sun, and, by
reasoning equally conclusive, proved that the attrac-
tion of that great central orb compels all the planets
to revolve about it in elliptical orbits, and holds them
with an irresistible power in their appointed paths.*

* " Do not understand me at all as saying there is no mystery about the planets' mo-
tion. There is just one single mystery,— gravitation ; and it is a very profound one.
How it is that an atom of matter can attract another atom, no matter how great the
distance, no matter what intervening substance there may be ; how it will act upon
it, or at least behave as if it acted upon it,— I do not know, I cannot tell. Whether they

24 THE HISTORY.

At last, he announced this grand Law of Gravita-
tion : Every particle of matter in the universe at-
tracts every other particle of matter with a force
directly proportional to its quantity of matter, and
decreasing as the square of the distance increases.

_^ II. SPACE.

We now in imagination pass into space, which-
stretches out in every direction, without bounds or
measures. We look up to the heavens, and try to
locate some object among the mazes of the stars.
Bewildered, we feel the necessity of some system of
measurement. Let us try to understand the one
adopted by astronomers.

The Celestial Sphere. — The blue arch of the sky,
as it appears to be spread over us, is termed the
Celestial Sphere. There are two points to be noticed
here.

First, that so far distant is this imaginary arch
from us, that if any two parallel lines from different
parts of the earth were drawn to this Sphere, they
would apparently intersect. Of course, this could
not be the fact ; but the distance is so immense, that
we are unable to distinguish the little difference of

are puslied together l)y means of an intervening ether, or what is the action, I cannot
und(T.stand. It stands with me along with the fact, that, when I will my arm to rise, it
rises. It is inscrutable. All the explanations that have been given of it seem to me
merely to darken counsel with words and no understanding. They do not remove the
difficulty at all. If I were to say what I really believe, it would be, that the motion of
the spheres of the material universe stand in some such relation to Him in whom all
things exist, the ever-present and omnipotent God, as the motions of my body do to my
will : I do not know bow, and never expect to know."— Pro/. Young.

SPACE. 25

four or even eight thousand miles, and the two lines
would seem to unite : so we must consider this great
earth as a mere speck or point at the center of the
Celestial Sphere.

Second, that we must neglect the entire diameter
of the earth's orbit, so that if we should draw two
parallel lines, one from each end of the earth's orbit,
to the Celestial Sphere, although these lines would
be nearly 186,000,000 miles apart, yet they would
appear to pierce the Sphere at the same point ; which
is to say, that, at that enormous distance, 186,000,000
miles shrink to a point. Consequently, in all parts
of the earth, and in every part of the earth's orbit,
we see the fixed stars in the same place.

This sphere of stars surrounds the earth on every
side. In the daytime, we cannot see the stars be-
cause of the superior light of the sun ; but, with a
telescope, they can be traced, and an astronomer will
find certain stars as well at noon as at midnight.

One half of the sphere is constantly visible to us ;
and so far distant are the stars, that we see just as
much of the sphere as we should if the upper part of
the earth were removed, and we were to stand four
thousand miles further away, or at the center of the
earth, where our view would be bounded by a great
circle of the earth.

On the concave surface of the Celestial Sphere,
there are imagined to be drawn three systems of
circles : the Horizon, the Equinoctial, and the
Ecliptic System. Each of these has (1) its Prin-
cipal Circle, (2) its Suhordinate Circles, (3) its
Points, and (4) its Measurements.

SPACE.

Fig. 7.

1. THE HORIZON SYSTEM.

(a) The Principal Circle is the Rational Horizon.
This is the great circle whose plane, passing through
the center of the earth, separates the visible from the
invisible heavens. The Sensible Horizon is the small
circle where the earth and the sky seem to meet : it
is parallel to the rational horizon, but distant from
it the semi-diameter of the earth. No two places
have the same sensible horizon : any two, on opposite
sides of the earth, have the same rational horizon.

(6) The Subordinate Cir-
cles are the Prime Verti-
cal circle, and the Merid-
ian. A vertical circle is
one passing through the
poles of. the horizon (ze-
nith, and nadir). The
Prime Vertical is a verti-
cal circle passing througl
the East and West points
The Meridian is a vertical
circle passing through the

E, center of earth ; Z, zenith ; Z', nadir ; , . .

PP', axis of earth; HAH', horizon; S, a Nortll aud SOUtll pOlUtS.
star; ZSZ', rertical circle passing through

a ■ AS, altitude of star ; ZS, zenith distance (q) The PolUtS are the
of star ; B.' A., azimuth of star. ^

Zenith, the Nadir, and the
K, S., E., and W. points. The Zenith is the point
directly overhead, and the Nadir, the one directly
underfoot. They are also the poles of the horizon —
i. e., the points where the axis of the horizon pierces
the Celestial Sphere. The N., S., E., and W. points
are familiar.

THE HORIZON SYSTEM. 2?

(d) The Measurements are Azimuth, Amplitude,
Altitude, and Zenith distance.

Azimuth is the distance from the meridian, meas-
ured east or west, on the horizon, to a vertical circle
passing through the object.

Amplitude (the complement of Azimuth) is the
distance from the Prime Vertical, measured on the
horizon, north or south.

Altitude is the distance from the horizon, meas-
ured on a vertical circle, toward the zenith.

Zenith Distance (the complement of Altitude) is
the distance from the zenith, measured on a vertical
circle, toward the horizon.

The Horizon system is one commonly used in
observations with Mural Circles, and Transit Instru-
ments.

2. THE EQUINOCTIAL SYSTEM.

(a) The Principal Circle is the Equinoctial. This
is the Celestial Equator^ or the earth's equator ex-
tended to the Celestial Sphere. At all places between
the equator and the pole, the celestial equator is in-
clined to the horizon at an angle equal to the dis-
tance of the zenith of the place from the pole. *

{h) The Subordinate Circles are the Hour Circles
(Right Ascension Meridians), the Colures, and the
Declination Parallels.

* Tlie latitude of a place is its distance from the equator, and this equals the distance
of the zenith of the place from the equinoctial. Hence, having given the latitude of a
place, to find the height of the celestial equator above its horizon, subtract the latitude
from 90°, and the remainder is the required angular distance. In like manner, the lati-
tude subtracted from 90° gives the co-latitude of the place— the complement of the
latitude.

28 SPACE.

The Hour Circles are thus located. The Equi-
noctial is divided into 360°, equal to twenty-four
hours of motion — thus making 15 equal to one hour
of motion. Through these divisions run twenty-four
meridians, each constituting an hour of motion
(time) or 15^ of space. The Hour Circles may be
conceived as meridians of terrestrial longitude (15"
apart) extended to the Celestial Sphere.

The Colures are two principal meridians ; the
Equinoctial Cohire is the meridian passing through
the equinoxes ; the Solstitial Colure is the meridian
passing through the solstitial points.

The Declination Parallels are small circles
parallel to the Equinoctial ; or they may be conceived
as the parallels of terrestrial latitude extended to the
Celestial Sphere.

The Celestial Poles are the points where the
axis of the earth extended pierces the Celestial
Sphere, and are the extremities of the celestial axis,
as the poles of the earth are the extremities of the
earth's axis. The North Pole is marked very nearly
by the Xorth Star, and every direction from that is
reckoned south, and every direction toward that is
reckoned north, however it may conflict with our
ideas of the points of the compass.

The Equinoxes are the points where the Equinoc-
tial and the Ecliptic (the sun's apparent path through
the heavens) intersect.

(c7) The Measurements are Right Ascension (R. A.),
Declination, and Polar Distance,

THE EQUINOCTIAL SYSTEM. S9

Right Ascension is distance from the Vernal
Equinox, measured on the equinoctial eastward
to the meridian which passes through the body.
R. A. corresponds to terrestrial longitude, and may
extend to 360° East, instead of 180'' as on the earth.
R. A. is never measured westward. The starting
point is the meridian passing through the vernal
equinox, as the meridian passing through Green-
wich is the point from which terrestrial longitude is
measured.

Declination is distance from the equinoctial,
measured on any Hour Circle or meridian north or
south. It corresponds to terrestrial latitude.

Polar Distance (the complement of Declination)
is the distance from either Pole, measured on an
Hour Circle.

The Equinoctial System is largely used by modern
astronomers, and accompanies the Equatorial Tele-
scope, Sidereal Clock, and Chronographs of the best
Observatories.

3. THE ECLIPTIC SYSTEM.

(a) The Principal Circle is the Ecliptic. This is
the apparent path of the sun in the heavens. It is
inclined to the equinoctial 23^° (23° 27' 15", Jan. 1,
1884), which measures the inclination of the Earth's
Equator to its orbit, and is called the obliquity of the
ecliptic. (See p. 58.)

The inclination of the ecliptic to the horizon, unlike
that of the equinoctial, varies at different times of
the year. The angle that the ecliptic makes with
the horizon is greatest when the vernal equinox is

30 SPACE.

on the western horizon and the autumnal on the
eastern ; it is least when the vernal equinox is on
the eastern horizon and the autumnal on the western.*

{b) The Subordinate Circles are Circles of Celestial
Longitude, and Parallels of Celestial Latitude.

The Circles of Celestial, Longitude are now
seldom employed. They are measured on the Eclip-
tic, as circles of Right Ascension (R. A.) are meas-
ured on the Equinoctial.

The Parallels of Celestial Latitude are little
used. They are small circles drawn parallel to the
ecliptic, as parallels of declination are drawn parallel
to the equinoctial.

The Poles of the Ecliptic are the points where the
axis of the earth's orbit meets the Celestial Sphere.

The Equinoxes are the points where the ecliptic
intersects the equinoctial. The place where the sun
crosses the equinoctial f in going north, which occurs
about the 21st of March, is called the Vernal Equinox.
The place where the sun crosses the equinoctial in
going south, which occurs about the 21st of Septem-
ber, is called the Autumnal Equinox. The Solstices
are the two points of the ecliptic most distant from
the Equator ; or they may be considered to mark the
sun's furthest declination north and south of the
equinoctial. The Summer Solstice occurs about the

* In the former instance, the angle is eqnal to the co-latitude, plus 23J° (the inclina-
tion of the ecliptic to the equinoctial) ; and, in the latter, the co-latitude minus 2;U°.
Thus, at the latitude of New York, it varies from 90° — 41* + ■2:i\ ° = 72J " ; to 90° —
41° — 231* = 25J'. In the one case, the summer solstice ia on the meridian of the
place, and, in the other, the winter.

t " This is commonly called ' crossing the line.' '

THE ECLIPTIC SYSTEM. 81

31st of June ; the Winter Solstice occurs about the
21st of December.

(d) The Measurements are Celestial Longitude and
Latitude.

Celestial Longitude is distance from the Vernal
Equinox measured on the ecliptic, eastward.

Celestial Latitude is distance from the ecliptic
measured on a Subordinate Circle, north or south.

THE ZODIAC.
A belt of the Celestial Sphere, 8° on each side of
the ecliptic, is styled the Zodiac. This is of very-
high antiquity, having been in use among the
ancient Hindoos and Egyptians. The Zodiac is
divided into twelve equal parts — of 30° each — called
Signs, to each of which a fanciful name is given.
The following are the names of the

SIGNS OF THE ZODIAC.

Libra a

Scorpio Ttj^

Sagittarius f

Capricornus . . V5>

Aquarius oji

Pisoes X

Aries HP

Taurus y

Gemini n

Cancer 25

Leo ^

Virgo nj

" The first, nn, indicates the horns of the Ram ; the
second, « , the head and horns of the Bull ; the barb
attached to a sort of letter, "1 , designates the Scor-
pion ; the arrow, i , sufficiently points to Sagitta-
rius ; V? is formed from the Greek letters, rp, the two
first letters of rpdyog, a goat. Finally, a balance,
the flowing of water, and two fishes, tied by a string,
may be imagined in ^, ^, and X, the signs of Libra,
Aquarius, and Pisces." (See pp. 210, 295.)

32 PRACTICAL QUESTIONS.

PRACTICAL QUESTIONS.

1. How high is the Xorth Star above your horizon ?

2. What is the sun's right ascension at the autumnal equinox ? At the
vernal equinox ?

3. What was the first discovery made by the telescojie ?

4. How high above the horizon of any place are the equinoctial points
when they pass the meridian ?

5. Jupiter revolves around the sun in 12 of our years. Assuming the
earth's distance from the sun to be 93,000,000 miles, compute Jupiter's dis-
tance by applying Kepler's third law.

6. The latitude of Albany is 42° 39' X ; what is the sun's meridian
altitude at that place when it is in the celestial equator ?

7. What is the co-latitude of a place ?

8. What is the declination of the zenith of the place in which you
reside?

9. Why are the stars generally invisible by day ?

10. Why is the ecliptic so called ?

11. Who fii-st taught that the earth is round ?

12. What is Astrology ?

13. How can we distinguish the fixed stars from the planets ?

14. How long was the Ptolemaic System accepted ?

15. In what respect did the Copernican System differ from the one now
received ?

16. For what is Astronomy indebted to Galileo ? To Newton ?

1 7. What is the amount of the obliquity of the ecliptic 1

18. Define Zenith. Xadir. Azimuth. Altitude. Equinoctial. Right
Ascension. Declination. Equinox. Ecliptic. Colure. Solstice. Polar
distance. Zenith distance. The Zodiac.

19. If the R. A. of the sun be 80", state in what sign he is then located ?
160° ? 280° ?

20. ^^^ly does the angle which the ecliptic makes with the horizon vary ?

21. "\Miy is the angle which the celestial equator makes with the horizon
constant ?

II.

THE SOLAR SYSTEM.

' In them hath He set a tabernacle for the sun,

" This world was once a Jluid haze of light,
Till toward the center set the starry tides
And eddied into sttns, that wheeling cast
The planets." — Tennyson.

L The Sun ,

1. Distance.

2. Ljght (t Heat.

3. Apparent Size,

4. Real Dimen-

sions.

5. Solar Spots..

<!
iJ
O
CO

IL The Planets.

' a. Discovery.
&. Number and Location.

c Size.

d. Constituents.

e. Motion across Disk.
/. Cliange iu Kate.
g. Prove the Rotation of Sun.
h. Synodic and Sidereal Rotation
i. Path of Spot-s.
,;. Individual Motion.
k. Change in Form.
I. Perio<licity of Spots.
m. Planetary Influence.
TO. Influence on Terrestrial Heat, etc
0. Heat of Spots.
p. Depression of Spots.
q. Brightness of Spots.
r. Faculw, rice-grains, etc.
a. Wilson's Theory.
h. Present Theory (Kirchhoff's).

How Solar Heat is Produced.

a. Common Characteristics.
6. Comparison of Planets.

c. Properties of the Ellipse.

d. Planetary Orbits.

e. Comparative Size of Planets.
/. Conjunction of.
g. Are Planets Inhabited ?

t A top. Division of Planets, etc

1. VCLCAS.

a. Description.
h. Motion in Space.

c. Distance from Earth.

d. Dimensions.

e. Seasons.
, /. Telescopic Features.

Vesds Repeat same Analysis as of Mercury.

/'a. Dimensions.
Rotundity.
Apparent & Real Motion.

f 1. Diumal Mo-
tion of Sun.

2. Unequal rate
of Motion.

3. Orbits of
Stars.

4. Unequal Ve-
locitiesofStars.

5. Appearance
I of Stars, etc.
( 1. Change in

appearance of
heavens.

Physical Con-
stitition . . . .

-iNTRODrCTION. . . ■

2. Mercurt ■

4. The Eakth .

—The Moon. -<

—Eclipses.

—The Tides.

a. Motion.
h. Dimens'ns.

c. Librations.

d. L'g-t&Ht.

e. Cen.ofGrav.
/. Atmosph're.
g. Lunarians.
h. Earth-shine,
i. Phases.
j. Harv'stM'n.
k. Wet Moon.
I. Nodes.
TO. Occulat'n.
n. Seasons.
0. Telescopic

Features.

2. Yearly path
of Sun.

3.MovesN.&S.
4. Change of
Seasons, etc. 20
points under
l^ this topic.

/. Precession of Equinoxes.

g. Nutation.

U. Refraction & Aberration.

i. Parallax.

5. Mars Same Analysis as Mercury.

6. The Minor Planets.

7. Jupiter Same Analysis as Mercury.

8. Saturn " "

9. Uranus " •'

^ 10. Neptune " "

III. Meteors, and Shooting Stars. ) Tlie subjects of the paragraphs may be inserted

IV. Comets r by the pupil, to complete these analyses, at

V. The Zodiacal Light ) the pleasure of the teacher.

Dium'l Mo-
tion of
Earth.

Yearly Mo-
tion of
Sun : its
C o n s e -
queiices.

THE SOLAR SYSTEM.

INTRODUCTION.

rr^HE Solar System is mainly comprised within
~L the limits of the Zodiac. It consists of —

1. The Sun — the center.

2. The major planets — Vulcan (undetermined), Mercury, Venus,

Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

3. The minor planets, at present (1884) two hundred and thirty-

seven in number.

4. The satellites, or moons, twenty in number, which revolve around

the different planets.

5. Meteors and shooting-stars.

6. Thirteen comets, which have now been found, by a second re-

turn, to move, like the planets, in elliptic paths, and to
revisit the sun periodically.

7. The Zodiacal Light.

How we are to imagine the solar system to our-
selves. — We are to think of it as suspended in space ;
being held up, not by any visible object, but in
accordance with the law of Universal Gravitation
discovered by Newton, whereby each planet attracts
every other planet and is in turn attracted by all.

First, the Sun, a great central globe, so vast as to
overcome the attraction of all the planets, and com-
pel them to circle around him; next, the planets,

36 THE SOLAR SYSTEM.

each turning on its axis while it flies around the sun

in an elliptical orbit : then, accompanying these, the
satellites, each revolving about its own planet, while
all whirl in a dizzy waltz about the central orb ;
next, the comets, rushing across the planetary
orbits at irregular intervals of time and space ; and
finally, shooting-stars and meteors darting hither
and thither, interweaving all in apparently inex-
tricable confusion.

To make the picture more wonderful still, every
member is flying with an inconceivable velocity, and
yet with such accuracy that the solar system is the
most perfect timepiece known.

I. THE SUN.

Sign, ©, a buckler with its boss.

Distance. — The sun's average distance from the
earth is nearly 93,000,000 miles.* Since the earth's
orbit is elliptical, and the sun is situated at one of its
foci, the earth is 3,000,000 miles further from the sun
in aphelion than in perihelion.

* The sun's distance from the earth is determined, as we shall learn hereafter (see
Celestial Measurements), by means of the solar parallax. In the former editions of this
work, the parallax of S".94 — deduced principally ftr)m observations upon the planet Mars
in 18'32— was accepted. This gave a solar distance of about 91| million miles, and has
been in general use among astronomers until recently. The obsen-ations of the last few
years have, however, shown that the true parallax is smaller, and that the sun is a
little further off than was supposed. Astronomers are not fully agreed upon the exact
Xtarallax that should be adopted, but there seems to be a general converging of opinion
toward 8".S0 as being, if not the exact parallax, at least as near it as we are able at
present to come. This new determination of the solar parallax renders necessary a cor-
responding change in the planetarj- distances, etc, as the sun's distance is the unit used
by astronomers in making all celestial measurements. In this chapter, the author has
followed the data given by Prof. Young in his work upon the Sun, as being the most
recent and authoritative view. (See p. 280.)

THE SUN. 37

As we attempt to locate the heavenly bodies in
space, we are startled by the enormous figures em-
ployed. The first number, 93,000,000 miles, is far
beyond our grasp. Let us, however, try to compre-
hend it. * If there were air to convey a sound from
the sun to the earth, and a noise could be made loud
enough to pass that distance, it would require over
fourteen years for it to come to us. Supi)Ose a raif-
road could be built to the sun. An express-train,
traveling day and night, at the rate of thirty miles
an hour, would require 353 years to reach its destina-
tion. Ten generations would be born and would die ;
the young men would become gray -haired, and their
great-grandchildren would forget the story of the
beginning of that wonderful journey, and would
read it in history, as we now read of Queen Elizabeth
or of Shakspere ; the eleventh generation* would
see the solar station at the end of the route. Yet
this enormous distance of 93,000,000 miles is used as
the unit for expressing celestial distances, — as the
foot-rule for measuring space ; and astronomers
speak of so many times the sun's distance as we
speak of so many feet or inches.

The Light of the Sun is equal to 5,563 wax-candles
held at a distance of one foot from the eye. It
would require 600,000 full-moons to produce a day as
brilliant as one of cloudless sunshine, f

* If a babe were born with an arm long enougli to reach the sun, and should touch
that fiery globe, the infant would grow to manhood and to old age and finally die, before
the sensation could traverse the nerve to his brain, and he feel the burn.

t According to Langley, the sun is blue, and to the inhabitants of other worlds may
shine as a bluer star than Vega. Tlie light from dill'erent parts of the solar disk, how-
ever, varies in color : while that from the center has a decidedly-blue tint, that from the
gd^e is of a chocolate hue. This difference is probably owing to the fact that the latter

204827

38 THE SOLAR SYSTEM.

The Heat of the Sun. — The amount of heat we
receive annually is sufficient to melt a layer of ice
110 feet thick, extending over the whole earth.* Yet
the sunbeam is only ^o o/o oo P^r^ ^s intense as it is at
the surface of the sun. Moreover, the heat and light
stream off into space equally in every direction. Of
this vast flood, only one twenty-three-hundred-
millionth part reaches the earth.

If the heat of the sun were produced by the burn-
ing of coal, it would require a layer sixteen feet in
thickness, extending over its whole surface, to feed
the flame a single hour. Were the sun a solid body
of coal, it would burn up at this rate in forty-six
centuries. Sir John Herschel says that if a solid
cylinder of ice 45 miles in diameter and 200,000 miles
long were plunged, end first, into the sun, it would
melt in a second of time.

Apparent Size. — The sun appears to be a little over
half a degree in diameter, so that 337 solar disks,
laid side by side, would make a half-circle of the
celestial sphere. It seems a trifle larger to us in
winter than in summer, as we are 3,000,000 miles
nearer it. If we represent the luminous surface of
the sun when at its average (mean) distance by 1,000,
the same surface will be represented when in aphe-
lion (July) by 967, and when in perihelion (January)
by 1,034.

passes through a greater thickness of the solar atmosphere, while our o\vn atmosphere
floes its part in strangling the blue rays of the sunlight, the red rays filtering through
with little loss.

* Recent experiments by Langley seem to increase this estimate to that of a sheet
of jce 180 feet thick (Popular Science Monthly, Sept., 1885).

THE SUN.

Dimensions.— Its diameter \^ about 866,000 miles.*
Let us try to understand this amount by comparison.

A mountain upon the surface of the sun, to bear
the same proportion to the globe itself as the loftiest
peak of the Himalayas does to the earth, would need
to be about 600 miles high.

Again : Suppose the sun were hollow, and the
earth, as in the cut (Fig. 8), placed at the center, not

Fig. 8.

only would there be room for the moon to revolve in
its regular orbit within the shell, but that would

* Pythagoras, whose theory of the universe was in so many respects very like the
one we receive, believed the sun to be 44,000 miles from the earth, and seventy-five
miles in diameter.

40 THE SOLAR SYSTEM.

stretch off in every direction nearly 200,000 miles
beyond.

Its volume is 1,300,000 times that of the earth —
i. e., it would take 1,300,000 earths to make a globe
the size of the sun. Its mass is 750 times that of
all the planets and moons in the solar system, and
330,000 times that of the earth. Its weight may be
expressed in tons, thus :

1,910,278,070,000,000,000,000,000,000.*

The Density of the sun is only about one-fourth
that of the earth, or 1.41 that of water, so that the
weight of a body transferred from the earth to the
sun would not be increased in proportion to the com-
parative size of the two. On account also of the vast
size of the sun, its surface is so far from its center
that the attraction is largely diminished, since that
decreases, we remember, as the square of the dis-
tance. However, a man weighing at the earth's
equator 150 lbs., at the sun's equator would weigh
about two tons, — a force of attraction that would in-
stantly crush him. At the earth's equator, a stone
falls 16 feet the first second ; at the sun's equator, it
would fall 144 feet, f

Telescopic Appearance of the Sun : Sun Spots. —
We may sometimes examine the sun at early morn-
ing or late in the afternoon with the naked eye, and

* This number is meaningless to our imagination, but yet it represents a force of
attraction that holds our own earth and all the planets steadily in their places; while it
fills the mind with an indescribable awe as we think of that Being who "made the sun,
and holds it in the very palm of His hand."

t A singular consequence of this has been suggested. "A cannon-ball could be
thrown only a short distance, since it would pass through a path of great curvature, 9n^
lyould fall to the sun within a few yards of the gun."

THE StfN.

at midday by using a smoked glass. The disk will
appear distinct and circular, and with no spot to dim
its brightness. If we use a telescope of moderate

Fig

The Swn seen through a Telescope.

power, taking the precai^tion to shield the eye with
a colored eye-piece, we shall find the surface of the
sun sprinkled with irregular spots (Fig. 9).*

• Tlie natural purity of the sun seems to have been formerly an article of faith among
astronomers, and therefore on no account to be called in question. Seheiner, it is said,
having reported to his superior that he had seen spots on the sun's face, was abruptly
dismissed with these remarks : " I have read ^Vristotle's WTitings from end to end many
times, and I assure you I do not find anything in them similar to that which you men-
tion. Go, my son, tianquillize yourself ; be assured tliat what you take for spots are the
faults of your glasses or your own eyes."

i2 THE SOLAR SYSTEM.

Discovery of the Solar Spots.— The solar spots
seem to have been noticed as early as 807 a.d., al-
though the telescope was not invented until 1610,
and Galileo is considered to have discovered them in
the following year. *

Number and Location. — Sometimes, but rarely,
the sun's disk is clear. During a period of ten years,
observations were made on 1982 days, on 372 of
which there were no spots seen. As many as two
hundred spots have been noticed at one time. They
are mostly found in two belts, one on each side of
the equator, within not less than 10" nor more than
SO'' of latitude. They seem to herd together, — the
length of the straggling group being generally par-
allel to the equator.

Size of the Spots. — It is not uncommon to find a
spot with a surface larger than that of the earth.
Schroter measured one more than 29,000 miles in
diameter. Sir J. W. Herschel calculated that one
which he saw was 50,000 miles in diameter. In
1843, one was seen which was 75,000 miles across,
and was visible to the naked eye for an entire week, f
On the day of the eclipse in 1858, a spot over 108,000
miles broad was distinctly seen, and attracted gen-
eral attention in this country. In 1839, Captain Davis
saw one which he computed was 180,000 miles long,
and had an area of twenty-four billion square miles.

If these are deep openings in the luminous atmos-

* We read in the l(^-book of tlie good ship Richard of Arundell, on a voyage, in

1590, to the coast of Guinea, that " on the 7, at the going downe of the snnne, we saw a

^reat black spot in the sunne ; and the 8 day, both at rising and setting, we saw the like,

-which si>ot to me seeming was about the bignesse of a shilling, being in 5 degrees of

ititude, and still there came a great billow out of the souther board."

t 1'' en the sun s surface = 430.3 miles. This spot was 2'47" acrosa {Scktoabt}.

THE SUN.

Fig. 10.

phere of the sun, what an abyss must that be at " the
bottom of which our earth could lie like a boulder in
the crater of a volcano ! "

Spots Consist of Distinct Paets. — From the ac-
companying repre-
sentation, it will be
seen that the spots
generally consist
of one or more dark
portions called the
umbra, and around
that a grayish por-
tion styled the pe-
numbra {pene, al-
most, and umbray
black). Sometimes,
however, umbrae
appear without a
penumbra,andvice
versa. The umbra itself has generally a dense black
center, called the nucleus. Besides this, the umbra
is sometimes divided by luminous bridges.

Spots are in Motion. — The spots change from day
to day ; but all have a common movement. About
fourteen days are required for a spot to pass across
the disk of the sun from the eastern side, or limb, to
the western ; in fourteen days, it reappears, changed
in form perhaps, but generally recognizable.

Spots apparently Change their Speed and Form

AS THEY PASS ACROSS THE DiSK. — A Spot is SCCU OU

the eastern limb ; day by day it progresses, with a
gradually-increasing rapidity, until it reaches the

&un-Spots.

44 THE SOLAR SYSTEM.

center ; it then slowly loses its rapidity, and finally
disappears on the western limb. The diagram illus-
trates the apparent change which takes place in the
form. Suppose at first the spot is of an oval shape ;
as it approaches the center it apparently widens and
becomes circular. Having passed that point, it be-
comes more and more oval until it disappears.

Fig. 11.

Change in Spots as they Cross the Disk.

This change in the Spots proves the Sun's Rota-
tion ON ITS Axis. — These changes can be accounted
for only on the supposition that the sun rotates on
its axis : indeed, they are the precise effects which
the laws of perspective demand in that case. About
twenty-seven days elapse from the appearance of a
spot on the eastern limb before it is seen a second
time. During this period the earth has gone forward
in its orbit, so that the location of the observer is
changed ; allowing for this, the sun's time of rotation
at the equator is about twenty-five days (25 d., 8 h.,
10 m. : Langier),

THE StTN.

Curiously enough, the equatorial regions move
more rapidly, and complete a rotation in less time,
than the rest of the siin. While a spot near the
equator performs a rotation in twenty-seven days,
one situated half-
way to either pole,
requires nearly
twenty -eight days.

Synodic and side-
real REVOLUTIONS
OF THE SPOTS. — We

can easily under-
stand why we make
an allowance for
the motion of the
earth in its orbit.
Suppose a solar spot
at a, on a line pass-
ing from the center
of the earth to the
center of the sun.
For the spot to pass
around the sun and
come into that same

Synodic and Sidereal Revolutions.

position again, requires about twenty-seven days.
But, during this time, the earth has passed on from
T to T'. The spot has not only traveled around to
a again, but also beyond that to a', or the distance
from a to a' more than an entire revolution. To do
this, requires about two days. A revolution from a
around to a' is called a synodic, and one from a
around to a again is called a sidereal, revolution.

46 the solar system.

Spots do not always move in straight lines. —
Sometimes their path curves toward the north, and

Fig. IS.

March. June. September.

sometimes toward the south, as in the figure. This
can be explained only on the supposition that the
sun's axis is inclined to the ecliptic (7"" 15').

Spots have a motion of their own. — Besides the
motion already named as assigned to the sun's rota-
tion, nearly every spot seems to have an individual
motion. Some spots circle about in small elliptical
paths, often quite regularly for weeks and even
months. Immense cyclones occasionally pass over
the surface with fearful rapidity, producing rotation
and sudden changes in the spots. At other times,
however, the spots seem "to set sail and move
across the disk of the sun like gondolas over a silver
sea."

Spots change their real form. — Spots break out
and then disappear under the eye of the astronomer.
WoUaston saw one that seemed to be shattered like
a fragment of ice when it is thrown on a frozen
surface, breaking into pieces, and sliding off in
every direction. Sometimes one divides itself into
several nuclei, while again several nuclei combine

THE SUN.

into a single nucleus. Occasionally a spot will re-
main for six or eight rotations, while often it will
last scarcely half an hour. Sir W. Herschel relates

Fig. lU.

Solar Cyclone, May 5th, 1857. (Secchi.)

that, when examining a spot through his telescope,
he turned away for a moment, and on looking back
it was gone.

Appearance of the spots is periodical.* — It is
a remarkable fact that the number of spots increases
and diminishes through a regular interval of about
11.11 years. These periodic variations are closely
connected with similar variations in the aurora
and magnetic earth-currents which interfere with
the telegraph.

Are the spots influenced by the planets ? —

* The regular increase and diminution in the number of the spots was discovered
by Schwabe of Prussia, who watched the sun so carefully that it is said "for thirty years
the sun never appeared above the horizon without being confronted by his imperturbable
telescope."

48 The solar system.

Many astronomers of high standing believe that the
solar spots are especially sensitive to the approach
of Mercury and Venus, on account of their nearness,
and of Jupiter, because of its size ; that the area of
the spots exposed to view from the earth is uniformly
greatest when any two of the larger planets come
into line with the sun ; and that when both Venus
and Jupiter are on the side of the sun opposite to us,
the spots are much larger than when Venus alone is
in that position. Most authorities, however, doubt
the accuracy of these observations, and deny this
planetary influence altogether.

Spots do not influence fruitfulness of the
SEASON. — Herschel first advanced the idea that years
of abundant spots would be years also of plentiful
harvest. This is not now generally received. What
two years could be more dissimilar than 1859 and
1860 ? Both abounded in solar spots, yet, in Europe,
one was a fruitful year and the other one of almost
famine. Whether the spots influence the weather is
still a mooted question.

Spots are cooler than the surrounding sur-
face. — It seems that the breaking out of a spot sen-
sibly diminishes the temperature of that portion of
the sun's disk. The faculae, on the other hand, do
not increase the temperature {Secchi).

Spots are depressions. — Careful observations
show that, in general, the " floor," so to speak, of the
umbra is sunk from two to six thousand miles below
the level of the luminous surface {Young).

Comparative brightness of spots and sun. —
If we represent the ordinary brightness of the

Photograxihic View of Spots and Faculte.

sun by 1,000, then that of the penumbra would
be about 800, and that of the umbra, 540 (Lang-
ley). There may be
much light and heat
radiated by^a spot,
which seems black as
compared with the sun ;
for we remember that
even a calcium light,
held between our eyes
and the sun, appears
as a black spot on the
disk of that luminary.

Appearance of the
sun's surface, — Even a
telescope of moderate
power will show the
?5urface of the sun to have a peculiar mottled appear'
3

Fig. 16.

'-"'"' '^^""

■S:-tS3^::rj

'^^

g
P

''k

FaculcB.

THE SOLAR SYSTEM.

ance not unlike that of an orange skin. But, under
favorable circumstances and with a telescope of high
power, the solar disk is found to be covered with
small, intensely bright bodies irregularly distributed.

Fig. 17.

WiOov-Leaf.

These are now known as rice-grains.'*' They are
often apparently crowded together in luminous
ridges, or streaks, termed faculce {facula, a torch) ;
while the rice-grains themselves, according to Prof.
Langley, are composed of granules. Minute as a

* Various obsen-ers describe the solar surface differently. A peculiar, elongated,
leaf-shaped appearance of the rice-grains, called the willow-leaf structure, is shown in
Fig. 17, as seen by Nasmyth. Newcomb compares the sun's appearance to that of a
plate of rice-soup. Young says it frequently resembles bits of straw lying parallel to
one another— the '• thatched-straw formation,"

Typical Sun-spot, uj Dec. 1S73, showing the filaments pointing to the center.

THE SUN. 61

granule seems, probably the smallest has a diameter
of, at least, 100 miles.

Physical Constitution of the Sun.* — Of the consti-
tution of the sun, and the cause of the solar spots,
very little is definitely known.

Wilson's Theory supposed that the sun is com-
posed of a solid, dark globe, surrounded by three
atmospheres. The first, nearest the black body of
the sun, is a dense, cloudy covering, possessing high
reflecting power. The second is called the photo-
sphere. It consists of an incandescent gas, and is
the seat of the light and heat of the sun, being the
sun that we see. The third, or * outer one, is trans-
parent — very like our atmosphere.

According to this theory, the spots are to be ex-
plained in the following manner. They are simply
openings in these atmospheres made by powerful
upward currents. At the bottom of these chasms,
we see the dark sun as a nucleus at the center, and
around this the cloudy atmosphere — the penumhra.
This explains a black spot with its penumbra. Some-
times the opening in the photosphere may be smaller
than that in the inner or cloudy atmosphere ; in that
case there will be a black spot without a penumbra.

It will be natural to suppose that when the heated
gas of the photosphere, or second atmosphere, is
violently rent asunder by an eruption or current
from below, luminous ridges will be formed by the
heaped-up gas on every side of the opening. This
would account for the faculce surrounding the sun-

* For the views of various authorities on the constitution of the sun, solar spots, etc.,
see Newcorab's Astronomy, third edition, p. 271,

THE SOLAR SYSTEM.

spots. It will be natural, also, to suppose that some-
times the cloudy atmosphere below will close up first
over the dark surface of the sun, leaving only an
opening through the photosphere, disclosing at the
bottom a grayish surface of penumbra. We can

Fig. 19.

WiUon's Theory.

readily see, also, how, as the sun revolving on its
axis brings a spot nearer and nearer to the center,
thus giving us a more direct view of the opening, we
can see more and more of the dark body. Then as
it passes by the center the nucleus will disappear,

THE SUN. 53

until finally we can see only the side of the fissure,
the penumbra, which, in its turn, will vanish.

The Present Theory* is deduced from the re-
sults of Spectrum Analysis, of which we shall here-
after speak. It is constantly being modified by new
discoveries. But we may, in general, believe the
sun to be a vast, fiery body, surrounded by an
atmosphere of substances volatilized by the intense
heat. Among these, we recognize familiar elements,
as iron, copper, &c.

The different portions of the sun are thought to be
arranged thus : (1). The nucleus, probably gaseous ; \
(2). The photosphere, an envelope several thousand
miles thick, which constitutes the visible part of the
sun ; (3). The chromosphere, composed of luminous
gas, mostly hydrogen, and the seat of enormous pro-
tuberances, tongues of fire, which dart forth, some-
times at the rate of 150 miles per second, and to a
distance of over 100,000 miles ; (^). The corona,X an
outer appendage of faint, pearly light, consisting of
streamers reaching out often several hundred thou-
sand miles. Of these solar constituents, the eye and
the telescope ordinarily reveal only the photosphere ;
the rest are seen during a total eclipse or by means
of the spectroscope.

The outer portion of the sun radiates its heat and

* As Kirchhoff, by his discoveries in Spectrum Analysis, laid the foundation of this
theory, it is often called after him.

t The interior of the sun, if gaseous, must be powerfully condensed, because of the
tremendous pressure of the atmosphere. The high temperature, however, prevents the
gas from liquefying. Tlie rain-.storms on the sun, if such ever occur, consist of drops
of molten iron, copper, zinc, &c., vaporized by the enormous heat ; and often a tempest
would drive before it this white-hot, metallic blast, with a speed of 100 miles per second.

t This is so called because, during a total eclipse, it forms around the moon a corona,
or glory, that is the most wonderful feature of this rare event. (See p. 141.)

54 THE SOLAR SYSTEM.

light, and, becoming cooler, sinks ; the hotter matter
in the interior then risQS to take its place, and thus
convection currents are established (Physics, p. 193).
The cooler, descending currents are darker, and the
hotter, ascending ones are lighter ; this gives rise to
the mottled look of the sun. At times, this occurs on
a grand scale, and the heated, up-rushing masses
form the faculse, and the cooler, down-rushing ones
produce the solar spots.

The Heat of the Sun is generally considered to be
produced by condensation, whereby the size of the
sun is constantly decreasing, and its potential energy
thus converted into kinetic. The dynamic theory
accounts for the heat and the solar spots by assum-
ing that there are vast numbers of meteors revolving
around the sun, and that these constantly rain down
upon the surface of that luminary. * Their motion,
thus stopped, is changed to heat, and feeds this great
central fire. Were Mercury to strike the sun in this
way, it would generate sufficient heat to compensate
the loss by radiation for seven years.

Doubtless, the solar heat is gradually diminishing,
and will ultimately be exhausted. In time, the sun
will cease to shine, as the earth did long since. New-
comb says that in 5,000,000 years, at the present
rate, the sun will have shrunk to half its present
size, and that it cannot sustain life on the earth
more than 10,000,000 years longer. Of this we may
be assured, there is enough to support life on our
globe for millions of years yet to come.

* Tlic heat of the sun could be maintained by an annual contraction of 220 feet in
its diameter, a decrease so insignificant as to be imperceptible with the best instru-
ments ; or by the annual impact of meteors equal in amount to y the mass of Mercury.

THE PLANETS. 55

II.— THE PLANETS.

INTRODUCTION.

The Planets will be described in regular order,
passing outward from the sun. In this journey, we
shall examine each planet in turn, noticing its dis-
tance, size, length of year, duration of day and
night, temperature, climate, number of moons, and
other interesting facts, showing how much we can
know of its world-life in spite of its wonderful dis-
tance. We shall encounter the earth in our imag-
inary wanderings through space, and shall explain
many celestial phenomena already partially familiar
to us.

In all these worlds, we shall find traces of the
same Divine hand, molding and directing in con-
formity to one universal plan. We shall discover
that the laws of light and heat are invariable, and
that the force of gravity, which causes a stone to fall
to the ground, acts similarly upon the most distant
planet. Even the elements of which the planets are
composed will be familiar to us, so that a book of
natural science published here might, in its general
features, answer for use in a school on Mars or
Jupiter.

Common Characteristics (Hind). — 1. The planets
move in the same direction around the sun ; their

56 THE SOLAR SYSTEM.

course, as viewed from the north side of the ecliptic,
being contrary to the motion of the hands of a
watch.

2. The}^ describe elliptical paths around the sun, —
not differing much from circles.

3. Their orbits are more or less inclined to the
ecliptic, and intersect it in two points — the nodes, —
one-half of the orbit lying north, and the other south
of the earth's path.

■i. They are opaque bodies, and shine by reflecting
the light they receive from the sun.

5. They rotate upon their axes in the same way
as the earth. Tliis we know by telescopic observa-
tion to be the case with many planets, and by anal-
ogy the rule may be extended to all. Hence, they
have the alternation of day and night.

6. Agreeably to the principles of gravitation, their
velocity is greatest at that part of their orbit nearest
the sun, and least at that part most distant from it ;
in other words, they move quickest in perihelion, and
slowest in aphelion.

Comparison cf the two G-roups of the Major
Planets. [Chambers.) — Separating the major planets
into two groups, if we take Mercury, Venus, the
Earth, and Mars as belonging to the interior, and
Jupiter, Saturn, Uranus, and Neptune to the exterior
group, we shall find that they differ in the following
respects :

1. The interior planets, with the exception of the
Earth and Mars, are not attended by any satellite,
while all the exterior planets have satellites.

2. The average density of the first group consider-

i'HE PLANETS.

ably exceeds that of the second, the approximate
ratio being 5:1.

3. The mean duration of the axial rotations, or the
mean length of the day of the interior planets, is
much longer than that of the exterior ; the average
in the former case being about twenty-four hours,
but in the latter only about ten hours.

Properties of the Ellipse. — In Fig. 20, S and S' are
the foci of the ellipse ; A C is the major axis ; B D,
the minor or conjugate axis; O, the center: or,
astronomically, O A is the semi-axis-major or mean

An Ellipse.

distance, O B the semi-axis-minor : the ratio of O S
to O A is the eccentricity ; the least distance, S A, is
the perilielion distance ; the greatest distance, S C,
the aphelion distance.

Characteristics of a Planetary Orbit. — It will not
be difficult to follow in the mind the additional
characteristics of a planet's orbit. Take two hoops,
and bind them into an oval shape. Incline one

58 THE SOLAR SYSTEM.

slightly to the other, as shown in Fig. 21. Let the
horizontal hoop represent the ecliptic. Imagine a
planet following the inclined hoop, or ellipse ; at a
certain point it rises above the level of the ecliptic : *
this point is called the ascending node, and the op-
Fig. SI.

Planetary Orbits.

posite point of intersection is termed the descending
node. A line connecting the two nodes is the line of
the nodes. The longitude of the node is its distance
from the first point of Aries, measured on the eclip-
tic, eastward.

Comparative Size of Planets (Chambers). — The following scheme
will assist in obtaining some notion of the magnitude of the planetary
system. Choose a level field or common ; on it place a globe two feet
in diameter for the Snn : Vulcan will then be represented by a small
pin's head, at a distance of about twenty-seven feet from the center
of the ideal sun ; Mercury by a mustard-seed, at a distance of eighty-two
feet ; Venus by a pea, at a distance of 142 feet ; the Earth, also, by a pea,
at a distance of 215 feet ; Mars by a small pepper-corn, at a distance of
327 feet ; the minor planets by grains of sand, at distances varying from
500 to 600 feet. If space wiU permit, we may place a moderate-sized

* Lockyer beautifully says : " We may imagine the earth floating aroiind the sun on
a boundless ocean, both sun and earth being half immersed in it. This level, this plane,
the plane of the ecliptic (because all eclipses occur in it), is used by astronomers as we
use the sea-level. We say a mountain is so far above the level of the sea. The astrono-
mer says the star is so high above the level of the ecliptic

THE PLANETS.

orange nearly one-quarter of a mile distant from the starting point to rep-
resent Jupiter ; a small orange two-fifths of a mUe for Saturn ; a full-sized
cherry three-quarters of a mile distant for Uranus ; and lastly, a plum
1| miles off for Neptune, the most distant planet yet known. Extending

Fig, ss.

Comparative Size of the rUinets.

this scheme, we should find that the aphelion distance of Encke's comet
would be at 880 feet ; the ajjhelion distance of Donati's comet of 1858 at
six miles ; and the nearest fixed star at 7,500 miles.

THE SOLAR SYSTEM.

According to this scale, the daily motion of Vulcan in its orbit would be
4| feet; of Mercury, 3 feet ; of Venus, 2 feet ; of the Earth, 1| feet ; of
Mars, 1^ feet ; of Jupiter, 10| inches ; of Saturn, 7^ inches ; of Uranus, 5
inches ; and of Neptune, 4 inches. This illustrates the fact that the orbital
velocity of a planet decreases as its distance from the sun increases. *

Conjunction of Planets. — The grouping together
of two or more planets within a limited area of the
heavens is a rare event. The earliest record we
have is the one of Chinese origin (p. G), stating that
a conjunction of Mars, Jupiter, Saturn, and Mercury

Fig. 23.

■r in Conjunction, January SO, ISOi.

occurred in the reign of the Emperor Chuenhio.
Astronomers tell us that this took place Feb. 28, 244G
B. c, between 10° and 18^ of Pisces. There is a very
general impression, however, that this conjunction
was afterward calculated and chronicled in their
records. In 1725, Venus, Mercury, Jupiter, and

♦ If we accept the Xelmlar Hj-pothesis (p. 255), we can easily understand the reason
of this; the exterior planets, being made earlier, had the motion of the nebula during
its earlier stage. The rotation-velocity of the nebula kept increasing, and so, of course,
each planet Dossessed a higher rate of orbital speed than the preceding one-

tHE PLANETS. 61

Mars appeared in the same field of the telescope. In
1859, Venus and Jupiter came so near each other
that they appeared to the naked eye as one object.

Are the Planets Inhabited? — This question is one
which very naturally arises, when we think of the
planets as worlds in so many respects similar to our
own. We can give no satisfactory answer. Many
think that the only object God can have in making
a world is to form an abode for man. Our own earth
was evidently fitted up, although perhaps not cre-
ated, for this express purpose. Everywhere about
us we find proofs of special forethought and
adaptation. Coal and oil in the earth for fuel and
light, forests for timber, metals in the mountains
for machinery, rivers for navigation, and level
plains for corn. The human body, the air, light, and
heat are all fitted to one another with exquisite
nicety.

When we turn to the planets, we do not know but
God has other races of intelligent beings who inhabit
them, or even entirely different ends to attain. Of
this, however, we are assured, that, if inhabited, the
conditions on which life is supported vary much
from those familiar to us. When we come to speak
of the different planets, we shall see (1) how they
differ in light and heat, from seven times our usual
temperature to less than -^^^^ ; (2) in the intensity
of the force of gravity, from 2^ times that of
the earth to less than | ; (3) in the constitution of
the planet itself, from a density ^ heavier than that
of the earth to one nearly that of cork.

The temperature may often sweep downward

THE SOLAR SYSTEM.

through a scale of 2,000° in passing from Mercury
to Neptune. No human being could reside on the
former, while we cannot conceive of any polar inhab-

Fig. %.

Size of tht' .S(t/t ay seen frtrni tke V in nets

itaat who could endure the intense cold of the latter.
At the sun, one of our pounds would weigh over 27
pounds ; on our moon, the pound weight would be-

fHE PLANETS. 63

Come only about two ounces ; while on Vesta, one of
the planetoids, a man could easily spring sixty feet
in the air and sustain no shock in falling. Yet, while
we speak of these peculiarities, we do not know
what modification of the atmosphere or physical
features may exist on Mercury to temper the heat,
or on Neptune to reduce the cold.

With all these diversities, we must, however, admit
the power of an all-wise Creator to form beings
adapted to the life and the land, however different
from our own. The Power that prepared a world
for us, could as easily and perfectly prepare one for
other races. May it not be that the same love of
diversity, that will not make two leaves after the
same pattern nor two pebbles of the same size, de-
lights in worlds peopled by races as diverse ? *

While, then, we cannot affirm that the planets are
inhabited, analogy would lead us to think that they
are, and that the most distant star that shines in
the arch of heaven may give light and heat to living
beings under the care and government of Him who
enlivens the densest forest with the hum of insects,
and populates even a drop of water with its teeming-
millions of animalcules.

Divisions of the Planets. — The planets are divided
into two classes : (1). Inferior, or those whose orbits
are within that of the earth — viz., Mercurj^, Venus ;
(2). Superior, or those whose orbits are beyond that

* Astronomers conceive the universe to contain worlds in every possible stage of
development, from the primary, gaseous nebula, to a worn-out, dead globe, like the
moon. At a certain period in its existence, each world may be fitted to support life.
Millions may now be in that condition ; others may be approaching, while others liave
passed it.

64 THE SOLAR SYSTEM.

of the earth — viz., Mars, Jupiter, Saturn, Uranus,
Neptune.

Motions of a Planet as seen from the Sun. — Could
we stand at the sun and watch the movements of the
planets, they would all be seen revolving with dif-
ferent velocities in the order of the zodiacal signs.
But to us, standing on one of the planets, itself in
motion, the effect is changed. To an observer at the
sun all the motions would be real, while to us many
are only apparent. The position of a planet, as seen
from the center of the sun, is called its heliocentric
place ; as seen from the center of the earth, its geo-
centric place. When Venus is at inferior conjunc-
tion, an observer at the sun would see it in the oppo-
site part of the heavens from that in which it would
appear to him if viewed from the earth.

Motions of an Inferior Planet — An inferior planet
is never seen by us in any part of the sky opposite
to the sun at the time of observation. It cannot
recede from him as much as 90°, or \ the circum-
ference, since it moves in an orbit entirely enclosed
by the orbit of the earth. Twice in every revolution
it is in conjunction ( 6 ) with the sun, — an inferior
conjunction (A) wlien it comes between the earth
and the sun, and a superior conjunction (B) when
the sun lies between it and the earth.

When the planet attains its greatest distance east
or west (as we see it) from the sun, it is said to be at
its greatest elongation.

When passing from B to A it is east of the sun,
and from A to B it is west of the sun. When east of
the sun, it sets later than the sun, and hence is

THE PLANETS.

evening star : when west of the sun, it rises earlier
than the sun, and hence is morning star. An inferior
■planet is never visible when in superior conjunction,
as its light is then lost in the greater brilliancy of
the sun. When in inferior conjunction, it some-

Conjunctions of Inferior Planet.

times passes in front of the sun, and appears to us as
a round, black spot swiftly moving across his disk.
This is called a transit.

Retrograde Motion of an Inferior Planet. —
Suppose the earth at A (Fig. 26), and the planet at B.

THE SOLAR SYSTEM.

Now, while the earth is passing to F, the planet will
pass to D, — the arc AF being shorter than BD, be-
cause the nearer a planet is to the sun the greater its
velocity. While the planet is at B, we locate it at
C on the ecliptic, in Gemini ; but at D, it appears to
us to be at G, in Taurus. So that the planet has

Betroffrade Motion.

retrograded through an entire sign on the ecliptic,
while its course all the while has been directly for-
ward in the order of the signs ; and to an observer at
the sun, such would have been its motion.

Phases of an Inferior Planet. — An inferior
planet presents all the phases of the moon. At supe-
rior conjunction, the whole illumined disk is turned
toward us ; but the planet is lost in the sun's rays :

THE PLANETS. 67

therefore neither Mercury nor Venus ever presents a
complete circular appearance, like the full moon. A
little before or after superior conjunction, an inferior
planet may be seen with a telescope ; but the whole
of the light side is not turned, toward us, and so the
planet appears gihhous, like the moon between the
first quarter and full. At its greatest elongation,
the planet shows us only one-half its illumined disk ;
this decreases, becoming more and more crescent
toward inferior conjunction, at which time the un-
illumined side is toward us.

Fig. 27.

Phases of an Inferior Planet.

Motions of a Superior Planet.— The superior planet
moves in an orbit which entirely surrounds that of the
earth. When the earth is at E (Fig. 28), the planet
at L is said to be in opposition to the sun ( S ). It
is then at its greatest distance from him— 180°. The
planet is on the meridian at midnight, while the sun
is on the corresponding meridian on the opposite side
of the earth ; or the planet may be rising, when the
sun is just setting. When the planet is at N, it is in

THE SOLAR SYSTEM.

conjunction, and being lost in the sun's rays is invis-
ible to us. When 90^ east or west of the sun, the
planet is said to be in quadrature (□).

Retrograde Motion of a Superior Planet. — Sup-
pose the earth to be at E and the planet at L, and
that we move on to G while the planet passes on to

Retrogrnde Motion ofaSujKrior Planet.

O— the distance EG being longer than LO, the re-
verse of what takes place in the movements of the
inferior planets ; at E, we should locate the planet at
P on the ecliptic, in the sign Cancer ; but at G, it
would appear to us at Q, in the sign Gemini, having

THE PLANETS. 69

apparently retrograded on the ecliptic the distance
PQ, while it was all the time moving on in the
direct order of the signs. Now, suppose the earth
passes on to I and the planet to U, we should then
see it at the point W, further on in the ecliptic than
Q, which indicates direct motion again, and at some
point near Q the planet must have appeared without
motion.

After this, it will continue direct until the earth has
completed a large portion of her orbit, as we can
easily see by imagining various positions of the earth
and planet, and then drawing lines as we have just
done, noticing whether they indicate direct or retro-
grade motion. The greater the distance of a planet
the less it will retrograde, as we can perceive by
drawing another orbit outside the one represented in
the cut, and making the same suppositions concern-
ing it as those we have already explained.

Sidereal and Synodic Revolution, — The interval
of time required by a planet to perform a revolution
from one fixed star back to it again, is termed a
sidereal revolution {sidus, a star).

1, The interval of time between two similar con-
junctions of an inferior planet with the earth and
the sun is termed a synodic revolution. Were the
earth at rest, there would be no difference between a
sidereal and a synodic revolution, and the planet
would come into conjunction twice in each revolu-
tion. Since, however, the earth is in motion, it fol-
lows that, after the planet has completed its sidereal
revolution, it must overtake the earth before they
can both come again into the same position with

70 THE SOLAR SYSTEM.

regard to the sun. The faster a planet moves, the
sooner it can do this. Mercury, traveling at a
greater speed and on an inner orbit, accomplishes
it much more quickly than Venus. The synodic
period always exceeds the sidereal,

2. The interval between two successive conjunc-
tions or oppositions of a superior planet is also
termed a synodic j^e volution. Since the earth moves
so much faster than anj' superior planet, it fol-
lows that, after it has completed a sidereal revo-
lution, it must overtake the planet before they can
again come into the same position with regard to
the sun. The slower the planet, the sooner this
can be done. Uranus, making a sidereal revolution
in eighty-four years, can be overtaken more quickly
than Mars, which makes one in less than two
years. It consequently requires over a second
revolution for the earth to catch up with Mars,
only ^ of a second one to overtake Jupiter, and
but little over ^hi of a second one to come up with
Uranus.

Planets as Evening and Morning Stars. — The in-
ferior planets are evening stars from superior to
inferior conjunction : and the superior planets, from
opposition to conjunction. During the other part of
their revolutions, they are morning stars.

Mercury is evening st

Venus

Mars

Jupiter

Saturn

FraoQUS

iir. , about 2 months.

H
13

THE PLANETS. 71

I. VULCAN (hypothetical).

Supposed. Discovery. — Le Vcrrier, having detected an error in tho
assumed motion of Mercury, suggested, in the autumn of 1859, that there
might be an interior planet, which was the cause of this disturbance. On
this being made public, M. Lescarbault, a French physician and au
amateur astronomer, stated that on Mai-ch 26 of that year lie had seen a
dark body pass across the sun's disk, which might have been the
unknown planet. Le Verrier visited him, and found his instruments
rough and home-made, but singularly accurate. His clock was a .simple
pendulum, consisting of an ivory ball hanging from a nail by a silk thread.
His observations were on prescription paper, covered with grease and
laudanum. His calculations were chalked on a board, which he planed olf
to make room for fresh ones. Le Verrier became satisfied that a new planet
had been discovered by this enthusiastic observer, and congratulated liim
upon his deserved success.

On March 20, 1862, Mr. Lummis, of Manchester, England, noticed a
rapidly-moving, dark spot, apparently the transit of an inner planet.
During the total eclipse of July 29, 1878, Professor Watson, of Ann Arlior
Observatory, and Dr. Lewis Swift, of Rochester, claimed to liave seen
two Intra-Mercurial planets. As yet, however, the existence of the planet
is not generally conceded. The name Vulcan and the sign of a hammer
have been given to it. Its distance from the .sun has been estimated at
13,000,000 miles, and its periodic time (its year) at twenty days.

II. MERCURY.

The fleetest of the gods. Sign, s , his wand.

Description. — Mercury is nearest to the sun of any
of the definitely-known planets. When the sky is
very clear, we maj sometimes see it, just after sun-
set, as a bright, sparkling star, near the western
jiorizpn. Its elevation increase's evening hj evening,

72 THE SOLAR SYSTEM.

but never exceeds 28°.* If we watch it closely, we
shall find that the planet again approaches the sun
and becomes lost in his rays. Some days after-
ward, just before sunrise, we can see the same planet
in the east, rising higher each morning, until its
greatest elevation equals that which it before at-
tained in the west. Thus the planet appears slowly
but steadily to oscillate like a pendulum, to and fro,
from one side to the other of the sun. The ancients,
deceived by this puzzling movement, failed to dis-
cover the identity of the two stars, and called the
morning star Apollo, the god of day, and the evening
star Mercury, the god of thieves, who walk to and
fro in the night-time seeking plunder, f

On account of the nearness of Mercury to the sun,
it is difficult to be detected. | It is said that Coper-
nicus, an old man of seventy, lamented in his last
moments that, much as he had tried, he had never
been able to see it. In our latitude and climate, we.
can generally easily find it if we watch for it at the
time of its greatest elongation, as commonly given
in the almanac.

Motion in Space. — Mercury revolves around the
sun at a mean distance of about 30,000,000 miles. Its

* This distance varies much, owing to the eccentricity of Mercury's orbit.

t The Greeks gave to Mercury the additional name of "The Sparkling One." The
astrologists looked upon it as the malignant jilanet. The chemists, because of its
extreme swiftness, applied the name to quicksilver. The most ancient account that we
have of this planet is given by Ptolemy, in his Almagest ; he states its location on the
15th of November, 205 b. c. The Chinese also state that on June 9, llS a. d., it was
near the Beehive, a cluster of stars in Cancer. Astronomers tell us tliat, according to
the best calculations, it was at that date witliin less than 1° of that group.

t An old English writer by the name of Goad, in 1680, humorously termed this planet,
" A squinting lacquey of the sun, who seldom shows his head in these parts, as if he
were in debt"

Mercury. 73

orbit is the most eccentric (flattened) of any among
the eight principal planets, so that, although when in
perihelion it approaches to within about 28,000,000
miles, in aphelion it speeds away 15,000,000 miles
further, or to the distance of over 43,000,000 miles.
Being so near the sun, its motion in its orbit is cor-
respondingly rapid, — viz., thirty miles per second.*

The Mercurial year comprises only about eighty-
eight days, or nearly three of our months. Mercury
is thought to rotate upon its axis in about the same
time as the earth, so that the length of the Mercurial
day is nearly the same as that of the terrestrial
one.

Though Mercury thus completes a sidereal revolu-
tion around the sun in eighty-eight days, yet to pass
from one inferior or superior conjunction to the next
(a synodic revolution) requires IIG days. The reason
of this is, that when Mercury comes around again
to the point of its last conjunction, the earth has
gone forward, and it requires twenty-eight days for
the planet to overtake us.

The Distance from the Earth varies still more
than the distance from the sun. At inferior con-
junction, Mercury is between the earth and the sun,
and its distance from us is the difference between
the distance of the earth and of the planet from the
sun : at superior conjunction, it is the stun of these
distances. Its apparent diameter in these different
positions varies in the same proportion as the dis-
tance, or nearly three to one. The greatest and least

♦ At this rate of speed, we could cross the Atlautic Ocean in two minutes.

74 THE SOLAR SYSTEM.

distances vary as either planet happens to be in
aphelion or perihelion.*

Dimensions. — Mercury is about 3,000 miles in di-
ameter. Its volume is about ^V that of the earth —
1. e., it would require twenty globes as large as Mer-
cury to make one the size of the earth, or 25,000,000
to equal the sun. It is \ denser than the earth, its
mass is nearly ^V that of the earth, and a stone let
drop upon its surface would fall 7^ feet the first
second. Its specific gravity is not far from that of
tin. A pound weight removed to Mercury would
weigh only about seven ounces.

Seasons. — As Mercury's axis is much inclined from
a perpendicular (perhaps 70°), its seasons are peculiar.
There are no distinct frigid zones ; but large regions
near the poles have six weeks of continuous day and
torrid heat, alternating with a night of equal length
and arctic cold. The sun shines perpendicularly
upon the torrid zone only at the equinoxes, while he
sinks far toward the southern horizon at one solstice,
and as far toward the northern horizon at the
other, t The equatorial regions, therefore, during
each revolution, are modified in their temperature
from torrid to temperate, and the tropical heat is
experienced alternately toward the north and the
south of what we call the temperate zones.

There is no marked distinction of zones as with us,
but each zone changes its character twice during the

* If at inferior conjunction Mercury is in aphelion and the earth in perihelion, its
distance from us is only 91,500,000 — 43,000,000 = 48,500,000 miles. If at superior con-
junction Mercury is in ai>helion and the earth in aphelion also, its distance from us
is 94,500,000 + 4:i,000,000= 137,500,000 miles.

t Read a chapter entitled " The Fiery World," in Proctor's Poetry of Astronomy.

MERCURY.

Mercurial year, or eight times during the terrestrial
one. An inhabitant of Mercury must be accustomed
to sudden and violent vicissitudes of temperature.
At one time, the sun not only thus pours down its
vertical rays, and in a few weeks after sinks far
toward the horizon, but, on account of Mercury's

Fig. 29.

Orhit and Seasons of Mermiry.

elliptical orbit, when in perihelion the planet ap-
proaches so near the sun that the heat and light are
ten times as great as ours, while in aphelion it re-
cedes so as to reduce the amount to four and a half
times. The average heat is about seven times that
of the earth, — a temperature sufficient to turn water
into steam, and even to melt zinc.

76 THE SOLAR SYSTEM.

The relative length of the days and nights is much
more variable than with us. The sun, apparently
seven times as large as it seems to us, must be a
magnificent spectacle, and illumine every object
with insufferable brilliancy. The evening sky is,
however, lighted by no moon.

Telescopic Features. — Through the telescope, Mer-
cury presents all the phases of the moon, from a slen-
der crescent to gibbous, after which its light is lost in
that of the sun. These phases prove that Mercury is
spherical, and shines by the light reflected from the
sun. Being an inferior planet, we never see it when
full, and hence the brightest, nor when nearest the
earth, as then its dark side is turned toward us.

t)wing to the dazzling light, and the vapors almost
always hanging around our horizon, this planet has
not of late received much attention ; the data here
given are mainly based upon the observations of the
older astronomers, and are, therefore, not universally
accepted. Mercury is thought by some to have a
dense, cloudy atmosphere, that materially dimin-
ishes the intensity of its heat and, perhaps, makes
it habitable, though others assert that the atmos-
phere is too insignificant to be detected. Some dark
bands about the planet's equator indicate, perhaps,
an equatorial zone. There are, also, lofty heights
which intercept the light of the sun. and deep valleys
plunged in shade. One mountain is claimed to be
over eleven miles high, or about ^ the diameter of
the planet.*

♦ Tlie height of the loftiest peak of the Himalayas is only 29,000 feet, or about ^Vos
part of the earth's diameter.

VENUS. ?7

III. VENUS.

The Queen of Beauty. Sign ? , a looking-glass.

Description. — Venus, the next in order to Mercury,
is the most brilliant of the planets.* She presents
the same appearances as Mercury. Owing, however,
to the larger size of her orbit, her greatest apparent
oscillations are nearly 48° east and west of the sun,t
or about 20^ more than those of Mercury. She is
therefore seen much earlier in the morning and
much later at night. She is morning star from in-
ferior to superior conjunction, and evening star from
superior to inferior conjunction.

Venus is the most brilliant about five weeks before
and after inferior conjunction, at which time the
planet is bright enough to cast a shadow at night.
If, in addition, at this time of greatest brilliancy,
Venus is at or near her highest north latitude, she
may be seen with the naked eye in full daylight. ;}:
This occurs once in eight years — the interval required
for the earth and planet to return to the same situa-
tion in their orbits ; eight complete revolutions of the

* When visible before sunrise, she was called by the aiicieiits Phosphorus, Lueifer,
or tl;e Morning Star, and when slie shone in the evening after sunset, Hesperus, Vesjier,
or the Evening Star.

t This distance varies only about 3", owing to the slight eccentricity of Venus's
orbit.

J Arago relates that Buonaparte, upon repairing to the Luxembourg, when the
Directory was about to give him a fete, was much surprised at seeing the multitude
paying more attention to the heavens above the palace than to him or his brilliant staff.
Upon inquiry, he learned that these curious persons were observing with astonisliment
a star which they supposed to be that of the Conqueror of Italy. The emperor himself
was not inditfiirent when his piercing eye caught the clear lustre of Venus smiling upon
him at midday.

t8 THE SOLAli SVSTEif.

earth about the sun occupying nearly the same time
as thirteen of Venus.

Motion in Space. — Venus has an orbit the most
nearly circular of any of the principal planets. Her
mean distance from the sun is about 67,000,000 miles,
which varies at aphelion and perihelion 1,000,000
miles, — a contrast to Mercury, which varies 15,000,000
miles.

Venus makes a complete revolution around the
sun in about 225 days, at the mean rate of twenty-
two miles per second ; hence her year is equal to
about seven and one-half of our months. This is a
sidereal revolution, as it would appear to an ob-
server at the sun ; a sijuodic re »'olution requires 584
days.

I\iercury, we remember, catches up with the earth
in twenty-eignt days after it reaches the point where
it left the earth at the last inferjr>r conjunction. But
it takes Venus nearly two and a half revolutions to
overtake the earth anu eome into the same conjunc-
tion again. This grows out of the fact that she
has a longer orbit than Mercury, and moves only
about one-sixth faster than the earth, while Mer-
cury travels nearly twice as fast as our planet.
Venus rotates upon her axis in about twenty-four
hours ; so the length of her day does not differ essen-
tially from ours.

Distance from the Earth. — Like that of Mercury,
the distance of Venus from the earth, when in in-
ferior conjunction, is the difference between the dis-
tances of the two planets from the sun ; when
in superior conjunction, the sum of these distances.

When nearest to us, Yenus is only about 25,000,000
miles away.

Figure 30 represents her apparent dimensions at
the extreme, mean, and least distances from us.
The variation is nearly as the numbers 10, 18, and 65.
It would be natural to think that the planet is the
brightest when the nearest, and thus the largest, but

Extreine, Mean, and Least Apparent Size of Venus ; and Iter Piloses.

we should remember that then the bright side is
toward the sun, and the unillumined side toward us.
Indeed, at the period of greatest brilliancy, of which
we have spoken, only about one-fourth of her light is
visible. At this time, however, observers have
noticed the entire contour of the- planet to be of a
dull gray hue, as seen in the cut.

Dimensions. — Venus is about 7, GOO miles in diame-
ter. The volume and density of the planet are each
about nine-tenths that of the earth. A stone let fall
upon her surface would fall fourteen feet in the first

THE SOLAR SYSTEM.

second : a pound weight removed to her equator
would weigh about fourteen ounces. From this we
see that the force of gravity does not decrease
exactly in proportion to the size of the planet, any
more than it increases with the size of the sun.
The reason is, that the body is brought nearer the
mass of the small planet, and so feels its attraction
more fully than when far out upon the circumfer-
ence of a large body, — the attraction increasing as
the square of the distance from the particles de-
creases.

Seasons. — Since the axis of Venus is very much
inclined from a perpendicular, her seasons are similar

Fig. 31.

V: iia.< lit ill I- SUstic

to those of Mercury. The torrid and temperate zones
overlap each other, and the polar regions have, alter-
nately, at one solstice a torrid temperature, and at
the other a prolonged arctic cold. The inequality of
the nights is very marked. The heat and light are

VENUS.

double that of the earth, while the circular form of
her orbit gives nearly an equal length to her four
seasons.

If the inclination of her axis is 75°, as some as-
tronomers hold, her tropics must be 75° from the
equator, and her polar circles 75° from the poles. The
torrid zone is, therefore, 150° in width. The torrid
and frigid zones interlap through a space of 60", mid-
way between the equator and the poles.

Telescopic Features. — Venus, being an interior
planet, presents, like Mercury, all the phases of the
moon. *

She is thought to have a dense, cloudy atmosphere.
This was suggested by the fact that at the transit of

Fig. SS.

i|lil!l!„,|i|,|i| i' ,1 |i||[f'i;i; i

ll'l. Illlll .'''Il'l

Crescent and S/iotj ./ I'l

Venus over the sun in 1761, 1769, and 1883, a faint
ring of light surrounded the black disk of the planet.

* This was discovered by Galileo, and was among the first achievements of his tele-
scopic observations. It had been argued against the Copeniican system that, if true,
Venus should wax and wane like the moon. Indeed, Copernicus himself boldly declared
that, if means of seeing the jdanets more distinctly were ever invented, Venus would be
found to present such phases. Galileo, with his telescope, proved this feet, ^pd
thus vindicated the Copemican theory.

82 THE SOLAK SYSTEM.

The evidence of an atmosphere, as well as of moun-
tains, however, rests upon the peculiar appearance
attending her crescent shape.

1. The luminous part does not end abruptly ; on
the contrary, the light diminishes gradually. This
diminution can be explained by a twilight caused by
an atmosphere which diffuses the rays of light into
regions of the planet where the sun is already set.
Thus, on Venus, as on the earth, the evenings are
lighted by twilight, and the mornings by dawn.

2. The edge of the enlightened portion of the
planet is uneven and irregular. This appearance is
doubtless the effect of shadows cast by mountains.

Spots have been noticed on her disk which are con-
sidered to be traceable to clouds. Herschel thinks
that we never see the body of the planet, but only her
atmosphere loaded with vapors, which may mitigate
the glare of the intense sunshine.

Satellites. — Venus is not known to have any
moon.

IV. THE EARTH.

Sign, ^, a circle with Equator and Meridian.

The Earth is the next planet we meet in passing
outward from the sun. To the beginner, it seems
strange enough to class our world among the heav-
enly bodies. They are brilliant, while it is dark and
opaque ; they appear light and airy, while it is solid
9,nd firm ,: we see in it no motion, while they ar^

THE EARTH.

constantly changing their position ; they seem mere
points in the sky, while it is vast and extended.
Yet, at the very beginning, we are to consider the

Fig. S3.

The Earth in Space.

earth as a planet shining brightly in the heavens,
^nd appe,g,ring to other worlds as a planet (ioes to us.

84 THE SOLAK SYSTEM.

We are to learn that it is in motion, flying througli
its orbit with inconceivable velocity ; that it is not
fixed, but hangs in space, held by an invisible
power of gravitation which it cannot evade ; * that
it is small and insignificant beside the mighty globes
that so gently shine upon us in the far-off sky ; that,
in fact, it is only one atom in a universe of worlds,
all firm and solid, and all, perhaps, equally fitted to
be the abode of life.

Dimensions. — The earth is not "round like a ball,"
but flattened at the poles. Its form is that of an
oblate spheroid. Its polar diameter is about 7,899
miles, and its equatorial about 7,925|. The com-
pression is, therefore, 26| miles. (See table in
Appendix.) If we represent the earth by a globe
one yard in diameter, the polar diameter would be
one-tenth of an inch too long. The circumfer-
ence of the earth is nearly 25,000 miles. Its density
is about 5 1 times that of water. Its weight is
6,069,000,000,000,000,000,000 tons.

The inequalities of the earth's surface, arising
from valleys, mountains, etc., have been likened to
the roughness on the rind of an orange. On a globe
sixteen inches in diameter, the land, to be in pro-
portion, should be represented by the thinnest writing
paper, the hills by very fine grains of sand, and ele-
vated ranges by thick drawing-paper. To represent
the deepest wells or mines, a scratch should be made
tha,t would be invisible except with a glass.

* Were the sun's attractive force upon the earth replaced by the largest steel tele-
graph wire, it would require nine wires for each square inch of the sunward side gf our
plobe, to hold the earth in her orbit.

THE EARTH. 85

The Rotundity of the Earth is proved in various
ways : (1) By the fact that vessels have sailed
around the earth ; * (2) when a ship is coming into
port, we see the masts first ; (3) the shadow of the
earth on the moon is circular ; (4) the polar star
seems higher in the heavens as we pass north ; and
(5) th^ horizon expands as we ascend an eminence, f
If we climb to the top of a hill, we can see further
than when on the plain at its foot. Our eyesight is
not improved ; it is only because ordinarily the cur-
vature of the earth shuts off the view of distant
objects, but when we ascend to a higher point, we
can see further over the side of the earth. The cur-
vature is eight inches per mile, 2^ x 8'"- = 32 inches
for two miles, 3^ x 8'"- for three miles, etc. An
object of these respective heights would be just
hidden at these distances.

Apparent and Real Motion. — In endeavoring to
understand the various appearances of the heavenly
bodies, it is well to remember how in daily life we

* It is curious, in connection with this well-known fact, to recall the arguments
urged by the Spanisli pliilosophers against the reasoning of Columbus, wlien he assured
them that he could arrive at Asia just as certainly by sailing west as east. " How,"
they asl<ed, " can the earth be round ? If it were, then on the opposite side the min
would fall upward, trees would grow with their brandies down, and everything would
be topsy-turvy. Every object on its surface would certainly fall off, and if a ship by
sailing west should get around there, it would never be able to climb up the side of the
earth and get back again. How can a ship sail up hill ? "

t " The history of aeronautic adventure affords a curious illustration of this same
principle. The late Mr. Sadler, the celebrated aeronaut, ascended on one occasion in a
balloon from Dublin, and was wafted across the Irish Channel, when, on his approach
to the Welsh coast, the balloon descended nearly to the surface of the sea. By this time
the sun was set, and the shades of evening began to close in. He threw out nearly all
his ballast, and suddenly sprang upward to a great height, and by so doing brought his
horizon to dip below the sun, producing tlie whole phenomenon of a western sunrise.
Subsequently descending in Wales, he, of course, witnessed a second sunset on the sa:ue
avening."

86 THE SOLAR SYSTEM.

transfer motion. On the cars, when in rapid move-
ment, the fences and the trees seem to gUde by us,
■while we sit still and see them pass. On a bridge,
when we are at rest, we watch the undulations of
the waves, until at last we come to think that they
are stationary and we are sweeping up the stream.

' ' In the cabin of a large Tcssel going smoothly before the ^vind on still
water, or drawn along a canal, not the smallest indication acquaints us
with the ' way it is making. ' We read, sit, walk, as if we were on land.
If we throw a ball into the air, it falls back into our hand ; if we drop it,
it alights at our feet. Insects buzz around us as in the free air, and smoke
ascends in the same manner as it would do in an apartment on shore. If,
indeed, we come on deck, the case is in some respects different ; the air,
not being carried along with us, drifts away smoke and other light bodies,
such as feathers cast upon it, apparently in the opposite direction to that
of the ship's progress ; but in reality they remain at rest, and we leave
them behind in the air. And what is the earth itself but the good ship we
are sailing in tlirough the universe, bound round the sun ; and as we sit
here in one of the ' berths,' we are unconscious of there being any ' way '
at all upon the vessel. On deck, too, out in the open air, it's aU the same
so long as we keep our eyes on the ship ; but immediately we look over
the sides — and the horizon is but the ' gunwale ' of our vessel — we see the
blue tide of the great ocean around us go drifting by the ship, and spark-
ling with its million stars as the waters of the sea itself sparkle at night
between the tropics."

Diurnal Rotation of the Earth aroimd its Axis.
— The earth, in constantly turning from west to east,
elevates our hcirizon above the stars on the west, and
depresses it below the stars on the east. As the
horizon appears to us to be stationary, we assign the
motion to the stars, thinking those on the west,
which it passes over and hides, to have sunk below it,
or set J and imagining those on the east, below which

THE EARTH.

it has dropped, to have moved above it, or risen. So,
also, the horizon is depressed below the sun, and we
call it sunrise ; it is elevated above the sun, and we
call it sunset.

We thus see that the diurnal movement of the sun
by day and the stars by night is an optical illusion,
— that here as elsewhere we simply transfer motion.
This seems easy enough for us to understand ; but it
was the "stone of stumbling" to ancient astrono-
mers for thousands of years. Copernicus himself, it
is said, first thought of the true solution while riding
on a vessel and noticing how he insensibly trans-
ferred the movement of the ship to the objects on
the shore. How much grander the beautiful sim-
plicity of this system than the cumbersome com-
plexity of the old Ptolemaic belief !

Diurnal Motion of the Sun. — The explanation
just given illustrates the apparent motion of the sun,
and the cause of day and night. Suppose S to be the
sun. The earth, E, turning upon its axis EF from

Fig. Sit.

Daily Motion of the Sun (Hind).

west to east, has only half its surface illuminated at
one time by the sun. To a person at D, the sun is in
the horizon and day commences, that luminary ap-

88 THE SOLAR SYSTEM.

pearing to rise higher and higher, with a westerly
motion, as the observer is carried forward easterly
by the earth's diurnal rotation to A, where he has
the sun in his meridian, and it is consequently noon.
The sun then begins to decline in the sky until the
spectator arrives at B. where it sets, or is again in
the horizon on the west side, and night begins. He
moves on to C, which marks his position at mid-
night, the sun being then on the meridian of places
on the opposite part of the earth, and he is brought
round again to D, the point of sunrise, when another
day commences.

Unequal Rate of Diurnal Motion. — Different
points upon the surface of the earth revolve with
different velocities. At the poles the speed of rota-
tion is nothing, while at the equator it is greatest, or
over 1,000 miles per hour. At Quito, the circle of
latitude is much longer than the one at the mouth of
the St. Lawrence, and the velocities vary in the
same proportion. The former place moves at the
rate of about 1,038 miles per hour : the latter, 682
miles. In our latitude (41°) the speed is about 780
miles per hour. We do not perceive this wonderful
velocity with which we are flying through the ether,
because the atmosphere moves with us.*

Were the earth suddenly to stop its rotation, the
terrible shock would, without doubt, destroy the

* " An ingenious inventor once suggested that we should utilize the earth's rota-
tion, as the most simple and economical, as well as rapid mode of locomotion that could
be conceived. This was to be accomplished by rising in a balloon to a height inacces-
sible to aerial currents. The balloon, remaining immovable in this calm region, would
simply await the moment when the earth, rotating underneath, should present the place
of destination to the eyes of travelers who would then descend. A well-regulated
watch and an exact knowledge of longitude would thus render traveling jiossible fh^m

THE EAETH. 89

entire race of man ; while we, with houses, trees,
rocks, and even the oceans, would be hurled, in one
confused mass, headlong into space. On the other
hand, were the rate of rotation to increase, the
length of the day would be proportionately shortened,
and the weight of all bodies decreased by the centrif-
ugal force thus produced. If the rotary movement
should become swift enough to reduce the day to
eighty-four minutes, the force of gravity would be
overcome, and, at the equator, all bodies would be
without weight; if the speed were still further in-
creased, loose bodies would fly off from the earth
like water from a swiftly-turned grindstone, while
we should be compelled constantly to "hold on"
to avoid sharing the same fate.* But against such
a catastrophe we are assured by the immutability
of God's laws. " He is the same yesterday, to-day, /
and forever."

Unequal Diurnal Orbits of the Stars. — In
figure 35, let O represent our position on the earth's
surface ; E Z B, our meridian ; E I B K, our horizon ;
P and P', the north and south poles ; Z, the zenith ;

east to west, all voyages north or south being interdicted. Tliis suggestion has only one
fault ; it supposes that the atmospherio strata do not revolve with the earth. Upon
tliat hypothesis, since we rotate (at London) with the velocity of 8:33 yards in a
second, there would result a wind in the contrary direction ten times more violent than
the most terrible hurricane. Is not the absence of such a state of things a convincing
proof of the participation of the atmospheric envelope in the general movement?" —

GUILLEMIN.

* Laplace concluded in 1709 that the inequalities of the earth's rotation were too
insignificant for measurement. But, more recently. Delaunay has shown from the moon's
acceleration that a minute change, caused by the friction of the sea and atmosphere
upon the earth's surface, has taken place, producing a variation in the length of the day.

The acceleration of the moon in its path, is, however, only seven feet per century,
or less than an inch per annum, and the time of the earth's rotation has increased but
xHeo of ^ second in 2,400 years.— B.ill.

THE SOLAR SYSTEM.

Z . the nadir : aud G I C K the celestial equator. Now
P B, it will be seen, is the elevation of the north pole
above the horizon, or the latitude of the place.

Suppose we should see a star at A, on the meridian
below the pole. The earth revolves in the direction
G I C ; the star will therefore move along A L to Z,
when it is on the meridian above the pole. It con-
tinues its course along the dotted line around to A
again, when it is on the meridian below the pole,
having made a complete circuit around the pole, but
not having descended below our horizon.

A star rising at B would just touch the horizon ;
one at I would move on the celestial equator, and
would be above the horizon as long a time as it is
below, — twelve hours in each case : a star rising at M
would come just above the horizon and set again at N.

I'HE EARTH. i)i.

Unequal Diurnal Velocities of the Stars. —
The stars appear to us to be set in a concave shell
which revolves daily about the earth. As differ-
ent parts of the earth really rotate with varying ve-
locities, so the stars appear to revolve at different
rates of speed. Those near the pole, having a small
orbit, revolve very slowly, while those near the
celestial equator move at the greatest speed.

Appearance of the Stars at Different Places
ON the Earth. — Were we placed at the north pole,
Polaris would be directly overhead, and the stars
would seem to pass around us in circles parallel to
the horizon, and increasing in diameter from the
upper to the lower ones. Were we placed at the
equator, the pole-star would be at the horizon, and
the stars would move in circles perpendicular to the
horizon, and decreasing in diameter, north and south
from those in the zenith, while we could see one
half of the path of each star. Were we placed in the
southern hemisphere, the circumpolar stars would
revolve about the south pole, and the others in
circles resembling those in our sky, only the points of
direction would be reversed to correspond with the
pole. Were we placed at the south pole, the appear-
ance would be the same as at the north pole, except
that no star is there to mark the direction of the
earth's axis.

Motion of the Earth in Space about the Sun. —
The earth revolves in an elliptical path about the
sun at a mean distance of 93,000,000 miles.

The eccentricity of this path, which is greater
than that of the orbit of Venus, changes about

92 THE SOLAR SYSTEM.

To^FFo per century. The orbit would, therefore,
finally become circular, were it not that, after the
lapse of some thousands of years, the eccentricity
will begin to increase again, and will thus vary
through all time within definite, although yet un-
determined limits. The circumference is nearl}"
600,000,000 miles, and the earth pursues this wonder-
ful journey at the rate of over eighteen miles per
second.

This revolution of the earth about the sun gives
rise to various phenomena, of which we shall now
proceed to speak.

1. Change in the Appearance of the Heavens
IN Different Months. — In Fig. 36, suppose A B C D
to be the orbit of the earth, and E F G H the
sphere of the fixed stars, surrounding the sun in
every direction. When our globe is at A, the stars
about E are on the meridian at midnight. Being
seen from the earth in the quarter opposite to
the sun, they are favorably placed for observa-
tion. The stars at G, on the contrary, will be in-
visible, for the sun intervenes between them and
the earth : they are on the meridian of the spec-
tator about the same time as the sun, and are hidden
in his rays.

In three months, the earth has passed over one-
fourth of its orbit, and has arrived at B. Stars
about F now appear on the meridian at midnight ;
those at E, which previously occupied their places,
have descended toward the west ; while those about
G are just coming into sight in the east.

In three months more, the earth is situated at C,

THE EARTH. 93

and stars about G shine in the midnight sky, those
at F having, in their turn, vanished in the west ;

Fig. 36.

^ *■

* *

•■*■*

* *

Appearance of the Heavens in Different Seasons (Hind).

stars at E are on the meridian at noon, and conse-
quently hidden in daylight ; and those about H are
just making their appearance in the east. One revo-
lution of the earth will bring the same stars again
on the meridian at midnight.

Thus the earth's motion round the sun as a center
explains the varied aspect of the heavens in the
summer and winter skies.

94: THE SOLAR SYStEYt.

2. Yearly Path of the Sun Through the
Heavens. — We have spoken of the diurnal motion
of the sun. We shall now speak of its second ap-
parent motion, its yearly path among the stars, — the
ecliptic* If we look at Fig. 37, we can see how the
motion of the earth in its orbit is transferred to the
sun, and causes him to appear to travel in a fixed
path through the heavens. When the earth is in any
part of its orbit, the sun seems to us to be in the
point directly opposite. For example, when the
earth is in Libra (=^)t — autumnal equinox — the sun
is in Aries (T) — vernal equinox; when the sun
enters the next sign, Taurus (d), the earth has
passed on to Scorpio (m). Thus, as the earth moves
through her orbit, the sun seems to pass along the
opposite side of the ecliptic, making the circuit of
the heavens in a year, and returning, at the end of
that time, to the same place among the stars. The
ecliptic crosses the celestial equator at two points,
called the equinoxes. (See page 30).

* Tliis yearly movement of the sun among the fixed stars is not so apparent to us as
his daily motion, because his superior light blots out the stars. But if we notice a star
at the western horizon just at sunset, we can tell what constellation the sun is in : then
wait two or three nights, and we shall find that this star has set, and others have taken
its jilace. Thus we can trace the sun through the year in his path among the fixed
stars in the horizon.

t When we say " the earth is in Libra," we mean that a spectator placed at the sun
would see the earth in that part of the heavens which is occupied by the sign Libra,
while a spectator on the earth would see the sun, at the same time, in that part of the
heavens which is occupied by the sign Aries. Just so, on June 21st, the earth enters
Capricorn, and the sun, Cancer. It is customary, however, having reference solely to
the sun's place, to locate the vernal equinox in Aries, and the autumnal equinox in
Libra ; the summer solstice in Cancer, and the winter solstice in Capricorn. In figure
37, the terms "summer solstice," "autumnal equinox," etc., refer to the season
upon the earth, and to the location of the sun in the ecliptic, but are not the names of
those points on tlie earth's orbit. The zodiacal signs are inserted for convenience of
illustration, to show where the earth would be located by a solar spectator : the pupil
should remember, however, that the signs belong to the ecliptic— which is the projec-
tion of the plane of the earth's orbit upon the celestial sphere, and not to the earth's path.

the earth. 95

3. Apparent Movement of the Sun, North and
HouTH. — Having now spoken of the apparent diurnal
and annual motions of the sun, tliere yet remains a
third motion. In summer, at midday, the sun is
high in the heavens ; in winter, he is low, near the
southern horizon. In summer, he is a long time above
the horizon ; in winter, a short time. In summer, he
rises and sets north of the east and west points ; in
winter, south of the east and west points. This
subject is so intimately connected with the next,
that we shall understand it best when taken in con-
nection with that topic.

4. Change of the Seasons. Variation in Length
OP Day and Night.— By studying Fig. 37, and
imagining the various positions of the earth in
its orbit, let us try to understand the following
points :

I. Obliquity of the ecliptic. — The axis of the earth
is inclined 23^° from a perpendicular to its orbit.
This angle is called the obliquity of the ecliptic.

II. Parallelism of the axis. — In all parts of the
orbit, the axis of the earth is parallel to itself, and
points almost exactly toward the North Star (p. 217).

Nature reveals to us nothing more permanent than
the axis of rotation in anything that is rapidly
turned. It is its rotation that keeps a boy's hoop
from falling. For the same reason, a quoit retains
its direction when whirled, and stays in the same
plane at whatever angle it may be thrown. A man
slating a roof wishes to throw a slate to the ground ;
he whirls it perpendicularly, and it will strike on the
edge without breaking. So long as a top spins there

Ihe Urbit of the Earth as seen by an Observer at the Sun. (See Note, p. dU-)

THE EARTH. 9?

is no danger of its falling, since its tendency to keep
its axis of rotation parallel is greater than the attrac-
tion of the earth. This wonderful law would lead
us to think that the axis of the earth always points
in the same direction, even if we did not know it
from direct observation,

III. The rays of the sun strike the various portions
of the earth, when in any position, at different
angles. — When the earth is in Libra, and also when
in Aries, the sun's rays strike vertically at the equa-
tor, and more and more obliquely in the northern
and southern hemispheres, as the distance from the
equator increases, until at the poles they strike
almost horizontally.

This variation in the direction of the rays pro-
duces a corresponding variation in the intensity of
the sun's heat and light at different places, and
accounts for the difference between the torrid and
polar regions.

IV. As the earth changes its position the angle at
which the rays strike any portion is varied. — Take
the earth when it is in Capricornus (V5) and the sun
in Cancer (s). He is now overhead, 23|° iiorth of
the equator. His rays strike less obliquely in the
northern hemisphere than when the earth was in
Libra, Let six months elapse : the earth is now in
Cancer and the sun in Capricornus ; and he is over-
head, 23J° south of the equator. His rays strike less
obliquely in the southern hemisphere than before,
but in the northern hemisphere more obliquely.
These six months have changed the direction of the
sun's rays on every part of the earth's surface. This

98 THE SOLAR SYSTEM.

accounts for the difference in temperature between
summer and winter.*

\V. Equinoxes. — At the equinoxes, one half of each
hemispht3re is illuminated : hence the name Equinox
{cequtis, equal ; and 7iox, night). At these points of
the orbit, the days and nights are equal over the
entire earth, f each being twelve hours in length.

VI. Xorthern and soiitheiii hemispheres unequally
illuminated. — While one-half of the earth is con-
stantly illuminated, the proportion of the northern
or the southern hemisphere that is in daylight or
darkness varies at all times, except at the equinoxes.
When more than half of a hemisphere is in the light,
its days are longer than the nights, and vice versa.

VII. The seasons and the comparative length of
the days and nights in the South Temperate Zone, at
any time, are the reverse of those in the North Tem-
perate Zone, except at the Equinoxes, ichen the days
and nights are of equal length.

VIII. Tlie Summer Solstice. — At the time of the
summer solstice, which occurs about the 21st of
June, the sun is overhead 23V" north of the equator,
and if his vertical rays could leave a golden line on
the surface of the earth as it rotates, they would
mark the Tropic of Cancer, The sun is at its fur-
thest northern declination ; he ascends the highest
he is ever seen above our horizon, and rises and sets
north of the east and west points. He seems now
to stand still in his northern and southern course.

♦ The long nights and short days of winter, and the sliort nights and long days of
summer, are also important factors in producing this difference of temperature,
t Except a small space at each pole.

THE EARTH. ^ 99

and hence the name Solstice {sol, the sun ; sto, I
stand). The days in the north temperate zone are
longer than the nights. It is our summer, and the
21st of June is the longest day of the year.

In the south temperate zone it is winter, and the
shortest day of the year. The circle that separates
day from night extends 23|° beyond the north pole,
and if the sun's rays could in like manner leave a
golden line on that day, they would trace on the
earth the Arctic Circle. It is the noon of the long
six-months polar day. The reverse is true at the
Antarctic Circle, and it is there the midnight of the
long six-months polar night (p. 117).

IX. The Autumnal Equinox. — The earth crosses
the aphelion point about the 1st of July. It is then
at its furthest distance from the sun, which each
day rises and sets a trifle further toward the south,
passing through a lower circuit in the heavens. At
the time of the autumnal equinox,* the 22nd of Sep-
tember, he is on the equinoctial, and if his vertical
rays could leave a line of golden light, they would
mark on the earth the circle of the equator. It is
autumn in the north temperate zone and spring in
the south temperate zone. The days and nights
are equal over the whole earth, the sun rising at
6 A.M. and setting at G p.m., exactly in the east and
the west, where the equinoctial intersects the horizon.

X. TJie Winter Solstice. — The sun after passing
the equinoctial — "crossing the line" — sinks lower
toward the southern horizon each day. At the

* The precise time of the equinoxes and solstices varies each year, but wtthin a small
limit.

100 THE SOLAR SYSTEM.

time of the winter solstice, about the 21st of De-
cember, the sun is directly overhead 23|^° south of
the equator, and if his vertical rays could leave a
line of golden light, they would mark on the earth's
surface the Tropic of Capricorn. He is at his furthest
southern declination, and rises and sets south of the
east and west points. It is our winter, and the 21st
of December is the shortest day of the year.

In the south temperate zone it is summer, and
the longest day of the year. The circle that separates
day from night extends 23^° beyond the south pole,
and if the sun's rays in like manner could leave a
line of golden light, they would mark the Antarctic
Circle. It is there the noon of the long six-months
polar day. At the Arctic Circle the reverse is true ;
the rays fall 23|^° short of the north pole, and it is
there the midnight of the long six-months polar
night. Here again the sun appears to us to stand
still a day or two before retracing his course, and it
is therefore called the Winter Solstice.

XI, The Vernal Equinox. — The earth reaches its
perihelion about the 31st of December. It is then
nearest the sun, which rises and sets each day fur-
ther and further north, and climbs up higher in the
heavens at midday. Our days gradually increase in
length, and our nights shorten in the same propor-
tion. About the 21st of March the sun reaches the
equinoctial, at the vernal equinox. He is overhead
at the equator, and the days and nights are again
equal. It is our spring, but in the south temperate
zone it is autumn.

XII. Yearly path finished. — The earth moves on in

AT LOS ANGELES

THE EARTH. 101

its orbit through the spring and the summer months.
The sun continues his northerly course, ascending
each day higher in the heavens, and his rays becom-
ing less and less oblique. About the 21st of June, he
again reaches his furthest northern declination, and
is at the summer solstice.

We have thus traced the yearly path, and noticed
the course of the changing seasons, with the length
of the days and nights. The same series has been
repeated through the ages of the past, and will be
through the future till time shall be no more.

XIII. Distance of the earth from the sun varies. —
We notice, from what we have just seen, that we are
nearer the sun in winter than in summer by 3,000,000
miles. The obliqueness with which the rays strike
the north temperate zone at that time prevents our
receiving any special benefit from this favorable
position of the earth.

XIV. Southern summer. — The inhabitants of the
south temperate zone have their summer while the
earth is in perihelion, and the sun's rays are about
^V warmer than when in aphelion, our summer-time.
This will perhaps partly account for the extreme
heat of their season.* The southern winters, for a
similar reason, are colder ; and this makes the aver-
age yearly temperature about the same as ours.

XV. Extremes of heat and cold not at the solstices.
— We do not have our greatest heat at the time of the
summer solstice, nor our greatest cold at the winter

* Captain Sturt, in speaking of tlie extreme heat of Australia, says tliat niatclies acci-
dentally dropped on the ground were ignited. A recent official rejicrt states that, in
South Australia, January. 188:1, the heat, in the sun, was 180°— only 32° below the
boiling-point-

102 THE SOLAR SYSTEM.

solstice. After the 21st of June, the earth, already
warmed by the genial spring days, continues to
receive more heat from the sun by day than it radi-
ates by night : thus its temperature still increases.
On the other hand, after the 21st of December, the
earth continues to become colder, because it loses
more heat during the night than it receives during
the day.

XVI. Summer longer than icinter. — As the sun is
not in the center of the earth's orbit, but at one of
its foci, the earth, from the time of the vernal to that
of the autumnal equinox, passes through more than
one-half of its orbit. The summer is, therefore, longer
than the winter. The difference is enhanced by the
variation in the earth's velocity at aphelion and at
perihelion.

XVII. Varying velocity of earth. — From the time
of the vernal equinox until the earth passes its
aphelion, the solar attraction tends to check its
speed ; thence until the time of the autumnal equinox,
the attraction is partly in the direction of its motion,
and so increases its velocity. The same principle
applies when going to and from perihelion.

XYIII. Curious appearance of the smi at the north jxilc. — " To a person
standing at the north pole, the sun appears to sweep horizontally around
the sky every twenty-four hours, witliout any perceptible variation in its
distance from the horizon. It is, however, slowly lising, until, on the 21st
of June, it is t\\'enty-three degrees and twenty-eight minutes above the
horizon, a little more than one-fourth of the distance to the zenith. This
is the highest point it ever reaches. From this altitude, it slowly descends,
its track being represented by a spiral or screw with a very fine thread ;
and in the course of three months it worms its way down to the horizon,
which it reaches on the 22nd of September. On this day it slowly sweej^

THE EARTH. 103

around the sky, with its face half hidden below the icy sea. It still con-
tinues to descend, and after it has entirely disappeared it is still so near the
horizon that it carries a bright twilight around the heavens in its daily
circuit. As the sun sinks lower and lower, this twilight grows gradually
fainter, till it fades away. December 21st, the sun is 23^° below the
horizon, and this is the midnight of the dark polar winter. From this
date, the sun begins to ascend, and after a time it is heralded by a faint
dawn, which circles slowly around tlie horizon, completing its circuit every
twenty-four hours. This dawn grows gradually brighter, and on the 22nd
of March the peaks of ice are gilded with the first level rays of the six-
months day. The bringer of this long day continues to wind his spiral
way upward, till he reaches his highest place on the 21st of June, and his
annual course is completed."

XIX. Results, if the axis of the earth were perpen-
dicular to the ecliptic. — The sun would then always
appear to move through the equinoctial. He would
rise and set every day at the same points on the
horizon, and pass through the same circle in the
heavens, while the days and nights would be equal
the year round. There Avould be near the equator a
fierce torrid heat, while north and south the climate
would change into temperate spring, and, lastly,
into the rigors of a perpetual winter.

XX. Results, if the equator of the earth ivere per-
pendicular to the ecliptic. — Were this the case, to
a spectator at the equator, as the sun leaves the
vernal equinox, he would each day pass through
a smaller circle, until at the summer solstice he
would reach the north pole, when he would halt for
a time, and then slowly return in an inverse man-
ner.

In our own latitude, the sun would make his
diurnal rotations as described, his rays shining

104 THE SOLAR SYSTEM.

past the north pole further and further, until we
were included in the region of perpetual day, when
he would seem to wind in a spiral course up to the
north pole, and then return in a descending curve to
the equator.

Precession of the Equinoxes. — We have spoken
of the equinoxes as if they were stationary. Over
two thousand years ago, Hipparchus (see page 8)
found that they are slowly falling back along the
ecliptic. Modern astronomers fix the rate at about
50" of space annually. If we mark either point in
the ecliptic where the days and nights are equal
over the earth — at which time the plane of the
earth's equator passes exactly through the center of
the sun— we shall find the sun comes back to that po-
sition the next year, 50" (50 m. 20 s. of time) earlier.
This remarkable effect is called the Precession of the
Equinoxes, because the position of the equinoxes in
any year precedes that which they occupied the year
before. Since the circle of the ecliptic is divided
into 360^, it follows that the time occupied by the
equinoctial points in making a complete revolution
at the rate of 50".2 per year is 25,817 years.

Results of the Precession of the Equinoxes. —
In Fig. 37, we see that the plane of the earth's
equator is inclined to that of the ecliptic. In order
that the plane of the terrestrial equator should pass
through the sun's center 50" earlier, it is necessary
that the plane itself should slightly change its place.
The axis of the earth is always perpendicular to
this plane, hence it follows that the axis is not rigor-
ously parallel to itself. It varies in direction^ so that;

'itt& fiABT H.

105

the north pole describes a small circle in the starry
vault twice 23|^° in diameter.

To illustrate this, let us suppose that, after a series
of years, the position of the earth's equator has
changed from efh to g Kl (Fig. 38). The inclina-
tion of the axis of the earth, CP, to CQ, the pole of
the ecliptic, remains unchanged ; but as it must kirn

Change of Earth's Eqitator and Axis.*

with the equator, its position is moved from CP to
CP', and the pole of the earth slowly traces the por-
tion of a circle, PP'. The direction of this motion is
the same as that of the hands of a watch, or the re-
verse of the revolution of the earth. The position of
the north pole in the heavens is gradually but almost
insensibly changing. It is now distant from the north
polar star about 1\°. It will continue to approach it

* See in the Appendix a description of a simple apparatus for illustrating this
subject

lot) THE SOLAR SYST.EM.

until they are not more than half a degree apart.
In 12,000 years, Lyra will be our polar star : 4,500
years ago the polar star was the bright star Alpha
in the constellation Draco. (See p. 217).

As the right ascension of the stars is reckoned
eastward from the vernal equinox along the equi-
noctial, the precession of the equinoxes increases the
R. A. of the stars 50" per year. On this account, star
maps should be accompanied by the date of their
calculation, that they may be corrected to corre-
spond Avith this annual variation.

The constellations of the zodiac (see p. 31) are
fixed in the heavens, while the signs are simply
abstract divisions which move with the equinox.
When named, the sun was in both the sign and
the constellation Aries, at the time of the vernal
equinox ; but since then the equinoxes liave retro-
graded nearly a whole sign, so that now, while the
vernal equinox is in the»sign Aries, this sign cor-
responds to the constellation Pisces, which is there-
fore the first constellation in the zodiac (Fig. 86).

Causes of the Precession of the Equinoxes.—
Before commencing the explanation of this phenom-
enon, it is necessary to impress upon the mind a few
facts. (1.) The earth is n^)t a perfect sphere, but is
swollen at the equator. It is like a sphere covered
with padding, increasing in thickness from the poles
to the equator ; this gives it a turnip-like shape.
(2.) The attraction of the sun is greater the nearer
a body is to it. (3.) The attraction is not for the
earth as a mass, but for each particle separately.

In the figure, the position of the earth at the time

The earth.

1U7

of the winter solstice is represented. P is the north
pole ; a b, the plane of the ecliptic ; C, the center of
the earth ; C Q, a line perpendicular to the ecliptic ;
the angle Q C P, the obliquity of the ecliptic. In this
position, the equatorial padding of which we have
spoken — the ring of matter about the equator — is not
turned exactly toward the sun, but is elevated above
it. Now the attraction of the sun pulls the part D
more strongly than the center ; the tendency of this

Fig. S9.

hijlit^nce oftlie Sun oh a Mountain near the Equator.

is to bring D down to a, and to lift I toward b. The
attraction for C is greater than for I, so it tends to
draw C away from I, and, as at the same time D
tends toward a, to pull I up toward b. The effect
of this, one would think, would be to change the
inclination of the axis C P toward C Q, and make it
more nearly perpendicular to the ecliptic. This
would be the result if the earth were not rotating
upon its axis.

Let us consider the case of a mountain near the
equator. This, if the sun did not act upon it, would
pass through the curve H D E in the course of a
semi-rotation of the earth. But, it is nearer the sun

108 THE SOLAR SYSTEM.

than is the center C ; the attraction therefore tends
to pull the mountain downward and tilt the earth
over, as we have just described ; so the mountain
will pass through the curve H/g, and, instead of
crossing the ecliptic at E, will cross at g, a little
sooner than it otherwise would. The same influence,
though in a less degree, obtains on the opposite side
of the earth. The mountain passes around the earth
in a curve nearer to h, and crosses the ecliptic a
little earlier.

The same reasoning will apply to each mountain
and to all the jjrotuberant mass near the equatorial
regions. The final effect is slightly to turn the
earth's equator so that it intersects the ecliptic
sooner than it would were, it not for this attraction.
At the summer solstice, the same tilting motion is
produced. At the equinoxes, the plane of the earth's
equator passes through the center of the sun, and
therefore there is no tendency to change of position.
As the axis C P must move with the equator, it
slowly revolves, keeping its inclination unchanged,
around C Q, the pole of the ecliptic, describing, in
about 2G,000 years, a small circle twice 23^° in diam-
eter.

Precession illustrated in the spinning of a top.
— This motion of the earth's axis is singularly illus-
trated in the spinning of a top, and the more so
because the forces are of an opposite character to
those which act on the earth, and thus produce an
opposite effect. We have seen that, if the earth had
no rotation, the sun's attraction on the "padding"
at the equator would bring C P nearer to C Q, but

THE EAflTH.

109

Fig. UO.

that, in consequence of this rotation, the effect
really produced, is that C P, the earth's axis, slowly
revolves around C Q, the pole of the heavens, in a
direction opposite to that of rotation.

In Fig. 40, let C P be the axis of a spinning top,
and C Q the vertical line.
The direct tendency of
the earth's attraction is
to bring C P further from
C Q (or to make the top
fall), and if the top were
not spinning this would
be the result ; but, in con-
sequence of the rotary
motion, the inclination
does not sensibly alter
(until the spinning is re-
tarded by friction), and so C P slowly revolves
around C Q in the same direction as that of rotation.
-^Nutation {nutatio, a nodding). — We have noticed
the sun as producing precession ; the moon has,
however, treble his influence ; for although her mass
is not ^-^.ooV.o-ro part that of the sun, yet she is 400
times nearer and her effect correspondingly greater.*
The moon's orbit does not lie parallel to the ecliptic,
but is inclined to it. Now the sun attracts the moon,
and disturbs its path, as he would that of the moun-
tain we have supposed, and the effect is the same.
The intersections of the moon's orbit with the ecliptic
travel backward, completing a revolution in about
18 years.

spinning of a To^).

See the Differential effect of the Sun and the Moon, under the head of the Tides.

110 'IHE SOLAR SYSTEM.

During half of this time, the moon's orbit is in-
clined to the ecliptic in the same way as the earth's
equator ; during the other half, it is inclined in the
opposite way. In the former state, the moon's at-
tractive tendency to tilt the earth is very small, and
the precession is slow ; in the latter, the tendency is
great, and precession goes on
..■^^^\:7<^ rapidly. The consequence of this

// ^x) is, that the pole of the earth is

\ ^ irregularly shifted, so that it

'^ \ travels in a slightly curved line,

/ . giving it a kind of "wabbling"

^ rr ox "nodding" motion, as shown

^''\:;yr:::^.../^ — though greatly exaggerated —

Path of the North Pole in the in Fig. 41. The obliquity of the

Heavens. ,. ,. -1*1 • t ,-,^^0

ecliptic, which we consider 23|
(23° 27' 15", Jan. 1, 1884. See p. 29), is the mean of
the irregularly curved line and is represented by
the dotted circle.

Periodical change in the obliquity of the
ECLIPTIC. — Although it is sufficiently near for all
general purposes to consider the obliquity of the
ecliptic invariable, yet this is not strictly the case.
It is subject to a small but appreciable variation of
about 46" per century. This is caused by a slow
change of the position of the earth's orbit, due to the
attraction of the planets. The effect of this move-
ment is gradually to diminish the inclination of the
earth's equator to the ecliptic (the obliquity of the
ecliptic). This will continue for a time, when the
angle will as gradually increase ; the extreme limit
of change being only 1° 21'. The orbit of the earth

THE EARTfl. , 111

thus vibrates backward and forward, each oscillation
requiring a period of 10,000 years.

The change is so intimately blended, in its effect
upon the obliquity of the ecliptic, with that caused
by precession and nutation, that they are separable
only in theory ; in fact, they all combine to produce
the waving motion we have already described. As
a consequence of this variation in the obliquity of
the ecliptic, the sun does not now come so far north
nor decline so far south as formerly ; while the
position of all the terrestrial circles — the Troj)ics of
Cancer, Capricorn, etc. — is constantly but slowly
changing. As the result of this variation in the
position of the orbit, some stars which were once
just south of the ecliptic are now north of it, and
others that were just north are now a little further
north ; thus the latitude of these stars is gradually
changing.

Change in the major axis (line of apsides) of
THE earth's orbit. — Besides all the changes in the
position of the earth in its orbit due to precession,
etc., the line connecting the aphelion and peri-
helion points of the orbit itself is slowly revolving.
The consequence of this is a variation in the length
of the seasons at different periods of time.

In the year 3958 b.c, the earth was in perihelion at the time of the au-
tumnal equinox, so that the summer and autumn seasons were of equal
length, but shorter than the winter and spring seasons, which were also equal. *

* Tliere is much (liscreiiancy in the views held concerning tlie Great Year of llie
a.stnmomers, as it is often called. (See Steele's Geology, pp. 272-3, note.) The state-
ment made in the text is that held by Loekyer, Hind, and others. The dates are those
given hy Chambers in his Descriptive Astronomy (3rd Edition), where the subject is fully
described.

112 THE SOLaH system.

In the year 1267 A.D., the earth was in j^erihelion at the time o^
the winter solstice, December 21, instead of January 1st, as now ; the
spring quarter was therefore equal to the summer one, and the autumn
quarter to the winter one, the former being the longer. In the year 6493
A. D. , the earth will be in perihelion at the time of the vernal equinox ;
summer will then be equal to autumn and winter to spring, the former
seasons being the longer. In the year 11719 A.D., the earth will be in
perihelion at the time of the summer solstice : finally, in 16945 A.D., the
cycle will be completed.and the autumnal equinox will again coincide with
the earth's perihelion.

Permanence in the Midst of Change. — We thus
see that the ecliptic is constantly modifying its ellip-
tical shape ; that the orbit of the earth slowly oscil-
lates upward and downward : that the north pole
steadily turns its long index-finger over a dial that
marks 26.000 years ; that the earth, accurately
poised in space, gently nods and bows to the attrac-
tion of sun, moon, and planets. * Thus changes are
taking place that would ultimately entirely reverse
the order of nature. But each of these variations
has its bounds, beyond which it cannot pass. The
promise made to man is that, ''while the earth re-
maineth, seed-time and harvest, and cold and heat,
and summer and winter, and day and night shall
not cease." The modern discoveries of astronomy
prove conclusively that the seasons are to be perma-
nent ; that the Creator, amid all these transitions,
has ordained the means of carrying out His promise
through all time.

Refraction. — The atmosphere extends above the

♦ These oscillations extend throughout the whole planetary system, the j^erioda
varjing from 50,000 to 2,000,000 years. " Great clocks of eternity, which beat ages as
ours beat seconds." — Kewcomb's Astronomy, page 95.

THE EAKTH.

113

earth about 500 miles (Physics, p. IIG). Near the
surface it is dense, while in the upper regions it is

Befraction.

exceedingly rare. The rays of light from the heav-
enly bodies passing through these different layers
are turned downward toward a perpendicular more
and more as the density increases. According to a
well-known law of optics (Physics, p. 150), if the ray
of light from a star were bent in fifty directions
before entering the eye, the star would nevertheless
appear to be in the line of the one nearest the eye.
The effect of this is, that the apparent place of a
heavenly body is higher than the true place. The
sun at S (Fig. 42), were it not for the atmosphere,
would send a direct ray to L. Instead, the ray at
A is refracted downward, and would then enter the
eye at N ; passing, however, through a layer of a (iif-

114 THE SOLAR SYSTEM.

ferent density at B, it is again bent, and meets the
eye of the observer at C. He sees the sun, not in the
direction of the curved line C B A S, but in that of
the straight line C B SI

The amount of refraction varies with the tempera-
ture, moisture, and other conditions of the atmos-
phere. It is zero for a body in the zenith, and in-
creases gradually toward the horizon (as the thick-
ness of the intervening atmosphere increases), where
it is sometimes as much as 35'.

Fi'j. hi-

Change of Place and Appearance of the Sun
AND. THE Moon. — The sun may be really below the
horizon, and yet seem to be above it. For example,
on April 20, 1837, the moon was eclipsed before the
sun had set. The mean diameter of both the sun
and the moon being about half a degree, it follows
that when we see the lower edge of either of these
luminaries apparently just touching the horizon, in

THE EARTH.

reality the whole disk is beloiv it, and would be
hidden were it not for the refraction. The day is
consequently materially lengthened.

Fig.iU.

Dejonnation qf tlie, Sun near tJie Horizon.

The sun and the moon often ai^^^ear flattened when
near the horizon. The rays from the lower edge
pass through a denser layer of the atmosphere, and
are therefore refracted more than those from the
upper edge : the effect of this is to make the vertical
diameter appear less than the horizontal, and so to
distort the figure of the disk into an oval shape.

The dim and hazy appearance of the heavenly
bodies when near the horizon is caused not only by
the rays of light having to pass a greater distance
through the atmosphere, but also by their traversing
the denser part. The intensity of the solar light is
so greatly diminished by going through the lower
strata, that we are then enabled to look upon the
s\in without being dazzled by his brilliant beams.

116 THE SOLAR SYSTEM.

Twilight. — The glow of light after sunset and
before sunrise, which we term twilight, is caused by
the refraction and the reflection of the sun's rays by
the atmosphere. For a time after the sun has really
set, the refracted rays continue to reach the earth ;
but when these have ceased, he still illuminates the
clouds and upper strata of the air, just as he may
be seen shining on the summits of lofty moun-
tains long after he has disappeared from the view of
the inhabitants of the plains below. The air and
clouds thus illuminated reflect back a part of the
light to the earth. As the sun sinks lower, less light
reaches us, until reflection ceases and night ensues.
The same thing occurs before sunrise, only in
reverse order.

Twilight is usually reckoned to last until the de-
pression of the sun below the horizon amounts
to 18° ; this, however, varies with the latitude,*
seasons, and condition of the atmosphere. In the
latitude of New York, twilight lasts from 1| to 2
hours, the shortest twilight being in winter, and the
longest in summer. Strictly speaking, in the latitude
of Greenwich there is no true night for a month
before and after the summer solstice, but constant
twilight from sunset to sunrise. The sun is then
near the Tropic of Cancer, and does not descend so
much as 18° below the horizon during the entire
nigi>t. At the equator the length of the evening
twilight is about 1^ hours, and remains almost con-

* When the sun's patli is very oblique to the horizon, a longer time is required for
the sun to descend or ascend the requisite vertical distance of IS" from the horizon j
and a shorter time, when his path is niore nearly perpendicular.

THE EARTH. 117

stant the entire year. The twilight is longest toward
the poles, where the night of six months is shortened
by an evening twilight of about fifty days and a
morning one of equal length.

Diffused Light, — The diffused light of day is pro-
duced in the same manner as that of twilight. The
atmosphere reflects and scatters the sunlight in
every direction. Were it not for this, no object
would be visible to us out of direct sunshine ; every
shadow of a passing cloud would be pitchy dark-
ness ; the stars would be visible all day ; no window
would admit light except as the sun shone directly
through it, and a man would require a lantern to go
around his house at noon.

The blue light reflected to our eyes from the at-
mosphere above us, or, more correctly, from the
vapor in the air, produces the optical illusion we
call the sky. Were it not for this, every time we
cast our eyes upward we should feel like one gazing
over a dizzy precipice ; while now the crystal dome
of blue smiles down upon us so lovingly and beauti-
fully that we call it heaven.

Aberration of Light. — We have seen that the
places of the heavenly bodies are apparently changed
by refraction. Besides this, there is another change
due to the motion of light combined with the motion
of the earth in its orbit. For example : the mean
distance of the earth from the sun is about 93,000,000
miles, and since light travels a little over 186,000
miles per second, it follows that the time occupied
by a ray of light in reaching us from the sun is
about 8^ min. (8 min. 18 sec.) ; so that, in fact,

118

THE SOLAK SYSTEM.

(1), we do not see the sun as it is, but as it was 8|
minutes ago. And since, during this time, the earth
has moved in its orbit about 20|" (2), we do not see
that luminary in the exact place it occupies at the
time of observation.

Illustration. — Suppose a ball let fall from a point
P, above the horizontal line A B, and a tube, of
which A is the lower extremity, placed to receive it.

AberrcUion of Light.

If the tube were fixed, the ball would strike it on the
lower side ; but if the tube were carried forward in
the direction A B, with a velocity properly adjusted
at every instant to that of the ball, while preserving
its inclination to the horizon, so that when the ball,
in its natural descent, reached B, the tube would
have been carried into the position B Q, it is evident
that the ball throughout its whole descent would be
found in the tube ; and a spectator referring to the

THE EARTH. 119

tube the motion of the ball, and carried along with
the former, unconscious of its motion, would think
that the ball had been moving in an inclined direc-
tion, and had come from Q.

A very common illustration may be seen almost
any rainy day. Choose a time when the air is quiet,
and the drops large. Then, if you stand still, you
will see that the drops fall vertically ; but if you
walk forward, you will see the drops fall as if they
were meeting you. If, however, you walk backward,
you will observe that the drops fall as if they were
coming from behind you. We thus see that the
drops have an apparent as well as a real motion.

The general effect of aberration is to cause
each star apparently to describe in the course of a
year a minute ellipse, the central point of which is
the place thes star would actually occupy were our
globe at rest.

Parallax is the difference in the direction of an ob-
ject as seen from two different places. For a simple
illustration, hold your finger before you in front of
the window. Upon looking at it with the left eye
only, you will locate your finger at some point on
the window ; on looking with the right eye only, you
will locate it at an entirely different point. Use your
eyes alternately and quickly, and you will be aston-
ished to see how your finger will seem to change its
place. Now, the difference in the direction of your
finger as seen from the two eyes is its parallax.

In astronomical calculations, the position of a
body as seen from the earth's surface is called its
apparent place, while that in which it would be seen

130

THE SOLAR SYSTEM.

from tke center of the earth is called its true
place. Thus, in Fig. 46, a star is seen by the ob-
server at in the direction OP; if it could be
viewed from the center R, its direction would be
in the line RQ. It is therefore seen from O at a
point in the heavens below its position in reference
to R. From looking at the cut, we can see (1)^ that

Parallax.

the parallax of a star near the horizon is greatest,
while it decreases gradually until it disappears alto-
gether at the zenith, since an observer at O, as well
as one at R, would see the star Z directly overhead ;
and (2), that the nearer a body is to the earth the
greater its parallax becomes.

It has been agreed by astronomers, for the sake of
imiformity, to correct all observations so as to refer

THE EARTH. 131

them to their true places as seen from the center of
the earth. Tables of parallax are constructed for
this purpose. The question of parallax is also of
great importance, because as soon as the parallax of
a body is accurately known, its distance, diameter,
etc., can be determined. (See Celestial Measure-
ments.)

Horizontal Parallax is the parallax of a body
when at the horizon. It is, in fact, the ea?'th's semi-
diameter as seen from the body. In Fig. 4G, the
parallax of the star S is the angle O S R, which is
measured by the line O R — the semi-diameter of the
earth. The su7i's horizontal parallax is the angle
subtended (measured) by the earth's semi-diameter
as seen from that luminary. As the moon is nearest
the earth, its horizontal parallax is greater than that
of any other heavenly body.

Annual Parallax. — The fixed stars are so dis-
tant from the earth that they exhibit no change of
place when seen from different parts of the earth.
The lines O S and R S are so long that they are
apparently parallel. Astronomers, therefore, instead
of taking the earth's semi-diameter, or 4,000 miles,
as the measuring tape, observe the position of the
fixed stars at opposite points in the earth's orbit. '
This gives a change in place of 186,000,000 miles.
The variation of position which the stars undergo at
these remote points is called their annual parallax.

122

THE SOLAR SYSTEM.

THE MOON.

New Moon, (9. First Quarter, «. Full Moon, ®. Last Quarter, #.

Motion in Space. — The orbit of the moon, consid-
ering the earth as fixed, is an ellipse of which our
planet occupies one of the foci. Her distance from

J'ath of the Moon.

the earth, therefore, varies incessantly. At perigee
(peri, near ; ge, the earth), she is 26,000 miles nearer
than in apogee {ajm, from ; ge, the earth) : the mean
distance is about 239,000 miles. To reach the moon.

THE MOON. 123

would require a chain of thirty globes equal in size
to the earth. An ordinary express-train would take
about a year to accomplish the journey.

The moon completes her revolution {sidereal)
around the earth in about 27| days ; but, as the
earth is constantly passing on in its orbit around
the sun, it requires over two days longer before the
moon comes into the same position with respect to
the sun and the earth, thus completing a synodic
revolution, or lunar month (29^ days).

The real path of the moon is the result of her
own motion and the onward movement of the earth.
The two combined produce a wave-like curve that
crosses the earth's path twice each month ; this,
owing to its small diameter compared with that of
the earth's orbit, is always concave toward the sun.
As the moon constantly keeps the same side turned
toward us, it follows that she must rotate on her
axis once each month.

Dimensions. — The moon's diameter is about 2,160
miles. To equal the earth, would require fifty globes
the size of the moon. The apparent size varies with
the distance ; the mean is, however, about one-half
a degree, nearly the same as that of the sun. The
moon always appears larger than she really is, on
account of her brightness. This is the effect of what
is termed in optics Irradiation.'^ For the same
reason it is often noticed that the crescent moon
seems to be a part of a larger circle than the rest of

* To illustrate this principle, cut two circular pieces of the same size, one of black
and the other of white paper. The white circle, wheu held in a bright light, will appear
much larger than the black cue.

124 tfiE SOLAR SYSTEM.

the moon. The moon appears larger on the horizon
than when high in the sky. This, however, is a
mere illusion. * By an examination of the cut, it is
easily seen that the moon is 4,000 miles nearer when
on the zenith than when at the horizon.

Besides these general variations in size, the moon
varies in apparent size to different observers. Much

Fig. i8.

The Distance of the Moon at fhe Horizon and at the Zenith.

amusement may be had in a large party or class by
a comparison of her apparent magnitude. The esti-
mates will differ from a small saucer to a wash-tub.

Librations {lihrans, swinging). — Though the moon
presents the same hemisphere to us, there are three
causes which enable us to see, in all, about 7^(10 of
her entire surface.

1. The axis of the moon is inclined a little to her
orbit, as also her orbit is inclined to the earth's orbit ;
so, when her north pole leans alternately toward and

• At the horizon we compare her with various terrestrial objects which lie between her
and us, while aloft we have no association to guide us in judging of her distance, and we
are led to underrate her size. If we look at her when near the horizon, through a roll of
paper, or the hands held tube-wise, this illusion will vanish.

fiHE MOON. 125

from the earth, we see sometimes past her north and
sometimes past her south pole. This is called lihra-
tion in latitude.

2. The moon's rotation on her axis is always per-
formed in the same time, while her movement along
her orbit is variable ; hence we occasionally see a
little further around each limh (outer edge) than at
other times. This is called libratioti in longitude.

3. The size of the earth is so much greater than
that of the moon, that an observer, by the rotation
of the earth, or by going north or south, can see fur-
ther around the limbs.

Light and Heat. — If the whole sky were covered
with full moons, they would scarcely make daylight,
since the brilliancy of the moon does not exceed
¥00^0-00 that of the sun. That portion of the moon's
surface which is directly exposed to the sun has
been thought to be highly heated, possibly to the
degree of boiling water,* but this is now considered
very improbable.

Whether or not the moon radiates any heat to the
earth has long been a mooted question. The best
authorities, at present, estimate the average heat of
the moonbeam at about -sso}^-^ that we receive from
the sun, or sufficient to raise the temperature of a
sensitive black-bulb thermometer j,-^q of a degree.

* Prof. Langley is now engaged in an exhaustive series of experiments upon this sub-
ject, using his famous "bolometer" — an instrument capable of detecting a difference of
.00001° C. The result of his observations upon Mount Whitney (18«1) showed that " mer-
cury would remain a solid under the vertical rays of a tropical sun were radiation into
space wholly unchecked, and that the temperature of a planet may, and not improlably
does, depend far less upon its neighborhood tf), or remoteness from, the sun, than upon
the constitution of its atmosphere." As the moon has no air-blanket, it is therefore very
doubtful whether its surface ever reaches a temperature of —100° F.

tflE SOLAR SYSTEM.

It would be absurd to suppose that this slight amount
of heat can ha-ve any appreciable effect upon tlie
weather.

Center of Gravity. — It is thought that possibly the
center of gravity of the moon is not exactly at her
center of magnitude, but about thirty-three miles
beyond, the lighter half being toward us. If that be
so, this side is equivalent to a mountain of that
enormous height ; and if water and air exist upon
the moon, they cannot remain on this hemisphere,
but must be confined to the side which is forever
hidden from our view.

Atmosphere of the Moon. — The existence of an
atmosphere upon our satellite is at present an open
question. If there be any, it must be extremely
rarefied, perhaps as much so as that in the vacuum
obtained in the receiver of our best air-pumps.
Appearance of the Earth to Lunarians. — If there

be any lunar inhabitants
on the side toward us, the
earth must present to
them all the phases which
their world exhibits to us,
only in a reverse order.
When we have a new
moon, they have a full
earth, a bright full-orbed
moon fourteen times as
large as ours. The lunar
inhabitants upon the side
opposite to us of course
never see our earth, unless they take a journey to

F'a. f'.

Appeararux of the Earth as seen frorn the
Moon.

THE MOON. 12'?

the regions from whence it is visible, to behold this
wonderful spectacle. Those living near the limbs of
the disk might, however, on account of the lihra-
tions, get occasional glimpses of it near their hori-
zon.

The Earth-Shine. — For a few days before and after
new moon, we may distinguish the outline of the
unillumined portion of the moon. In England, it is
popularly known as "the old moon in the new
moon's arms." This reflection of the earth's rays
must serve to keep the lunar nights quite light, even
in new earth.

Phases of the Moon. — The phases of the moon
show conclusively that it is a dark body, which
shines by reflecting the light it receives from the
sun. Let us compare its various appearances with
the positions indicated in the figure.

(1.) We see the moon as a delicate crescent in the
west just after sunset, as she emerges from the sun's
rays at conjunction. She soon sets below the hori-
zon. Half of the surface is illumined, but only a
slender edge with the horns turned from the sun is
visible to us. Each night the crescent broadens, the
moon recedes about l'S° further from the sun, and
sets correspondingly later, until at quadrature half
of the enlightened hemisphere is turned toward us,
and the moon is said to be in her first quarter.

(2.) The moon, continuing her eastern progress
round the earth, becomes gibbous* in form, and,
about the fifteenth day from new moon, reaches the
point in the heavens directly opposite to that which

* Gibbous means more than a half and less tliau the whole of a circle.

i2J

THE SOLAR SYSTEM.

the sun occupies. She is then in opposition, the
whole of the illumined side is turned toward us, and
we have a full moon. She is on the meridian at mid-

Fig. BO.

Phases of the Moon.

night, and so rises in the east as the sun sets in the
west, and vice versa.

THE MOON. 129

(3.) The moon, passing on in her orbit from oppo-
sition, presents phases reversed from those of the
second quarter. The proportion of the illumined
side visible to us gradually decreases ; she becomes
gibbous again ; rises nearly an hour later each even-
ing, and in the morning lingers high in the western
sky after sunrise. She now comes into quadrature,
and is in her third quarter.

(4). From the third quarter, the moon turns her
enlightened side from us and decreases to the
crescent form again ; as, however, the bright hemi-
sphere constantly faces the sun, the horns are
pointed toward the west. She is now seen as a
bright crescent in the eastern sky just before sun-
rise. At last, the illumined side is completely turned
from us, and the moon herself, coming into conjunc-
tion with the sun, is lost in his rays. To accomplish
this journey through her orbit from new moon
to new moon again, has required 29|^ days — a lunar
month.

Moon Runs High or Low. — All have, doubtless,
noticed that, in the long nights of winter, the full
moon is high in the heavens, and continues a long
time above the horizon ; while in midsummer she is
low, and remains a much shorter time above the
horizon. This is a wise plan of the Creator, which is
seen yet more clearly in the arctic regions. There,
the moon, during the long summer day of six
months, is above the horizon only her first and fourth
quarters, when her light is least ; but during the
tedious winter night of equal length, she is continu-
ally above the horizon for lier second and third

130 THE SOLAR SYSTEM.

quarters. Thus, in polar regions, the moon is never
full by day, but is always full every month in the
night.

We can easily understand these phenomena when
we remember that the new moon is in the same
quarter with, and the full moon is in the opposite
quarter from, the sun. AVhen, therefore, the sun
sinks low in the southern sky the full moon rises
high, and when the sun rises high the full moon
sinks low.

Harvest Moon. — While the moon rises, on the
average, 50 m. later each night, the exact time
varies from less than half an hour to a full hour.
Near the time of the autumnal equinox the moon, at
her full, rises about sunset for a number of nights
in succession. This produces a series of brilliant
moonlight evenings. It is the time of harvest in
England, and hence has there received the name of
the Harvest Moon. In the following month (October),
the same occurence takes place ; it is then termed the
Hunter's Moon.

The cause of this phenomenon lies in the fact that
the moon's path is variously inclined to the horizon
at different seasons of the year.* When, at the time
of rising, the full moon is near the vernal equinox,
the angle her path makes with the horizon is least,
and when she is near the autumnal equinox it is
greatest. In the former case, the moon, moving
eastward each day about 13°, will descend but little
below the horizon, and so for several successive

* Besides this reason, we should remember that the motion of the moon is slowest
at apogee and fastest at perigee. (See note, p. 302.)

THE MOON.

131

Fig. 51.

evenings will rise at about the same hour. In the

latter, she will descend much further each day and
thus will rise much later each night. The least pos-
sible variation in the hour of rising is 17 minutes, —
the greatest is 1 hour and 16 minutes.

In Figure 51, let S represent the sun ; E, the earth ;
M, the moon ; C F, the moon's path around the earth
when the autumnal equinox is in the eastern horizon ;
E D, when the vernal equinox is in the eastern hori-
zon ; A M B S, the horizon ; and M cZ =M & = 13°, the
distance the moon
moves each day.
When passing
along the path C
F, the moon sinks
below the horizon
the distance a h,
and when moving
along the path E
D, only the dis-
tance c d. It is
obvious that be-
fore the moon can
rise in the former
case, the horizon

must be depressed the distance a b, and in the latter
only c d ; and the moon will rise each evening cor-
respondingly later in the one and earlier in the other.

Cause of "Dry Moon," and "Wet Moon." — At new
moon, when the bright crescent lies nearly perpen-
dicular to the horizon, the moon is popularly called
a wet moon, and when it is almost horizontal, the

The Harvest Moon.

132 THE SOLAR SYSTEM.

moon is termed a dry moon. The cause of this
change in the crescent is astronomical, and not
meteorological. The form of the crescent has there-
fore no connection with the weather. A little reflec-
tion will show us that the horns, or cusps, of the
new moon must point from the sun. As the ecliptic
(from which the moon's path varies but slightly) is
differently inclined to the horizon at various times
of the year, this will give the crescent a different
position with reference to the horizon (p. 29).

Nodes. — The orbit of the moon is inclined to the
ecliptic about 5°, the points where her path crosses
it being termed nodes. The ascending node ( 8 ) is
the place where the moon crosses in coming above
the ecliptic, or toward the north star ; the descending-
node ( 3 ) is where it passes below the ecliptic. The
imaginary line connecting these two points is called
the ''line of the nodes."

Occultation. — The moon, in the course of her
monthly journey round the earth, frequently passes
in front of the stars or planets, which disappear on
one side of her disk and reappear on the other. This
is termed an occultation, and is of practical use in
determining the difference of longitude between
various places on the earth.

Lunar Seasons; Day and Night, Etc. — As the
moon's axis is so nearly perpendicular to her orbit,
she cannot have any change of seasons. During
nearly fifteen of our days, the sun pours down his
rays unmitigated by any atmosphere to temper them.
To this long, torrid day succeeds a night of equal
length and polar cold.

Fig. es

134 THE SOLAK SYSTEM.

How strange the lunar appearance would be to us !
The disk of the sun seems sharp and distinct. The
sky is black and overspread with stars even at mid-
day. There is no twilight, for the sun bursts
instantly into day, and, after a fortnight's glare, as
suddenly gives place to night ; no air to conduct
sound ; no clouds ; no winds ; no rainbow ; no blue
sky ; no gorgeous tinting of the heavens at sunrise
and sunset ; no delicate shading ; no soft blending
of colors, but only sharp outlines of sun and shade. *

The nights of the visible hemisphere must be
brilliantly illuminated by the earth, whose phases
"serve well as a clock — a dial all but fixed in the
same part of the heavens, like an immense lamp,
behind which the stars slowly defile along the black
sky."

Telescopic Features. — Even with the naked eye,
we see on the moon's surface bright spots (the sum-
mits of lofty mountains, gilded by the first rays of
the sun), and darker portions — Ioav plains yet lying
in comparative shadow. The telescope reveals to
us a region torn and shattered by fearful though
now ext"^ctt volcanic action. Everywhere the

* The moon is a fossil world, an ancient cinder, a ruined habitation perpetuated only
to admonish the earth of her own impending fate, and to teach her occupants that another
home must be provided, whicli frost and decay can never invade. The moon was once
the seat of all the varied and intense activities that now characterize the surface of our
earth. At one time its physical condition was like that of tlie parent eartli from which
it had just been seiiarated : but, being smaller, it cooled faster, and its geologic periods
were coiTcsjiondingly shorter. Its life-age was perhaps reached while the earth was yet
glowing.— Read Wini-hell's Geology of tlie Stars.

t Several distinguished astronomers assert, however, that the crater Linnx-us lias
undergone noticeable transformations. Its sides seem to liave fallen in, and the interior
to have become filled up, as if by a new eruption. It is said to present an appearance
similar to that of the Sea of Serenity. Other marked changes are said to have been
discovered from time to time, on the moon's surface^ but they are not generally ac^

THE MOON.

135

Fig. 53.

Telescopic View of the Mootj,

136

THE SOLAR SYSTEM.

crust is pierced by craters, whose irregular edges
and rents testify to the convulsions our satellite has
undergone.

Mountains. — The heights of more than 1,000 of
the lunar mountains have been measured, some of
which exceed 25,000 feet. When the sun's rays strike
one of these mountains obliquely, the shadow is as
distinctly perceived as that of an upright staff when
placed opposite the sun. Some of the elevations are
insulated peaks that shoot up from the center of
circular plains ; others are mountain ranges extend-

CopenUtug.

ing hundreds of miles. Most of the lunar heights
have received names of men distinguished in science.
Thus we find Plato, Aristarchus, Copernicus,* Kepler,

credited. For an interesting discussian of this subject, read a chapter entitled "A Xew
Crater in the Moon," in Proctor's Poetry of Astronomy.

* This is one of the grandest of the lunar craters. It is situated on the tip of the nose
of the " Man in the Moon." Its diameter is forty-six miles, and its encircling rampart
rises 12,000 feet above the interior plateau, in tjie midst of which $t^nds a group ci
fpnps, one 2,400 feet in heighj.

$HE MOON. 131?

and Kewton, associated, however, with the Apen-
nines, Carpathians, etc.

Gray Plains, or Seas. — These are analogous to
our prairies. They were formerly supposed to be
sheets of water, but they exhibit the uneven ap-
pearance of a plain, instead of the regular curve of a
sea. The former names have been retained, and we
find on lunar maps the Sea of Tranquillity, the Sea
of Nectar, Sea of Serenity, etc.

Rills, Luminous Bands. — The latter are long,
bright streaks, irregular in outline and extent, which
radiate in every direction from Tycho, Kepler, and
other mountains ; the former are similar, but are
sunken, and have sloping sides, and were at first
thought to be ancient river-beds. Their nature is a
mystery.

Craters constitute the most curious feature of the
lunar landscape. They are of volcanic origin, and
usually consist of a cup-like basin, with a conical
elevation in the center. Some of the craters have a
diameter of over 100 miles, and are great walled plains,
sunk so far behind huge, volcanic ramparts that the
lofty wall surrounding an observer at the center
would be beyond his horizon. Other craters are
deep and narrow, — as Newton, which is said to be
about four miles in depth, — so that neither earth nor
sun is ever visible from a great part of the bottom.
The appearance of these craters is strikingly shown
in the accompanying view (Fig. 53) of the region to
the southeast of Tycho.

138

THE SOLAR SYSTEM.

ECLIPSES.

Eclipse of the Sun. — If the moon should pass
through either node at or near the time of conjunc-
tion, or new moon, she would necessarily come be-
tween the earth and the sun, for the three bodies
are then in the same straight line. This would cause

Fig. ''!">.

Theory of a Total and a Partial Eclipse of the Sun.

an eclipse of the sun. If the moon's orbit were in
the same plane as the ecliptic, an eclipse of the sun
would occur at every new moon ; but as the orbit is
inclined, it can occur only at or near a node.

Fid. 5^.

iln-oii/ uj an. Aiiaalar Kdipic uf l/ie 6u:t.

The eclipse may be partial, total, or annular.
-In Fig. 55, we see where the dark shadow {umbra)

ECLIPSES. 139

of the moon falls on the earth and obscures the
entire body of the sun. To the persons within that
region, there is a total eclipse; the breadth of this
space is not large, averaging only 140 miles. Be-
yond this umbra, there is a lighter shadow, penum-
bra, {pene, almost; umbra, a shadow), where only a
portion of the sun's disk is obscured. Within this
region, there is a partial eclipse. To those persons
living north of the equator and of the umbra, the
eclipse passes over the lower limb of the sun ; to
those south of the umbra, it passes over the upper
limb.'"' When the eclipse occurs exactly at the node,
it is said to be centixil. If the eclipse takes place
when the moon is at apogee, her apparent diameter
is less than that of the sun ; as a consequence, her
disk does not cover the disk of the sun, and the vis-
ible portions of that luminary appear in the form of
a ring (annulus) ; hence there is an annular eclipse
in all those places comprised within the limits of the
cone of shadow prolonged to the earth.

General facts concerning a solar eclipse. — The
following data may guide in understanding the
phenomena of solar eclipses.

(1.) The moon must be new.

(2.) She must be at or near a node.

(3.) When her distance from the earth is less than
the length of her shadow, the eclipse will be total or
partial.

(4.) When lier distance is greater than the length
of her shadow, the eclipse will be annular or partial.

* South of tljc yquatiir the revi'i'.se of these ijlieiioineiia wmilcl Ijaiipeii.

140 THE SOLAR SYSTEM.

(5.) There can be no eclipse at those places where
the sun himself is invisible.

(6.) An eclipse is not visible over the whole illu-
mined side of the earth. As the moon's diameter is
less than that of the earth, her cone of shadow is too
small to enshroud the entire globe, so that the region
in which it is total cannot exceed 180 miles in
breadth. As, however, the earth is constantly rotat-
ing on its axis during the duration of the eclipse,
the shadow may travel over a large surface.

(7.) If the moon's shadow fall upon the earth when
she is nearing her ascending node, it will sweep

Fig. 57.

^^^^^^-- ^^J%J

Solar Ecliptic Limit (17°).

across the south polar regions : if, when nearing her
descending node, it will graze the earth near the
north pole. The nearer a node a conjunction occurs,
the nearer the equatorial regions the shadow will
strike.

(8.) At the equator, the longest possible duration
of a total solar eclipse is about eight minutes ; of an
annular, twelve minutes. One reason of the greater
length of the latter is, that then the moon is in
apogee, when she always moves slower than in
perigee. The duration of total obscuration is great-
est when the moon is in perigee and the sun in
apogee ; for then the apparent size of the moon is
greatest, and that of the sun is least.

ECLIPSES.

141

Fir;. .'S.

(9.) There cannot be more than five nor less than
two solar eclipses per year. A total or an annular
eclipse, in its recurrence at any place^ is exceedingly
rare. There has been (according to Halley) only one
total eclipse visible at London since 1140.

(10.) A solar eclipse conies on the western limb, or
edge of the sun, and passes off on the eastern.

(11.) The disk of the sun is divided into twelve
digits, and the amount of the eclipse is estimated by
the number of digits which it covers. Thus an
eclipse of six digits is one in which half the diam-
eter of the disk is concealed.

Curious phenomena attend a total eclipse. Around
the sun is seen a
beautiful corona,
or halo of light,
like that wliich
painters give to the
head of the Virgin
Mary. Flames of
a rose-red color
play around the
disk of the moon.
When only a mere
crescent of the sun
is visible, it seems
to resolve itself
into bright spots
interspersed with

dark spaces, having the appearance of a string of
glittering beads (Baily's Beads).

The attendant circumstances of a total eclipse

Eclipse of 1858.

THE SOLAR SYSTEM".

Fid. 50.

are of a peculiarly impressive character. The dark-
ness is so dense that the brighter stars and planets
are seen, birds cease their songs and fly to their
nests, flowers close, and the face of nature assumes
an unearthly, cadaverous hue, while a sudden fall

of the temperature
causes the air to
feel damp, and the
grass to be wet as
if from excessive
dew. Orange, yel-
low, and copper
tints give objects
a strange appear-
ance. ''Men look
at eacl> other, and
behold, as it were,
corpses. "

The ancients re-
garded a total
eclipse with feel-
ings of indescribable terror, as an indication of the
anger of an offended Deity, or the presage of some
impending calamity.* Even now, when the causes

Annular Eclipse of 1SS5, skormaig Baily's Beads.

* William of Malinesbury thus connects tlie eclipse of August '2, 1133, with Henrj- I.,
who left England on that day, never to return alive : " Tlie elements manifested their
sorrows at this great man's last departure. For the sun on that day, at the Oth hour,
shrouded his glorious face, as the poets say, in hideous darkness, agitating the hearts
of men by an eclipse : and on the 6th day of the week, early in the morning, there was
so great an earthriuake that the ground appeared suddenly to sink down ; an horrid
noise beiiiR first heard beneath the surface."

The same writer, sjieaking of the total eclipse of March 20, 1140, says : " During this
year, in Lent, on the 13th of the kalends of April, at the Oth honr of the 4th day of the
week, there was an eclipse, tliroughout England, as I have heard. With us, indeed, and
with all our neighbours, the obscuration of the Sun also was so remarkable, that i)ersons
sitting at table, as it then happened almost every where, for it was Lent, at first feared

148

Corona seen in 1S71.

are fully understood, and the time of the eclipse can
be predicted within the fraction of a second, the

that Chaos was come again : afterwards learning the cause, they went out and beheld
the stars around the Sun. It was thought and said by many, not untruly, that the king
(Steplien) would not continue a year in the government."

Columbus made use of an approacliing eclipse of tlic moon, which took place March 1,
1504, to relieve his fleet, tljen in great distress from want of supplies. As a punishment
to the islanders of Jamaica, who refused to assist him, he threatened to deprive them
of the light of the moon. At first they were indifferent to his threats, but " when the
eclipse actually commenced, tlie barbarians vied with each othur in the production of
the necessary supplies for the Spanish fleet."

Among the Hindoos a singular custom is said to exist. When, during a solar eclipse,
the black disk of our satellite begins slowly to advance over the sun, the natives believe
that some terrific monster is gradually devouring it. Thereupon they lieat gongs, and
rend the air with screams of terror and shouts of vengeance. For a time their frantic

144 THE SOLAR SYSTEM.

change from broad daylight to almost instantaneous
gloom is overwhelming, and inspires with awe even
the most careless observer. (See note, p. 303.)

The Saros. — The nodes of the moon's orbit are con-
stantly moving backward. They complete a revolu-
tion around the ecliptic in about 18^ years. Now
the moon makes 2-23 synodic revolutions in 18 years
and 10 days ; the sun makes 19 revolutions with re-
gard to the lunar nodes in about the same time.
Hence, in that period, the sun, the moon, and the
nodes will be in nearly the same relative position.
If, then, we :qeckon 18 years and 10 days from any
eclipse, we shall find the time of its repetition.

This method was discovered, it is said, by the Chal-
deans. The ancients were enabled, by this means,
to predict eclipses, but it is considered too inaccurate
by modern astronomers.

Metonic Cycle. — The Metonic Cycle (sometimes
confounded with the Saros) was not used for fore-
telling eclipses, but for ascertaining the age of the
moon at a given period. It consists of nineteen
tropical years,* during which time there are 235 new
moons ; so that, at the end of this period, the new
moons will recur at seasons of the year correspond-
ing to those of the preceding cycle. By registering,
therefore, the exact days of any cycle at which the

efforts seem futile and the eclipse still progresses. At length, however, the increasing
uproar reaches the voracious monster ; he appears to pause, and then, like a fish reject-
ing a nearly swallowed bait, gradually disgorges the fiery mouthful. Wlien the sun is
quite clear of the great dragon's mouth, a shout of joy is raised, and the poor natives
disperse, delighted to think that they have so successfully relieved their deity from his
impending peril.

* A tropical year is the interval between two successive returns of the sun to the
vernal equinox.

PLATE in.

Various Forms qf Solar Prominmces. (See pp. 53, 141, 262.)

ECLIPSES. 145

new and full moons occur, such a calendar shows on
what days these events will happen in succeeding
cycles.

Since the appointment of games, feasts, and fasts
has been made very extensively, both in ancient and
modern times, according to new or full moons, such
a calendar becomes very convenient for finding the
day on which the required new or full moon takes
place. Thus, if a festival were decreed to be held in
any given year on the day of the first full moon after
the vernal equinox : find what year it is of the lunar
cycle, then refer to the corresponding year of the
preceding cycle, and the day will be the same. The
Golden Number, a term still used in our almanacs,
denotes the year of the lunar cycle. Four is the
golden number for 1884.

Fig. 6S.

Ecli2Jse oftlie Moon.

An Eclipse of the Moon is caused by the passing
of the moon into the shadow of the earth, and hence
can take place only at full moon — opposition. As
the moon's orbit is inclined to the ecliptic, her path
is partly above and partly below the earth's shadow ;
thus an eclipse of the moon can take place only at
or near one of the nodes. In Fig. 63, the umbra is
represented by the space between the lines IS, c and
7

146 THE SOLAR SYSTEM.

I b ; outside of this is the penumbra, where the earth
cuts off the light of only a portion of the sun. The
moon enters the penumbra of the earth at a, — this is
termed her first contact ivith the penumbra ; next she
encounters the dark shadow of the earth at b, — this is
called the first contact ivith the umbra j she then
emerges from the umbra at c, — the second contact
tcith the umbra; finally, she touches the outer edge
of the penumbra at d, — the second contact tcith the
penumbra. Since the earth is so much larger than
the moon, the eclipse can never be annular; as,
however, the eclipse may occur a little above or be-
low the node, the moon may only partly enter the
earth's shadow, either on its upper or lower limb.
From the first to the last contact with the penumbra,
five hours and a half may elapse.

Total eclipses of the moon are rarer events than
those of the sun, since the lunar ecliptic limit is only
about 12° ; yet they are more frequently seen by us,
(1) because each one is visible over the entire unil-
lumined hemisphere of the earth, and also (2) be-
cause by the diurnal rotation during the long dura-
tion of the eclipse, large areas may be brought
within its limits. So it will happen that while the
inhabitants of one district witness the eclipse through-
out its continuance, those of other regions merely
see its beginning, and others only its termination.

The moon does not completely disappear even in
total eclipses. The cause of this lies in the refraction
of the solar rays in traversing the lower strata of
the earth's atmosphere : they are analyzed, and
purple our moon with the tints of sunset, The

THE TIDES. 147

amount of refraction and the color depend upon the
state of the air at the time.

THE TIDES.

Description. — Twice a day, at intervals of about
twelve hours and twenty-five minutes, the water be-
gins to set in from the ocean, beating the pebbles
and the foot of the rocky shore, and dashing its
spray high in air. For about six hours, it climbs far
up on the beach, flooding the low lands and trans-
forming creeks into rivers. The instant of high-water
or flood-tide being reached, the water begins to
descend, and the ehh succeeds the flow. The water,
however, falls somewhat slower than it rises.

Fig. 63.

Spring Tide.

The Tides are Caused by a great wave, which,
raised by the moon's attraction, follows her in her
course around the earth.* The sun, also, aids some-

* Prof. Ball, Royal Astronomer of Ireland, elainis that once the moon was nearer the
earth than now ; the day and the month were equal, eacli three hours long. At 40,000
miles distance, the moon was a greater tide iiroducer l>y 216 times. As the moon receded
from the earth, both revolved more slowly. At the ju'csont time, 27 eartli rotations
equal one moon rotation. This lias remained, and will remain, sensibly tnie, for thou-
sands of years. But the friction of tlie tides will, in the far future, lengthen the day to
pqual 57 of our present days, — a condition tliat will then last for ages,

148 THE SOLAR SYSTEM.

what in producing this effect ; but as the moon is 400
times nearer the earth, her influence is far greater. *

As the waters. are free to yield to the attraction of
the moon, she draws them away from C and D and
they become heaped up at A. The earth, being
nearer the moon than the waters on the opposite
side, is more strongly attracted, and so, being drawn
away from them, they are left heaped up at B. As
the result, high-water is produced at A by the water
Doing pulled from the earth, and at B by the earth
being pulled from the water.

The influence of the moon requires a little time to
produce its full effect ; hence high- water does not
occur at any place when the moon is on the me-
ridian, but a few hours after. As the moon rises
about fifty minutes later each day, there is a corre-
sponding difference in the time of high-water.
While, however, the lunar tide-wave thus lags about
fifty minutes every day, the solar tide occurs uni-
formly at the same time. They therefore steadily
separate from each other. At one time, they coin-
cide, and high-water is the sum of the lunar and
solar tides ; at other times, high-water of the solar
tide and low-water of the lunar tide occur simul-
taneously, and high-water is then the difference be-
tween the lunar and solar tides, f

* The whole attraction of the moon is only tJo that of the sun : yet her influence in
producing the tides and precession is greater, because tliat depends not upon the entire
attraction either exerts, but upon the difference between their attraction upon the
earth's center and upon the earth's nearest surface. For the moon, on account of her
nearness, the proportion of the distance of these parts is treble that of the sun, and
hence her greater eflect.

t We should bear in mind the philosophical truth, that the tide in the open sea does
not consist of a progressive movement of t)ie water itself, but, only of the form of the
>vave.— P^j/siw, p- lOlt

THE TIDES. 149

Causes that modify the tides. — At new and at
full moon (the syzygies) the sun acts with the moon
(Fig. 63) in elevating the waters ; this produces the
highest, or Spring-tide. In quadrature (Fig. 64), the
sun tends to diminish the height of the water : this
is called Neap-tide. When the moon is in perigee,
her attraction is stronger ; hence the flood-tide is
higher, and the ebb-tide is lower than at other times.

Neap Tide.

This remark applies also to the sun. The height of
the tide also varies with the declination of the sun
and the moon, — the highest or equinoctial tides tak-
ing place at the equinoxes, if, when the sun is over
the equator, the moon also happens to be very near
it : the lowest occur at the solstices. The force and
the direction of the winds, the shape of the coast,
and the depth of the sea greatly complicate the ex-
planati'on of local tides.

Height of the tide at different places. — In the
open sea, the tide is hardly noticeable, the water
sometimes rising not higher than a foot ; but where
the wave breaks on the shore, or is forced up into

150 THE SOLAR SYSTEM.

bays or narrow channels, it is very conspicuous.
The difference between ebb and flood neap-tide at
New York is over three feet, and that of spring-tide
over five feet ; while at Boston it is nearly double
this amount. A headland jutting out into the ocean
will diminish the tide ; as, for instance, off Cape
Florida, where the average height is only one and a
half feet. A deep bay opening up into the land like
a funnel will converge the wave, as at the Bay
of Fundy, where it rolls in, a great, roaring wall
of water sixty feet high, frequently overtaking and
sweeping off men and animals.* The tide sets up
against the current of rivers, and often entirely
changes their character ; for example, the Avon
at Bristol is a shallow ditch, but at flood-tide it
becomes a deep channel navigable by the largest
Indiamen.

V. MARS.

The god of war. Sign, S , shield and spear.

Description. — Passing outward in our survey of the
solar system, we next meet with Mars. This is the
first of the superior planets, and the one most like
the earth. It appears to the naked eye as a bright
red star, rarely scintillating, and shining with a
steady light, which distinguishes it from the fixed

The tide-wave ascends the Hudson Kiver at about the same speed as the steam-
boats ; at Albany it reaches a height of a little over two feet.

MARS.

151

stars.* At conjunction its apparent diameter is only
about 4"; but once in about two years it conies into
opposition with the sun, when its diameter may in-
crease to 30". At intervals of nearly 15 years, this
occurs when the planet is in perihelion and the earth
in aphelion. Mars then shines with a brilliancy
rivalling that of Jupiter himself, f

Fit}. 65.

DlaiiiAiUr q/ Mars at Exlienic, Mean, and Leant Distances.

Motion in Space. — Mars revolves around the Sun
at a mean distance of about 141,000,000 miles. Its
orbit is sufficiently flattened to bring it at perihelion
26,000,000 miles nearer that luminary than when in
aphelion. Its motion varies in different portions of
its orbit, but the average velocity is about fifteen
miles per second. The Martian day is 37 min. longer
than ours, and the year contains about 608 Martian
days, equal to 087 terrestrial days (nearly two years).

Distance from Earth. — When in opposition, the

* Its ruddy appearance has led to its being celebrated among all nations. The Jews
gave it the appellation of " blazing," and it bore in other languages a similar name.

\ The next favorable opposition will occur in 1892.

152 THE SOLAR SYSTEM.

distance of Mars is (like that of all the superior
planets) the difference between the distance of the
planet and that of the earth from the Sun : at con-
junction, it is the sum of these distances. If the
orbits were circular, these distances would be the
same at every revolution. The elliptical figure, how-
ever, occasions much variation. Thus, if Mars, at
opposition, be in perihelion while the earth is in aphe-
lion, it is removed from us about 34,000,000 miles.

Dimensions. — The diameter of Mars is nearly 4,200
miles. * Its volume is about 1 and its density i that
of the earth. A stone let fall on its surface would
fall six feet the first second. It is somewhat flat-
tened at the poles, and bulged at the equator, like our
globe.

Seasons. — The light and heat of the sun at Mars
are less than one-half that which we enjoy. Its axis
is inclined about 27', therefore its zones and seasons
do not differ materially from our own : its days,
also, as we have seen, are of nearly the same length.
Since, however, its year is equal to nearly two of
our years, the seasons are lengthened in proportion.

There must be a considerable difference between
the temperature of its northern and southern hemi-
spheres, as the former has its summer when 20,000,000
miles further from the sun than the latter : an in-
creased length of 76 days may, however, be sufficient

* Some autliors place the diameter of Mars at about 5,000 miles. There is, also, a
discrepancy as to the other data of this planet. Prof. Hall, as the result of his
observations, gives the density = .770 ; force of gravity = .37 ; fall of a body, 1st
sec. = feet. With the discovery of the satellites we have now the means of securing
exact results. The difficulty of observation, however, is shown from the fact that " the
light which falls upon the earth from one of these moons is about what a man's hand on
which the sun sliines at Washington would reflect to Boston."

MARS.

153

Fig. 66.

compensation. It lias an atmosphere like our own,
loaded with clouds.

Mars has two moons. * Our earth and its moon pre-
sent in the Martian evening sky a beautiful pair of
planets, constantly remaining in close proximity to
each other, and exhibiting
all the phases which Mer-
cury and Venus present to
us.

Telescopic Features. — Un-
der the telescope. Mars ex-
hibits slight phases. Its
surface is covered M'^ith red-
dish spots, which are be-
lieved to be continents.!
Other portions, of a green-
ish tint, are considered to
be bodies of water. The proportion of land to water
on the earth is reversed in Mars. '^^ Here every con-
tinent is an island ; there every sea is a lake : but

w vj Mar

* Tlie satellites of Mars were discovered in August, 1S77, by Prof. Hall of the Nava)
Observatory, Washington. The outer one revolves about the planet in 30 hr. 18 min.,
at a distance of about 12,300 miles ; and the inner one in 7 hr. 40 min., at a distance of
3,600 miles (less than that of remote cities on our own continent). The inner moon
moves so much faster than the rotation of Mars that to an inhabitant of that planet, the
moon would seem to rise in the west and set in the east, passing through all the phases
of our moon during a single night. The moons have been named Deimos and Phobus,
or Dread and Terror— the sons of Mars. The diameter of tliese little globes is probably
less than 15 miles. For an amusing description of such a world, read " Living in Dread
and Terror," a chapter in Proctor's " Poetry of Astronomy."

t So carefully has the surface of this planet been studied, that a globe of Mars has
been prei)ared which is said to be in some respects more perfect than any globe of the
earth. The different bodies of land and water have been named after distinguished
astronomers. A characteristic feature of the seas is the long, narrow cliannels. Schi-
aparelli, the Italian astronomer, claims to have discovered a number of singular dark
lines, now known as " canals." They seem to connect different bodies of water, and,
though without sufficient reason, have been by some considered as the work of the Mar-
tian inhabitants.

154 THE SOLAR SYSTEM.

these, like our own continents, are chiefly confined
to one hemisphere, so that the habitable area of the
two globes may not differ so much as the size of the
planets."

The ruddy color is thought by Herschel to be due
to an ochery tinge in the soil ; by others it is attrib-
uted to peculiarities of the atmosphere and clouds.
Lambert suggests that the color of the vegetation on
Mars may be red instead of green. There are con-
stant changes going on in the brightness of the disk,
owing, it is supposed, to the variation of the clouds
of vapor in its atmosphere. No mountains have yet
been discovered.

In the vicinity of the poles are brilliant white
spots, which are considered to be masses of snow.
The " snoiv zoiies" apparently melt and recede with
the return of summer in each hemisphere, and in-
crease on the approach of winter. We can thus from
the earth watch the formation of polar ice and the
fall of snow, — in fact, the changes of the seasons — on
the surface of a neighboring planet.

VI. THE MINOR PLANETS.

Discovery.— Beyond Mars there is a wide interval
that was not filled until the present century. The
bold, imaginative Kepler conjectured that there was
a planet in this space. This supposition was cor-
roborated by Titius's discovery of what has since
been known as

Bode's Law.— Take the numbers 0, 3, G, 12, 24, iS,

THE Minor planets, or asteroids. 155

96, 192, 384, each of which, after the second, is double
the preceding one. If we add 4 to each of these
numbers, we form a new series ;

4, 7, 10, 16, 28, 52, 100, 196, 388.

A.t the time this law was discovered, these numbers
represented very nearly the proportionate distance
from the sun of the planets then known, taking the
earth's distance as ten, except that there was a blank
opposite 28. This naturally led to inquiry, and a
systematic effort to solve the mystery. *

On the 1st day of January, 1801, the nineteenth
century was inaugurated by Piazzi's discovery of
the small planet Ceres, at almost the exact distance
necessary to fill the gap in Bode's series. The an-
nouncement of other new planets soon followed,
until now (1885) there are two hundred and forty-
seven, with a probability of more being found. In-
deed, Leverrier has calculated that there may be
perhaps 150,000 in all.

Description. — These minor worlds, or "pocket
planets," as Herschel styled them, are diminutive
indeed. The largest of them is Vesta, which shines
at times as a star of the 6th magnitude, and can then
be seen with the naked eye. f Those recently discov-

* It is a curious fact that the discovery of Ceres sliould have been made by an out-
sider, as Piazzi did not belong to the society of 24 astronomers tlien searching for the
planet. The publication of Bode's law had little to do with the result. In fact, the
direct cause was an error of the press in putting an extra star in Wollaston's Catalogue,
and while Piazzi was looking for this star he found Ceres.

t The small size of the disks of the minor planets defies exact measurement. New-
comb makes Ceres and Vesta the largest of the group, with diameters between 200 and
400 miles. Echo has been assigned a diameter of 17 miles, or not far from the size of the
miniature moons of Mars. S.n'eral of these little worlds have been found but to be lost
again ; while the mere labor of tracing the movements of so many tiny globes already
surpasses the probable worth of the results.

156 THE SOLAR SYSTEM.

ered are so small that it is difficult to decide which

is the smallest. A good walker could easily m.ake the
tour of one in a day ; a prairie farmer would need to
pre-empt a whole such world for a cornfield. *'A
man placed on one of these tiny globes could leap
60 feet high, and, in his descent, would sustain no
greater shock than he does on the earth from jump-
ing or leaping a yard. *" These planets revolve around
the sun in regular orbits, comprising a zone about
100,000,000 miles in width. Their paths are variously
inclined to the ecliptic; Massalia's is only 41, while
that of Pallas rises 34".

Origin. — A conjecture concerning the origin of these
bodies is, that they are the fragments of a large
planet that, in a remote antiquity, was shivered to
pieces by some terrible catastrophe. " One fact
seems above all others to confirm the idea of an inti-
mate relation between these planets. It is this : if
their orbits consisted of solid rings, they would be
found so entangled that it would be possible, by
taking up any one at random, to lift all the rest."
The more probable view is given under the " Nebular
Hypothesis."

Names and Signs.— Ceres, the first discovered, re-
ceived the symbol 9 , a sickle, as that goddess was
supposed to preside over harvests. Pallas, the
second, named from the goddess of wisdom and sci-
entific warfare, obtained the sign $ , the head of a
spear. Of late, a simple circle with the number
inclosed has been adopted ; thus O represents Ceres,
® is the sign of Pallas.

JUPITER. 15"?

VII. JUPITER.

The king of the gods. Sign 7i, a liieroglyphic representation of an eagle,
" tlie bird of Jove."

Description. — From the smallest members of the
solar system we now pass to the largest planet — the
colossal Jupiter. Its peculiar splendor and brilliancy
distinguish it from the fixed stars, and vie even with
the lustre of Venus. It is one of the five planets dis-
covered in primitive ages.*

Motion in Space. — Jupiter revolves about the sun
at a mean distance of about 483,000,000 miles. His
movement among the fixed stars is slow and majestic,
comporting well with his vast dimensions and the
dignity conferred by four attendant worlds. He
advances through the zodiac at the rate of one
sign yearly ; so that if we locate the planet
now, a year hence we shall find it equally advanced
in^he next sign. Yet slowly as he seems to travel
through the heavens, he is bowling along through
space at the enormous speed of nearly 500 miles per
minute. The Jovian day is equal to only about ten
of our hours, while the year is lengthened to about
12 of our years, comprising near 10,000 of his days.

Distance from. Earth. — Once in thirteen months
Jupiter is in opposition, and his distance from the
earth is measured by the difference of the distances
of the two bodies from the sun. At the expiration

' In tliose early times, Jupiter was supposed to be the cause of storm and tempest
Pliny thought that lightning owed its origin to this planet. An old almanac of 1368,
foretelling the harmless condition of Jupiter for a certain month, says, " Jubit es hote
and moyste and does weel til al thyiiges and noyes nothing."

TttE SOLAR SYSTEM.

Fig. €7

of half this time he is in conjunction, and his dis-
tance from us is measured by the sum of these
distances.

Dimensions.— The diameter of this planet is about
90,000 miles. Its volume is 1,4:00 times that of the
earth, and much ex-
ceeds that of all the
other planets com-
bined. Seen at the
distance of the moon,
this immense globe
would embrace 1,000
times the space of the
full moon. Its den-
sity is only one-quar-
ter that of the earth :
moreover, its rapid ro-
tation upon its axis,
whereby a particle on
the equator revolves with a velocity of 473 miles per
minute against the earth's 17 miles per minute, must
produce a powerful centrifugal force which materi-
ally diminishes the weight of objects near its
equator. Consequently, a stone let fall on Jupiter
would pass through only about -42 feet the first
second. As a result of this rapid rotation, the
planet is one of the most flattened of any in the
solar system, the equatorial diameter exceeding the
polar by 5,000 miles.

Seasons. — As the axis of Jupiter is but slightly in-
clined from a perpendicular to the plane of . its orbit,
there is little difference in the length of his days and

I ic of Jin

JUPITER. 159

nights, which are each of about five-hours duration.
At the poles, the sun is visible for nearly six years,
and then remains set for the same length of time.
The seasons are but slightly varied. Summer reigns
near the equator, while the temperate regions enjoy
perpetual spring. The light and heat of the sun are
only 2r of what we receive ; yet peculiarities of soil
or atmosphere may compensate this difference. The
evening sky on Jupiter must be magnificent ; besides
the glittering stars which adorn our heavens, four
moons, waxing and waning, each with its diverse
phase, illuminate his night. All the starry exhi-
bition sweeps through the sky in five hours.

Telescopic Features. — Jupiter's Moons. — Through
the telescope* Jupiter presents a beautiful Copernican
system in miniature. Four small stars — moons — ac-
company him in his twelve-yearly revolutions. From
hour to hour their positions vary, and they seem to
oscillate from one side to the other of the planet. At
one time, there will be two on each side ; and again,
three on one side, while the remaining star is left
alone. They are also frequently found to disappear,
one, two, or even three at a time, and, more rarely,
all four at once.

These moons are called by the ordinal numbers,
reckoning outward from the planet. With an ordi-
nary glass, there is nothing to distinguish them
from small stars. The Illrd., being the largest and

* There are well-authenticated instances on record of their having been seen by the
naked eye. Among others, the follo\i'ing singular case is mentioned. Wrangle, the
celebrated Russian traveler, states that, when in Siberia, he once met a hutiter, who
said, pointing to Jupiter, " I have just seen that star swallow a small one and then
vomit it up again."

160

THE SOLAR SYSTEM.

brightest, will generally be identified the most easily.
The 1st. satellite appears to the inhabitants of the
planet almost as large as our moon to us ; the Ilnd.,
and Illrd., about half as large.

SATELLITES OF JUPITER.

Mean distance
from Jupiter.

Diameter.

Density.
Water as 1.

Sidereal period.

I. lo

267,380

425,156

678,393

1,192,823

2,352 m.
2,000 "
3,436 "
2,929 "

1.12
2.14
1.87
1.47

I). 11. .M.

1 IS 23
3 13 4

in. Ganymede

IV. Callisto

7 3 43
19 16 32

It is noticeable that here are four satellites revolv-
ing about Jupiter, one of them larger than the
planet Mercury, and each surpassing in size the
minor planets between Mars and Jupiter. The
moons are not only distinguised by their various
dimensions, but also by the variety of their color.
The 1st. and Ilnd. have a bluish tint, the Illrd. a
yellow, and the IVth, a reddish shade. The space
occupied by this miniature system is about two and
a half million miles in diameter.

Eclipse of the Moons. — Jupiter, like all celestial
bodies not self-luminous, casts into space a cone of
shade. The 1st., Ilnd., and Illrd. satellites revolve
in orbits but very little inclined to the plane of the
planet's orbit. During each revolution, they pass
between the Sun and Jupiter, producing a solar
eclipse ; and also, by passing through the shadow of
the planet itself, cause to themselves an eclipse
of the sun, and to Jupiter an eclipse of a moon.
The IVth. moon passes through a path more in-
clined, and therefore its eclipses are less frequent ;

JUPITER.

161

instead of being fully eclipsed, it sometimes just
grazes the shadow. Through a telescope, we can dis-
tinctly watch the disappearance, or immersion, of the

Fig. 68.

Eclipses and Occnltations of Jupiter's Moon.'i

satellites in the planet's shadow, their reappearance,
or emersion, and also the transits of their shadows
as round black dots moving across the disk of Jupiter.

162 THE SOLAR SYSTEM.

In Fig. 68, we see various positions of the moons :
the 1st. is eclipsed ; the Ilnd. is passing across the
disk of the planet on which its shadow is also
thrown ; the Illrd. is just behind the planet, and so
occulted or concealed, while it has not yet entered
the shadow; the IVth. is in view from the earth.

These satellites revolve with great rapidity, as is
necessary in order to overcome the superior attrac-
tion of the planet and prevent their being drawn to
its surface. The 1st. goes through all its phases in
If days: the IVth., in less than twenty days. A
spectator on Jupiter might witness, during the
Jovian year, 4,500 eclipses of the moon (moons), and
about the same nimiber of the sun.

Velocity of Light — By an attentive examination
of the eclipses of Jupiter's moons. Romer (a Danish
astronomer), in 1617, discovered that the motion of

light is not instantaneous, as was then believed. He
noticed that the observed times of the eclipses were
sometimes earlier and sometimes later than the
calculated times, according as Jupiter was nearest
or furthest from the earth. In Fig. 69, let J represent
Jupiter ; e, one of the moons : S, the sun ; and T and t,
different positions of the earth in its orbit. When

JUPITER. 163

the earth is at T, the eclipse occurs 16 min. and 36
sec. earlier than at t. That interval of time is
required for the light to travel across the earth's
orbit, giving a velocity of about 186,000 miles per
second.

Jupiter's Belts are dusky streaks of varying
breadth and number, lying more or less parallel to
the planet's equator. A brighter, often rose-colored,
space marks the equatorial regions. The belts are
not permanent, but change sometimes in the course
of a few hours. Occasionally, only two or three
broad belts are visible ; at other times, a dozen
narrow ones appear. Often, spots are seen that are
more lasting than the dark stripes.* It is now sup-
posed that the planet is enveloped in dense clouds,
through which light cannot penetrate, and that the
globe itself is heated to a high degree, and gives off
vapors, t The parallel appearance is doubtless due
to strong equatorial currents, analogous to our trade-
winds.

* In 1878, a "Great Red Spot" appeared in the southern hemisphere of Jupiter.
Its length was estimated at 8,000 miles, and its breadth at 2,000 miles. This curious
phenomenon is still visible, but much diminished in brightness (1884).

t Jupiter and Satimi are older planets than the earth and Mars, but, being so large,
they have cooled more slowly, and are yet only partially solidified, so that Jupiter, at
least, still shines with much of its primeval flre. Mars tyiiifies the middle age ; Saturn
and Jupiter, the youth; and Uranus and Xeptune, the infancy of planetary existence.
In the case of Saturn and Jupiter, we never see the real planets, but only the outline of
their atmospheres. If this theory be true, Jupiter and Saturn now rejiresent the con-
dition in which our earth existed ages ago, before a solid crust had beeu formed upon its
surface. — (Geology, p. 17.)

164 THE SOLAR SYSTEM.

VIII. SATURN.

Tie god of tiire. Sign % , an anvient scythe.

Description. — We now reach, in our outward jour-
ney from the sun, the most remote world known to
the ancients. It shines with a steady pale yellow
light, which distinguishes it from the fixed stars.
Its orbit is so vast that its movement among the
constellations may be easily traced through one's
lifetime. It requires two and a half years to pass
through a single sign of the zodiac ;* hence, when
once known, it may be readily found again. The
earth leaves it at conjunction, makes a yearly rev-
olution about the sun, comes to its starting point,
and overtakes Saturn in about thirteen days there-
after.! It is smaller than Jupiter, but more gor-
geously attended. Besides a retinue of eight satel-
lites, it is surrounded by a system of rings, some
shining with a golden light, and others transparept,
— a spectacle as wonderful as it is unique.

Motion in Space. — Saturn revolves about the sun
at a mean distance of nearly 886,000,000 miles. The
eccentricity of its orbit is a trifle more than that of
Jupiter, so that while it may, at perihelion, come
fifty million miles nearer than its mean distance, at
aphelion it swings off as much beyond. We can
form some estimate of the size of its immense orbit,
when we remember that it is moving 22,000 miles

* Because of its slow, dreary pace, Saturn was chosen by the ancients as the symbol
for lead.

t From this the year of Saturn may be determined. As 13 : 378 days : : Earth's
year : Saturn's year = 30 yr. nearly.

SATURN.

165

Salum.

per hour, and yet, from night to night, we can

scarcely detect any rig. wi

change of place.

The Saturnian year

is equal to about

thirty of ours, and

comprises nearly

25,000 Saturnian

days, each about

10;^ hours long.

The Distance
from the Earth is
found in the same
manner as that of the other superior planets, being
least in opposition and greatest in conjunction. Ac-
cording as the earth and Saturn occupy different
portions of their orbits, the distances between them
at different times may vary nearly 300,000,000 miles.

Dimensions. — The diameter of Saturn is about
73,000 miles. Its volume is 700 times that of the
earth. Its density is about f that of water, or a
little more than that of pine wood. The Saturnian
force of gravity is therefore scarcely greater than
the terrestrial, so that a stone would fall toward
the surface of that immense globe only about seven-
teen feet the first second.

Seasons, — The light and heat of the sun at Saturn
are only .-jJjt that which we receive. The axis of
Saturn is inclined from a perpendicular to the plane
of its orbit about 3V.* The seasons therefore are

* Proctor says 26° ; others, 28°. Let the pupil adapt the paragraph to each of tliese
estimates.

166 THE SOLAR SYSTEM.

similar to those of the earth, but on a larger scale.
The sun climbs in summer about 8° higher above the
horizon, and sinks corresponding!}^ lower in winter.
The tropics are 16° further apart, and the arctic and
antarctic circles 8° further from the poles. Each of
Saturn's seasons lasts more than seven of our years.
There is an interval of fifteen years between the
autumn and spring equinoxes, and between the sum-
mer and winter solstices. For fifteen years, the sun
shines On the north pole, and a night of the same
length envelopes the south pole. The atmosphere is
doubtless very dense, as the belts seem to indicate.

Telescopic Features. — Saturn's Rings. — Galileo
first noticed something peculiar in the shape of
Saturn. Through his imperfect telescope it seemed
to have on each side a small planet, like a supporter,
to help old Saturn on his way. Galileo therefore an-
nounced to his friend Kepler the curious discovery,
that '' Saturn is threefold." As the planet, however,
approached its equinoxes, these attendants vanished
from his instrument. This was a great perplexity to
the philosopher, and he never solved the mystery.
When the rings were afterward seen, their real
form was not known. They were supposed to be a
kind of handle attached to the planet.

Description of the Rings. — The series consists of
three rings of unequal breadth, surrounding the
planet at the equator. The exterior ring is separated
from the middle one by a distinct break, while the
interior ring seems joined to the middle one. They
differ in their brightness ; the exterior ring is of a
grayish tint ; the middle one is the most brilliant^

SATURN. 167

being more luminous than Saturn itself ; the interior
one is darker and has a purple tinge. The two outer
rings are known as the bright rings, and the inner
one is called the dusky ring. The exterior and mid-
dle rings are both opaque and cast on the planet a
distinct shadow ; while the interior one is so trans-
parent that it appears upon the globe of Saturn as a
dark band through which the surface of the planet
is readily seen.

Saturn's Rings. (Proctor.)

Miles.

Diameter of exterior ring 166,920

Breadth of exterior ring 10,000

Diameter of middle ring 144,300

Breadtli of middle ring 17,000

Distance between exterior and middle ring 1,700

Diameter of interior ring 92,000

Breadth of interior ring 8,600

Distance of interior ring from the pljftiet 10,000

Entire breadth of ring system 37,570

Thickness of rings, less than 100

Rotation.— T[\Q rings revolve around Saturn in
about 10| hours, in the same direction as the planet
rotates on its axis. The globe of Saturn is not
exactly at the center of the rings. This fact, com-
bined with the rotary motion, is essential to the
stability of the rings, preventing them from being
precipitated upon the planet.

Phases of the Rings. — The plane of the rings is
inclined about 28° to the ecliptic. In its revolution
about the sun, the axis of Saturn remaining parallel
to itself, the sun sometimes illumines the northern
and sometimes the southern face of the rings. At
JSaturn's equinoxes, only the edge receives the light,

168

THE SOLAR SYSTEM.

and the rings are invisible to us, except with the
most powerful telescopes, and then only as a line of
light. The body of the planet constantly cuts off
the sun's rays from a portion of the rings, and also
serves to conceal from our view some of the lumin-

Fig. 71.

iji.ii:,iJilJii;iiJ;:lilLi:li;.,!Jiiilljlii!iilli];!iilililliti!lil!lllli

Phases of Saturn's Rings.

ous part. By a careful study of the cut, these various
positions of the planet and rings, with the favorable
times for observation, may be understood.

Composition of the Rings. — It is now generally
believed that the rings consist of a cloud of tiny
satellites, — too small to be seen with the telescope, —
revolving about the planet (see Nebular Hypoth-
esis).

JBelts.— The surface of Saturn is traversed by f^int

SATtJRM.

dusky belts of a fstr less distinct and definite appear-
ance than those upon Jupiter. The equatorial re-
gions are more strongly marked than the other parts
of the disk.

Composition of the Planet.— It is quite probable
that Saturn, like Jupiter, has no solid crust, but con-
sists of molten matter surrounded by vapor that con-
tinually rises from the heated interior (note, p. 163).

Satellites. — Saturn has eight satellites.

Names of Saturn's
fr'atellites.

I I Jliinas

II i Eiu'elaau.s.,

III. .
IV...
V. . . .
VI...
VII..
VIII.

Tethys. ..

Dioiie

Rliea ....

Tit^ui ....
Hyperion
Japetus. .

Distance from

t<aturn in

miles.

120,800
15.5,015
191,248
245,87(5
343,414
796,157
1,006,656
2,313,835

.\p))ro,\imate

diameter in

miles.

1,000

•>

500

1,200

3,300

1,800

fiidereal

Period in

days.

0.94
1.37
1.88
2.73
4.51
16.94
21.29
79. to

Titan is the largest, and in size exceeds Mer-
cury. Enceladus and Mimas are the faintest of
twinklers, and can be seen only with a powerful
telescope. They were first detected by Herschel,
''threading like pearls the silver line of light," to
which the ring, then seen edgewise, was reduced, —
advancing off it at either end, returning, and then
hiding themselves behind the planet. The first
three of these moons are nearer to Saturn than our
moon is to the earth, but Japetus is nearly ten times
as distant : so that the diameter of the Saturnian
system is nearly four and a half million miles.

Saturnian Scenery. — The magnificence of the

170

THE SOLAR SYSTEM.

scenery upon Saturn must surpass anything with
which we are famihar. In the cut, is given an ideal
view of a landscape located upon the planet at a lati-
tude of about 28°, taken at midniglit. The rings form
an immense arch, which spans the sky and sheds a

Fig. m.

Ideal Landscape on Saturn, supposing a solid crust to exist.

soft radiance around ; while, to add to the strange
beauty of the night, eight moons in all their different
phases — full, new, crescent, or gibbous — light up the
starry vault.

IX. URANUS.

" Heaven," the most ancient of the gotls. Sign, IJl ; H, the initial letter of Hersehel,
with a planet suspended from the cross-bar.

Description. — On the 13th of March, 1781, between
10 and 11 p. M., Sir William Hersehel was examining

URANUS. 171

with his great telescope some stars in the constella-
tion Gemini. A small star attracting his attention,
he observed it with a higher magnifying power,
when, unlike the fixed stars, its disk widened.
Watching it for several nights, he detected its mo-
tion in space ; but, mistaking its true character, he
V announced the discovery of a new comet. A few-
months examination revealed the error, and the new
^ body — new to us, but older perhaps than our own
^ world* — was admitted to be a member of the solar
system.

Uranus may be seen in a dark sky, by a person of
strong eyesight, if he previously knows its exact
position among the stars. Its faintness is due to its
great., distance from the earth. Were it as near as
the sun, it would appear twice as large as Jupiter.

Motion in Space. — Uranus revolves about the sun
at a mean distance of nearly 1,782,000,000 miles. Its
year exceeds eighty-four of ours.

Dimensions. — Its diameter is about 33,000 miles.
Its density is about equal to that of the water from
the Dead Sea. The force of gravity upon the surface
of the planet is ^^ that upon the earth.

Seasons. — We know little of the seasons of Uranus.
If its axis lies in the plane of its orbit, the sun
must wind in a spiral form around the planet. The
light and heat are less than x.^xttt of that which we

* It is now known that Uranus had been previously observed by other astronomers.
Le Monier at Paris had watched it for twelve successive nights, but pronounced it a
fixed star. He had also seen it on previous occasions, and had he been an orderly ob-
server, he would doubtless have detected its planetary character ; but he was extremely
careless, as may be inferred from the fact related by Arago, that he had been shown one
of Le Monier's observations of this planet written on a paper bag which originally con-
taiued hair-powder puivliased at a perfumer's.

l7^ THE SOLAR SYSTEM.

receive; the light has been estimated to be about the
quantity that would be afforded by three hundred
full moons. The inhabitants of Uranus, if any such
exist, can see Saturn, and perhaps Jupiter, but none
of the planets \vithin the orbit of the latter.

Telescopic Features. — Xo spots or belts have been
discovered. The time of rotation and the other
features so familiar to us in the nearer planets are
therefore unknown with regard to Uranus.

Satellites. — Uranus has four moons, of which
little is known except the curious fact that their
orbits are nearly perpendicular to the plane of the
planet's orbit, and that their movements are appar-
ently retrograde — /. e., in the same direction as the
hands of a watch.

X. NEPTUNE.

The god of the sea. Sign, "f , his trident.

Description- — Xeptune is the far-off sentinel at the
outpost of the solar system, being the most distant
planet of which we have any knowledge. It is in-
visible to the naked eye, and appears in the telescope
as a star of the sixth magnitude.

Discovery. — For many years, the motions of Ura-
nus had been such as to baffle the most perfect calcula-
tions. While far-distant Saturn, after his journey of
thirty years, came around to his place true to the min-
ute, Uranus defied arithmetic, and refused to con-
form to the time set down for him on the heavenly
dial.

KEPTUNjbi. 173

At length it was suggested that there was another
planet exterior to Uranus, whose attraction produced
these perturbations. So marked was this impression
with Herschel, that he writes : " We see it as Colum-
bus saw America from the shores of Spain. Its
movements have been felt trembling along the far-
reaching line of our analysis with a certainty not
far inferior to ocular demonstration."

Finally, two young mathematicians, Leverrier, of
Paris, and Adams, of Cambridge, England, each un-
known to the other, set about the task of finding the
place of this new planet. The problem was this :
Given the disturbances produced by the attraction oj
the unknown planet, to find its orbit and its place in
the orbit.

Adams, after assiduous labor for nearly two years,
completed his calculations and submitted them to
Prof. Airy, the Astronomer Royal, in 1845. In the
summer of 184G, Leverrier laid a paper before the
Academy of Sciences in Paris, announcing the
position of the unknown planet. Prof. Airy, hear-
ing of this, was so impressed with the value of
Adams's calculations, that he wrote to Prof. Challis,
of Cambridge, to search that quarter of the heavens.
Prof. Challis did as requested, and saw a star which
afterward proved to be the planet so anxiously
sought for, although at that time he failed to ascer-
tain its true character. In September, of the same
year, Leverrier wrote to Berlin, asking for assistance
in searching for the planet. Dr. Galle, on receiv-
ing the request, turned the large telescope of the
Observatory to the place indicated, and almost im-

IH THE SOLAR SYSTEM.

mediately detected a bright star not laid down in the
maps. This proved to be the predicted planet, found
within less than a degree of the spot described by
Leverrier.

Such is the history of one of the grandest achieve-
ments of the human mind. It stands as an ever
fresh and assuring proof of the exactness of astro-
nomical calculations, and the power of the intellect
to understand the laws of the God of Nature.

Motion in Space. — Neptune revolves about the sun
at a mean distance of about 2,790,000,000 of miles.
The Neptunian year is equal to nearly 165 terrestrial
ones. Its motion in its orbit is the slowest of any of
the planets, since it is the most remote from the sun.
The velocity decreases from Mercury, which moves
at the rate of about 105,000 miles per hour, to Nep-
tune, whose rate is only 12,000 miles.

Dimensions. — Neptune's diameter is about 37,000
miles. Its volume is nearly 100 times that of the
earth. Its density is a little less than that of Uranus.

Seasons. — As the inclination of its axis is un-
known, nothing can be ascertained concerning its
seasons. The sun gives to Neptune but xio^ the
light and heat which we receive.

Though Neptune is at the extreme of the solar
system, 2,790,000,000 miles beyond us, the same
heavens bend above, the Milky Way is no nearer to
the eye, and the fixed stars shine no more brightly.
The planets, however, are all too near the sun to be
seen, except Saturn and Uranus. The Neptunian
astronomers, if there be any, are well situated for
measuring the annual parallax of the stars, since

METEOfiS AND SHOOTING STaHS. 175

NeptuDe has an orbit of 5,580,000,000 miles in
diameter, and hence the angle must be thirty times
as great as that which the terrestrial orbit affords.

Telescopic Features. — On account of the recent
discovery of this planet and its immense distance,
nothing is known of its rotation or physical features.

Satellites. — Neptune has one moon, at nearly the
same distance from it as our own moon is from the
earth. The revolution of this body about the planet,
which is accomplished in about six days, has fur-
nished the materials for calculating the mass of
Neptune.

III. METEORS AND SHOOTING
STARS.

Description. — All are familiar with those luminous
bodies that flash through our atmosphere as if the
stars were indeed falling from heaven. Different
names have been applied to them, although the dis-
tinction is not very definite.

(1) Aerolites are those stony or iron masses
which descend to the earth.

(2) Meteors are luminous bodies which have a
sensible diameter and a spherical form. They fre-
quently pass over a great extent of country, and are
seen for some seconds. Many leave behind them a
train of glowing sparks ; others explode with reports
like the discharge of artillery,— the pieces either
continuing their course, or falling to the earth as

176 THE SOLAR SYSTEM.

aerolites. Some meteors pass on into space ; some
are vaporized ; while others are burned, and the ashes
and fragments fall to the ground.
(3) Shooting Stars are those evanescent, brilliant

Fig. 73.

A Meteor vriih its Train.

points that suddenly dart through the higher regions
of the air, leaving a fiery train behind.
1. Aerolites. — The fall of aerolites is frequently men-

METEORS AND SHOOTING STARS. 177

tioned and well authenticated. Chinese records tell
of one as long ago as 616 B.C., that, in its fall, broke
several chariots and killed ten men. A block of
stone, equal to a full wagon-load, fell in the Helles-
pont, B.C. 465. By the ancients, these stones were
held in great repute. The Emperor Jehangire, it is
related, had a sword forged from a mass of meteoric
iron which fell in the Punjab in 1620. In 1795, amass
was seen, by a ploughman, to descend not far from
where he was standing. It threw up the soil on
every side, and penetrated some distance into the
solid rock beneath. In 1807, there was a shower of
stones, one weighing 200 lbs., at Weston, Connect-
icut. A mass once fell in South America, that was
estimated to weigh fifteen tons. When first dis-
covered, it was so hot as to prevent all approach.
Upon its cooling, many efforts were made, by some
travelers who were present, to detach specimens,
but its hardness was too great for the tools that they
possessed. In Yale College cabinet, there is a mass
of meteoric iron, weighing 1,635 lbs.

Aerolites consist of elements which are famil-
iar. The analysis of these stellar objects gives us
names as commonplace as if they had known a far
less romantic origin, — iron, tin, copper, nickel,
cobalt, lime, magnesia, oxygen, sulphur, phos-
phorus ; in all, about twenty elements have been
found. This fact is interesting as revealing some-
thing of the chemistry of the region of space, concern-
ing which we otherwise know little. The compounds,
however, are so peculiar as to distinguish an aerolite
ITOQi other substances. For example, meteoric iron,

178

THE SOLAK SYSTEM.

a prominent constituent of aerolites, is an alloy that
has never been found in terrestrial minerals.

2. Meteors. — The records of meteors are even more
wonderful than those of aerolites. It is related that
at Crema, Italy, one day in the loth century, the sky
at noonday became dark, — a cloud of appalling black-
ness overspreading the heavens. Upon this cloud,
appeared the semblance of a great peacock of fire

Fig. 7i.

Copy of a Print Showing the Peculiar Crystalline Sti~ucture of Meteoric Iron.

flying over the town. This suddenly changed to a
huge pyramid, that rapidly traversed the sky.
Thence arose awful lightnings and thunderings,
amid which there fell upon the plain rocks, some
of which weighed 100 lbs. In 1803, a brilliant fire-
ball was seen traversing Normandy with great
velocity, and some moments after, frightful explo-
sions, like the noise of cannon, were heard coming
from a black cloud hanging in the clear sky; they

METEORS AND SHOOTING STARS. 179

were prolonged for five or six minutes. These dis-
charges were followed by a shower of heated stones,
some weighing over 24 lbs. In 1819, a meteor was
witnessed in Massacb.usetts and Maryland, the
diameter of which was estimated at half a mile. In
July, 1860, a brilliant fireball passed over the State
of New York, from west to east, and was last seen
far out at sea. On the evening of Feb. 12th, 1875, a
magnificent meteor " illumined the entire State of
Iowa, and parts of Missouri, Illinois, Wisconsin, and
Minnesota. The aerolites that have been collected
show its weight to have been fully 5,000 lbs."

3. Shooting Stars. — One of the earliest accounts of
star-showers is that which relates how, in 472, the
sky at Constantinople appeared to be alive with fly-
ing stars and meteors. In some Eastern annals we
are told that in October, 1202, "the stars appeared
like waves upon the sky. They flew about like
grasshoppers, and were dispersed from left to right."
It is recorded that in the time of King William II.
there occurred in England a wonderful shower of
stars, which "seemed to fall like rain from heaven.
An eye-witness, seeing where an aerolite fell, cast
water upon it, which was raised in steam, with a
great noise of boiling." *

Showers of 1799 and 1833.— The most remark-
able accounts are those of the showers of November
12th, 1799, and November 13th, 1833. Humboldt, in
describing the former, says the sky was covered

* Rastel says concerning it : " By the report of the common people in this kynge's
time, diverse great wonders were seene, and tlierefore the kynge was told by diverse pjf
his familiars that God was not content with his lyvyng."

180 THE SOLAR SYSTEM.

with innumerable fiery trails, which incessantly
traversed the sky. From the beginning of the phenom-
enon, there was not a space in the heavens three
times the diameter of the moon that was not filled
every instant with the celestial fireworks, — large
meteors blending constantly their dazzling brilliancy
with the long phosphorescent paths of the shooting
stars. (See notes, p. 305.)

The latter shower was most brilliant on this conti-
nent, and was visible from the lakes to the equator.
Phosphoric lines swept over the sky like the flakes
of a snow-storm. Large meteors darted across the
heavens, leaving luminous trains behind them that
were visible sometimes for half an hour : they gen-
erally shed a soft white light : occasionally, how-
ever, yellow, green, and other colors varied the
scene. Irregular fireballs, almost stationary, glared
in the sky ; one especially, larger than the moon,
hung in mid-air over Niagara Falls, and mingled its
light with the foam and mist of the cataract. In
many sections, the people were terror-stricken by
the awful spectacle, and supposed that the end of
the world had come.

Inferior showers were seen in 1831, and 1832, and
in the succeeding years, until 1839. These did not
compare in brilliancy with the remarkable phenom-
enon of 1833. There was an interval of about 3-4
years between the great showers of 1799 and 1833 ; this
seemed to indicate another shower in 1866 or 1867.

In November, 1866, the people of both hemispheres
were literally awake to the subject. Newspapers
aroused the mpst sluggish imagination with thrilling

METEORS AND SHOOTING STARS. 181

accounts of the scenes presented in 1790 and 1833.
Extempore observatories were established at every
convenient point. Watchmen were stationed, and
the city bells were to be rung on the appearance of
the first wandering celestial visitor. The exact
night was not definitely known, but, for fear of a
mistake, the 11th, 12th, and 13th were generally ob-
served. The anxious vigils, the fruitless scannings
of the sky, the disappointment, the meteors that
were dimly thought to be seen, — all these were re-
corded in the memory of the temporary astronomers
of that year.

While, however, the people of America were thus
disappointed, there was enacted in England a dis-
play brilliant indeed, though inferior to the one of
1833. The staff at Greenwich Observatory counted
about 8,000 meteors.

In November, 1867, the long-expected shower was
seen in this country, but it failed to satisfy the pub-
lic anticipation. The sky was, however, illumined
with shooting stars and meteers, some of which ex-
ceeded Jupiter or Venus in brilliancy.

Number of Meteors and Shooting Stars. — Prof,
Newton estimates that the average number of me-
teors that traverse the atmosphere daily, and which
are large enough to be visible to the eye on a dark,
clear night, is 7,500,000 ; and if to these the tele-
scopic meteors be added, the number would be in-
creased to 400,000,000. In the space traversed by
the earth, there are, on the average, in each volume
the size of our globe (including its atmosphere), as
many as 13,000 small bodies, each one capable of

182 THE SOLAH SYSTEM.

furnishing a shooting star visible under favorable
circumstances to the naked eye.

Annual Periodicity of the Star-Showers. — On al-
most any clear night,, from five to seven shooting
stars may be seen per hour, but in certain months
they are much more abundant. Arago names the
following principal dates :

April 4-11 ; 17-25. October (about) 15.

August 9-11. November 13-14.

Origin. — Aerolites, meteors, and falling stars are
produced by small bodies — planets in miniature —
revolving, like our earth, about the sun. Their or-
bits intersect the orbit of the earth, and if, at any
time, they reach the point of crossing exactly with
the earth, there is a collision. Their mass is so
small, that the earth is not jarred any more than
a railway train would be by a pebble thrown
against it.

These small bodies may come near the earth and
be drawn to its surface by the power of attraction ;
or they may sweep through the higher regions of
the atmosphere, and then escape its grasp : or,
finally, they may, under certain conditions, be com-
pelled to revolve many times around the earth as
satellites.

The November " meteoroids " (as these bodies are
called before igniting) move at the rate of 26 miles
per second in a direction nearly opposite that of the
earth. They, therefore, meet our atmosphere with a
relative velocity of 44 miles per second. As they
sweep through the air, the friction partly arrests

METEORS AND SHOOTING STARS. 183

their motion, and converts it into heat and light.
The body thus becomes visible to us. Its size and
direction determine its appearance. If very small,
it is consumed in the upper regions, and leaves only
the luminous trail of a shooting star. If of large
size, it may sweep along at a high elevation, or
plunge directly toward the ground. Becoming
highly heated in its course, it sheds a vivid light,
while, unequally expanding, it explodes, throwing
off large fragments which fall to the earth as
aerolites, or continue their separate course as me-
teors. The cinders of the consumed portion rain
down on us as fine meteoric dust. *

Meteoric Rings. — These little bodies, it is thought,
do not generally revolve individually about the sun,
but myriads of them are collected in a ring.
"When the earth passes through one of these floating
girdles, a star-shower follows. This would account
for their regular appearance at certain seasons of
the year. The November meteoroids are not, like
the August ones, uniformly distributed through the
ring, but are principally collected in a swarm that
has a period of 33^ years ; hence the August shower
occurs quite regularly each summer, while the great
November one happens only three times in a cen-
tury. The orbit of the November stream extends
beyond that of Uranus. The point where it crosses
the earth's orbit moves forward about 50" per annum,
and thus that star-shower occurs about a day later
at each return. It takes three or four years for this

♦ Prof. Young estimates that 100 tons of meteoric matter fall upon the earth daily
from outer space.

184

THE SOLAR SYSTEM.

Fig. 75.

swarm to pass the node, showing that the shoal of
meteoroids occupies about -^ of its orbit. The earth

in its annual revolution about
the sun is supposed to en-
counter several hundred of
these meteoric rings.

The Physical Relation be-
tween meteoroids and comets
is now generally acknowl-
edged. The orbit of the Au-
gust meteors is known to be
identical with Comet III 1862
(Swift's), and that of the
November 14th shower cor-
responds with Comet I 1866
(Tempel's). The small show-
ers of November 24 and 27
are thought to be produced
by meteors traveling in the
path of the two dissevered
parts of Biela's comet.

The grand problem of me-
teoric astronomy to-day is to
identify the numerous mete-
oric rings, and to detect their
allied comets. Being thus
intimately associated, they
must have a common history.
Prof. Newton, the great ad-
n 7, , r,* . . ,r . vocate of this theory, broadly

Orhit of the August Meteors. •' ' -^

asserts that every meteoric
stone was once a part of a comet, and every mete-

COMETS. 185

oric shower consists of broken fragments of some
known or unknown comet.

Radiant Point. — The meteoroids are, of course,
moving in parallel lines, but, by an optical illusion,
they seem to radiate in all directions, the radiant
point being in that part of the heavens which the
earth is then approaching. * A star (m) in the blade
of the sickle is the point from which the stars in the
November shower radiate, while one in Perseus (y)
is the radiant point of the August shower.

Height. — Herschel estimates the average height of
shooting stars above the earth to be seventy-three
miles at their appearance, and fifty -two at their
disappearance.

Weight.— Prof. Harkness calculates that the aver-
age weight of shooting stars does not differ much
from one grain.

IV. COMETS.

We come now to notice a class of bodies the most
fascinating, perhaps, of any in astronomy. The
suddenness with which comets flame out in the
sky, the enormous dimensions of their fiery trains,
the swiftness of their flight, the strange and mys-
terious forms they assume, their departure as un-
heralded as their advent, — all seem to bid defiance
to law, and partake of the marvellous. Superstitious

* The same illusion is seen if, looking upward, we watch snow-flakes falling during a
calm. Those coming directly toward our eyes seem to be motionless, and the rest to
separate from them in diverging lines. This is the effect of perspective, and the " rad-
iant point" is really the "vanishing point" of the parallel lines through which the
meteors are moving. See Newcomb's Astronomy, p. 399.

186 THE SOLAR SYSTEM.

fears have been excited by their appearance, and
they have been looked upon in every age as

" Threatening the world with famine, plague, and war ;
To princes, death ; to kingdoms, many curses ;
To all estates, inentable losses ;
To herdsmen, rot ; to plowmen, hapless seasons;
To sailors, storms ; to cities, civil treasons." •

Description. — The term comet signvQ.es a.hairy body.
A comet consists usually of three parts ; — the nucleus,
a bright point in the center of the head ; the cotna
(hair), the cloud-like mass surrounding the nucleus ;

Fig. 76. Fig. 77.

Comet Kithout a Nufleus. Comet mth a Nitdeus.

and the tail, a luminous train extending generally in
a direction opposite to the sun. There ai-e comets
without the tail, and others with several tails, while
some are deprived of even the nucleus. The last
consist merely of a fleecy mass, known to be a
comet from its orbit and rapid motion.

• Thas the comet of 43 b. c, which appeared just after the assassination of Julias
CiBsar, was looked upon by the Romans as a celestial chariot sent to convey his soul
heavenward. An old English writer observes : " Cometes signifle corruptions of the
a\Te. Tliey are signes of earthquakes, of warres, of changyng kjTigedomes, great
dearthe of com, yea, a common death of man and beast." Another remarks : " Expert
ence is an eminent e\'idence that a comet, like a sword, portendeth war ; and a hairy
comet, or a comet with a beard, denoteth the death of kings, as if God and nature in-
tended by comets to ring the knells of princes, esteeming bells in churches upon earth
Dot sacred enough for such illustrious and eminent performances."

eoMEtg. 187

Comets are not confined, like the planets, to the
limits of the zodiac, but appear in every quarter of
the heavens, and move in every conceivable direc-
tion. When first seen, the comet resembles a faint
spot of light upon the dark background of the sky:
as it approaches the sun the brightness increases,
and the tail begins to show itself. Generally it is
brightest near perihelion, and gradually fades away
as it recedes, until it is finally lost, even to the
telescope.*

The Time of Greatest Brilliancy depends some-
what on the position of the earth. If, as represented
in the figure, the earth is at

° ' Fig. 78.

a when the comet, moving to-
ward perihelion, is at r, the
comet will appear more dis-
tinct than when it is more dis-
tant at P, although at the latter
point it is really brighter. If,
however, the earth is at c omt o/ coraet.

at the time of perihelion, the

comet will be much more conspicuous. Again, if
the earth is passing from a to 6 during the time

* While a comet remains in regions beyond the planets, where the temperature is be-
low — 140° C, its matter must be chiefly solid or liquid. On its approach to the sun, its
enveloping atmosphere (if none existed, one will now be formed) will expand, and the
nucleus will appear, surrounded by a blaze of light, feeble at first, but becoming more
and more brilliant, and so producing the head, or coma, of the comet. Many comets do
not go beyond this first phase, and, being exposed only to a moderate heat, remain
telescopic. Others, piercing further the solar system, and reaching a higher tempera-
ture, develop a more abundant atmosphere. The sun, while attracting to himself the
nucleus, has power to repel some of the matter of the atmosphere ; how or why, we
know not. Enough, that certain parts fly ofl" as if driven by a gale, so making the tail,
which increases more and more until the atmosphere is exhausted. Meanwhile, remark-
able changes take place in the nucleus. Eruptions occur. Pieces are sometimes thrown
off large enough to form a new comet, and showers of spark-like particles, with oecasion-
ally stony masses, fill the orbit of the comet with meteoroids.— Sc/iiaporeUi.

188 THE SOLAR SYSTEM.

the comet is near the sun, it will appear less brill-
iant than if the earth were moving from c to d, as
we should then be much nearer it during its greatest
illumination.

Number of Comets. — Kepler remarks that "there
are as many comets in the heavens as fish in the
sea." Arago, basing his calculations on the number
known to exist between the sun and Mercury, has
estimated that there are 17,500,000 within the solar
system. Of this vast number, few are visible to the
naked eye, and a still less number attract observa-
tion, owing to their inferior size and brilliancy.
Many are doubtless lost to our sight by being above
the horizon in the daytime. During the eclipse of
1882, Lockyer, who was in Egypt to take observa-
tions, saw a brilliant comet near the sun.

Orbits of the Comets. — Comets form a part of the
solar system, and are subject to the laws of gravita-
tion. Like the planets, they revolve around the sun,
though they differ in the form of their orbits. While
the planets move in paths varying but little from
circular, and thus never depart so far from the sun as
to be invisible to us, the comets travel in extremely
elongated (flattened) ellipses, so that they can be ob-
served by us through only a small portion of their
paths.

In Fig. 79 are represented the three general classes
of cometary orbits. A comet traveling along an
elliptical orbit, though it may pass far from the sun,
will yet return within a fixed time ; one pursuing
either a parabolic or hyperbolic curve cannot return,
as the two sides separate from each other more and

COMETS.

169

more. Many of the comets of the first class have been
calculated, and they have repeatedly visited our por-
tion of the heavens ; while those of the other classes,
having once visited our system, go away forever,

Fig. 79.

Three Forms of Cometary Orbits.

seeking perhaps in the far-off space another sun,
which in turn they will abandon as they have our own.
Calculation of a Comet's Return. — As we can c""
serve so small a proportion of the entire orbit, ^^ p^^-
very difficult, indeed oftentimes impossible, to d*

idO

THE SOLAR SYStEiM.

whether it is an hyperbola, an ellipse, or a parabola.
A few are known to move in elliptical paths, and
their orbits have been so accurately computed that it
is possible to predict the time of their appearance.
The other comets may never return, or at least not

Fig. 80.

Projections of a few Cometary Orbits on the Plane of the Ecliptic,

for centuries hence. They may be paying our sun
their first visit ; or, if they have swept through the
"^vlar system before, it may have been at so remote a
eitie that no record is preserved, even if it were not
as tre the creation of man. Under these circum-

COMEfS. 131

stances, it is difficult to determine the place of these
apparently erratic wanderers ; yet, in spite of all
these obstacles, some have been tracked into space
far beyond the telescopic view. For example, the
comet of 1844 is announced to pay a visit to the
astronomers of the year of our Lord 101,844. The
period of the comet of 1744, is fixed at 123,683
years.

Distance from the Sun. — Some comets at their peri-
helion sweep near the sun. Thus the one of 1680 came
where the temperature was estimated by Newton to
be about 2,000 times that of red-hot iron. * The near-
est approach known is that of the comet of 1843,
whose perihelion distance was but about 30,000 miles
from the surface of the sun ; in fact, it doubled
around that body in two-hours time. (Guillemin.)f
The greatest aphelion distance yet estimated is that
of the comet of 1844, which is over 400,000,000,000
miles. The velocity varies, of course, with the
position in the orbit. The comet of 1680 moved in
perihelion at the rate of over two hundred and
seventy-seven miles per second ; while in aphelion
its velocity is only about six miles per hour.

Density of Comets. — The quantity of matter con-
tained in a comet is exceedingly small. Even tele
scopic stars are visible through the densest part. The
comet of 1770 became entangled among Jupiter's

* The comet of 1680 excited such terror in Europe that a medal was struck, to quiet
the fears of the people. The inscription read thus: "The star threatens evil things;
trust only ! God will turn them to good." Newton calculated the oi bit of this comet
and proved that the comet moves around the sun in obedience to the law of gravity.

t The comet of 1843 excited much interest in this country since one Miller had pre-
dicted that the end of the world would come in that year; his followers imagined this
comet presaged the destruction of all things.

192 THE SOLAR SYSTEM.

moons, and remained there four months without in
terf eriug with their movements ; indeed, so far from
that, its own orbit was so much changed by their
proximity, that, from a periodical return of 5f years,
it has not been seen since. We have good reason to
suppose that the earth, in 1861, passed through the
tail of a comet, its presence being indicated only by
a peculiar phosphorescent mist. So that even should
our earth run full-tilt against a comet, the shock
might be quite imperceptible.* Still, however
lightly we may speak of the probability of such a
collision, we must remember that there are comets
of greater solidity. Donati's, for instance, is esti-
mated by some to be about yws the mass of the earth.
The concussion of such a body, moving with the
speed of a cannon-ball, would undoubtedly produce
a very sensible effect.

It is not determined whether comets shine by their
own or by reflected light. If, however, their nuclei
consist of white-hot matter, a passage through such
a furnace would be anything but desirable or satis-
factory. After all the calculations of Astronomy,
our only safety lies in that Almighty Power which
traces the path and guides the course alike of
planets and comets : He, whose eye marks the fall

* " However dangerous might be the shock of a comet, it might be so slight that it
would onlr do damage to that part of the earth where it actuaUy struck ; perhaps,
even, we might cry quits, if, wliile one kingdom were devastated, the rest of the earth
were to enjoy the rarities which a body coming from so far might bring to it. Perhaps
we should be very surprised to find that the debrU of these masses that we despised
were formed of gold or diamonds ; but who would be the more astonished — we or the
comet-dwellers who would be cast upon our earth? What strange beings each would
find the other?" Lettre sur la Comete, par M. De Manpertuis.

Young says, " It seems, on the whole, more probable that a comet is only a cloud of
dust and vapor- a smcike-wreath — than that there is at the center any solid kernel. A
comet is a mere airy nothing."

COMETS. 193

of the sparrow, sees as well the flight of* the worlds
He has created.

Variations in Form and Dimensions. — Comets ap-
pear to be subject to constant variations. They are
now thought generally to decrease in brilliancy at
each successive revolution about the sun. The same
comet may present itself sometimes with a tail, and
sometimes without. When the comet first appears,
there is commonly no tail visible, and the light is
faint. As it approaches the sun, however, its bright-
ness increases, the tail shoots out from the coma,
and grows daily in length and splendor. Supernu-
merary tails, shorter and less distinct than the prin-
cipal one, dart out, but they generally soon disap-
pear, as if from lack of material. The tail of the
comet of 1843, just after the perihelion, increased in
length 5,000,000 miles per day. As the tail thus ex-
tended, the nucleus was correspondingly contracted,
so that this comet actually " exhausted its head in
the manufacture of its own tail."

Remarkable Comets. — Among the many comets
celebrated in history, we shall notice only some of
those that have appeared in the present century.
The great comet of 1811 was a magnificent spectacle. *
The head was 112,000 miles in diameter ; the nucleus
was 400 miles ; while the tail, of a beautiful fan-
shape, stretched out 112,000,000 miles. "The aphelion
distance of this comet is fourteen times that of Nep-
tune, or 40,000,000,000 miles. It is announced to re-
turn in thirty centuries ! " To what profound depths

* This was considered by tlie Russians to presage Napoleon's Invasion.

194 THE SOLAR SYSTEM.

of space, beyond the solar system, beyond the reach
of the telescope, must such a journey extend !

Fig. 81.

Coggia's Comet, 1S7L

The Comet of 1835 is known as Halley's comet.
This is remarkable as being the first comet whose
period of revolution was satisfactorily established.
Dr. Halley, on examining the accounts of the great
comets of 1531, 1607, and 1682, suspected that they
were the reappearances of the same comet, whose
period he fixed at about 75 years.* He finally ven-

• The history of this comet, as it has been traced back by it^ period of seventy-five
years, is quite eventful. It was seen in England in 1066, when it was looked upon with
dread as the forerunner of the \ictorj' of William of Normandy. It was then equal to
the full moon in size. In 1456, its tail reached from the horizon to the zenith. It was

COMETS. 195

tured to predict the return of the coined at near the
end of 1758 or beginning of 1759. Although Halley
did not live to see his prophecy fulfilled, great in-
terest was felt in the result. It was not destined,
however, for a professional astronomer to be the

Fig. 8S.

Donati's Coniel.

first to detect the comet. A peasant near Dresden
saw it on Christmas night, 1758.

supposed to indicate the success of Mahomet II., who had already taken Constantinople,
and then threatened the whole Christian world. Pope Calixtus III., therefore, ordered
extra Ave Marias to be repeated by everybody, and also the church bells to be rung
daily at noon (whence originated the custom now so universal). A prayer was added as
follows : "Lord, save us from the devil, the Turk, and the comet." In 1223, it was con-
sidered the precursor of the death of Philip Augustus of France. The first recorded ap-
pearance of Halley's comet was b. c. 130, when it was supposed to herald the birth of
Mithridates.

19G THE SOLAR SYSTEM.

The Comet of 1843 was so brilliant that it was
visible in full daylight. It was so near the sun at
perihelion as " almost to graze his surface."

Encke's Comet has a period of only 3^ years. A
most interesting discovery has been made from ob-
servations upon its motion. The comet returns each
time to its perihelion about 3^ hours earlier than the
calculations indicate. Hence, Prof. Encke has been
led to conjecture chat space is filled with a thin,
ethereal medium capable of diminishing the centri-
fugal force, and thus contracting the orbit of a comet.

DoNATi's Comet (1858) was the subject of universal
wonder. When first discovered, in June, it was
240,000,000 miles from the earth. In August, traces
of a tail were noticed, which expanded in October to
about 50,000,000 miles in length. This comet, though
small, has never been exceeded in the brilliancy of
the nucleus and the graceful curvature of the tail.
It will return in about 2,000 years.

The " Great Comet of 1882 " had, soon after pass-
ing its perihelion, a nucleus as bright as a star of the
1st magnitude, and a tail 60,000,000 miles long. The
aphelion of its orbit is six times further than Nep-
tune from the sun, and the comet's period is esti-
mated at between eight and nine centuries.

V. ZODIACAL LIGHT.

Description. — If we watch the western horizon in
March or April, just after sunset, we shall sometimes
see the short twilight of that season illuminated by

ZODIACAL LIGHT.

197

a faint nebulous light, of a conical shape, flashing
upward, often as high as the Pleiades. In Septem-
ber and October, at early dawn, the same appearance

Fig. SS.

Zodiacal Light.

can be detected near the eastern horizon. The light
can be seen in this latitude only on the most favor-
able evenings, when the sky is clear and the moon
absent. Even then, it will be frequently confounded
with the Milky Way or auroral lights. At the base.

198 THE SOLAR SYSTEM.

it is of a reddish hue, where it is so bright as often
to efface the sraaller stars. In tropical regions, the
zodiacal light is perpetual, and shines with a bril-
Uancy sufficient, says Humboldt, to cast a sensible
glow on the opposite part of the heavens.

Origin. — The commonly-received opinion is, that it
is caused by a faint, cloud-like ring, perhaps a
meteoric zone, that surrounds the sun, and becomes
visible to us only when the sun himself is hidden
below the horizon. Others maintain that, since it
has been seen in tropical regions in the east and the
west simultaneously, it can be explained only on the
theory of a " nebulous ring that surrounds the earth
within the orbit of the moon."

PRACTICAL QUESTIONS.

1. Would the earth rise and set to a Lunarian ?

2. Could there be a transit of Neptune ?

3. Why does Mars's inner-moon rise in the Avest ?

4. In what part of the sky do you always look for the planets ?

5. Show how it was impossible for the darkness that occurred at the
time of the Cmcifixion of Christ to have been caused by an eclipse of the
sun.

6. Is there any danger of a collision between the earth and a comet ?

7. How are aerolites distinguished ?

8. "When do we see the old moon in the west after sunrise ?

9. When do we see the moon high in the eastern sky in the afternoon
before the sun sets ?

10. When is a planet morning, and when evening, star ?

11. Is the sun really hotter in summer than in winter?

12. Why is a planet invisible at conjunction ?

13. Must an inferior planet always be in the same part of the sky as tb?
sun ? A superior planet J

PRACTICAL QUESTIONS. 199

14. Why, in summer, does the sun, at rising and at setting, shine on
the north side of certain houses ?

15. What effect does the volume of a planet have upon the force of
gravity at its surface ?

16. In what part of the heavens do we see the new moon ? The old
inoon ? The crescent moon ?

17. What is the Golden Number in the almanac ?

18. Why do we have more lunar than solar eclipses ?

19. In what direction do the horns of the moon turn ?

20. Is the " tidal- wave " an actual movement of the water ?

21. Why does the sun " cross the line " in some years on March 21, and,
in others, on March 22 ?

22. Do we ever see the sun where it really is ?

23. At Edinburgh, Scotland, there are times when the sun rises at 3^
o'clock A. M. and sets at 8^ o'clock p. M., and the twilight lasts the entire
night. When and why is this ?

24. Which is the longest day of the year ?

25. Is the moon nearer to us when it is at the horizon, or at the zenith ?

26. How many solar eclipses would happen each year if the orbits of the
sun and the moon were in the same plane ?

27. Is there any heat in moonlight ?

28. Can we see the moon during a total eclipse ?

29. Which of the planets are repeating a portion of the earth's history ?

30. How many times does the moon turn on its axis each year ?

31. Can you explain the different signs used in the almanac ?

32. Show how the moon is a prophecy of the earth's future.

33. Does the sun really rise and set ?

34. Are the bright portions of the moon mountains or plains ?

35. Which of the heavenly bodies are self-luminous ?

36. VVliy is not a solar eclipse visible on the whole earth ?

37. What is meant by the " mean distance " of a planet ?

38. What keeps the earth in motion around the sun ?

39. Do we ever see the sun after it sets ?

40. When does the earth move the most rapidly in its orbit ?

41. Have we conclusive evidence that any planet is inhabited ?

42. When is the twilight the longest? The shortest ? Whj^ ?

43. What is a moon "j

200 PRACTICAL QUESTIONS.

44. To a person in the south temperate zone, wliere would the sun be
at noon ?

45. Is it correct to say that the moon revolves about the earth, when we
know that, according to the law of Physics, they must both revolve about
their common center of gravity ?*

46. During a transit of Venus, do we see the body of the planet itself on
the face of the sun ?

47. How many real motions has the sun ? How many apparent ones ?

48. How many real motions has the earth ?

49. Can an inferior planet have an elongation of 90° ?

50. How do we know the intensity of the sun's light on the surface of
any of the planets ?

51. Why is the Tropic of Cancer placed where it is ?

52. What planets would float in water ?

53. How must the moons of Jupiter appear during their transit across
the disk of that planet ?

54. "The shadow of the satellite precedes the satellite itself when Ju-
piter is passing from conjunction to oi)position, but follows it between
opposition and conjunction." Explain.

55. What facts point to the conclusion that Mars may, perhaps, have
passed his planetary prime ?

56. Why may we conceive that Saturn and Jupiter are yet in their
planetary youth ?

57. Show how, if the Nebular Hypothesis (p. 256) be accepted, the
fashioning of a planet must require an enormous length of time.

58. Do we know the cause of gravitation ?

* "Strictly speaking, the moon does not revolve around the earth, any more than
the earth around the moon ; but, by the principle of action and reaction, the center of
each body moves around the common center of gravity of the two bodies. The eartli
being eighty tiniis as heavy as the moon, this center is situated within the former,
about three-quarters of the way from its center to its surface."— i^ewcomh's Astronomy,
p. 91.

III.
THE SIDEREAL SYSTEM.

•' He telleth the number of the stars / He calleth them all by theit
names,"

Psalm cxlvii. 4.

L The Stabs.

f 1. fixED Staks KOt SEEir.

2. Parallax ksi> Distawce.

3. Motion.

4. Stars ake Srss.

5. OcB Si-s A Star.

6. Solar System ix Monos.

7. Number of Stars.

8. Scintillation.

9. Magnitude.

10. Cause of Difference in Brightness.

11. Sames.

VI. The Constellations.

13. Invention of Constellations.

14. Signs and Constellations not agreeing

15. Perma>"ence of Constellations.
Ifi. Y.^lit; of Stars.

17. Ancient Views.
IS. Three Zones.

IL The Constel-
lations.....

( 1. How traced.

•2. Ursa Major.

3. Ursa Minor.

1. Northern Cir- )

;

CCMPOLAR Con- '

STELLATIONS.for

4. Draco. >

Latitude of New

5. Cepheus.

York. I <J. Cassiopeia.

1. How TRACED.

2. Perseu:^.

3. AiJDROMEDA.

4. Aries.

5. Taubcs.

6. Auriga.

7. Pisces.

8. Cetus.

9. Gemini.

10. Orion.

11. Canis.

12. Leo.

2. EtJCATORIAL

13. Cancer.

Constellations.

14. YiRoo.

15. Hydra.

16. Canes Venatcci.

17. Berenice's Hair,

IS. Bootes.

19. Hercitles.

20. Corona.

21. Serpentarius.

22. Libra.

23. SAonT.uiius.

24. Capricornvs.

3. The SoiTHERN

25. Cygnus.

, Con-stell-\tion-s.

l,2t<. Lyra.

^ a. Description.
! 6. Principal Stare.
V c. Mythological Hist-
' d. Distance of Polaris.

) t. Latitude.

a. Description.
6. Principal Stare,
c. M>-thological Hist

in. Double St.uis, St.vr Clusters,
Colored Stars, etc

IV. Celestial Chemistry

VL Celestial MEASUBEMt-vre .

r 1-^. Double Stars, Colored Stars, Variabli
I Stars, Temporary Stars, Star Clu-s

J TERS, NEBUL.t:.

\ 7. Magellanic Clouds.

I S. The Milky Way.

V9. The Nebular Hypothesis,

1. Spectrum Analysis.

2. Spectroscope.

3. Revelations Concerning Srs.

4. Concerning Stars.

1 5. Concerning Nebul.e.
V^6. Concerning Solar Fl.oie3.
/ 1. Sidereal.
J 2. Solar.
1 3. Mean Solar.
I 4. Sun-dial, etc.

' 1 To Find Distance of Pl-Ocets from sun.
I •'! To Fint) Moons Distance from Earth.
^ 3. To Find Sun's Distance from Earth.
L 4. To FiifD LoKcrruDK op a Place, etc.

THE SIDEREAL SYSTEM.

I. THE STARS.

IN our celestial journey we have reached Nep-
tune, the sentinel outpost of the solar system.
We are now nearly 2,800,000,000 miles from our
sun. Yet we are apparently no nearer the fixed stars
than when we started. They twinkle as serenely
there in the far-off sky as to us here on the earth.
The heavens by night, with the exception of a few
changes in the planets, look familiar. Between them
and us there is still a vast chasm which no imagi-
nation can bridge ; a distance so immense that figures
are meaningless, and we can only call it space, — so
profound that to us it is limitless, though beyond we
see other worlds twinkling, like distant lights over a
waste of waters.

We never see the Stars. — This assertion seems
paradoxical, yet it is strictly true. So far are the
stars removed from us, that we see only the light they
send, but not the surface of the worlds themselves.
They are merely glittering points of light. The most
powerful telescope fails to produce a sensible disk.
This constitutes a marked difference between a planet
and a fixed star.

204 THE SIDEREAL SYSTEM.

The Annual Parallax of tne Fixed Stars. — When
speaking of this subject on page 121, we said that
186,000,000 miles, or the diameter of the earth's orbit,
is the unit for measuring the parallax of the fixed
stars. Yet when the stars are viewed from even
these extreme points, they manifest so slight a
change of place, that to estimate it is one of the most
delicate feats of astronomy.

At the present time, it is considered that the star
Alpha (a) Centauri in the southern heavens is the
nearest to the earth. Its parallax is judged to be
about 1". Its distance is more than 200,000 times that
of the earth from the sun, or twenty trillions of miles.
This is probably by no means its actual distance,
but merely the limit within which it cannot be, but
beyond which it must be.*

These figures convey to our mind no idea of dis-
tance. Our imagination fails to grasp the thought,
or to picture the vast void across which we are gaz-
ing. We remember that light moves at the rate of
186,000 miles per second. A ray at that speed would,
in one day, plunge out into the abyss beyond Nep-
tune six times the distance of that planet from the
sun. Yet it must sweep on at this prodigious speed,
day and night, for over 3^ years to span the gulf

* David Gill, the Royal Astronomer at the Cape of Good Hope, has recently deter-
mined the parallax of a Centauri to be 0".75. This would make its distance 275,0CC
astronomical units. 275,000 x 93,000,000 miles = over 25^ trillion miles. Light
would require about i\ years to travel this enormous distance. Vega's parallax is
placed at not far from 0".2, which indicates a distance of about 1,000,000 astronomical
units. Hence, Vega shines upon us from the inconceivable distance of niiiety-three
trillion mile.i.' The parallax of Sirius has been variously estimated at from 0".16 to
0".38. Newcomb places this star at more than a million radii of the earth's orbit away
from us, yet its light is four times as brilliant as that of any other star. The difficulty
of measuring the stellar parallax may be judged from the fact that 1" measures the angle
at wliich a globe three-tenths of an inch in diameter would be seen when a mile away.

. THE STARS. 205

and reach a stopping point at the nearest fixed star.
It has been estimated that the average time re-
quired for the light of the smallest stars which are
visible to the naked eye to reach the earth is about
125 years. What, then, shall we say of those far-
distant ones, whose faint light appears as a mere
fleecy whiteness even in the most powerful tele-
scopes? The conclusion is irresistible, that the light
we receive set out on its sidereal journey far back in
the past, perhaps before the creation of man!

Motion of the Fixed Stars. — It will aid us still
further in comprehending the immense distances of
the stars, to learn that, though they seem to be fixed,
they are moving much more swiftly than any of
the planets. Thus, Arcturus flies through space at
the astonishing rate of 200,000 miles per hour, or
nearly twice that of Mercury, and more than three
times that of the earth. Yet, through all our life-
time, we shall never be able to detect any change in
its position. " It requires three centuries for it to
move over the starry vault a space equal to the
moon's apparent diameter."

The Stars are Suns.— The vast distance at which
the stars are known to be, precludes the thought of
their shining, like the planets or the moon, by reflect-
ing back the light of our sun. They must be self-
luminous, and are doubtless each the center of a
system of planets and satellites.

Our Sun a Star. — As we see only the suns of these
distant systems, so their inhabitants see only the sun
of our system, and that as a small star.

Our System in Motion. — Like all the other stars.

THE SIDEREAL SYSTEM.

our sun is in motion. It is sweeping onward, with
its retinue of worlds, 150,000,000 miles per year, toward
a point in the constellation Hercules. The Pleiades
has been thought to be the center around which this

Mg. 8L

A part of the Constellation of the Twins.

great movement is taking place, but most astron-
omers consider the idea as a mere speculation.

The Number of the Fixed Stars.— When we look at
the heavens on a clear night, the stars seem innumer-

tH£ STAflg. ^0?

able. To count them, one would think almost as in-
terminable a task as to number the leaves on the
trees. It is, therefore, somewhat startling to learn
that the entire number visible to the most piercing
eyesight does not exceed 6,000, while few can dis-
cern more than 4,000.* The number, however, which
may be seen with a telescope is marvellous. In Fig.
84, is shown a portion of the heavens where the
naked eye sees but six stars. Could we examine the
same region of the sky with more powerful instru-
ments, new constellations would doubtless be des-
cried in the infinite depths of space.

Scintillation. — The twinkling of the fixed stars is
due to what is termed in Physics the "Interference
of Light." The air, being unequally dense, warm,
and moist in its various strata, transmits very irregu-
larly the different colors of which white light is com-
posed. Now one color prevails over the rest, and
now another, so that the star appears to alter its hue
incessantly. As the purity and density of the air
vary, the twinkling of the stars also changes, and,
therefore, it is always greatest near the horizon, t

Magnitude of the Stars. — As the telescope reveals
no disk of even the nearest stars, we know nothing
of their comparative size. The finest spider's thread,
placed at the focus of the instrument, hides the star
from the eye. When the moon passes in front of a

* This illusion may be easily exiilained, when we remember how the impression of a
bright light remains upon the retina, as in the whirling of a firebrand.

t Humboldt says that at Cumana, in South America, where the air is remarkably
pure and uniform in density, the stars cease to twinkle after tliey have risen 15° above
the horizon. This gives to the celestial vault a peculiarly calm and soft appearance. — It
should he noticed that interference occurs only when the light emanates from a point.
A body that subtends a visual angle, i. e., has a sensible disk, like a planet, cannot twinkle.

208

THE SIDEREAL SYSTEM.

star, the occultation is instantaneous, and not gradual,
as in the case of the planets. Classification depends,
therefore, merely upon their relative brightness.
The most conspicuous are termed stars of the first

Fin. sr,.

magnitude; of these there are about twenty. The
number of second-magnitude stars in the entire
heavens is sixty -five ; of the third, about 200 ; of the
fifth, 1,100 ; of the sixth, 3,200 ; of the seventh, 13,000 ;
of the eighth, 40,000 ; and of the ninth, 142,000. Few
persons can see smaller stars than those of the fifth
or sixth magnitude.

The DiflFerence in the Brightness of the stars may
result from a dilBference in their distance, size, or
intrinsic brightness. Hence it follows that the faint-
est stars may not be the most distant from the earth.

Names of Stars. — Many of the brightest stars
received proper names at an early date ; as Sirius,
Arcturus. The chief stars of each constellation are
distinguished by the letters of the Greek alphabet ;

The Greek Alphabet.

Alpha

Iota

P p Rlio

Beta

Kappa

2 <T Sigma

Gamma

Lambda

T T Tau

Delta

Y V Upsilon

Epsilon

* <t> Phi

Zeta

X X Chi

Eta

Omicron

♦ ^ Psi

Theto

Q 0} Omega

THE STAKS. 209

the brightest one being usually called Alpha (a), the
next Beta (j"^), etc., — the name of the constellation, in
the genitive case, being put after each. Ex., a Arie-
tis, (3 Lyrse.*

Star catalogues are issued, containing the stars
arranged in the order of their Right Ascension, and
numbered for convenience of reference. Argelan-
der's Charts have 334,188 stars marked in the north-
ern hemisphere.

The Constellations. — From the earliest ages, the
stars have been arranged in constellations, for the
purpose of more readily distinguishing them. Some
of these groups were named from their supposed re-
semblance to certain figures, such as perching birds,
pugnacious bulls, or contorted snakes, while others
do honor to the memory of classic heroes.

"Thus monstrous forms, o'er heaven's nocturnal arch,
Seen by the sage, in pomp celestial march ;
See Aries there his glittering bow unfold,
And raging Taurus toss liis horns of gold ;
With bended bow the sullen Archer lowers,
And there Aquarius comes with all his showers ;
Lions and Centaurs, Gorgons, Hj'dras rise.
And gods and lieroes blaze along the skies."

With a few exceptions, the likeness is purely fan-
ciful. Not only are the figures uncouth, and the
origin often frivolous, but the boundaries are not
distinct. Stars occur under different names ; while
one constellation encroaches upon another, f Though,

* This means a of Aries, /3 or Lyra ; the genitive case in Latin being equivalent to the
preposition of.

t Chambers well remarks, "Aries should not have a horn in Pisces and a leg in Cetus,
nor should 13 Argos pass through the Unicorn's flank into the Little Dog. 51 Oamelopar-
dali might with proiiriety be extracted from the eye of Auripa, a)!"! the ribs of Aquarius
Released froin 4C Capricorni."

210 THE SIDEREAL SYSTEM.

however, the constellations are thus rude and im-
perfect, there seems little hope of any change. Age
gives them a dignity that insures their perpetua-
tion.

The Invention of the Constellations goes back into
ages of which no record remains. By some it has
been ascribed to the Greeks. When the signs of the
zodiac were named, they doubtless coincided with
the constellations. Aries (the ram) was so called
because it rose with the sun in the spring-time, and
the Chaldean shepherds named it from the flocks,
their most valued possession. Then followed, in
order, Taurus (the bull) and Gemini (the twins),
called from the herds, which were esteemed next in
value. At the summer solstice, the sun appears to
stop, and, crab-like, to crawl backward ; hence the
name Cancer (the crab). When the sun is in Leo,
the brooks being dry, the lion leaves his lurking-
place and becomes a terror to all. Virgo comes
next, when the virgins glean in the summer harvest.
At the autumnal equinox, the days and nights are
equally balanced, and this is beautifully represented
by Libra (the scales). The vegetation decays in the
fall, causing sickness and death ; the Scorpion, which
stings as it recedes, is suggestive of this Parthian
warfare. Sagittarius (the archer) tells of the hunt-
ing month. Capricornus (the goat, which delights
in climbing lofty precipices) denotes how at the
winter solstice the sun begins to climb the sky on
his return north. Aquarius (the water-bearer) is a
natural emblem of the rainy season. Pisces (the
fisHes) is the month for fishing.

THE STAES. 211

Signs and Constellations do not Agree. — By the

precession of the equinoxes, as we have before de-
scribed on page 106, the signs have fallen back along

Fig. 86.

The Signs and Constellations, us they now Compare in the Heavens, the former having
fallen back, and the latter apparently advanced, SO" each.

the ecliptic about 30°, so that those stars which were,
during the infancy of astronomy, in the sign Aries
{v) are now in Taurus (a), and those which were in
the sign Pisces (X) are now in Aries {^)*

Permanence of the Constellations. — The general
appearance of the constellations and the figures

* If the teacher will put a pin at the center of Fig. 86, and then draw a sharp knife
between the signs and the constellations, so as to detach the middle of the cut, and
cause the inner part to revolve, the signs may be turned before any constellation, an(}
|;hijs this change be clearly apprehended.

212 THE SIDEREAL SYSTEM.

which the stars form are due to the position we
oecupy. Could we cross the gulf of space beyond
Neptune, the stars now so familiar to us would look
strangely enough in their new groupings. As one in
riding through a forest sees the trees apparently
increase in size and open up to view before him.
while they decrease in size and close in behind him.
forming clusters and groups which constantly change
as he passes along, so, as our earth travels -^vith the
solar system on its immense sidereal journey,
the stars will gradually grow larger and brighter in
front, while those behind us will appear smaller and
dimmer.

Since, in addition to this, the stars themselves are
in motion with varying velocity and in different
directions, the constellations must change still more
rapidly, so as ultimately to transform the appear-
ance of the heavens. In time, the " Bands of Orion "
will be loosened, and the '' Seven Sisters " will glide
apart. Such are the distances, however, that, al-
though these movements have been going on con-
stantly, no variation has occurred, since the crea-
tion of man, that is perceptible, save to the watchful
astronomer. Nothing in nature is so invariable as
the stars. They are the standards of time. Myriads
of years must elapse before new maps of the constel-
lations will be required.

Value of the Stars in Practical Life.—" The stars
are the landmarks of the universe." They seem to
be placed in the heavens by the Creator, not alone to
elevate our thoughts and expand our conceptions of
the infinite and eternal, but to afford us, amid the

THE STARS. 313

constant fluctuations of our own earth, something
unchangeable and abiding. Every object about us
is constantly shifting, but over all shine the " eternal
stars/' each with its place so accurately marked,
that to the astronomer and the geographer no decep-
tion is possible. To the mariner, the heavens be-
come a dial-plate, the figures on its face set with
glittering stars, along which the moon travels as a
shining hand that marks off the hours with an accu-
racy no watch can ever rival. Standing on the deck
of his vessel, far out at sea, a single observation of
the sun or the stars decides his location in the waste
of waters as accurately as if he were at home, and
had caught sight of some old landmark he had
known from his boyhood. In all the intricacies of
surveying, the stars furnish the only immutable
guide. Our clocks vainly strive to keep time with
the celestial host. Thus, by an evident plan of the
Creator, even in the most common affairs of life, are
we compelled to look for guidance from the shifting
objects of earth up to the heavens above.

Ancient Views. — Anaximenes (500 b. c.) thought
that the stars were for ornaments, and were nailed
like bright studs into the crystalline sphere. Anaxa-
goras considered that they were stones whirled up
from the earth by the rapid motion of the ether, and
that its inflammable properties set them on fire and
caused them to shine as stars. Some schools of the
Grecian philosophers — the Stoics, Epicureans, etc. —
believed that they were celestial fires kept alive
by matter that constantly streamed up to them from
the center of the heavens. The stars were at one

214 THE SIDEREAL SYSTEM.

time said to feed on air ; at another, to be the breath-
ing holes of the universe.

Three Zones of Stars. — If we recall what was said
on page 90, concerning the paths of the stars and the
appearance of the heavens at different seasons of the
year, we shall see that the constellations are nat-
urally divided into three zones. The first embraces
those which are visible through the entire year ; the
second, those whose paths can be seen only in part
on any given night ; and the third, those whose paths
just graze our southern horizon, or never pass above
it.

II. THE CONSTELLATIONS.

I. The Northern Circumpolar Constellations are

visible in our latitude every night. They may be
easily traced by holding the book up toward the
northern sky in such a way that Polaris and the Big
Dipper on the map and in the heavens agree in posi-
tion, and then locating the other constellations by
comparison.

As the stars revolve about Polaris, their places will
vary with every successive night through the year.
The cut represents them as they are seen at midnight
of the winter solstice. At 6 P. M. of that day, the
right-hand side of the map should be held downward,
and the Big Dipper will be directly below the north
star. At 6 a. m., the left-hand side should be at the
bottom, and the Dipper will be above Polaris. From

THE CIRCUMPOLAR CONSTELLATIONS.

215

day to day, this aspect will change, each star coming
a little earlier to the meridian, or to its position on
the preceding night. The rate of this progression is
six hours, or 90°, in three months.

{Map No. 1.) Fig. S7.

^''.,it*

.^ i I R A F F j,- • ^ \ ^

R A

Northern Circumpotar Constellations.

Ursa Major is represented under the figure of a
great bear. It contains 133 stars visible to the naked
eye. This constellation has been celebrated among
all nations. It is remarkable that the shepherds of
Chaldeain Asia and the Iroquois Indians of America
gave to it the same name.

216 THE SIDEREAL SYSTEM.

Principal Stars. — A noticeable cluster of seven
stars — six of the second and one of the fourth mag-
nitude — forms what is familiarly termed the Dipper.
In England it is styled Charles's "Wain, from a
fancied resemblance to a wagon drawn by three
horses tandem. Mizar {^) has a ininute companion,
Alcor, which Humboldt tells us could be rarely seen
in Europe. A person with good eyesight may now
readily detect it. Megrez {6), at the junction of the
handle and the bowl, is to be marked particularly,
since it lies almost exactly in the colure passing
through the autumnal equinox. Dubhe and Merak
are termed the Pointers, because they point out the
polar star. The bear's right fore-paw and hind-paw*
are each marked by two small stars, as shown in the
cut ; a similar pair nearly in line with these denote
the left hind-paw (see £, Fig. 90).

Mythological History. — Diana had a beautiful attendant named
Callisto. Juno, the queen of heaven, becoming jealous of the maid, trans-
formed her into a bear.

" The prostrate wretch lifts up her head in prayer,
Her arms grow shaggy, and deformed with hair ;
Her nails are sharpened into pointed claws.
Her hands bear half her weight and turn to paws.
Her lips, that once would tempt a god, begin
To grow distorted in an ugly grin.
And lest the supplicating brute might reach
The ears of Jove, she was deprivetl of speech.
How did she fear to loflge in woods alone.
And haunt the fields and meadows once her own !
How often would the deep-mouthed dogs pursue,
Whilst from her hounds the frighted hunters flew."

* It is well to notice that Dubhe and Merak are about 5" apart ; Dubhe and Benet-
nasch are about 25" ajart ; the jmws of the Beai are 15" apart ; while Polaris is about
30° distont.

THE CIRCUMPOLAR CONSTELLATIONS. 21?

Some time afterward, CalJisto's son, Areas, being out hunting, pursued
his mother, and was about to transfix her with his uplifted spear, when
Jupiter in pity transferred them both to the heavens, and placed them
among the constellations as Ursa Major and Ursa Minor.

Ursa Minor is represented under the figure of a
small bear. It contains twenty-seven stars, of which
only three are of the third, and four of the fourth
magnitude.

Principal Stars. — A cluster of seven stars forms
the Little Dipper. Three of them are small, and are
seen with difficulty. Polaris, at the extremity of the
handle, has been known from time immemorial as
the North Polar Star. Until the mariners compass
came into use, it was the star

" Whose faithful beams conduct the wandering ship
Through the wide desert of the pathless deep."

Polaris does not mark the exact position of the pole,
since that is about 1^° toward the Pointers. This
distance will gradually diminish,* until in time
(2130 A. D.) it will be only |° : then it will increase
again, until, in the lapse of ages, 12,000 years hence,
the brilliant star Vega (a Lyrse) will fulfill the office of
polar star for those who shall then live on the earth, f
The Distance of Polaris is so great, that, though
the star is moving through space at the rate of ninety

* Five stars of the Dipper itself are drifting away from tlie sim, at tlie rate of 17
miles per second, .seeming to form a family or group by themselves. Proctor's Easy
Star Lessons gives charts representing the appearance of the Dipper for 100,000 years.

\ Of the nine Pyramids which are standing at Gizeh, Egypt, six have openings facing
the north. These lead to straight passages which descend at a uniform angle of about
26° and are parallel with the meridian. If we supi)o.se a person, 4,000 years ago, stand-
ing at the lower end of one of these passages, and looking out, his eye would strike the
sky near the st:ir Tliuban, which was then the jiolar star. The supposed date of the
building of these Pyramids (the Great Pyramid, 2123 b. c.) agrees with that epoch,
and naturally suggests that the builders had some special design in this peculiar con-
struction.

218 THE SIDEREAL SYSTEM.

miles per minute, this tremendous speed is impercep-
tible to us. It requires nearly fifty years for its light
to reach the earth ; so that, when we look at Polaris,
we know that the ray which strikes our eye set out
on its journey through space half a century ago.
We cannot state positively that the star is now in
existence, since if it were destroyed to-day it would
be fifty years before we should miss it. *

Calculation of Latitude from Polaris.— By an
observer at the equator, Polaris is seen at the horizon.
If he goes north, the horizon is depressed, and Polaris
seems to rise in the heavens. When it has reached
the height of a degree, the observer is said to have
passed over a degree of latitude on the earth's sur-
face. As he moves further north, the polar star con-
tinues to ascend ; its distance above the horizon
denoting the latitude of each place in succession,
until at the north pole, if one could reach that point,
Polaris would be seen directly overhead.

Drftco is represented under the figure of a long
sinuous serpent, stretching between Ursa Major and
Ursa Minor, nearly encircling the latter constellation,
and finally reaching out its head almost to the body
of Hercules.

Principal Stars. — Four small stars form a quad-
rilateral figure at the head ; a fifth, of the fourth
magnitude, which is scarcely visible, marks the end
of the nose ; several scattered groups and little
triangles of small stars denote the position of the
various coils of the body ; thence, an irregular line
of stars traces the dragon's tail around between Ursa

* Some recent observations seem to reduce this to 42 years.

THE CIUCUMPOLAR constellations. 219

Major and Ursa Minor. Tliuban, lying midway be-
tween y of the Little Dipper and ^ of the Big Dipper,
is noted as the polar star of forty centuries ago.

Mythological History. — Jupiter had carried off Europa. 7\genor,
her father, sent her brother Cadmus in pursuit of his lost sister, bidding
him not to return until he was successful in his search. After a time,
Cadmus, weary of his wanderings, inquired of the oracle of Apollo concern-
ing the fate of Europa. He was told to cease looking for his sister, to fol-
low a cow as a guide, and when she rested, there to build a city. Hardlj'
had Cadmus stepped out of the temple, when he saw a cow slowly walking
along. He followed her until slie came upon the broad plains where
Thebes afterward stood. Here she stopjied. Cadmus, wishing to offer a
sacrifice to Jupiter in gratitude for the delightful location, sent his servants
to seek for water. In a dense grove near by, was a fountain guarded by a
fierce dragon {Draco), and sacred to JIars. The Tyrians, approaching this
and attempting to dip up some water, were attacked, and many of them
killed, by the enormous serpent, whose head overtopped the tallest trees.
Cadmus, becoming impatient, went in search of his men, and, on arriving
at the spring, saw the sad disaster. He forthwith fell upon the mon-
ster, and after a severe battle succeeded in slaying him. While standing
over his conquered foe, he heard a voice from the ground bidding him take
the dragon's teeth and sow them. He obeyed. Scarcely had he finished
when the earth began to move and the points of spears to prick through
the surface. Next, nodding plumes shook off the clods, and the heads of
armed men protruded. Soon a great harvest of warriors covered the entire
plain. Cadmus, in terror at the appearance of those giants, whom he
termed Sparti {the soion), prepared to attack them, when suddenly they
turned upon themselves, and never ceased their warfare till only five of
the crowd survived. These, making peace with one another, joined Cadmus,
and assisted him in building the City of Thebes.

Cepheus is represented as a king in regal state,
with a crown of stars on his head, while he holds in
his hand a scepter which is extended toward his wife,
Cassiopeia. The constellation contains thirty-five
stars visible to the naked eye.

220 THE SIDEREAL SYSTEM.

Principal Stars. — The brightest star is Alderamin
(a), in the right shoulder. Alphirk (;^), in the girdle, is
at the common vertex of several triangles, which point
out respectively the left shoulder (i), the left knee (y),
and the right foot. The head, which lies in the Milky
Way, is marked by a little triangle of three stars.
This forms, with a, (3, and i, quite a regular quad-
rilateral figure. A bright star of the fifth magnitude,
close to Polaris, points out the left foot.

Cassiopeia^ is represented as a queen seated on
her throne. On her right, is the king ; on her left,
Perseus, her son-in-law ; above her, Ajidromeda,
her daughter. The constellation contains sixty-seven
stars visible to the naked eye.

Principal Stars. — A line drawn from Megrez ((J),
in Ursa Major, through Polaris and continued an
equal distance, will strike Caph {13) in Cassiopeia.
This star is noticeable as marking, with the others
named, the equinoctial colure, and as being on the
same side of the true pole as Polaris. The principal
stars form the figure of an inverted chair, which is
very striking and may be easily traced.

II. Equatorial Constellations. — The constellations
we shall now describe lie south of the circumpolar
groups. Only a portion of their paths is above our
horizon. In using the maps, the observer is supposed
to stand with his back toward Polaris, and to be look-
ing directly south. Commencing with the constella-
tion Perseus, so intimately connected with the other

* Fo» tlie mythological history of Cassiopeia, see Perseus and Andromeda. The names
of the principal stars in the Chair make a mnemonic word ,—0(iy ^e, hagdc. The student
can often form such an association of the letters, and will fiud the device an aid to his
memory. (Compare Virgo, page 230.)

EQUATORIAL CONSTELLATIONS. 221

members of the royal family just described, we pass
eastward in our survey, and notice the various con-
stellations as they slowly defile in silent march across
the sky.

The first map represents the constellations on or
near the meridian at nine o'clock in the evening- of
the winter solstice. On the evening- of the autumnal
equinox, the left-hand side of the map should be
turned downward toward the eastern horizon. On
the evening of tlie vernal equinox, the right-hand
side should be turned to the western horizon. At
these different times, the stars, though keeping
their relative positions, will be diversely inclined to
the horizon. As the stars apparently move westward
at the rate of lo"" per hour, the time of the evening
determines what stars will be visible, and also their
distances above the horizon.

Perseus is represented as brandishing an enor-
mous sword in his right hand, while in his left he
holds the head of Medusa. The constellation com-
prises eighty-one stars visible to the naked eye.

Principal Stars. — The most prominent figure is
called the Segment of Perseus. It consists of several
stars arranged in a line curving toward Ursa Major.
Algenib (a), the brightest of these, is of the second
magnitude. Algol (p. 242), in the midst of a group of
small stars, marks the head of Medusa. Between the
bright stars of Perseus and Cassiopeia, is a beautiful
star-cluster visible to the naked eye.

MythologicaIj HisToiiY. — Perseus, from whom this constellation was
named, was the son of Jupiter and Danaii. His grandfather, Acrisius, liav-
ing been iuformed by the oracle that his grandson would be the instru-

3XJ2

THE SIDEREAL SYSTEM,
(Map No. 8}— Fig. 88.

Aj\-D R o■^^^

f I S H ,E-S

C n ^,,«.

ment of his death, put the mother and child in a coffer and set them adrift
on the sea. Fortunately, they floated near tlie island Seriphus, where they
were rescued and kindly treated by a brother of Polydectes, king of the
country. "When Perseus had grown up, he was ordered by Polydectes
to bring him, as a marriage gift, the head of Medusa. Xow Medusa was
once a beautiful maiden, who dared to compare her ringlets with those of
Minerva ; whereupon, the goddess changed her locks into hissing serpents,
and made her features so hideous, that she turned to stone every living
object upon which she lixed her Gorgon gaze. Perseus was at first over-
powered at the thought of undertaking this enterprise; but Mercury
promised to be his guide, and to furnish him with his winged .shoes ; Mi-
nerva loaned him her wonderful shield, that was bright as a mirror ; and
the Nymphs gave him, in addition, Pluto's helmet, which made the bearer
invisible. Thus equipped, Perseus mounted into the air and flew to the
ocean, where he found the three Gorgons, of whom Medusa was one, asleep.
Fearing to gaze in her face, he looked upon tlie image reflected in Minerva's
shield, and with his sword severed Medusa's head from her body. The
blood gushed forth, and with it the winged steed Pegasus. Grasping the
head, Perseus flew away. The other Gorgons awaking, pursued him, but
he escaped their search by means of Pluto's helmet As he flew over the wilds

EQUATORIAL CONSTELLATIONS. 223

of Libya, in his aerial route, the blood dripping from the gory head of the
monster produced the innumerable serpents for which that country was
afterward noted.

Andromeda is represented as a beautiful maiden
chained to a rock.

Principal Stars. — Algenib and Algol in Perseus
form, with Almach (y) in the left foot of Andromeda,
a riglit-angled triangle opening toward Cassiopeia.
This figure is so perfect, that the stars may be easily
recognized. The girdle is pointed out by Merach (/3),
and two other stars which form a line slightly curv-
ing toward the right foot. The breast is denoted by
a very small triangle composed of three stars, — 6 of
the fourth magnitude, another of the fifth magnitude
just south, and an exceedingly minute star a little at
the west. Alpheratz («), in the head of Andromeda,
belongs also to Pegasus. This star, with three others
—Algenib (y), Markab (a), and Scheat (^3),— all of
the second magnitude, constitute the Great Square
of Fegasus. The brightest stars of these two con-
stellations form a figure strikingly like the Big Dip-
per. Algenib and Alpheratz lie in the equinoctial
colure which passes through Caph.

Mythological History. —Cassiopeia had boasted that her daughter
Andromeda was fairer than the Sea-nymphs. They appealed, in great
indignation, to Neptune, who sent a sea-monster {Cetus) to devastate the
coast of Ethiopia. To appease the deities, her father Cepheus was directed
by the oracle to bind his daughter to a rock, to be devoured by Cetus.
Perseus, returning from the destruction of Medusa, saw Andromeda in her
forlorn condition. Struck by her beauty and tears, he offered to liberate
her at the price of her hand. Her parents joyfully consented, and, in
addition, offered a royal dower. Perseus slew the terrible monster, and.

^24 THE SIDEREAL SYSTEM.

freeing Andromeda, restored her to hsr parents. All the promiuent actors
in this scene were honored with seats among the constellations. The Sea-
nymphs, it is said, in petty spite of Casoioj>eia, prevailed that she should
be placed where for half of the time she hangs with her head do^v-nward, — a
fit lesson of humility. Cephcus, her husband, shares in her punishment.

Aries, the irim, was anciently the first constella-
tion of the zodiac. It is now the first sign, but the
second constellation. On account of the precession
of the equinoxes, the constellation Pisces occupies
the first sign.

PRI^'CIPAL Stars. — The most noted star is a Arietis
(Alpha of Aries, more commonly called simply Arie-
tis), in the right horn. This lies near the path of the
moon and is one of the stars from which longitude is
reckoned. A line di'awn from Almach to Arietis
will pass through a beautiful figure of three stars
called TJie Triangles.

Mythological History. — Phrixns and Helle were the children of
Athamas, king of Thessalj*. Being persecuted by Ino, their step-mother,
they were com}>elled to flee for safety, ilercury provided them a ram
which bore a golden fleece. The children were no sooner placed on his back
than he vaulted into the heavens. In their aerial jouniey, Helle becoming
dizzy fell off into the sea, which was afterward called the Hellespont, now
the Dardanelles, Phrixus having reached Colchis in safety, offered the ram in
sacrifice to Jupiter, and gave the golden fleece to Aetes, his protector. The
Argonautic exjiedition in pursuit of this golden fleece, by Jason and his
followers, is one of the most romantic of mythological stories. It is,
undoubtedly, a fanciful account of the first important maritime expedition.
Rich spoils were the prizes to be secured.

Taurus consists of the head and shoulders of a
bull, which is represented in the act of plunging at
Orion.

Principal Stars. — The Hyades, a beautiful cluster

EQUATORIAL CONSTELLATIONS. 225

in the head, forms a distinct V. The brightest of
these is Aldebaran, a fiery red star of the first mag-
nitude.* The Pleiades (Job, xxxviii, 31), or the Seven
Sisters, is the most conspicuous group in the sky
(p. 206). It contains a large number of stars, six of
which are visible to the naked eye. There were said
to have been seven anciently, but that Electra left
her place in order not to behold the ruin of Troy,
which was founded by her son Dardanus. Other
myths relate that the ''Lost Pleiad'''' was Merope,
who married a mortal. Alcyone is the brightest
Pleiad. El Nath (r^) and ^ point out the horns of
Taurus.

Mythological Hlstoiiy. — This is the animal whose form Jupiter
assumed when he bore off Europa. The Pleiack-s were the daughters of
Atlas, and Nymphs of Diana's train. They were distinguished for their
unblemished virtue and mutual affection. The hunter Orion having pur-
sued them one day, iu their distress they prayed to the gods, when Jupiter,
in pity, transferred them to the heavens.

Aurlf/a, the Charioteer or Wagoner, is represented
as a man resting one foot on a horn of Taurus, and
holding a goat and kids in his left hand and a bridle
in his right.

The Principal Stars are arranged in an irregular
five-sided figure. Capella, the goat-star, is of the
first magnitude. It travels in its orbit 1,800 miles per
minute ; seventy years — a long lifetime —are required
for its light to reach the earth. Near by is a tiny
triangle, formed of three small stars, called the Kids.
Menkalinan (,'?) is in the right shoulder, d in the right

* Aldebaran is estimated to move through the heavens at tlic rate of 55 miles per
second. (See pp. 205, 201.) A numl)er of the liright stars between Aldel^aran and the
Pleiades have a common motion of alxjut 10" per century toward the east.

226 THE SIDEREAL SYSTEM.

hand, /3 (common to Auriga and Taurus) the right
foot, and i the left foot. Capella, /3, and 6 (a star in
the head) form a triangle. The origin of this con-
stellation is unknown.

I^isces, the fishes, is represented by two fishes tied
together bv a long ribbon. It consists of small stars,
which can be traced only upon a clear night, and in
the absence of the moon.

CefKS, the ichale, is a huge sea-monster, slowly
ploughing his way eastward, midway between the
horizon and the zenith. It may easily be found, on
a clear night, by means of the numerous figures
given in the map.

(Map So. S)—Fxg. S9.

lUu^'fs . n . ^

. -U R 1 G A

.Ram

Castor •-

'"•*'"

;' """"--- ^

• ; » -^'U,^ -L;'

L /U.'

Polhi . ■■•

• • * ^^^ ' .'

,-*r----

■ .^~.^T W I

, S ' ' ' .

'"7^~~~

l^lTTLE. Dot
Pj'ocyon

■-J D *~" "f-

Sirius ■ /\^

\ V '

HARE

1 a

^"^AT,,

.''-

• .^• ''og

^c-

%6 *c.

;>::t; . ; _ .^*

,/> •

Gemini^ the Ticins, represents the twin brothers
Castor and Pollux.

• " Castor is resolved by the telescope into two stars, whose angular distance from
each other is 5"— the angle that one inch would subtend 1,146 j-ards off."— Ball. '

EQUATORIAL CONSTELLATIONS. 227

The Principal Stars are Castor'-' and Pollux, which
are of the first and second magnitudes. The latter
is one of the stars from which longitude is reckoned
by means of the Nautical Almanac. The constella-
tion is clearly distinguished by two nearly parallel
rows of stars, that by a slight effort of the imagina-
tion may be extended into the constellations Taurus
and Orion.

Mythological History. — Castor and Pollux were noted, — the former
for his skill in training horses, the latter for boxing. They were tenderly
attached to each other, and were inseparable in their adventures. They ac-
companied Jason on the Argonautic expedition. A storm having arisen
during this voyage, Orpheus played on his wonderful lyre and prayed to the
gods ; whereupon the tempest was stilled, and star-like flames shone upon
the heads of the twin-brothers. Sailors, therefore, considered them as
patron deities,* and the balls of electric flame seen on masts and shrouds,
now called St. Elmo's fire, were named after them. Afterward, Castor was
slain. Pollux being inconsolable, Jujuter off'ered either to take him up to
Olympus, or to let him share his immortality with his brother. Pollux pre-
ferred the latter, and so tlie brothers pass alternately one day under the
earth, and the next in the Elysian Fields. Not only did sailors thus con-
fide in their watch over navigation, but soldiers believed them to return,
mounted on snow-white steeds and clad in rare armor, to take part in the
hard-fought battle-fields of the Romans.

" Back comes tlie chief in triumph,
Who in the hour of flglit
Hath seen the great Twin Brethren,

In harness on his right.
Safe comes the ship to haven,

Through billows and through gales.
If once the great Twin Brethren
Sit shining on the sails."— Lays of Ancient Rome.

Orion is represented under the figure of a hunter
assaulting Taurus. He has a sword in his belt, a

* We remeinbci- that Paul saileil for Italy in a ship whose sign was Castor and Pollux.
■^Acts, xxviii. 11.

228 THE SIDEREAL SYSTEM.

club in his right hand, and the skin of a lion in his
left. This is one of the most clearly defined and con-
spicuous constellations in the heavens.

Principal Stars. — Four brilliant stars, in the form
of a parallelogram, mark the outlines of Orion. Betel-
geuse, a beautiful ruddy star of the first magnitude,
is in the right shoulder; Bellatrix (>), of the second
magnitude, is in the left shoulder ; Rigel, of the first
magnitude, is in the left foot ; and Saiph (>«), of the
third magnitude, is in the right knee. Two small
stars near /- form with it a small triangle, which is
itself the vertex of a larger triangle composed of /-,
•/, and Betelgeuse. Near the center of the parallel-
ogram are three stars forming the Belt of Orion.
This group is also called the Bands of Orion (Job,
xxxviii, 31), Jacob's rod, and the Yard. It received
the last name because it forms a line 3' long, divided
in equal parts by a star in the center. These divi-
sions are useful for measuring the distances of the
stars. Running from the belt southward, is an
irregular line of stars which marks the sword ; west
of Bellatrix is a curved line denoting the lion's skin.
South of Orion are four stars forming a beautiful
figure styled The Hare,

Mythological Histohy. — Orion was a famous hunter. Becoming
enamored of Merope, he desired to marry her. (Enopion, her father,
opposing the choice, put out the eyes of the unwelcome suitor. The
blinded hero followed the sound of a Cyclop's hammer until he came to
Vulcan's forge. Vulcan, taking pity, instructed Kedalion to conduct him
to the abode of the sun. Placing his guide on his shoulder, Orion pro-
seeded to the east, and at a favorable place

" Clinibint; up a narrow gorge.
Fixed his blank eyes upon the sun."

EQUATORIAL CONSTELLATIONS. 229

The healing beams restored him to sight. As a punishment for having
profanely boasted that he was able to conquer any animal the earth could
produce, he was bitten in the heel by a scorpion. Afterward, Diana placed
him among the stars ; where Sirius and Procyon, liis dogs, follow liim, the
Pleiades fly before him, and far remote is the Scorj/km, by whose bite he
perished.

Canis Major and Ca^iis Minor contain each a
single star of the first magnitude, Sirius, and Pro-
cyon.* These two, with Betelgeuse, Phaet in the
Dove, and Naos in the Ship, form a huge figure
known as the Egyptian X. Sirius, the dog-star, is
the most brilliant star in the heavens. It is reced-
ing from the earth at the rate of 20 miles per second
(Huggins). Seventeen years are required for its light
to reach us.f (See note, p. 308.)

Leo is represented as a rampant lion. It is one of
the most beautiful constellations in the zodiac.

The Principal Stars are arranged in the form of
a sickle. Regulus, in the handle, is a brilliant star of
the first magnitude. It is one of the stars from
which longitude is reckoned. It is almost exactly in
the ecliptic. Zosma {6) lies in the back of the lion,
6 in the thigh, and Denebola, a star of the second
magnitude, in the brush of the tail.

Cancer includes the stars that lie irregularly
scattered between Gemini, Head of Hydra, Procyon,
and Leo. In the midst of these, is a luminous spot,
called Praesepe, or the Bee-hive, which an ordinary
glass will resolve into stars.

* Procyon, like Sirius, was formerly considered a star of evil omen, and as bringing
bad weather. " Who that is learned in matters astronomioal," said Digges, the astrol-
oger, " noteth not the great efl'ects at the rising of the star called the Litel Dogge."

t In 1802. Alvan G. Clark, son of the famous telescope-maker, discovered a companion
of Sirius, "distant from the star 28 times the Sun's distance from the Earth."

230 THE SIDEREAL SYSTEM.

Virgo is represented as a beautiful maiden with
folded wings, bearing in her left band an ear of corn.

The Pri>-cipal Stae, Spica, in the ear of corn, is of ^
the first magnitude, and is used for determining Ion--
gitude at sea. Denebola, Cor Caroli (a), Arcturus,
and Spica form a figure about 50= in length, called

(Map No. Uy Fig. 00.

the Diamond of Virgo. Five third-magnitude stars,
e 6 y, 7], .^, (the mnemonic word is begde) make a
corner known among the Arabian astronomers as
" The retreat of the howling dog."

Mythological History. -Virgo was the Goddess Astr^a. According
to the poets, the early history of man ..as the golden age. It was a time
of innocence and truth. The gods dwelt among men, and perpetual spring
delighted the earth. Next, came the sUver age, less tranquil and serene.

EQUATORIAL CONSTELLATIONS. 231

but still the gods lingered and happiness prevailed. Then followed the
brazen and iron ages, when wickedness reigned supreme. The earth was
wet with slaughter. The gods left the abodes of men, one by one, Astrsea
alone remaining ; until finally she too, last of all the immortals, bade
the earth farewell. Jupiter thereupon placed her among the constellations.

Hifdra is a long, straggling serpent, having its
head near Procyon and extending its tail beyond
Virgo, a total distance of more than 100°.

The Principal Star is Cor Hydrse, of the second
magnitude. It is a lone star, and may be easily
found by a line drawn from y Leonis through Regu-
lus, and continued about 23°. The head is marked
by a rhomboidal figure of four stars of the fourth
magnitude lying near Procyon. Several little
triangles may be formed of them and other small
stars lying near. The Crater, or Cup, is a beautiful
and very striking semicircle of six stars of the fourth
magnitude directly south of Leonis. Corvus {S, e,
y, (5), the raven, lies 15° east of the Cup. e Corvi is in
the equinoctial colure.

Mythological History. — Hydra was a fearful serpent which in ancient
times infested the lake Lerna. Its destruction constituted one of the
twelve labors of Hercules. Tlie Crow was formerly white, it is said, but
was changed to its raven tint on account of its proneiiess to tale-bearing.

Canes Vetiaticlf the hunting dogs. This constel-
lation contains the bright star. Cor Caroli (a), which is
found by a line passing from Benetnasch (??) through
Berenice's Hair to Denebola (/3).

Berenice's ILiir is a beautiful cluster midway
between Cor Caroli and Denebola. Near by is a
single bright star of the fourth magnitude.

232

THE SIDEREAL SYSTEM.

Mythological History.— Berenice was the wife of Ptolemy. Her
husban.l going upon a dangerous expedition, she promised to consecrate
her beautiful tresses to Venus if he should return in safety. Soon after the
fulfilment of this vow the hair disappeared from the temple where it had
been deposited. Berenice being much disquieted at this loss, Conon, the
astronomer, announced that tlie locks had been transferred to the heavens,
in proof of which he pointed out this cluster of hitherto unnamed stars.
All parties were satisfied with this happy termination of the difficulty.

Bootes, the bear-driver, is represented as a hunts-
man grasping a club in his right hand, while in his

(Map No. Sy-Fig. 91.

R E A' T ,-'B E

^ 'c . ; "V JO •

g • BERENICE' „ . . \

li ■^''' ' ':■. 8« *L 1 N.."

-7 t .• . ' \ -'-'

~- ^. ^ . . ,>'' e

•: . ^\ > • • • /

left he holds by the leash his two greyhounds {Canes
Venatici), with which he is pursuing the Great Beai
continually around the north pole.

Principal Stars.— Arcturus (Job, ix, 9), a mag-
nificent star of the first magnitude, is in the left
knee. It forms a triangle with Denebola and Spica,
and also one with Denebola and Cor Caroli. It
travels in its orbit fifty-five miles per second, qv

EQUATORIAL CONStELLAflONg. 333

three times as fast as the earth (p. 205). Its light
reaches the earth in twenty-five years. Mirach (t) lies
in the girdle, 6 in the right shoulder, Alkaturops (//)
in the club, /3 in the head, and Seginus (y) in the left
shoulder. Seginus forms with Cor Caroli and Arc-
turus a triangle, right-angled at Seginus. Three
small stars in the left hand of Bootes lie near
Benetnasch.

Mythological History. — Bootes is supposed to have been Areas, the
son of Callisto. (See Ursa Major.)

Hercules is represented as a warrior clad in the
skin of the Nemsean lion, holding a club in his right
hand and the dog Cerberus in his left. His foot is
near the head of Draco, while his head lies 38° south
and his club reaches 10 degrees beyond.

The Principal Star is Ras Algethi (a Herculis).
This forms a triangle with j3 and S. A peculiar
figure of four stars (tt, ?/, ^, e), north of these, marks
the body. (See Maps, Nos. 5, 6, and 7.) The left
knee is pointed out by 0, and the left foot by y.

Mythological History. — This constellation immortalizes the name of
one of tlie greatest heroes of antiquity. Hercules was the son of Jupiter
and Alcmena. "While he was yet lying in his cradle, Juno, in her jealousy,
sent two serpents to destroy him. The precocious infant, however,
strangled them with his hands. By the cunning artifice of Juno, Hercules
was made subject to Eurystheus, his elder half-brother, and compelled to
perform all his commands. Eurystheus enjoined upon him a series of the
most difficult and dangerous enterprises that could be conceived, which have
been termed the " Twelve Labors of Hercules. " Having completed these
tasks, he afterward achieved others equally celebrated. Near the close of
his life he killed the centaur Nessus. The dying monster charged Dejanira,
the wife of Hercules, to preserve a portion of his blood as a charm to use

234 tHE SIDEREAL SYSTEM.

ill case the love of her husband should ever fail her. In time, Dejanira
thought she needed the potion, and Hercules having sent for a white robe
to wear at a sacrifice, she steeped the garment in the blood of Xessus. Xo
sooner had Hercules put on the fatal robe than the venom stung his bones
and boiled through his veins. He attempted to tear it off, but in vain.
It stuck to his flesh, and tore off" great pieces of his bodj\ The hero,
finding he must die, ascended Mount (Eta, where he erected a funeral pyre,
spread out the skin of the Xemaean lion, and laid himself down uiion it.
PhUoctetes applied the torch. With perfect serenity of countenance
Hercules awaited approaching death —

" Till the gofl, the earthly part forsaken,

From the man in flames asunder taken.

Drank the heavenly ether's purer breath.

Joyous in the new unwonted lightness

Soared he upward to celestial brightness,

Earth's dark, hea\-y burden lost in death." — Schiller.

Corona consists of six stars arranged in a semi-
circular form. The brightest of these is Alphecca.
This makes a triangle, with Mirach (^) and (J in Bootes.
It forms a similar figure with Mirach and Arcturus.

SerjjentariiiSf or Ophiuch tis^ the serpent-bearer,
is represented under the figure of a man grasping in
both hands a prodigious serpent, which is writhing
in his grasp.

Principal Stars. — Ras Alhague {a), in the head, is
of the second magnitude. It is about 5° from Ras
Algethi. They form a pair of stars conspicuous like
the pairs in Gemini, Canis Minor, Canis Major, etc. ;
3 marks the right shoulder, and k the left. There is
a small cluster near 3, called Taurus Poniatowskii.
An irregular square of four stars, near y Herculis,
denotes the head of the serpent.

Mythological History. — This constellation perpetuates the memory
of JEsculapius, the father of medicine. He was so skilful that he restored
several persons to life ; whereupon Pluto complained to Jupiter that his

fiQUATOHlAL CONSTELLATIONS.

235

kingdom was in clanger of being depopnlateil. Therefore Jupiter struck
him with a tliunderbolt, but afterward placed him among the constel-
lations. Serpents were sacred to ^sculapius, because of the superstitious idea
that they have the power of renewing their youth by changing their skin.

Libra represents the scales of Astraea (Virgo), the
goddess of justice. It may be recognized by the

(Map No. G)—Fig. 92.

quadrilateral figure formed by its four principal
stars.

Scorpio is represented under the figure of a huge
scorpion, stretching through 25°. It is a most in-
teresting constellation.

Principal Stars. — Antares («) is a fiery red star of
the first magnitude. It marks the heart of the Scor-
pion. The head is indicated by several stars, the

^36 THE siDeHeal system.

most prominent of which is §, arranged in a line
slightly curved. The tail may be easily traced by a
series of stars which winds around through the Milky
Way in a beautiful manner. *

Mythological Hlstory. — This is the scorpion that sprang out of the
earth at the command of Juno, and stung Orion. Scorpio and Orion are
so placed among the constellations that they never appear in the heavens
together.

Sagittarius^ the archer, is represented as a cen-
taur with his bow bent, as if about to let fly an arrow
at Scorpio.

Principal Stars. — A row of stars from ju to /? marks
the bow : another from }- eastward points out the
arrow and the right arm drawn back in bending the
bow. North of r, two stars of the fourth magnitude
denote the head of the centaur. The 3Iilk Dipper,
so called because the handle lies in the Milky Way,
is a very striking figure.

Mythological History. — This constellation is named in honor of
Chiron, one of the centaurs. These monsters were represented as men from
the head to the loins, while the remainder of the body was that of a horse
— the ancients having so high an opinion of that animal that the union
was not considered in the least degrading.

Chiron was renowned for his skill in music, medicine, and prophecy.
The most distinguished heroes of mythology were among his pupils. He
taught j^sculapius phj'sic ; Apollo, music ; and Hercules, astronomy.

Cajyricoriias contains no very conspicuous stars.
The Southern Fish (No. 6) has one star of the first
magnitude, Fomalhaut («, No. 7), which on a clear
summer evening may be seen in the south, midway
to the zenith. Antinous and the Eagle is a double

* Antares (anti, like ; Ares, Mars) was so named because it rivalled Mars in brightness
and color.

EQUATORIAL CONSTELLATIONS.

Constellation. It contains a beautiful star of the
first magnitude, Altair. This is conspicuous, as be-
ing the center one in the row of three bright stars.
A similar row denotes the tail of the eagle ; the first
star of which is named C, and the last star lies in
Cerberus. The Dolphin contains a pretty cluster in
the form of a diamond. It is sometimes called Job's
Coffin.

(Map No. 7)-Fig. 93.

Pe"g ASU S'

.-...

. y

Ta ;

^EADVot

'; ' -.

s ^

K-N-

••'s

«> R a'c

~«'<1

■\1

■ ye

-• '

,'r

■ V .

: F 6

X .

'fi

■ 1 W'

-, ,.

ABROW

/' »J*

• •

/•

DOLPHIN' •

/' to :

r-Y- ■■

' i

^(a

".' •

*f>'

waterbe'arer

S A G

. * •' .■■

•
c

. •'a

ANT I'N U S .

'.*::•

CygnuSf the swan, is a remarkable group of stars.
the principal ones being so arranged as to form a
large and beautiful cross. The upright piece lies
along the Milky Way. It is composed of four stars,
three of which, Deneb (a), y, and S, are bright, while
the fourth is a variable star. No. 61, a minute star,
scarcely visible to the naked eye, is noted as being
the nearest to the earth of any of the fixed stars in
the northern hemisphere (p. 241).

238 THE SIDEREAL SYSTEM.

LyrOf the harp, contains one brilliant blue star,
Vega (p. 217). Close by it is a parallelogram of four
smaller stars, by which it may be easily recognized.

.Mythological History.— This is the celestial lyre upon which Orph-
eus discoursed such ravishing music that Avild beasts forgot their fierceness
and gathered about him to listen, while the rivers ceased to flow, and the
very rocks and trees stood entranced.

III. Southern Constellations.— AVe now imagine
ourselves viewing the stars visible to a person far

(Map No. 8)— Fig. 91,.

SOUTHERN
■--.-HYDRUS . .■..;;. CROSS

-?>

1^ O -R K^ '

^Jp/ak'6V

^^eat DOG*

south of the equator. The constellations are re-
versed with reference to the horizon. The two stars
which, in the northern hemisphere, compose the base
of the parallelogram in Orion, form here the upper
side. Sirius is above Orion. All the northern cir-
cumpolar constellations are hidden from view. At
the southern pole there is no conspicuous star, but
the richness and number of the neighboring stars
compensate this deficiency, and give to the heavens

THE SOUTHERN CONSTELLATIONS. 23d

an incomparable splendor. Here is the magnificent
constellation Argo, in which we find Canopus, looked
upon anciently as next to Sirius in brilliancy : rj, a
variable star, now surpasses it in brightness.

Nearly at the height of the south pole, blazes the
Southern Cross; below is the Centaur, containing
two stars of the first magnitude and five of the
second ; and above is Hydrus, where shines Acher-
nar, another beautiful star of the first magnitude.

III. DOUBLE STARS, COLORED
STARS, NEBUL^^, ETC.

1. Double Stars.— To the naked eye, all the stars
appear single. With the telescope, over 10,000 have
been found to be double. Thus, Polaris consists of
two stars about 18" apart ; Rigel has a companion
about 10" from it ; and Sirius, one distant 7". A

240 THiJ SIDEflEAL SYSTEM.

good opera-glass will separate £ Lyrse into two
components.

In case two stars happen to lie in the same straight
line from us, though at immense distances from each
other, their light will blend. They will be seen by
the naked eye as a single star, and by the telescope
as a double star. They are called optical double
stars. Many, however, of the double stars have been
found to be physically connected. Each double star
of this class forms a binary system of two suns re-
volving in an elliptical orbit about their common
center of gravity, like the planets in the solai* system,
in accordance with Newton's law of gravitation. In
a few instances, there are combinations of triple,
quadruple, and even septuple stars. Thus e Lyrae is
a double-double star, while Orionis is a system of
six suns. The components of a double star com-
monly differ in brightness ; so that frequently the
fainter one is nearly lost in the brilliancy of its com-
panion sun.

The Periods of some systems have been ascer-
tained. Thus, I Ursae Majoris is a double star, and
the two stars of which it is composed have performed
an entire revolution about each other since they
were found to be connected. There are only eleven
binary stars now known whose periods are less
than a century, while the others have periods which
seem to extend, in some cases, beyond a thousand
years.

Orbits. — It is not possible to estimate the dimen-
sions of the orbits of the double stars, until their dis-
tances from us are definitely known. ''Taking the

DOUBLE STARS. 241

estimated distance of 61 Cygni (550,000 times the
sun's distance from the earth)* as a basis, the com-
panions of that system cannot cultivate a very
intimate acquaintance, since they must be over a
billion miles apart. From these data, astronomers
have attempted even to calculate the mass of some
of the double stars. 61 Cygni, although scarcely
visible to the naked eye, and known to be the sec-
ond nearest to us of any of the fixed stars, is esti-
mated to weigh one-third as much as our sun." (See
p. 308.)

II. Colored Stars. — We Kave already noticed that
the stars are of various colors. f Sirius is white;
Antares, red; and Capella, yellow ; while Lyra has a
blue tint, and Castor has a green one. In the pure
transparent atmosphere of tropical regions, the
colors are far more brilliant. There, oftentimes,
the nocturnal sky is a blaze of jewels, — the stars
glittering with the green of the emerald, the blue
of the amethyst, and the red of the topaz.

In the double and multiple stars, every color is
presented in all its richness and beauty ; while there
are also combinations of colors complementary to
each other. Here is a green star with a blood-red
companion ; here an orange and a blue sun ; there a
yellow and a purple one. The triple star y Andro-
medse is formed of an orange-red sun and two others
of an emerald green.

Every tint that blooms in the flowers of summer,

♦ Recent measurements of this star seem to indicate its probable distance from the
Bun to be 400,000 radii of the earth's orbit.

t The theory has been advanced that the color indicates the intensity of the heat oj
(he st?r. A white star is tlierefore hotter than a red star ; and a blue, than a jellow one

243 THE SIDEREAL SYSTEM,

flames out in the stars at night. "The rainbow
flowers of the footstool and the starry flowers of the
throne," proclaim their common Author ; while rain-
bow, flower, and star alike evince the same Divine
love of the beautiful.

We can hardly conceive the effects produced in a
system having colored suns. Take a planet revolv-
ing about tp Cassiopeise for instance. This is illu-
minated by a red, a blue, and a green sun. Some-
times, by the succession of these suns, a cheerful
green day would present a charming relief to a fiery
red one ; and that might be still further subdued by
a gentle blue one. The odd contrast of color and the
vicissitudes of extreme heat and cold that obtain on
such a world, present a picture which our fancy can
sketch better than words can paint.

The colors of the stars change. Sirius was ancient-
ly red. It is now unmistakably white. There are
two double stars which were described by Herschel
as white ; each is now composed of a golden-yellow
and a greenish star.

III. The Variable Stars have periodic changes of
brilliancy. The following are most conspicuous :

Algol, in the head of Medusa, is a star of the
second magnitude for about two and a half days,
when it suddenly decreases, and in three-and-a-half
hours descends to the fourth magnitude. It then re-
kindles, and in three-and-a-half hours is again as
brilliant as ever.

MiRA, the wonderful, a star in the Whale, has a
period of eleven months. It is ordinarily of the
second magnitude for about fifteen days. It then

DOUBLE STARS. 243

decreases for three months, until it becomes invisible
to the naked eye. This period of darkness lasts five
months ; it then rebrightens for three months, until
it regains its former lustre. Occasionally, however,
it fails to brighten at all beyond the fourth magni-
tude, while on one occasion its light was almost equal
to that of Aldebaran. Sometimes no perceptible
change takes place for a month ; then again, there is
a sensible alteration in a few days.

The Reason of this Variability is not under-
stood. It has been suggested, in the case of Mira,
that it may be a globe rotating on its axis, and that
different portions of its surface, illuminated to differ-
ent degrees of intensity, are thus presented to us.
Others have conceived that there may be satellites
revolving about these suns, and that when their dark
bodies interpose between the stars and our earth,
they eclipse the light wholly or in part.

IV. The Temporary Stars suddenly blaze out in
the heavens, and then gradually fade away. The
most celebrated one burst forth in Cassiopeia, in the
year 1572. Tycho Brahe says : " One night as I was
examining the celestial vault, I saw with unspeak-
able astonishment a star of extraordinary brightness
in Cassiopeia. Struck with surprise, I could scarcely
believe my eyes. To convince myself that there was
no illusion, 1 called the workmen of my laboratory
and the passers-by, and asked them if they saw the
star which had so suddenly made its appearance.
It could be compared only with Venus at her quad-
rature, being seen distinctly at midday." Its color
was at first white, then yellow, and finally red. Its

244 THE SIDEREAL SYSTEM.

brightness decreased gradually until the spring of
1574, when the star disappeared from view and has
not since been seen. As two brilliant stars had pre-
viously appeared in Cassiopeia, at intervals of about
three centuries, they have been thought, by some, to
be identical, and that it is only a variable star of long
period.

Since this discovery by Tycho Brahe, numerous in-
stances are recorded of stars which have suddenly
burst forth, and have then either faded out entirely,
or remain as faint telescopic objects. In the latter
case, they are termed New stars. One of this kind
appeared in Corona Borealis, in 1866. At first it was
of the second magnitude, but in a week changed to
the fourth, and in a month diminished to the ninth.
Strangely, too, some stars have disappeared from the
heavens, and are styled Lost stars. The changes
which are thus constantly taking place are calculated
to make the term *' eternal stars" seem a very in-
definite phrase.

Explanation. — ^These phenomena are as yet little
understood. A rotation about an axis would fail
to explain the changes in color. Some think that
these stars revolve in enormous orbits of such eccen-
tricity that at their most distant points they fade out
of sight. Arago has shown, in reply to this, that for
a star to decrease in brightness from the first magni-
tude to the second by moving directly from us, even
with the velocity of light, would require six years.
As we have just seen, the star of 1866 underwent this
change in brilliancy in a week.

The mind cannot help wondering if they are oot

DOUBLE STARS. 245

instances of enormous conflagrations in which a
world is overwhelmed in ruin ! The investigations
of spectrum analysis indicate that the star of 1866
consisted of burning hydrogen gas. We can suppose
that the gas was evolved by some convulsion, and,
taking fire, wrapped the entire globe in flames.
This need not involve the idea of destruction, but
only a change of form. A dark star may thus
become luminous, or a bright one may be extin-
guished.*

5. The Star Clusters are groups of stars so massed
together as to present a hazy, cloud-like appearance.
Several of them have been already named, — the Plei-
ades, the Beehive in Cancer, Berenice's Hair, the
Hyades, and the group in the sword-handle of Per-
seus. The principal stars of which they are com-
posed can generally be distinguished by the naked
eye, although by the use of a small opera or spy-
glass the number is increased.

In the southern sky, there are clusters still more
remarkable. In the Cross, is a group of 110 stars of
various colors, red, blue, and green, so that looking
on it, says Herschel, is "like gazing into a casket of
precious gems." A cluster in Toucan is compact in
the center, where it is of an orange-red color ; the
exterior is composed of pure white stars, making a
border of exquisite contrast.

It is generally conceded that there is some close

* The x>rocess of apparent creation and destruction is going on in the heavens im-
mediately before the eye of the astronomer. New stars flash light, old stars are lost,
worlds burst into flame, and their glowing embers fade into darkness. Are they re-
created into new worlds? We know not. We only perceive that the same Almighty
power which fitted up this earth for our home is yet at work among the worlds about
us, and we are thus witnesses of His eternal presence.

246

THE SIDEREAL SYSTEM.

Fig. 9e.

Star-duster in Toucan.

physical relation existing between the stars compos-
ing such an ''archipelago of worlds," but its nature
is a mystery. They seem generally crowded together
toward the center, blending into a continuous blaze
of light. Yet, although they appear so densely com-
pacted, it is probable that, if we could change our
stand-point and penetrate one of these gi'oups *of
suns, we should find it, on our approach, opening up
and spreading out before us, until, in the midst, the
suns would shine down upon us from the heavens as
the stars do in our own sky.

6. Nebulae are faint, misty objects, like specks of
luminous clouds. A few are visible to the naked eye,
but the telescope reveals thousands. They differ
from clusters in not being resolvable into stars when
viewed through the largest telescopes. With the con-

NEBUL.^. 247

stant improvement made in these instruments, how-
ever, many so-called nebulae have been resolved, and
thus the number of clusters has been increased,
while new nebulae have been discovered.

Until of late, it was thought that all nebulae were
simply groups of stars, which would be ultimately
discerned in the more powerful telescopes yet to be
made. Spectrum analysis shows, however, that
many of these luminous clouds are gaseous, and are
not composed of stars.

Since all the nebulae maintain the same position
with respect to the stars, their distance must be
inconceivably great, and, in order to be visible to us,
their magnitude must be proportionately vast. They
are most abundant at the two poles of the Milky
Way, but are more uniformly distributed over the
heavens lying near the south pole.

It is now generally believed that nebulae constitute
the material for making stars, — are, in fact, sun-
germs ; that all stars originally existed as nebulae ;
and that every nebula will, in time, be changed into
stars.

Nebulae are divided, according to their form, into
six classes — elliptic, annular', spiral, planetary , irreg-
ular nebulce, and nebulous stars, f

The Elliptic or merely oval nebulae are the most
abundant. Under this head is classed the Great
Nebula in Andromeda, which was discovered over

t This division of the nebulae is purely arbitrary, and used only to introduce some
order of arrangement. The shape of the nebulae changes with the power of the telescope
through which they are seen. Thus the Great Nebula in Andromeda, as resolved by
Bond, is no longer oval, but irregular in form. The Ring- Nebula of Lyra, seen through
the large telescope of to-day, is egg-shaped; while the Dumb-bell Nebula assumes the
outline of a chemical retort

248

THE SIDEREAL SYSTEM.

a thousand years ago, and is visible to the naked
eye. Prof. Bond, of the Cambridge Observatory,
jr,g 97 has partly resolved it into stars,

of which he has counted 1500,
although its nebulous appear-
ance was still retained. Through
the telescope, it is one of the
most glorious objects in the
heavens. "If we suppose this
nebula to be one continuous bed
of stars of different sizes for its
entire extent, it must comprise
the enormous number of 30,000,000."

The distance of such nebulse from the earth passes
our comprehension. Some astronomers have esti-
mated that a ray of light would require 800,000 years
to span the gulf that intervenes. Imagination wearies
itself in the attempt to understand these figures.
They teach us something of the limitless expanse of
that space in which God is working the mysterious
problem of creation.

Fid. 9S.

Nebula in Andromeda.

ycbula in Lyra.

The Annular 2s ebulje have the form of a ring.
There are four of these **ring universes." In the cut

NEBULA.

249

is a representation of one in Lyra, — first, as seen by
Herscliel, having in the center a nebulous film like a

Fig. 99.

Spiral Cluster in Cane* Venatki.

250

THE SIDEREAL SYSTEM.

"bit of gauze stretched over a hoop;" second, as
sho-wn in Lord Rosse's telescope (p. II), which re-
solves the filmy parts of the nebula into minute stars,
and reveals a fringe of stars along the edge.

The Spiral or Whirlpool Xebul^ are exceedingly
curious. The most remarkable one is in Canes Vena-
tici. It consists of brilliant spirals sweeping outward
from a central nucleus, and all overspread with a
multitude of stars.* One is lost in attempting to
imagine the distance of such a mass, and the forces
which produce such a "tremendous hurricane of
matter — perhaps of suns."

Planetary Nebula, by their circular form and
pale, uniform light, resemble the disks of the distant
planets of our system. Their
edges are generally well de-
fined, though sometimes slightly
furred. There is one in Ursa
Major, which, if located at the
distance of 61 Cygni, would
"fill a space equal to seven
times the entire orbit of Nep-
tune."

Planetary Nebula IRREGULAR NEBUL^ are thoSO

which have no definite form. Many present the
irregularities of clouds torn by the tempest. Some
of the likenesses which may be traced are strangely
fantastic : for example, the Dumb-bell Xebula, in the
constellation Vulpecula, and the Crab Xebula, near
the southern horn of Taurus. There is also one known

Fig. 100.

* Columbus discovered a new continent, and so immortalized his name ; what shall
we say of the astronomer who discovers a system of worlds ?

NEBULA.

261

as the Great Nebula ""''■ '"'■

in the Sword-handle
of Orion, which bears
a faint resemblance
to the wings of a bird.
Nebulous Stars
are so called because
they are enveloped
by a faint nebula,
usually of a circular
form. The star is
generally seen at the
center, although
some nebulae sur-
round two stars, Dumb-bell Nebula.

having one in each focus. It is thought that these
may be suns possessing immense atmospheres, which
are rendered visible somewhat as that of our sun is in
the zodiacal light ; and that in like manner our sun
may present to other worlds the appearance of a
nebulous star.*
Variable Nebulje. — Certain changes take place

♦ Nothing in all nature is more suggestive of the magnificence and immensity of
Creation, than are the nebulous star-clusters, many of which are at such an inconceiv-.
able distance, that the most powerful telescopes show them only as a confused mass of
light. A casual observer, — even though when led by scientific analogy to resolve each
little patch of star-dust into a host of separate suns, and to provide each sun with a
retinue of inhabited planets,— might think of them as little colonies of suns, set on the
very outskirts of world-creation, and moving in such close proximity, that the peoples
of the various worlds might communicate with one another. Yet, were he transported
to some planet whirling about one of those far-otf star-suns,— a multitude of which
blend as a single point of light to our human eyes,— he would see the other suns only as
fixed stars in the firmament above him ; and though many of them might surpass in
splendor the glory of our own Sirius, yet all would still remain at such an immense
distance as to baffle the research of the most powerful telescopic instruments. Thus, too,
he would probably find each planet revolving at such a distance from its sister planets,
as to render the certain knowledge of other inhabited worlds as elusive there as here.

952

THE SIDEREAL SYSTEM.

Fig. IVS.

Crab Nebula,

among the nebulae which can be accounted for only
under the supposition that they, like some of the
stars, are variable. Mr. Hind tells us of a nebula in
Taurus which, in 1852, was distinctly visible with a
small telescope, but, in 1862, had vanished entirely out
of the reach of a powerful instrument. The Great
Nebula in Argo, when observed by Herschel in 1838,

had in the center a vacant space containing a star ot
the first magnitude, enshrouded by nebulous matter.
In 1863, the nebulous matter had disappeared, and
the star was only of the sixth magnitude. These
facts as yet defy explanation. They illustrate the
vast and wonderful changes constantly taking place
in the heavens.

Double Nebula. — There seems to be a physical
connection existing between some of the nebulae,
similar to that already noticed in respect to certain
stars. In the case of the latter, this inter-relation
has been proved, since, even at their distances, their
movements can yet be traced in the lapse of years.
'* But, owing to the almost infinite depths in the abyss
of the heavens at which these nebulae exist, thou-
sands of years, perhaps thousands of centuries,
would be necessary to reveal any movement." —
(Guillemin.)

7, Magellanic Clouds. — ITot far from the southern
pole of the heavens, there are two cloud-like masses,
distinctly visible to the naked eye, known to navi-
gators as Cape Clouds. Sir John Herschel describes
them as consisting of swarms of stars, clusters, and
nebulae, seemingly grouped together in the wildest
confusion. In the larger, he found 582 single stars,
46 clusters, and 281 nebulae.

8. The Milky Way. — Via Lactea, or the Galaxy, is
a luminous, cloud-like band that stretches across the
heavens in a great circle. It is inclined to the celes-
tial equator about 63°. This stream of suns is
divided into two branches from a Centauri to Cyg-
nus. To the naked eye, it presents merely a diffused

254 iHEl SIDEREAL SYSTEM.

light ; but with a large telescope it is found to con-
sist of myriads of stars densely crowded together.*

These stars are not uniformly distributed through
the entire extent. In some regions, within the space
of a single square degree we can discern as many as
can be seen with the naked eye in the entire heavens.
In other parts, there are broad, open spaces. A
remarkable instance of this occurs nears the South-
ern Cross. There is a dark, pear-shaped vacancy,
with a single bright star at the center, glittering on
the blue background of the sky. In viewing it, one
is said to be impressed with the idea that he is looking
through an opening into the starless depths beyond
the Milky Way.

The northern galactic pole is situated near Coma
Berenices, and the southern in Cetus. Advancing
from either pole toward the Milky Way, the number
of stars increases, at first slowly and then more
rapidly, until the proportion at the galaxy itself is
thirty-fold.

Fig. 103.

HerscheVa Theory of the Stellar System,

* Herschel states that 258,000 stars once passed across tlie field of his great reflector
in 41 minutes. With the powerful instruments now making, it is probable that many
more could be seen.

Nebula. S55

Herschel's Theory.* — Sir W. Herschel has con-
jectured that the stars are not indifferently scattered
through space, but are collected in a stratum some-
thing like that shown in the cut, and that our sun
occupies a place at S, near where the stream
branches, A and E being the galactic poles. It is evi-
dent that, to an eye viewing the stratum of stars in
the direction SB, SC, or SD, they would seem much
denser than in the direction SA or SE. Thus are we
to think of our own sun as a star of the second or
third magnitude, and of our little solar system as
plunged far into the midst of this vortex of worlds, a
mere atom along that

" Broad and ample road
Whose dust is gold and ])avenient stars."

9. The Nebular Hypothesis is a theory advanced
by Laplace, to show how the solar system may have
been formed, f As since modified, its outlines are as
follows: In the "beginning," all the matter which
now composes the sun, and the various planets with
their moons, was in a gaseous and highly heated
state. It filled the space at present occupied by the
system, and extended far beyond the orbit of Nep-
tune. In other words, the solar system was simply
an immense nebula. The heat, which is the repel-

* other theories have been advanced by astronomers, but we are as yet ignorant of the
real structure of the universe outside of our own system.

t We should remember that this theory aims to tell only the way in which our system
was developed. The parent nebula must have confciined a potential energy equal to all
the manifestations of force since made in tlie entire system. Nothing could be devel-
oped from a mass of nebulous matter the germs of which had not been put in it origin-
ally by the Creator. The analogies of nature all go to show that the Creator's plan is,
in general, not to produce any object in a perfect and matured state ; but rather, by a
gradual growth, to unfold its full form and functiou.

256 THE SIDEREAL SYSTEM.

lant force, overcame the attraction of gravitation.
Gradually the mass cooled by radiation. As centu-
ries passed, the repellant force becoming weaker, the
attractive force drew the matter and condensed it
toward one or more centers. The nebula then pre-
sented the appearance of a nebulous star — a nucleus
enveloped by a gaseous atmosphere.

According to a well-known law in physics, seen
in every -day life, wherever matter seeks a center —
as in a whirlpool, in a whirlwind, or even in water
poured through a funnel — a rotary motion was estab-
lished. As the rotary motion of the nebula increased,
the centrifugal force finally overcame at the exte-
rior the attraction of gravitation. A ring of condensed
vapor was then left behind.* Centuries elapsed, and
again, under the same conditions, a second ring was
detached. Thus, one by one. concentric rings were
separated from the parent nebula, all revolving in
the same plane and in the same direction. These
different rings, becoming gradually consolidated,
formed the planets. Generally, however, in this
process, while still in the vg^porous state and slowly
condensing, the rings themselves detached other
rings that were in turn consolidated into satellites.

In the case of Saturn, several of these secondary
rings did not condense into globes, but still remain
as rings which revolve about the planet, f Mitchell

*A eonadenbie modification of the Xelnilar Hypothesis is possible, leaving its
geneal ides, hcfwever, intact. It is dow generally conceded that the several planets
■w&t xtot "tfciowB oS," bat merely detached and left behind. Proctor thinks that the
solar system is tbe resolt at mteteork a^gregaiion as well as gaseous condensation : the
f*««»**g jM tikdr iahiMCiy beong so large, gathered immense quantities of meteoroids,

t b tte eaae attke miaor plaaeta and Ote rings of Saturn, we may suppose tfaat the

NEBULAR HYPOTHESIS. 257

naively remarks, " Saturn's rings were left unfinished
to show us how the world was made." The ring
which formed the minor planets broke up into small
fragments, none large enough to attract the rest and
thus form a single globe.

The central mass of vapor finally condensed itself
into the sun, which remains the largest member of
the system. According to this theory, the sun may
yet give off a few more planets, whose orbits will not
exceed its present diameter. After a time, all its
heat will be radiated into space, its fire will become
extinct, and life on the planets will cease. We know
not when this remote event may occur. We can-
not fathom the purpose of God in creating and main-
taining this system of worlds, nor can we foretell
how soon it may complete its mission. We are as-
sured, however,

*' That nothing walks with aimless feet,
That not one life shall be destroyed,
Or cast as rubbish to the void,
When God hath made the pile complete."

In Memokiam.

rings were composed of matter uniformly distributed ; while in the case of the rings
that consolidated into planets, there was a nucleus that attracted the rest of the matter
to itself. It is possible that the rings of Saturn may yet break up and form new satel-
lites for that planet. Indeed, some hold that one at least of the rings has thus been
resolved into small meteorites. These may be attracted, and .so picked up, one by one,
in succession by the larger, until they form another moon, which will continue to re-
volve about the planet as the ring does now. — " The present state of the .solar system is
a living picture of the entire history of a single planet. From the sun's fire-mist, to
ring-girt Saturn ; from Saturn, to storm-beaten Jupiter ; from Jupiter, to the sunny
summer-time of our own planet ; from Earth, to autumn-browned Mars ; and from
Mars, to the wintry silence and desolation of the dark gulches of the moon,— there is a
series of stages that carries the thought back into the eternity long passed, as well as
onward into the measureless depths of the future, and confers upon human intelligence
a sort of exemption from the limitations of finite existence."— Pro/. Winchell.

258 THE SIDEREAL SYSTEM.

IV. CELESTIAL CHEMISTRY.

Spectrum Analysis. — The rainbow — that child of
the sun and shower — is familiar to all. The brilliant
band of colors, seen when the sunbeam is passed
through a prism, is scarcely less beautiful. The ray
of light containing the primary colors is here spread
out fan-like, and each tint reveals itself. This var-
iously-colored band is called a spectrum (plural,
spectra). There are three different kinds of spectra —

1st. When the light of a solid or liquid bod} , as
iron white-hot, is passed through a prism, the spec-
trum is continuous, and consists of a series of distinct
colors, varying from red on one side to violet on the
other.

2nd. If the light of a burning gas containing any
volatilized substance be passed through a prism, the
spectrum is not continuous, but is ornamented by
bright-colored lines, — sodium giving two yellow lines ;
strontium, a red one ; silver, two beautiful green ones.
Each element produces a definite series which can
be recognized as its test.

3rd. If a light of the first kind be passed through
one of the second, the spectrum is crossed by dark
lines. Thus, if the white light of an electric lamp
be passed through a flame containing sodium, instead
of the vivid yellow lines so characteristic of that
metal, two black lines exactly occupy their place.
A gaseous flame absorbs the rays of the same color that
it emits. (See note, \>. 310.)

O T3

1 n

CELESTIAL CHEMISTRY.

259

The Spectroscope. — This instrument consists of
two small telescopes, with a prism mounted between
their object-glasses (Fig. 106). The rays of light enter
through a narrow slit at A, and are rendered parallel
by the object-glass. They then pass through the
prisms at C, are separated into the different colors,
and, entering the second telescope at D, fall upon the

Fig. 106.

eye at B. A third telescope is sometimes attached,
which contains a minutely-accurate scale for meas-
uring the distances of the lines. In addition, a
mirror may throw in at one side of the slit a ray of
sunlight or starlight, and so we can compare the
spectrum of the sunbeam with that of any flame we
desire.

260

THE SIDEREAL SYSTEM.

Revelations of the Spectroscope Concerning
THE Sun. — The spectrum of the sunbeam is not con-
tinuous, but is crossed by a large number of dark
lines, called, from their discoverer, Fraunhofer's
lines. It is therefore concluded that the sun's light
is of the third class just named, and that it is pro-
Fig. 106.

A Spectruscope.

duced by the vivid light of a highly heated body
shining through a flame full of volatilized sub-
stances.

But not only does spectrum analysis thus shed light
on the physical constitution of the sun, but these
lines are so distinctive, so marked and varied, that
the elements of which the sun is composed may be
discovered.* Thus, for example, iron gives a spec-
trum of some 450 lines, differing in intensity and

* The following twenty-two elements have been detected : sodium, calcium, liarium.
magnesium, iron, chromium, nickel, cobalt, hydrogen, manganese, aluminium, titanium,
palladium, vanadium, molybdenum, strontium, lead, uranium, cerium, strontium,
cadmium, oxygen, and a probability of several more, such as carbon, silver, tin, etc.

CELESTIAL CHEMISTRY. 261

relative length. These are bright when iron vapor
is burning, and dark when white light is passed
through such burning vapor. In the solar spectrum
we have such a coincidence of dark lines, as to make
the conclusion irresistible that iron is contained in
the sun's atmosphere.*

Stars are Suns. — The same method of analysis has
been applied to the stars. The spectra are marked
by dark lines. Their constitution is therefore like
our sun, and they also exhibit familiar elements.
Betelgeuse, for example, contains many substances
known to us, but, as is thought, no hydrogen. What
a world that must be without water ! We thus trace
in the faintest star that trembles in the measureless
depths of space, the elements that compose the com-
mon objects of our own life. We know that we are
akin to nature everywhere, — a part of a system vast
as the universe.

The Motion of a Star may be resolved into two
components : one representing its motion at right
angles, and the other its motion parallel to the line
of vision. The former component can be determined
by the telescope ; the latter is revealed by the spectro-

* Recent researches in spectroscopy present important problems. On elevating the
temperature, it has been found that not only the lines of the spectrum of a substance
vary, but new ones appear. Certain substances have apparently common lines. A
molecule containing a few atoms gives a line-spectrum ; increase the number of atoms
and it presents a finted-spectrum (i. e., one composed of bands, each made up of lines,
and having a sharp boundary on one side but fading away on the other) ; increase the
number yet more, and it yields a continuous sx)ectrum. New queries have therefore
arisen in Solar Physics. How many atoms are there really in a specified molecule t
What is the meaning of certain unfamiliar lines seen in the solar spectrum ? Why do we
not detect in the sun many of those substances that form so large a part of the earth's
crust? Lockyer supposes that the so-called elements are really compounds whose mol-
ecules may be "dissociated" by intense heat, so that in the sun we see only the germs
of our familiar chemical forms. Bead Lockj^er's " Spectrum Analysis," and Young's
t'fheSun."

262 THE SIDEREAL SYSTEM,

scope. If the star is moving towards us, the number
of vibrations producing any color will be increased,
and hence the dark lines corresponding to that color
in the spectrum will be pushed beyond its usual
place toward the violet end ; if going from us, the
number will be decreased, and the dark lines be
pushed toward the red end of the spectrum.* The
amount of displacement once determined, the velo-
city of the star can be reckoned by means of well-
known laws of optics.

Spectra of Nebulae, — Instead of being marked
with dark lines, as are the si)ectra of the stars, many
of the nebulae exhibit bright lines. Their spectra
are, therefore, of the 2nd kind. This proves such
nebulae to consist, not, like the stars, of an intensely-
heated nucleus shining through a luminous atmos-
phere, but of a glowing mass of gas.f Out of 60
nebulse examined by Mr, Huggins, 20 exhibited the
bright lines belonging to the gases, and all contained
nitrogen.

The Solar Flames, which were formerly seen only
during an eclipse, can now be examined by means of
the spectroscope, at any time. J The sun has thus

* The same result is produced in the case of sound. The whistle of an approaching
train sounds shriller than when it is receding. See Physics, p. 133,

t Tlie Dumb-bell nebula is said to emit a light only about one twenty-thousandth
part that of a common wax-candle. If this matter be a " sun-germ," how immensely
must it become condensed before its rushlight glimmering can rival the dazzling bril-
liancy of even our own sun !

J " The red portion of the spectrum will stretch athwart the field of view like a scarlet
ribbon with a darkish tend across it ; and in that band will appear the prominences,
like scarlet clouds, so like our own terrestnal clouds, indeed, in form and texture, that
the resemblance is quite startling. One might almost think he was looking out
through a partly-opened door upon a sunset sky, e.vcept that there is no variety or con-
trast of color ; all the cloudlets are of the same pure scarlet hue. Along the edge of the
op)ening is seen the chromospliere, more brilliant than the clouds which rise from it or
float above it. and^ for the most i>art, made up of minute tongues and filamente. "—youn^.

TIME. 263

been found to be a sea of fire swept by the most vio-
lent storms.* Flames travel over its surface with a
velocity of which we can form no conception ; "one
jet shot out 80,000 miles and disappeared in ten min-
utes." Young describes a protuberance that reached
the enormous height of 350,000 miles and then faded
entirely away, all within two hours.

V. TIME.

Sidereal Time. — A sidereal day is the exact interval
of time in which the earth rotates on its axis. It is
found by marking two successive passages of a star
across the meridian of any place. This is so abso-
lutely uniform, that, as recent investigations seem
to show, the length of the sidereal day has not varied
more than gV ^f ^ second in 2,400 years, (note, p. 89).

The sidereal day is divided into twenty-four equal
portions, which are called sidereal hours, and each
of these hours into sixty portions, termed sidereal
minutes, etc.

Astronomical clocks are regulated to keep si-
dereal time. The day commences when the vernal
equinox is on the meridian. Therefore, the time by
a sidereal clock does not point out the hour of the
ordinary day. It indicates only how long it is since
the vernal equinox crossed the meridian, and thus
shows the right ascension of any star which may

* Such a storm " coming down upon us from the north would in 30 seconds after it
had crossed the St. Lawrence be in the Gulf of Mexico, carrying with it the whole sur-
face of the continent in a mass, not of ruin simply, but of glowing vapor, in which the
vapors arising from the dissolution of the materials composing the cities of Boston, New
7ork; and Chicago would be mixed in a single undistinguishable cloud."— Newoomb,

364 THE SIDEREAL SYSTEM.

happen to be on the meridian at that moment. The
hours of the clock are easily reduced to degrees (p.
28). The astronomer always reckons the hour of the
day consecutively up to twenty-four.

Solar Time. — A solar day is the interval between
two successive passages of the sun across the meri-
dian of any place. If the earth were stationary in
its orbit, the solar day would be of the same length
as the sidereal ; but, while the earth is turning
around on its axis, it is going forward at the rate of
360° in a year, or about 1° per day. When the earth
has made a complete rotation, it must therefore
perform a part of another rotation through this
additional degree, in order to bring the same meri-
dian vertically under the sun.

One degree of diurnal rotation is equal to about
four minutes of time. Hence the solar day is four
minutes longer than the sidereal day. For the con-
venience of society, it is customary to call the solar
day 24 hours long, and make the sidereal day only
23 hr. 56 min. 4 sec. in length, expressed in mean
solar time. A sidereal day being shorter than a
solar one, the sidereal hours, minutes, etc., are
shorter than the solar ; 24 hours of mean solar time
being equal to 24 hr. 3 min. 56 sec. of sidereal time.

From what has been said, it follows that the earth
makes 366 rotations around its axis in 365 solar days.

Mean Solar Time. — The solar days are of unequal
length. To obviate this difficulty, astronomers sup-
pose a mean sun moving through the equator of the
heavens (which is a circle and not an ellipse) with a
perfectly uniform motion. When this mean sujj

TIME. 265

passes the meridian of any place, it is mean noon ;
and when the true sun is in the same position, it is
apparent noon. This mean day is the average length
of the solar days in the year. The clocks in common
use are regulated to keep mean time.* V/hen it is
twelve by the clock, the sun may be either a little
past or a little behind the meridian.

The difference between sun-time (apparent solar-
time) and clock-time (mean time) is called the
*^ Equation of time.'' This is the greatest about the
first of November, when the sun is over sixteen and
a quarter minutes in advance of the clock. The sun is
the slowest about February 10th, when it is about
fourteen and a half minutes behind mean time.

Mean and apparent time coincide four times in the
year — namely, April 15th, June 14th, September 1st,
and December 24th. On these days, the noon-mark
on the sun-dial coincides with twelve o'clock.

The Sun-Dial. — The apparent time of the dial may
be readily changed to mean time, by adding or sub-
tracting the number of minutes given in the almanac
for each day in the year, under the heading "sun
slow" or "sun fast." A noon-mark is thus a very
convenient method of regulating a timepiece, f

♦ In France, until 1816, apparent time was used ; and the confusion was so g^eat,
that Arago relates how the town clocks would differ thirty minutes in striking the same
hour. As the time varied every day, no watchmaker could regulate a watch or clock to
keep it

t The following manner of obtaining one without a transit instrument may be use-
ful. Select a level hard surface which is exposed to the sun from about 9 a. m. to 4 p. m.
Upon this carefully describe, with compasses, a circle of eight or ten inches in diameter.
Take a piece of heavy wire, six or eight inches in length, one end of which is sharpened.
Drive this perpendicularly into the center of the circle, leaving it just high enough to
allow the extreme end of its shadow to fall upon the circle about 9J or 10 a. m. Mark
this point, and also the place where the shadow touches the circle in the afternoon.
Take a point half-way between the two, and, drawing a line from that to the center of
the circle, it will be the meridian liiie, or noou-mark.

266

THE SIDEREAL SYSTEM.

Why the Solar Days are of Unequal Length- — There
are two reasons for this, — the unequal orbital motion
of the earth, and the obliquity of the ecliptic. First :
the orbit of the earth is an ellipse ; and thus the ap-
parent yearly motion of the sun along the ecliptic is
variable. In perihelion, in January, the sun appears
to move eastward daily 1° 1' 9' .9 ; while at aphelion,
in July, only 57' 11". 5. As the earth in its diurnal
motion rotates uniformly from west to east, and the
sun passes eastward irregularly, this must produce a
corresponding variation in the length of the solar
day. The sun, therefore, comes to the meridian
sometimes earlier and sometimes later than the
mean noon, and they agree only at perihelion and
aphelion.

Second : as we have just seen, the mean sun is sup-
posed to move in a circle and not an ellipse. This

would make the
motion along
the ecliptic uni-
form, but the
obliquity of the
ecliptic would
still cause an ir-
regularity in the
length of the
day. The mean
sun is therefore supposed to pass along the equi-
noctial, which is perpendicular to the earth's axis :
while the ecliptic is inclined to it 23" 27'. Let A
represent the vernal equinox ; I, the autumnal ; AEI,
the ecliptic ; AI, the equinoctial ; PK, PL, PM, etc.,

TIME. 26?

meridians. Let the distances AB, BC, CD, etc., be
equal arcs of the ecliptic, which are passed over by the
sun in equal times. Next, on the equinoctial, mark
off distances Aa, ah, he, etc., equal to AB, BC, etc.
These are equal arcs of right ascension, or hour-
circles, through which the earth, rotating from
west to east, passes in equal times. Now, meridians
drawn through these divisions, would not agree with
those drawn through equal divisions on the ecliptic.
Hence, a sun moving along the ecliptic, which is in-
clined, would not make equal days, even though the
ecliptic were a perfect circle.

Let us see how the mean and apparent solar days
would compare. Let the real sun pass in its east-
ward course from A to B in a certain time ; the mean
sun moving the same distance would reach the point
a, since the latter travels on the base and the former
the hypothenuse of a triangle. The earth, rotating
from west to east, would cause the real sun to cross
any meridian earlier than the mean sun ; hence, ap-
parent time would be faster than clock-time. By
holding the figure up above us toward the heavens,
we can see how a westerly sun would cross the meri-
dian earlier than an easterly one. Following the
same reasoning, we can see that at the solstice, solar
and mean time would agree ; while beyond that point
the mean time would be faster.

The Civil Day is the mean solar day. It extends
from midnight to midnight.* The method of divid-

* Until recently, very many nations terminated one day and commenced the next at
sunset. Under this plan, 10 o'clock on one day would not mean the same as 10 o'clock
on another day. The Puritans commenced the day at 6 p. m. The Babylonians, Per-
sians, and Assyrians began the day at sunrise.

368 THE SIDEREAL SYSTEM.

ing the day into two portions of twelve hours each,
is said to have been adopted by Hipparchus, 150
years b. c, and is now in use over the civilized
world. The astronomical method of reckoning the
hours consecutively up to twenty-four is much more
convenient, and is therefore coming into general
favor. The names of the days are derived as follows :

1. Dies Solis Latin . • . .Sun's day.

2. Dies Lunse . . . . *' ... Moon's day.

3. Tius daeg. Saxon . . . .Tius's day.

4. Wodnes daeg... " ... .Woden's day.

5. Thurnes daeg.. " . . .Thor's day.

6. Friges daeg " ... .Friga's day.

7. Dies Satumi.... Latin . . . .Saturn's day.

The Year. — The sidereal year is the interval of a
complete revolution of the earth about the sun, meas-
ured by a fixed star. It comprises 365 d., 6 hr., 9
min., 9.6 sec. of mean solar time. The mean solar
year (tropical year) is the interval between two suc-
cessive passages of the sun through the vernal equi-
nox. It comprises 365 d., 5 hr., 48 min., 46.7 sec. If
the equinoxes were stationary, there would be no
difference between the sidereal and the tropical year.
As the equinoxes retrograde along the ecliptic 50" of
space annually, the former is 20 min., 20 sec. longer.

The anomalistic year is the interval between two
successive passages of the earth through its perihe-
lion, which moves eastward about 11". 8 annually. It
is 4 min., 40 sec. longer than the sidereal year.

The Ancient Year. — The ancients ascertained the
length of the year by means of the gnomon. This
was a perpendicular rod standing on a smooth plane

TIME. 269

on wliicli was a meridian line. When the shadow
cast on this line was the shortest, it indicated the
summer solstice; and when it was the longest, the
winter solstice. The number of days required for
the sun to pass from one solstice back to it again de-
termined the length of the year. This they found to
be 365 days. As that is nearly six hours less than
the true solar year, dates were soon thrown into con-
fusion. If, at a certain date, the summer solstice
occurred on June 20th, in four years it would fall
on the 21st; and thus it would gain one day every
four years, until in time the summer solstice would
happen in the winter months.

Julian Calendar. — Julius Csesar first attempted to
make the calendar year coincide with the motions of
the sun. By the aid of Sosigenes, an Egyptian
astronomer, he devised a plan of introducing every
fourth year a leap-year, which should contain an
extra day. This was termed a bissextile year, since
the sixth (sextilis) day before the kalends (first day)
of March was then counted twice.

Grregorian Calendar. — Though the Julian calen-
dar was nearly perfect, it was yet somewhat defec-
tive. It considered the year to consist of 3G5^ days,
which is 11 minutes in excess. This accumulated
year by year, until in 1582 the difference amounted
to ten days. In that year, the vernal equinox oc-
curred on the 11th of March, instead of the 21st.
Pope Gregory undertook to reform the anomaly, by
dropping ten days from the calendar and ordering
that thereafter only centennial years which are
divisible by 400 should be leap-years. The Gregorian

270 TflE SIDEREAL SYSTEM.

calendar was generally adopted in Catholic countries.
Protestant England did not accept the change until
1752. The difference had then amounted to 11 days.
These were suppressed and the 3rd of September was
styled the 14th.* Dates reckoned according to the
Julian calendar are termed Old Style (O.S.); and
those according to the Gregorian calendar, New
Style (N.S.).

Commencement of the Year. — The Jews began
their civil year with the autumnal equinox ; but
their ecclesiastical year, with the vernal equinox.
When Caesar revised the calendar, the Romans com-
menced the year with the winter solstice (Dec. 22)^
and it is probable he did not intend to change it
materially. He ordered it to date from January 1, in
order that the first year of his new calendar should
begin with the day of the new moon immediately
succeding the winter solstice.

The Earth our Timepiece. — The measure of time
is, as we have just seen, the length of the mean
day. This is estimated from the length of the
sidereal day. Hence, the standard for time is the
rotation of the earth on its axis. All weights and
measures are based on time. An ounce is the weight
of a given bulk of distilled water. This is measured

* This sweeping change was received in England with great dissatisfaction. Prof
De Morgan narrates the following : " A worthy couple in a conntrj' town, scandalized by
the change of the calendar, continued for many years to attempt the obsen'ance o(
Good Friday on the old day. To this end they walked seriously and in full dress to the
church door, on which the gentleman rapi)ed with his stick. On finding no admittance,
they walked as seriously back again and read the service at home. There was a wide-
spread superstition that, when Christmas day began, the cattle fell on their knees In their
stables. It was asserted that, refusing to change, they continued their prostrations
according to the Old Style. In England, the members of the Government were mobbed
in the streets by the crowd, which demanded the eleven days of which they had been
illegally deprived."

CELESTIAL MEASUREMENTS. 2tl

by cubic inches. The inch is a definite part of the
length of a pendulum which vibrates seconds in the
latitude of London. Arago remarks, a man would
be considered a maniac who should speak of the in-
fluence of Jupiter's moons on the cotton trade. Yet
there is a connection between these incongruous
ideas. The navigator, travelling the waste of waters
where there are no paths and no guide-boards, may
reckon his longitude by the eclipses of Jupiter's
moons, and so decide the fate of his voyage. We
can easily see how the rotation of the earth on its
axis influences the cost of a cup of tea.

VI. CELESTIAL MEASURE-
MENTS.

Many persons read the enormous figures which
indicate the distances and dimensions of the heaven-
ly bodies with a questioning, indefinite idea, entirely
unlike the feeling of certainty with which they read
of the distance between two cities, or the number
of square miles in a certain State. Many, too, imag-
ine that celestial measurements are so mysterious in
themselves that no common mind can hope to grasp
the methods. Let us attempt the solution of a few
of these problems.

1st, To Find the Distances of the Planets from
the Sun.— In Fig. 108, E represents the earth; ES,
the earth's distance from the sun; V, the planet
Venus; and VES, the angle of elongation (a right-
angled triangle). It is clear that, as Venus swings

272 THE SIDEREAL SYSTEM.

apparently east and west of the sun, this angle may
be easily measured ; also, that it will be the greatest
when Venus is in aphelion and the earth in peri-
helion at the same time, for then VS will be the
longest and ES the shortest. Now in every right-
Fig. 108. angled triangle the propor-

tion between the hypothe-
nuse, ES, and the side op-
posite, VS, changes as the
angle at E varies, but with
the same angle remains the
same whatever may be the
length of the lines them-
selves. This proportion be-
tween the hypothenuse and

Comparative Distance of Venus and the the sido OppOSitC any angle

is termed the sine of that
angle. Tables are published containing the sines for
all angles. In this way, the mean distance of Venus
is found to be jVn that of the earth; Mars, f times;
Jupiter, 5| times, etc.*

2iid. To Measure the Moon's Distance from the
Earth. — (1.) The Ancient Method.— As the moon's
distance is so much less than that of the other
heavenly bodies, it is measured by the earth's semi-
diameter. The method, an extremely rough one,
which was in use among the ancients, was something

• If the pupil haa studied Trigonometry, lie may apply here the simple proportion —
ES : VS : : Radius : Sine of 47° 15" = greatest elongation of Venus

The same result would be obtained by the use of Kepler's third law ; and on page
19, we saw how the distances of the planets themselves could be determined by the
periodic times, if the distance of the earth from the sun is first known. So that when
we have accurately determined the sun's distance from us, we can then decide by either
of the methods named the distance of all the planets. Indeed the sun's distance is, as
already remarked, the " foot-rule " for measuring all celestial distances.

CELESTIAL MEASUREMENTS. 273

like the following : In an eclipse of the moon, that
body passes through the earth's shadow in about
four hours. If, then, in four hours, the moon travels
along its orbit a distance equal to the diameter
of the earth, in twenty-four hours it would pass over
six times, and in a lunar month (about thirty days)
one hundred and eighty times, that distance. The
circumference of the lunar orbit, then, must be one
hundred and eighty times the diameter of the earth.
The ancients supposed the heavenly orbits to be
circles, and, as the diameter of a circle is about ^ of
the circumference, they deduced the diameter of the
moon's orbit as 120 times, and the distance of the
moon from the earth as 60 times, the semi-diameter
of the earth.

(2.) Modern Method by the Lunar Parallax. —
Under the head of parallax, we saw how, in common
life, we obtain a correct idea of the distance of an
object by means of our two eyes. We proved that
one eye alone gives no notion of distance. Just,
then, as we use two eyes to find how far from us an
object is, so the astronomer uses two astronomical
eyes, or observatories, located as far apart as pos-
sible, to find the parallax of a heavenly body. In
Fig. 109, M represents the moon ; G, an observatory
at Greenwich ; and C, another at the Cape of Good
Hope. At the former, the distance from the north
pole to the center of the moon, measured on a
meridian of the celestial sphere, is .found to be 108°.
At the latter station, the distance from the south
pole to the moon's center is measured in the same
Wdij, aijd foun(J to be 73^°. The sum of these angles

274 THE SIDEREAL SYSTEM.

is 1811^''. Now, the entire distance from the north
pole around to the south pole, measured on a meridian,
can be only half a great circle, or 180°. This differ-
ence of 1|° must be the difference in the position of
the moon, as seen from the two observatories. For
the observer at the former station will see the moon
projected on the celestial sphere at G', and in meas-
uring its distance from the north pole will measure
an arc bQ' further than if he were located at E, the
center of the earth. The observer at the latter sta-
tion will see the moon projected on the celestial sphere
at C, and in measuring its distance from the south
pole will measure an arc bC more than if he were
located at E, the center of the earth. The sum of
bG' and bC = G'C is the difference in the position of
the moon as seen from the two stations. In other
words, it is the moon's parallax. The arc GC,
measures the angle C'MG'; that angle is equal to the
opposite angle GMC = 1^°. Now, in the four-sided
figure GECM, the sides GE and CE are both equal
radii of the earth = 3,956 miles; while the distance
from G to C is the difference in the latitude of the
two places. The angles ZGM and Z'CM, being the
zenith distances of the moon, are known, and so the
angles MGE and MCE are easily found. EM, the
moon's distance from the center of the earth, is
thus readily computed by a simple trigonometrical
formula.

(3.) The Horizontal Parallax of the Moon is
most commonly found by estimating its distance,
not from the north and south poles, as just explained
under the general meaning of the term parallax, but

CELESTIAL MEASUREMENTS.

275

from a fixed star. The moon's horizontal parallax is
now estimated at 57', which makes its distance about
sixty times the earth's semi-diameter (p. 273).*

Fig. 109.

P' z

Measuring Moon's Distance from the Earth.

3. To Find the Sun's Distance from the Earth. —
This might be estimated by obtaining the solar paral-
lax in the same manner as the lunar parallax. It
would be necessary only to take the sun's distance
from the north and south poles respectively at Green-
wich and the Cape of Good Hope, and then subtract-
ing 180° from the sum of the two angular distances,
the remainder would be the solar parallax. The dif-
ficulty in this method lies in the fact that when the
sun shines the air is full of tremulous motion. This
increases refraction — that plague of all astronomical
calculations — to such an extent that it becomes im-

* In figure 110, let S represent the moon, sun, or any other heavenly body ; AB, the
semi-diameter of the earth ; and ASB, the "horizontal parallax" of the body. Then, by
tthe following trigonometrical formula, the distance fro.ip thp earth may be easily p*lci)r
Jated— AS : AB : : Radius : Sin of AS?

276 THE SIDEREAL SYSTEM.

possible to calculate so small an angle with any
accuracy. Neither can the parallax be estimated, as
in the case of the moon, bj'^ measuring the distance

Fig. 110.
B

from a fixed star, since when the sun shines the stars
near by are invisible even in a telescope. Astrono-
mers have therefore been compelled to resort to other
methods.

(1.) Calculation of Solar Parallax by Observ-
ing Mars. — We have already seen that the distance
of Mars from the sun is | that of the earth from the
sun. If, therefore, we can find Mars's distance from
the earth, we can multiply it by three, and so obtain
the distance of the sun from the earth. In 1862, when
Mars was in opposition, it came very near us, for it
was in perihelion while the earth was in aphelion,
so that its distance (as since ascertained) was only
about 34,000,000 miles. Astronomers at Greenwich
and the Cape, and at various American and European
observatories, calculated the distance of the planet
from the north and south poles, as well as from seve-
ral fixed stars, in the manner just explained for
obtaining the lunar parallax. The result of these
observations fixed the solar parallax at 8". 94,* mak-
ing the sun's distance 91,430,000 miles.

* 3j' the formula on page 27"), wc have, AS : AB : : Radius : Sin 8".94,

CELESTIAL MEASUREMENTS.

277

(2.) Calculation of Solar Parallax by Obser-
vation OF the Transit of Venus.— In the figure, let
A and B represent the position of two observers sta-

Fig. 111.

Transit of Vemte.

tioned at opposite sides of the earth. At the time of
the transit, the one at A will see the planet Venus as
a round black spot at V" on the sun's disk, while the
one at B will see it at V. The distance V'V" is the
difference in the position of Venus as seen from the
two stations on the earth. The distance AB is the
diameter of the earth. The distance V'V" is as much
greater than AB as VV" is greater than VA. The
distance of Venus from the sun is known, by Prob.
I., to be .72 that of the earth. The distance of Venus
from the earth must, then, be 1.00— .72 =.28. Hence,
VV", the distance from the sun to Venus, =.72-^.28=
2. 5 times the length of AV, the distance of Venus from
the earth. Therefore, V'V" is equal to 2^ times AB,
the earth's diameter, or 5 times the solar paral-
lax. Knowing the hourly motion of Venus, it is
necessary only for each observer to find when the
planet's disk enters upon and leaves the sun's disk,
to determine the length of the path (chord) it
traces. A comparison of the length and direction

278 THE SIDEREAL SYSTEM.

of these chords will give the length V V" in seconds
of space.

The advantage of this method is that, as the dis-
tance V V" is two and half times that of AB, an
error in measuring that chord affects the solar par-
allax less than one-fifth.

Time of a Transit of Venus.* — This is an event of
rare occurrence. It happens only at intervals of 8,
105^ ; 8, 131^, years, &c. Were the planet's orbit in
the same plane as the ecliptic, a transit would take
place during each synodic revolution ; but as it is
inclined about 3|°, the transit can occur only when
the earth is at or near one of the nodes at the same
time with the planet when in inferior conjunction.
As the nodes of Venus now fall in that part of the
earth's orbit which we pass in the beginning of
June and December, transits always occur in those
months.

The Transit of June 3rd, 1769, excited great inter-
est. King George III. fitted out an expedition to
Tahiti, under the coromand of the celebrated naviga-
tor, Capt. James Cook. In order to make the angle
as great as possible, and so increase the length of the
chords, or paths of the planet across the sun, astron-
omers were sent to all the most favorable points of
observation — St. Petersburg, Pekin, Lapland, Cali

* Tlie first transit ever seen was witnessed by Horrox, a young amateur astronomei
residing near Liverpool. His calculations fixed upon Sunday, Nov. 24, 1639 (0. S.)-
He, however, commenced his watch of the sun on Saturday preceding. The following
day he resumed his ob3er\'ation at sunrise. Tlie hour for church arriving, he repaired
to service as usual. Returning to his labor Immediately afterward, he says : "At this
time an opening in the clouds, which rendered the sun distinctly vnsible, seemed as if
Divine Pro\idence encouraged my aspirations ; when — oh most gratifying spectacle ! the
object of so many earnest wishes — I perceived a new spot of perfectly round form that
had just entered upon the left limb of the sun."

CELESTIAL MEASUREMENTS. 279

fornia, etc. They fixed the solar parallax at 8". 58,
making the sun's distance 95,393,055 miles.*

The transits of Dec. 8, 1874, and Dec. 6, 1882, were
carefully observed by several government expedi-
tions ; the results have not yet been fully announced.

The next transits will happen,

June 8 2004.

June 6 2012.

December 11 2117.

December S 2125.

June 11 2247.

The transits of Mercury are more frequent ; but
owing to the nearness of the planet to the sun, they
are of little value in determining the solar parallax.

Changes in the Estimate op the Solab Paral-
lax. — About 1824, Encke deduced 8". 58 as the prob-
able result of the observations upon the transit of
1769. This conclusion held the ground for nearly
thirty years, and the corresponding solar distance of
95,293,000 miles is found in all the older text-books.
About 1860, Le Verrier announced that he could
reconcile the theories regarding certain of the plan-
ets only by assuming a greater solar parallax. As
the result of various calculations, together with the
material furnished by the observations upon the

♦ Le Gentil, sent out by the French Academy to observe the transit of 1761 in the
East Indies, was prevented from making liis first i)ort by the war with England. High
winds afterward kept him out at sea till the transit was over. He then resolved to re-
main abroad until after the transit of 1769. Eight long years passed, and the morning
of June 3, 1709, dawned bright and beautiful. Le Gentil, with his instruments all in
place, was counting the moments for the long-awaited transit to begin ; when, suddenly,
the sky grew black with clouds, and a tropical storm, the first in days, swept by.
Meantime, Venus came and went, and the ill-fated Le Gentil had again lost the oppor-
tunity of years. Prostrated by his bitter disappointment, it was two weeks before he
could hold his pen to write the story of his second failure.

280 THE SIDEREAL SYSTEM.

planet Mars in 1862, a new parallax of 8". 94 was
obtained. This has been accepted by all until
recently, and was used in former editions of this
work. It is now known to be too large, and astron-
omers are making every effort to determine this most
important factor in celestial measurements. As al-
ready stated on page 36, the parallax at present re-
ceived is about 8". 80, which represents a mean solar
distance of 92,885,000; in round numbers, 93,000,000,
as given in the present edition.

The difficulty of determining the solar parallax
accurately will be seen, when one is told that the
correction from the old value of 8". 58 to the recent
one of 8 '.94, was a change in the angle equal to that
which the breadth of a human hair would make
when seen at a distance of 125 feet. Yet this reduced
the estimated distance of the sun from 95,293,000
miles, to 91,430,000 miles.

4. To Find the Longitude of a Place.* — (1.) The
Solar Method. — If the sailor can see the sun, he
watches it closely with his sextant ; and when the
sun ceases to rise any higher in the heavens it is ap-
parent noon. By adding or subtracting the equation
of time (as given in his almanac), he obtains the true
or mean noon. He then compares the local time thus
determined, with the Greenwich time as kept by the

* It is pleasant to notice tliat tlie astronomer can preiUct witli the utmost precision.
He announces that on such a year, niontli, day, hour, and second, a celestial body will
occupy a certain position in the heavens. At the time indicated, we point our telescope
to the ])lace, and, at tlie instant, true beyond the accuracy of any timepiece, the orb
sweeps into view ! A prediction of the Nautical Almanac is received with as much
confidence as if it were a fact contained in a book of history. " On the trackless ocean,
this book is the mariner's trusted friend and counsellor; daily and nightly its revela-
tions bring safety to ships in all parts of the world. It is something more than a mere
book. It is ao ever-present manifestation of the order and harmony of the universe."

CELESTIAL MEASUREMENTS. 281

ship's chronometer. The difference in time reduced
to degrees, gives the longitude.

(2.) The Lunar Method.— On account of the diffi-
culty in obtaining a watch which will keep the exact
Greenwich time through a long voyage, the moon is
more generally relied upon than the chronometer.
The Nautical Almanac is always published, for the
benefit of sailors, three years in advance. It gives
the distance of the moon from the principal fixed
stars which lie along its path, at every hour in the
night. The sailor has only to determine with his
sextant the moon's distance from any fixed star, and ■
then, by referring to his almanac, find the correspond-
ing Greenwich time. By comparing this with the
local time, and reducing the difference to degrees,
etc., he obtains the longitude.

5. To Find the Latitude of a Place.— (1.) By
means of the sextant find the elevation of the pole
above the horizon, and this gives the latitude direct-
ly. (Fig. 35.)

(2.) In the same manner, determine the height of
the sun above the horizon at noon. The sun's decli-
nation for that day (as laid down in the almanac),
added to or subtracted from this, gives the height of
the equinoctial above the horizon. Subtract this
result from 90°, and the remainder is the latitude.

" Place an Astronomer on board a sliip ; blindfold him ; carry him by
any route to any ocean on the globe, whether under the tropics or in one
of the frigid zones ; land him on the wildest rock that can be found ;
remove his bandage, and give him a chronometer regulated to Greenwich
or Washington time, a transit instrument Anth the proper appliances, and
the necessary books and tables, and in a single clear night he can tell his
position mthin a hundred yards by observations of the stars. "

282 THE SIDEREAL SYSTEM.

6. To Find the Circumference of the Earth. — If

the earth were a perfect sphere, it is obvious that
degrees of latitude would be of the same length
wherever measured on its surface. Each would be
^^0^ of the entire circumference. If, however, a per-
son sets out from the equator, and travels along a
meridian toward either pole, and, when the polar star
has risen in the heavens one degree above the horizon,
he marks the spot, and then continues his journey,
marking each degree in succession, he will find that
the degrees are not of equal length, but increase
gradually from the equator to the pole. If, now, the
length of a degree be measured at different places,
the rate of variation can be found, and then the
average length be estimated. Measurements for this
purpose have been made in Peru (almost exactly at
the earth's equator), Lapland, England, France,
India, Russia, etc. So great accuracy has been at-
tained, that Airy and Bessel, who have solved the
problem independently, differ in their estimate of
the equatorial diameter but 77 yards, or only y^o of
a mile.

7. To Find the Relative Size of the Planets. — The
volumes of two globes are proportional to the cubes
of their like dimensions. The diameter of Mercury
is 3,000 miles, and that of the earth 7,925 ; then,

The volume of Mercury : the volume of the earth : : 3000^ : 7925".

The same principle applied to the volume or bulk of
the sun gives —

The bulk of the sun : bulk of the earth :: 866,0003 : 7925»,

PRACTICAL QUESTIONS. 283

8. To Find the Diameter of the Sun. — (1.) A very
simple method is to hold up a circular piece of paper
before the eye at such a distance as exactly to hide
the entire disk of the sun. Then we have the pro-
portion,

As dist. of paper disk : dist. of sun's disk : : diam. of paper d. : diam. sun's d.

(2.) The apparent diameter of the sun, as seen
from the earth, is about 32': the apparent diameter
of the earth, as seen from the sun, is twice the solar
parallax, or 17". 60 (p. 36). Thence, the

Ap. diam. of earth : ap. diam. of sun : : real diam. of earth : real diam. of sun.

(3.) Knowing the apparent diameter of the sun,
and its distance from the earth, the real diameter is
found by Trigonometry. In figure 110, let S represent
the earth ; AB, the radius of the sun ; and ASB, half
the apparent diameter of the sun. We shall then
have the proportion,

AS : AB : : radius : sin. 16' (half mean diam. of sun).

By a similar method the diameters of the planets are
obtained.

PRACTICAL QUESTIONS.

1. In what constellation is Job's Coffin ? The Letter Y ? The Scalene
Triangle ? The Dipper ? The Kids ? The Triangles ?

2. Name some facts in the solar system for which the nebular hypo-
thesis fails to account.

3. Which is probably hotter, a yellow or a red star ?

4. Are any of the stars likely to collide with each other ?

5. Is the real day longer or shorter than the apparent one ?

6. Do we ever see the stars '(

284 PRACTICAL QUESTIONS.

7. What fixed star is nearest the earth ?

8. How often is Polaris on the meridian of a place ?

9. How do we know that the stars are suns ?

10. Can a watch keep apparent time ?

11. How could a child be 8 years old before a return of its birthday ?
12 ^^"^leu will a watch and a sun-dial agree ?

13. "What star will be the Pole Star next after Polaris ?

14. Why is the birthday of Washington celebrated on Feb. 22, when he
was born Feb. 11, 1732 (0. S.) ?

15. Does the tide have any effect on the length of the day ?

16. Will the Big Dipper always look as it does now ?

17. How many times does the earth turn on its axis every year ?

18. Does the spectroscope tell us anything concerning the constitution
of the moon, or any of the planets ?

19. When the United States bought Alaska from Russia, the calendar
used there was found to be one day ahead of our reckoning. Why was
this ?

20. "Why do the dates of the solstices and equinoxes vary a day in differ-
ent years ?

21. Why are not forenoon and afternoon of the same day, as given in
the almanac, of equal length ?

22. In what part of the heavens do the stars apparently move from west
to east ?

23. What year was only nine months and six days long ?

24. What day will be the last day of the Xineteeuth Century ?

25. If one should watch tlie sky, on a winter's evening, from 6 v. si. to
6 A. M., what portion of the celestial sphere would he be able to see ?

26. How do we know tliat the moon has little, if any, atmosphere ?

27. In Greenland, at what part of the year will the midnight sun be seen
due north ?

28. Can you give any other proof of the rotundity of the earth, besides
that named in the text ?

29. Point out the error in the following passage from Byron's ' ' Darkness "
where the poet, in describing the effect of the sun's destruction, says —

■' I had a dream, » » *

* * * which was not all a dream,
The bright sun was extinguislied, and the stars
Did wander darkling in the external space
Ray less aud pathless."

PRACTICAL QUESTIONS. 285

30. Explain the remark of the First Carrier in Scene i, Act ii, King
Henry IV : " An't be not four by the day, I'll be hanged : Charles' wain
is over the new chimney."

31. Why does not the earth move with equal velocity in all parts of its
orbit ?

32. How many Jovian-years old are you ?

33. Why is the sky blue ?

34. At what season of the year does Cliristmas occur in Australia ?

35. What causes tlie apparent movement of the sun north and south ?

36. On what part of the earth is the twilight the longest f The shortest I

37. Name the causes which make our summer longer than winter.

38. Why is not total darkness produced when a dense cloud passes be-
tween us and the sun ?

39. Why does the time of the tide vary each day ?

40. Why is an annular, longer than a total, eclipse ?

41. Why is it colder in winter than in summer '

42. Do the solar spots affect our weather ?

43. Can the moon be eclipsed in the day-time ?

44. Why are the sidereal days of uniform length ?

45. Why are not the solar days of uniform length ?

46. What does the moon's phases prove ?

47. Why do the sun and moon a})pear flattened when near the hori-
zon?

48. How many stars can we see with the naked eye ?

49. Is there ever an annular eclipse of the moon <

50. "While the sun rises and sets 365 times, a star rises and sets 366
times." Explain.

51. How many moons are there in the solar system ?

52. What causes the twinkling of the stars (

53. Name some of the uses of the stars. *

* " To the astronomer, the fixed stars are immovable lioundary-stones by which he
determines the courses of the wandering lieavenly bodies. To the geographer, they are
the signal-stations according to which he surveys the chart of the earth by the heavens.
To the mariner, they are the lights that direct him over the dark paths of the seas. To
the hunter, the herdsman, the wanderer, they are a clock. To the farmer, they are a
calendar. The historian finds in them many a memorable event in the oldest Grecian
history. The poet reads in them the charming Grecian mythology, which has furnished
such rich materials to dramatic art ; and every person of sensibility receives from them
au impulse to worship, meditation, and liope."

286 PRACTICAL QUESTIONS.

54. Describe the methods by which we determine the distance of the sun
from the earth.

55. Why do not the signs and the constellations of the Zodiac agree ?

56. When we look at the Xorth Star, how long since the light that enters
our eye has left that IxkIv ?

57. In what diiection does a comet's tail generally point ?

58. ^Vhat is the cause of shooting stars ?

59. Why docs the crescent moon appear larger than the dark body of the
moon ?

60. What is the i-eal path of the moon ?

61. What would be the result if the axis of the earth were parallel to the
plane of its orbit ?

62. Do we see the same stai-s at different seasons of the year ?

63. Why do we not jierceive the earth's motion in space ?

64. Did the earth ever shine as a star ? Does it now shine as a planet ?

65. What is the nebular hyjx)thesis ?

66. What is the cause of the solar spots ?

67. Would it make the new moon "drier" or "wetter" if the moon's
path ran north of, instead of on, the ecliptic at the time of new moon '.

68. Under what conditions are we accustomed to transfer motion ?

69. ^^'^ly do not the planets twinkle 1

70. Why is the horizon a circle ?

71. What causes are gradually increasing the length of the day ?

72. What distance does the moon gain in her orbit each year ?

73. State the general argument which renders it probable that other
worlds are inhabited.

74. Illustrate the uniformity of Xature. \Miat thought does this
suggest ?

75. At what rate are we traveling through space ? How is this determ-
ined?

76. ^^^ly does the length of a degree of latitude increase in going from
the equator toward either pole of the earth ?

77. How can you detect the yearly motion of the sun among the stars ?

78. Have you actually traced the movement of any one of the planets, so
as to understand its jjecuUar and irregular wandering among the stars ?

79. How do you explain the varied aspect of the heavens in the different
seasons of the year ?

PRACTICAL QUESTIONS. 287

80. How Joes the sjjiiiniug of a top illustrate the subject of precession ?

81. Why do solar eclipses come on from the west and cross to the east,
while lunar eclipses come on from the east and cross to the west !■

82. Newcomb, in his Astronomy, says that, " If, when the moon is near
the meridian, an observer could in a moment jump from New York to
Liverpool, keeping his eye fixed upon that body he could see her apparently
jump in the opposite direction about the same distance." Explain.

83. When, and by whom, was the basis of the calendar we now use fully
established ?

84. How much is the Russian reckoning of time ]>ehind ours ?

85. Is there any gain in having the astronomical and the calendar year
agree ?

86. What religious festival is fixed each year by the motion of the moon ?

87. Why can we, at different times, see both poles of the planet Mai-s <

88. What famous astronomical discovery was made on the tii-st day of
this century ?

89. Do the stars rise and set at the poles ?

90. Name and locate the stars of the first magnitude which are seen in
our sky.

91. Name three bright stars which lie near the first meridian.

92. What events were transpiring in our history a Saturnian century ago ?

93. What is the sun's declination at the winter solstice ? At the autunmal
equinox ?

94. Will the u-idth of the terrestrial zones always remain exactly
as now ?

95. Is it always noon at 12 o'clock ?

96. ^^^len the sun's declination is 23^ N., in what sign is he then
located, and what is his R. A. ?

97. What is the apparent diameter of the sun ?

98. How can a sailor find his latitude and longitude at sea ?

99. How many miles on the solar disk represent a second of apparent
diameter ?

100. At what latitude will there be twilight during the entire midsummer
Vight ?

Fig. lis.

Cambridge Equatorial Telescope.

IV.

APPENDIX

APPENDIX.

TABLE ILLUSTEATING KEPLER'S THIRD LAW. (CHA>rBERS.)

In the first column are the relative distances of
the planets from the sun ; in the second, the periodic
times of the planets ; and in the third, the squares
of the periodic times divided by the cubes of the
mean distances. The decimal points are omitted in
the third column for convenience of comparison.
The want of exact uniformity is doubtless due to
errors in the observations.

Vulcan ?
Mercury
Venus •
Earth -
Mars -
Jupiter
Saturn
Uranus
Neptune

.143

19.7

1.32 716

.38710

87.969

133 421

.72333

224.T01

133 413

365.256

133 403

1.52369

686.979

133 410

5.20277

4,a32.585

1.33 294

9.53858

10,759.220

133 375

19.18239

30,686.821

133 422

30.03627

60,126.722

133 413

Arago, speakinsr of Kepler's Laws, says : " These interesting laws, tested for every
planet, have been found so perfectly exact, that we do not hesitate to infer the dis-
tances of the planets from the sun from the duration of their sidereal periods ; and it
is obvious that this method possesses considerable advantages in point of exactness."

MEASUREMENTS OF THE EARTH'S DIAMETER.

Polar diameter
Equatorial diameter
Compression - -

7899.17

7925.64

26.47

Bessel.

7899.11

79i5.60

26.49

293

APPENDIX.

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QUESTIONS FOR CLASS USE.

These are the questions which the author has used in his own classes
for review and examination. In the historical portion, he has required
his pupils to write articles upon the character and life of the various
persons named, gathering materials from every attainable source. He
has also introduced whatever problems the class could master, taking
topics from the article on Celestial Measurements and the various
mathematical treatises.

Introduction. — Define Astronomy. Is the earth a planet? Is the
moon a planet? What is the sky? Why does it seem concave ? What
gives it its color? What is the difference in the appearance of a fixed
star and a planet? What is the Milky Way? In what direction does
it span the heavens? In what season of the year is it most brilliant ?

I. THE HISTORY.

5-6. What can you say of the antiquity of astronomy ? How far
back do the Chinese records extend ? Name some astronomical phe-
nomena they contain. Why were these astronomers at fault in failing
to announce the eclipse ? (Ans. Certain religious ceremonies were
performed on such occasions, and their omission was believed to expose
the nation to the anger of the gods.) Why should the Chaldeans have
become versed in this study ? How ancient are their records ? What
discoveries did they make? How does the Asiatic differ from the Euro-
pean mind ?

7. What Grecian philosopher early acquired a reputation in thi?
science? What other discovery did Thales make (Physics, p. 251)?
What did he teach? What memorable eclipse did he predict? What
did Anaximander teach? In what century did Pythagoras live? What
was his characteristic trait ? Did he advance any proof of his system ?
Explain his theory. How does it differ from ours? What strange
views did he hold ?

8. When did Anaxagoras live? What did he teach? What theory
did Eudoxus advance? What is the theory of the cr3'stalline spheres?

294 QUESTIONS FOR CLASS USE.

What has Hipparchus been styled ? What addition did he make to
astronomical knowledge ? How man}- stars in our present catalogue
(p. 207;? How did Egypt rank in science at an early day? What
preparation did the Grecian philosophers make to fit themselves for
teachers? How long did Pythagoras travel for 'his purpose?

9. What can you say of the School at Alexandria ? What great work
did Ptolemy write? What theory did he expound? Was it original?
What discovery did Eratosthenes make ? Describe that method (p. 282).
Show how the movements of the planets puzzled the ancients.

10. What was the theory of " cycles and epicycles " ? Did the
ancients believe in the reality of this cumbrous machinery? Did this
theory possess anj* accuracy ?

11. Could it be adapted to explain any new motion ? What was the
remark of Alfonso ? Describe the progress of learning among the
Saracens.

12. Where was the first Observatorj' in Europe built? W^hen did
Spain lose her prominence in scientific studies?

13. What is astrology ? What was its association with astronomy?
State something of the repute in which astrolog)- was held. Tell what
you can of the system. W'hat use did it subserve ?

14. What theory displaced the Ptolemaic? W'hen? Was the system
of Copernicus original ? What credit is due him? Describe his idea
of apparent motion. How did he apply this to the heavenly bodies ?
What crudity did he retain ?

15. Who was Tycho Brahe ? What was his theory? How did it
differ from Ptolemy's and Copernicus's ? What good did Brahe
accomplish? Could he generalize his facts? Had he a telescope ?
How did Kepler differ from Brahe? What were the two prominent
characteristics of Kepler ?

16-19. State his three laws. Tell how he discovered the first. The
«econd. The third. Describe (he ellipse. Define focus, perihelion,
and aphelion. What remarkable statement did Kepler make? When
did Galileo live ?

20. What discoveries did he make in Physics? In astronomy?
WTiat advantage did he have over his predecessors? Give an account
of his observations on the moon. On Jupiter's moons.

21. Why did this settle the controversy between the Ptolemaic and
the Copernican system? How were Galileo's discoveries received?
Give some of Sizzi's arguments. Who discovered the law of gravita-
tion ?

23. Repeat it. How was this idea suggested ? WTiat familiar laws
of motion aided Newton? How did he apply these to the motion ot
the moon? Repeat the storj- of his patient triumph.

THE SOLAR SYSTEM. 295

24, 25. What is the celestial sphere ? Give the two illustrations which
show its vast distance from the earth. Why can we not see the stars
by day, as by night? What portion of the sphere is visible to us?
Name the three systems of circles.

26-30. Name and define (i) the principal circle, (2) the secondary
circles, (3) the points, and (4) the measurements of each system. De-
fine especially, because in common use, zenith, nadir, azimuth, alti-
tude, equinoctial, right ascension, declination, equinox, ecliptic, colure,
and solstice. What is N or S in the heavens ?

31. What is the Zodiac? How wide is it? How ancient? How
is it divided ? State the names and signs.* State the meaning of each
(p. 210.)

II. THE SOLAR SYSTEM.

What bodies compose the solar system ? Describe how we are t&
picture it to ourselves.

The Sun. — What is its sign? Its distance from us? Illustrate.
What is the solar parallax (see pp. 121, 275)? What change has recentl>'
been made in the estimate of the parallax of the sun, and of its distance
from the earth? (See p. 279.)

37. How are celestial distances measured? What is the color of the
sun ? To what is the sun's light equal ? To how many moons ?

38. To what is its heat equal? Illustrate. What proportion of the
sun's heat reaches the earth? What is the apparent size of the sun?
How does this vary?

39. 40. State the solar dimensions, (i) diameter— illustrate ; (2)
volume ; (3) mass ; (4) weight ; (5) density. How large did Pythagoras
think the sun is? Tell something about the force of gravity on the
sun. How much would you weigh if carried to its surface? (The force
of gravity on the sun as compared with the earth is 27.6.) How does
the sun appear to the naked ej'e?

41. How can we seethe spots? What were formerly the views of
astronomers with regard to the sun's face ?

42. When were the spots discovered? Tell something about the
number of the spots. Their location." Size. What number of miles
subtend a second of arc at the distance of the sun?

* "The Ram, the Bull, the Heavenly Twins,
And ne.\t the Crab, tlie Lion shines,

The Virgin and the Scales,
The Scorpion, .Archer, and He-goat,
The Man that bears the watering-pot,

And Fish with glittering tail?,"

296 QUESTIONS FOR CLASS USE.

43. Describe the parts of which the spots are composed. Describe
the motion of the spots.

44. How do the spots change in form as they pass across the disk?
What does this prove ? What is the length of a solar axial rotation ?

45. 46. Explain a sidereal and a synodic revolution of a spot. Why
do not the spots move in straight lines? Show how they curve. Tell
what you can about the irregular movements of the spots.

47. Illustrate how suddenly they change. What can j-ou say about
their periodicity? Who discovered this? Is there any connection be-
tween the solar spots and the aurora ?

48. Tell about the influence of the planets on the spots. Do the
spots aflfect the truitfulness of the season ? Does the temperature of the
spots diflfer from that of the rest of the sun ? Are the spots depressions
in the sun ?

49. How much darker are they than the adjacent surface? Is the
sun brighter than the Drummond light? {Ans. "The sun gives out
as much light as one hundred and forty-six lime-lights would do, if
each were as large as the sun and were burning all over.'')

50. What are the faculae ? Describe the mottled appearance of the
sun. What is the shape of the bright masses? What is a granule?
What is its size ?

51. 52. Describe the constitution of the sun according to Wilson's
theor\'. How are the spots produced ? The faculse ? The penumbra ?
The nucleus ? The umbra ?

53,54. What is the present theor)' (" KirchhofTs Theory ") ? Name
the four different portions of the sun. Define the nucleus. The photo-
sphere. The chromosphere. The corona. What are the protuber-
ances? How are the spots produced ? The umbra? The penumbra?
Upon what discoveries does this theory depend (p. 262)? What is the
cause of the heat of the sun ? Will the heat ever cease ?*

The Planets. — Name the six characteristics common to all the
planets. Compare the two groups of the major planets.

57, 58. Draw an ellipse, and name the various parts. Define the
ecliptic. The plane of the ecliptic. Why is the ecliptic so called?
Define the ascending node. The descending node. Line of the
nodes. Longitude of the node. Tell what )'ou can with regard to
the comparative size of the planets.

* If we accept the Nebular hypothesis (p. 256), we must suppose that the heat is
produced by the condensation of the nebulous matter and consequent chemical
changes. The sun is radiating its heat constantly, and, at some time, its light will go
out, in turn, as that of the earth and the planets has before it. This theory is of especial
mterest, as it shows that the sun, as well as the solar system, has a certain fixed exist-
ence; and that, " like all natural objects, it passes through its regular stages of birtt).
vigor, decay, and death, in ope order of progress." — Newcomb,

THE SOLAR SYSTEM. W

60. What is a conjunction ? Name the earliest that are recorded.

61-3. What do you say concerning the probability of the planets being
inhabited ?* State the conditions of life on the different planets. What
are the two divisions of the planets?

64. What causes the apparently irregular movements of the planets ?
Define heliocentric and geocentric places. Illustrate. In what part
of the sky is an inferior planet always seen? Define inferior and
superior conjunction. Greatest elongation.

65. Why is a star atone time " evening " and, at another, " morning
star " ? What is a transit ^

65, 66. Explain the retrograde motion of an inferior planet. (This
motion, it will be remembered, was one that sorely puzzled the
ancients.) Describe the phases of an inferior planet.

67. Why does an inferior planet have phases ? Define gibbous.
Explain the opposition and conjunction of a superior planet.

68. Explain its retrograde motion. Must a superior planet always
be seen in the same part of the sky as the sun? Define quadrature.
Can an inferior planet be in quadrature?

69. 70. Which retrogrades more, a near or a distant planet? Define
a sidereal and a synodic revolution of an inferior and a superior planet,
and tell what you can about each. In what case would there be no
difference between a sidereal and a synodic revolution ? Why is a planet
invisible when in conjunction? When is a planet evening, and when
morning star ?

71. Tell what you can about the supposed discovery of a planet in-
terior to Mercury. What name and sign have been given to it?

Mercury. — Definition and sign? Describe the appearance of Mer-
cury, and where seen. What was the opinion of the ancients ? Of the
astrologists ? Of chemists ? Why is it difllicult to see this planet ?
When can we see it best ?

73. What is the peculiarity of its orbit? What is Mercury's greatest
elongation from the sun? Why does this vary? What is Mercury's
distance from the sun? What is its velocity? What is the length of

* The uniformity of Nature is a most effective argument in this direction. Light
travels everywhere through the universe at the same rate. The elements of star
planet, and the earth are the same. The sun, which may be considered as the mother
of the earth, is composed of the same materials. The laws of gravitation rule so
absolutely that the satellite ofSiriuswas not discovered until after it was observed
that an unknown influence afiected the star. " The uniformity of law and mattei is
proof that there must be through the universe organizations similar to those of our
system. We see the result of these laws in the world we inhabit, and we cannot
doubt that the same powers and the same materials have produced organizations
similar to these of the earth in millions of other places, though we can only
philosophically suppose their existence, not practically prove it." — IV. Meytr,

298 QUESTIONS FOR CLASS USE.

its day? Of its year? What is the difference between its sidereal and
synodic revolutions ? What is its distance from the earth?

74-6. Show why its greatest and least distances vary so much. What
is its diameter? Volume? Density? Force of gravity? Specific
gravity? How much would you weigh on Mercury? (Mercury's foice
of gravity as compared with that of the earth is 46.) Describe its sea-
sons. (If the pupil does not understand pretty well the subject of the
terrestrial seasons, it would be well here to read carefully page 95,
et seq.) What is the temperature? The appearance of the sun ? Has
Mercury any moon? What is the appearance of the planet through a
telescope? What do these phases prove? What do we know of the
mountains and valleys upon Mercurj'? The atmosphere? Have we
any recent observations ?

77. Venus.* — Definition and sign? Ancient names? Appearance
to us? When brightest ? Can Venus be seen by daj-? Illustrate.

78. Describe the orbit of Venus. What is the distance of Venus
from the sun? Velocity? Length of the year? Day? Difference
between the sidereal and synodic revolutions? Distance from the
earth?

79. How near may Venus approach us? How does her apparent
size vary? When is Venus the brightest? What is her diameter?
Volume? Density?

So. Force of gravity ? (The force of gravity on the surface of Venus
is .82 that of the earth.) Docs the force of gravity increase or decrease
with the mass or volume of the body? Describe the seasons upon
Venus.

81,82. Describe the telescopic appearance of Venus. Who dis-
covered the phases of Venus ? What was the effect of this discovery?
What proof have we that Venus possesses a dense atmosphere? Has
Venus a moon ?

83. E.\RTH. — Sign ? What is the appearance of the earth from the
other planets? Do we, then, live on a star? Is it probable that the
earth was always dark and dull as it now seems to us?* How does

♦ Venus Is the only planet mentioned by Homer —

O'os S'a.<rTr\p elcri ncr' dcTTpaffi wKxdi a/xoXyw
*E(J7r€po5 b? KdAAio"To? €v ovpavio itTTaTcu. atTTrjp

Iliad^ xxii. 317.

* Probably not. The earth was doubtless once a glowing star, like the sun. Its
crust is only the ashes and cinders of that fearful contJagration. The rocks are all
burnt bodies. The atmosphere is only the gas left over after the fuel was all con-
sumed. Every organic object has been rescued by plants and the sunbeam from the
grasp u! oxygen.

The solar system. 299

the size of the earth compare with that of the other planets? What is
the shape of the earth ? What is its exact diameter ? (See Table in
the Appendix.)

84. Circumference? Density? Weight? What comparison may be
made to illustrate its inequalities? How do you prove the rotundity
of the earth ? *

85. Why can we see further from the top of a hill than from its base ?
Why is the horizon a circle ?

86-7. Give some illustrations of apparent motion. Is it, then, natu-
ral for us to transfer motion ? Under what conditions do you think
this occurs ? Explain the cause of the rising and setting of the sun
and stars. Who first explained these phenomena in this manner?
What do you say of its simplicity ?

88. What is the cause of day and night? Do all places on the
earth revolve with equal velocity ? Illustrate. At what rate do we
move? Wh}' do we not perceive our motion?

89. What would be the effect if the earth were to stop its rotation?
Is there any danger of this catastrophe? How is the length of the day
increasing? Is the amount appreciable ?

90-1. Draw the figure, and show how the stars move daily through
unequal orbits and with unequal velocities. Describe the appearance
of the stars at the N. Pole. At the Equator. At the S. Pole.

92-3. Describe the path of the earth about the sun. Defin eccentri-
cit}\ What is the amount of the eccentricity of the earth's orbit? Is
this stable? Do we see the same stars at different seasons of the year ?
Why not? If we should watch from 6 p. M. to 6 .k. m., what portion of
the sphere would we see?

94. What do we mean by the yearly motion of the sun among the
stars ? How can we see it ? What is the cause ? What is the ecliptic ?
Why so called ? What are the equinoxes? What do you understand
when you see in the almanac the statement that *' The earth is in
Aries?" " The sun is in Sagittarius?" etc. How many apparent
motions has the sun ? Name them, and give the cause and effects of
each. Has the sun any real motions?

95. Describe the apparent motion of th sun, N. and S. How is it
that the sun in summer shines on the north side of some houses both
at rising and setting, but in winter never does? Define the obliquity
of the ecliptic. The parallelism of the earth's axis. What do you say
of its permanence ? Why will a top stand while spinning, but will fall
as soon as it ceases ?

97. Show how the rays of the sun strike the various parts of the

* It is said that aeronauts, at a proper height, can distinctly see the curving form
of the earth's sui£ice.

300 QUESTIONS FOR CLASS USE.

earth at different angles at the same time. Show how the angles vary
at different times. Is the sun reall}' hotter in summer than in winter?
Why does it seem to be ? Why is it warmer in summer than in win-
ter ? What effect upon the temperature has the difference in the length
of the summer and the winter day?

98-100. Explain the cause of equal day and night at the equinoxes.
\Vh)' are our days and nights of unequal length at all other times?
Why does the length vary at different seasons of the year ? How do
the seasons, &c., in the N. Temperate Zone compare with those in the
S. Temperate Zone? Describe the yearly path of the earth about the
sun — (i), at the summer solstice ; (2), at the autumnal equinox ; (3), at
the winter solstice ; (4), at the vernal equinox ; (5), the yearly path
finished back to the starting-point. Is the division of the earth's
surface into zones an artificial or a natural distinction? Who in-
vented it?

loi. How much nearer are we to the sun in winter than in summer?
Why is it not warmer in winter? How is it in the South Temperate
Zone? W^hen do the extremes of heat and cold occur? Why do they
not occur exactly at the solstices?

102. Why is summer longer than winter ? Does the earth m ve
with the same velocity in all parts of its orbit? Describe the curious
appearance of the sun at the North Pole. In Greenland, at what part
of the year will the midnight sun be seen due north ? What is the
length of the days and nights at the Equator?

103. Describe the results if the axis of the earth were perpendicular
to the ecliptic. If the equator were perpendicular to the ecliptic.

104-5. Define the precession of the equinoxes. Who discovered
this fact ? At what rate does this movement proceed ? What time will
be required for the equinoxes to make an entire revolution ? What
are the results of precession? What star was formerly the Polar Star?
(See p. 219.)

106-9. Explain the cause of precession. How does the spinning of a
top illustrate ihis subject ? In what way is the force which acts on a
spinning-top opposite 10 that which produces precession ? What is
Nutation ? What is the cause of the nodding motion ? How does the
moon's influence compare with that of the sun ? W'hat is the effect of
Nutation?

no. What is the real path of the N. Pole through the hea%'ens ? Is
the obliquity of the ecliptic invariable ? What is the period of this
oscillation ?

III. What causes combine to produce this nodding motion we have
described ? Why are the tropics located where they are ? Is their
position on the earth permanent? What effect does precession have

THE SOLAR SYSTEM. 301

on the latitude of the stars ? What is meant by the line of apsides of
the earth's orbit? Is this permanent ?* What is the Great Year of
the Astronomers ?

112. At what season of the year is the earth now in perihelion ?
When was it in perihelion in the autumn? When :n the winter?
When will perihelion occur in the spring? When in summer?
When will the cycle be completed ? What provision is there for per-
manence in the midst of these changes?

113-14. What is refraction? Its effect? Upon what principle of
Optics is this based ? How does refraction vary ? Are the sun and
moon ever where they seem to be ? Is the real day longer or shorter
than the apparent one ?

115. Describe the apparent deformation of the sun and moon near
the horizon. Explain. Why are not these bodies apparently deformed
in the same way when they are high m the heavens ? Why do they
appear smaller in the latter case ? (See Fig. 48, p. 124.) What causes
the hazy appearance of the heavenly bodies near the horizon ?

116. What is the cause of twilight ? How long does it last? Is it
the same at all seasons of the year? In all pans of the earth ?

117. Where is it the longest? Shortest? State the cause of this
variation. What is diffused light? What would be the effect if the
atmosphere did not act in this way? Is there really any sky in the
heavens ? What is the cause of the appearance ? What is aberration
of light ?

118. Illustrate this phenomenon. State two reasons why we never
see the sun where it really is.

119. What is the general effect of aberration ? Define parallax. Il-
lustrate.

120-21. Define true and apparent place. How does parallax vary ?
What is the practical importance of this subject (pp. 36, 278)? Define
horizontal parallax. What is the sun's horizontal parallax ? What is
the annual parallax?

The Moon. — Signs ? Describe the moon's orbit. What is the
moon's distance from the earth? Illustrate. What is the difference
between her sidereal and synodic revolutions ? What is the real path
ot the moon? (Imagine a pencil fastened to the spoke of a wheel, and
the wheel rolled by the side of a wall on which the pencil is constantly
marking.) How often does the moon turn on her axis ? What is the

* "The line of equinoxes of the earth's orbit, as we have seen, has a slow left-
handed retrograde motion of so".2 eacli year, called the precession of the equinoxes \
and the line of apsides has a still sXowftx right-handed direct motion oi \\".2<)\ and
in consequence of the motion of both these lines, the an^le formed by them changes
through 6i".49 each year, so as to complete an entire revolution in 21,077 years."

302 QtJESTlONS FOR CLASS USE.

moon's diameter? Volume? How does her apparent size vary?
Why does she appear larger than she really is ?

124. Why does the crescent moon appear larger than the dark body
of the moon? When ought the moon to appear the largest? Do all
persons think the moon to be of the same apparent size?

125. Explain the three librations of the moon. How does moonlight
compare with sunlight? Is there any heat in moonlight?

126. Does the center of gravity in the moon coincide with that of
magnitude ? Has the moon any atmosphere ? What proof have we of
this? (Ans. (i). We see but slight, if any, appearance of twilight on
the moon. (2). When the moon passes between us and a star, it does
not refract the light of a star, so that the atmosphere cannot be suflS-
cient to support more than y§ij of an inch of the mercurial column.)
What must be the effect of this lack upon the temperature of the moon's
surface? State Langley's observations upon Mount Whitney. How
does the earth appear from the moon?

127-9. What is the earth-shine ? How is it caused ? What is it called
in England ? Describe the path of the moon around the earth, and the con-
sequent phases. Why is new moon seen in the west and full moon in the
east ? Why can we sometimes see the moon in the west after the sun rises,
and in the east before the sun sets? What is the length of a lunar
month?* What do we mean by the moon's running high or low?
What is the cause of this variation? Is it of any use?

130-1. What is harvest moon ? What is the cause ?f

132. Explain the cause of "Dry Moon" and "Wet Moon." What
are nodes? How much is the moon's orbit inclined to the ecliptic — •
our ideal sea-level? What is an occultaiion? What use does it sub-
serve? Describe the seasons, heat, etc., on the moon.

135-7. Describe the telescopic appearance of the moon. Are the
mountains the light or the dark portions? What canyon say about
them? The gray plains? The rills? The craters? What are the pecu-
liar features of the lunar landscapes? Are the lunar volcanoes extinct?

* " The moon's sidereal period is not constant, and a comparison of modern with
ancient observations shows that it has undergone an acceleration since tlie period ot"
the Chaldean observations of eclipses made 720 b. c. Several explanations have been
given by Laplace and others, of the supposed cause of the acceleration of the moon's
mean motion ; but it is highly probable that it is a pseudo-phenomenon^ that owes its
origin to a real lengthening of the time of rotation of the earth (which is the unit
of astronomical time), caused by the friction of the sea and atmosphere."

+ It will aid in understanding the cause of harvest moon, if one gets clearly in
mind the fact that the moon when full is always in the opposite part of the heavens
from the sun. At the time of the autumnal equinox, i. e. when the sun is at the
autumnal equinox, (or in Libra, note, p. 94.) the moon must be at the vernal equinox,
(or in Aries.) The least retardation of the moon, which occurs at this time, happens,
therefore, in September.

SOLAJEl SYSTEM. 303

138. Eclipses. — When can an eclipse of the sun occur ? Show how
a solar eclipse may be total, partial, or annular. Define umbra. Pe-
numbra. Central eclipse. State the general principles of a solar
eclipse. What curious phenomena attend a total eclipse?* What
are Daily's Beads ? What is the corona ? Describe the effect of a total
eclipse. What curious custom prevails among the Hindoos? What
is the Saros? Is it now of any value? What is the metonic cycle?
Explain its use. What is the golden number? What is the cause of a
lunar eclipse ? Why are lunar eclipses seen oftener than solar ones?
How were total eclipses formerly regarded ?

147. The Tides. f — Define ebb. Flow. How often does the tide
happen? Explain the cause. Why does the tide occur about fifty
minutes later each day? Why is there a tide on the side opposite the
moon? The sun is much larger than the moon ; why does it not pro
duce the larger tide? What effect has the friction of the tides produced
upon the earth? What theory upon this topic has Professor Ball
advanced ? What is meant by the differential effect of tlie moon ? Why
is not the tide felt out at sea? What is spring-tide ? Neap-tide? Why
does the tide differ so much in various localities? Tell about the
height of the tides at different points. Why is there no tide on a lake ?
Is the tidal wave a forward movement of the water ?

150. Mars. — Definition and sign? Describe the appearance of this
planet. When is it brightest? What is its distance from the sun?
Velocity? Day? Year? Distance from the earth? What is the pecu-
liarity of its orbit? What is the diameter of Mars? Its volume and
density as compared with the earth? How far would a stone fall on
its surface the first second? Who discovered its moons ? What is the
peculiarity of these tiny globes? What are the peculiar telescopic
features of Mars? What is the cause of its ruddy color? Wliat are
the snow-zones? Can we watch the change of its seasons?

* Lockyer, describing the beginning of a total eclipse, says : " One seems in a new
world— a world filled with awful sights and strange forebodings, and in which still-
ness and sadness reign supreme ; the voice of man and the cries of animals are hushed ;
the clouds are full of threatenings and put on unearthly hues ; dusky, livid, or purple,
or yellowish crimson tones chase each other over the sky irrespective of the clouds.
The very sea is responsive and turns lurid red. All at once the moon's shadow comes
sweeping over air, and earth, and sky, with frightful speed. Men look at each other
and behold, as it were, corpses, and tlie sun's light is lost."— Gillis. in his observa-
tions upon the eclipse of 1859, as witnessed by him m Peru, remarks: "At 1.54, the
moment of totality, the attendants, catching sight of the corona, dropped on theii
knees, and shouted, '• La Gloria ! La Gloria ! "

t As the tidal wave does not move as rapidly as the earth does, the water has an
apparent backward motion. It has been suggested that this (as well as the friction
of the atmosphere) acts as a break on the earth's diurnal revolution. It has been
shown that the moon's true place can be best calculated if we suppo-e that the side-
real day is shortening at the rate of iHU of a second in 2,400 years. (See page 89.)

304 QUESTIONS FOR CLASS USE.

154. Minor Planets (Asteroids).* — Give Bode's law. Telthow the
first of these planets was discovered. How many are now known?
Are the)- probably all discovered ? Describe some of these " pocket
planets". Do they all lie within the Zodiac? What is their origin ?
{Ans. According to the nebular hypothesis, the ring of matter broke
up into numberless small bodies, instead of aggregating into one large
planet.) Give some of the names and signs.

157. Jupiter. — Definition and sign? Describe his appearance. De-
scribe his orbit, \yhai is his distance from the sun? Velocity? Day?
Year? Distance from the earth? Diameter? Volume? Density?
Centrifugal force ? Force of gravity ? Figure? Describe his seasons.
Upon what does the change of seasons in any planet depend? What
must be the appearance of the Jovian sky? Describe the telescopic
features of Jupiter. Are Jupiter's moons visible to the naked eye?
What are their names? What is their size? What space do they
occupv? Describe the eclipse of Jupiter's moons. Define immersion,
emersion, and transit. How rapidly do the satellites revolve? What
can you say of the frequency of eclipses on Jupiter? Describe the belts.
Why are they parallel to its equator? How was the velocity of light
discovered? Does Jupiter emit light? Is it probable that a solid
crust has formed over this planet? In what waj- is Jupiter repro-
ducing the earth's histori"?

164. Saturn. — Definition and sign ? Describe Saturn's appearance
in the heavens. How nipidly does this planet move through the sky?
What is its distance from the sun ? What is the peculiarity of its
orbit ? What is its velocity ? Year ? Day ? Distance from the earth ?
Diameter? Volume? Density? Force of gravity ? Describe its sea-
sons. Has it any atmosphere ? Who discovered the rings of Saturn ?
Describe them. Which are the Bright Rings ? Which is the Dusky
Ring? Are they stationary? E.\plaiu their phases. Of what are
they composed? Does Saturn emit !ight? Describe Saturn's belts.
Describe Saturn's moons. The scenerj' on Saturn.

170. Ur.\nus. — Definition and sign ? How was this planet dis-
covered ? Tell of its previous observation by Le Monier. Is Uranus
visible to the naked eye ? What is its distance from the sun ? Year ?

* " It may surprise some persons to learn that the total mass of the two or three
hundred small planets which have been discovered between the orbits of .Mars and
Jupiter, is sufficient only to make a globe a little over 403 miles in diameter. In other
words if our globe were divided into 8,000 equal parts, one of these parts would equal
in bulk and in weight the toUl of all these asteroids. Or, cut the earth through the
equator, then take a section of about three-fourths cf a mile in thickness, and it would
furnish material for all these small planets and something remaining. It would seem
that the solar system could' not be much damaged, if some of these small planets
should drop out of their courses and join some of the larger ones."

THE SOLAR SYSTEM, 305

Diameter? Density? Describe its seasons. Telescopic features.
Satellites. Peculiarity of its moons.

172. Neptune.— Definition and sign? What is the appearance of
this planet in the sky ? Give an account of its wonderful discovery.
What is its distance from the sun? Year? Velocity? Diameter?
Volume ? Density ? Do we know anything of its seasons ? Why not ?
What is the appearance of the heavens? What are the telescopic
features of Neptune? Has Neptune any moon? What advantage
have the Neptunian astronomers ?

175. Meteors, Aerolites, and Shooting-Stars.— Define an aero-
lite. A shooting-star. A meteor. Give some account of the fall of
aerolites. What elements are found in aerolites? How can an aero-
lite be distinguished? Give an account of wonderful meteors. Of
shooting-stars.

176. Describe the showers of 1799 and 1833.* The shower of 1866. At
what intervals did these showers occur ? Why was not the shower of
1866 seen in this country ? (Ans. Our side of the earth was not turned
toward the meteors.) What is the average number of meteors and
shooting stars daily? Why do we not see more of them? In what
months are they most abundant ?f Describe the origin of meteors and
shooting-stars. What is their velocity? What causes the light? The
explosion often heard ? What is the theory of meteoric rings? What
is their shape ? How do these streams of meteoroids account for the
showers at regular intervals? What is the period of the November
ring? Why is the August shower so uniform, while the November
one is periodic ?:J: What is the relation between meteors and comets?

* A southern planter, describing the effect of the star-shower of 1833, says: ''I
was suddenly awakened by the most distressing cries that ever feU on my ears.
Shrieks of horror and cries for mercy I could hear from most of the negroes of three
plantations, amounting in all to about 600 or 800. While earnestly listening for the
cause, I heard a faint voice near my door calling my name. I arose, and taking my
sword, stood at the door. At this moment I heard the same voice still beseeching me
to rise, and saying, ' Oh, my God, the world is on fire !' I then opened the door,
and it is difficult to say which excited me most, the awfulness of the scene or the
cries of the distressed negroes. Upwards of one hundred lay prostrate on the ground,
some speechless, and some with the bitterest cries, with their hands raised, implor-
ing God to save the world and them. The scene was truly awful, for never did rain
fall much thicker than the meteors towards the earth : east, west, north, and south, it
was the same."

+ It has been noticed, from very early times, that the night of the 10th of August
(St. Laurence's Day) is especially favorable ff)rthe occurrence of shooting-stars; and
in Catholic Ireland, these stars, on the toth of August, are always called the " tears
of St. Laurence the Martyr," who was put to death by being broiled upon a gridiron.

t The fact that the November meteoroids are collected in a shoal instead of be-
ing distributed uniformly through the orbit gives color to the idea that this stream
has not been long a member of the solar systcip, " In 1867, Leverrier stated his be-

306 QUESTIONS FOR CLASS USE.

What do 3-ou mean by the radiant point ? What is the height of
meteors ? Weight ?

185. Comets. — How were comets looked upon by the ancients? Il-
lustrate. Define the term comet. What is the tail?* The nucleus?

lief that the November meteors form the remains of some comet that had been re-
cently introduced into the solar system by the attraction of one of the large outer
planets. He lound that the year a. d. 126 would give a position to the planet Uranus
capable of producing such an eflFect, by converting the parabolic path of a comet into
the path now described by the November meteors. In the year a. d. 137, the changed
path of the comet for the first time came near the earth in her orbit round the Sun,
since which year the petrified comet or shower of stones has completed 52 entire re-
volutions, the last of which terminated on the 13th of November, 1866. Theophanes
of Byzantium relates that in November, a. d. 472, the sky at Constantinople appeared
to be on Are with flying meteors. This corresponded with the tenth revolution of
the November meteors. — Conde, in his history of the dominion of the Arabs, speak-
ing of the year a. d. 902, states that in the month of October (13th), on the night of
the death of King Ibrahim Ben Ahmed, an immense number of falling stars were
seen to spread themselves over the face of the sky like rain, and tliat the year in ques-
tion was thenceforth called the ' Year of Stars' This year corresponded to the
twenty-third revolution of the November meteors. — A similar shower of stars took
place on the 17th of October, a. d. 934. — On the 14th of October, a. d. 1002, a remark-
able shower of shooting-stars is noted by the Arab astronomers and historians, cor-
responding with the completion of the twenty-sixth revolution of the November
meteors. — It is related in the annals of Cairo that on the 19th of October, a. d. 1202,
the stars appeared like waves upon the sky, towards the east and west ; they flew
about like grasshoppers, and were dispersed from left to right. This shower corres-
ponded with the thirt3'-second revolution of the November meteors.— On the 22nd of
October, a. d 1366, a shower of stars was noted, corresponding with the thirty-
seventh revolution of the November meteors.— A similar phenomenon (forty-second
revolution) was observed on the 25th of October, a. d. 1533. — The forty-seventh
revolution was noted on the 9th of November, a. d. 1698. — The fiftieth revolution,
observed by Humboldt and Boupland, on the 12th of November, a. d. 1799, as already
remarked, first led modern astronomers to speculate on the true nature of these re-
markable periodic phenomena. — The early observations of this meteoric shower were
dated on the 12th of October, and during 52 revolutions the intersection of its orbit
with that of the earth has moved on to the 14th of November. — Mr. Adams hasshown
this movement of nodes to be a consequence of the attractions of the superior planets,
and has finally demonstrated the truth of the cometary origin of the November
meteors."' — Houghton.

* " Comets are almost alwaj-s accompanied by tails, which are placed in the line
joining the Sun and Comet, and on the side opposite to the Sun. Exceptions to this
rule, though rare, sometimes occur. For e.xample. the tail of the Comet of 1577 de-
viated 21 ° from the line joining the Sun and the Comet, and the tail of the Comet of
1680 diverged 5^ from the same line. Comets have been occasionally observed with
two tails, one in the usual position, and the other in nearly an opposite direction, or
towards the Sun. The angle between the two tails, when such a phenomenon has
been observed, has always been very considerable, varying from 140° to 170°.
This rare phenomenon of two tails is supposed to be connected with certain rapid
changes which the gaseous substance of the Comet is obser\'ed to undergo on ap-
proaching the Sun. There are many instances on record, in which the tails of Comets
were observed to stretch through 100° of the celestial sphere, and the apparent

THE SIDEREAL SYSTEM. 307

The head? The coma? Does each comet necessarily possess all
these parts ? How would a mere round, fleecy mass be known to be
a comet ? What mistake did Herschel make in looking, as he sup-
posed, at one of this kind (p. 171)? Where do comets appear? In
vvhat direction do they move? How does a comet look when first
seen? Describe the approach of a comet to the sun. Upon what does
the time of greatest brilliancy depend ? What do you say of the num-
ber of the comets ? What was Kepler's remark ? Why do we not see
them oftener? Where did Lockyer see one? Describe the orbits of
comets. Which class has been calculated ? Which classes never re-
turn? Describe the difficulty of calculating a comet's orbit. Name
the periods of some comets. What has been the distance from the sun
of some noted comets ? Velocity ? What do you say of the density of a
comet? Illustrate. Is there any danger of our running against a
comet ? Do comets shine by their own or by reflected light ? Tell what
you can of their variation in form and dimensions. Give some account
of the comets of 1811, 1835, ^"d 1843. For what is Biela's comet
noted ? (Ans. " A very remarkable phenomenon attended the perihelion
passage of this comet in the latter end of 1845. It became divided
into two comets, which did not again re-unite, but traveled along to-
gether in similar orbits. This unique phenomenon was noticed for
the first time in America on the 29th of December. The greatest dis-
tance observed between these two fragments of Biela's comet, before
their final disappearance, was about iivo-thirds of the moon's distance
from the earth.") For what is Encke's comet noted ? What is its period ?
Give some description of Donati's comet. The comet of 1882.

196. Zodiacal Light. — Wherecan this be seen ? What is its appear-
ance? Where is it brightest? What is its origin ?

III. THE SIDEREAL SYSTEM.

203. Tell something of the appearance of the heavens at Neptune's
distance from the sun — our starting-point. Do we ever see the stars?
What do we see, then ? Whicli star is nearest the earth ? What is its
parallax? Its distance? How long would it take light to reach the
nearest star? How would the earth's orbit appear at that distance ? Our
sun? How long does it take for the light of the smaller stars to reach
the earth? What can you say of the motion of the fixed stars? Illustrate.

length of the tail is known to undergo most rapid changes. We shall mention only
one case as an example of this phenomenon. The Comet of 1618 presented to the
Danish astronomer, Longomontanus, a tail of 104° in length, while it had been
measured by Kepler a few days previous, and ascertained to be only 70° long,"

308 QUESTIONS FOR CLASS USE.

What proof have we that the fixed stars are suns ?* Describe the motion
of the solar system. Is the center known ? How many stars can
we see with the naked eye ? With a telescope ? Have all the stars
been discovered ? What is the cause of the twinkling of the stars?
Do we know anything of the magnitude of the stars ? Name the points
of difference between a planet and a fixed star. What do you mean
by a star of the first magnitude? How many are there ? Of the second
magnitude ? How many sizes can one see with the naked eye ? What is
the cause of the difference in the brightness ? How are the stars named ?
Describe the division of the stars into constellations. Is there any real
likeness to the mythological figures? Name any figure which seems
to you well founded. Are the boundaries distinct? Who invented
the system ? State the possible meaning of the signs of the Zodiac and
their origin. Explain why the signs and the constellations of the
Zodiac do not agree. What causes the appearance of the constella-
tions? Would they appear as they now do, if we should go out into
space among them? Are the present forms permanent? State the value
of the stars in practical life. What were the views of the ancients with
regard to the stars? Describe the division of the stars into three zones.
214. The Constell.\tions. — The questions on these are uniform :
( i) description. {i\ principal stars, and (3) mythological history. Therefore,
they need not be repeated with each constellation. What are the
pointers? Does Polaris mark the exact position of the North Pole?
How man}- times per day is Polaris on the meridian of any place?
Explain how this applies in navigation or surveying. State how the
amount of the variation from the true north will change through the
ages. What star will ultimately become the pole-star? What curious
facts are stated concerning the Pyramids? What do you say ot the dis-
tance of Polaris ? How razy latitude be calculated by means of Polaris ?
What stars never set in our sky? What stars never rise?f Will the

* Sinus shines at least 200 times as brightly as our sun would shine if set beside
it. Assuming its surface to be equally brilliant, this would imply, in comparison
with our sun, a diameter 14 times and a volume 3.000 times as great. Its luster,
however, seems higher than the sun's, but, even making allowance for that, we must
stUl consider this giant sun to be at least 1,000 times as large as our own orb. Re-
cent evidence tends to show that its rate of recession from us is diminishing, so that
we may e.xpect this to change into a motion of approach. Here is a hint that Sirius
is travelling in a mighty orbit with movements carrying it alternately from and toward
us — Proctor.

+ All stars whose north polar distance is less than the latitude of any place, will
never set at that place, and all stars whose south polar distance is less than the latitude,
will never rise. The Greeks and the Romans were familiar with the fact that cer-
tain stars never descend below the horizon. The following quotations are interesting :
'• Immunemque aequoris Arcton."'

Ovid, Met.\m. xiii. 293.
■• Arctos
^quoris expertes." Jo. 726—7.

THE SIDEREAL SYSTEM. 309

Big Dipper always appear as now ? Name three bright stars near the
first meridian. (Ans. a Andromedse, >■ Pegasi, and ,i CassiopeiiE.) How
many degrees of longitude correspond to an hour of time? At what
rate is Sirius receding from the earth? How has this motion been dis-
covered? (See page 261.)

239. Double Stars, etc. — Does any star appear double to the naked
eye ? How many have been found by the use of the telescope ? What
is an optical double star? Are all double stars of this class ? Describe
the revolution of a binary system. What other combinations have been
discovered? What are their periods? Orbits? Mass? Are these
companion stars as close to each other as they seem ?

241. Name some prominent colored stars. Do their colors ever
change? Which colors would indicate the hottest star? What is the
probable effect in a system having colored suns ?

242. What are variable stars ? Describe the changes of Algol. Of
Mira. What is the cause ?

243. What are temporary stars? Describe the one seen in Cassio-
peia. The one in Corona Borealis, in 1866 ? What are lost stars ?
Can you give any explanation of this phenomenon? Of what did the
star of 1866 consist? Are these stars destroyed? Is the process of
creation now complete ?

245. What are star clusters ? Name several. Is such a grouping a
mere optical effect ? Are they probably as closely compacted as they
seem to be ?

246. What are nebulse ? How do they differ from clusters? Is it
probable that all nebulse will be resolved into clusters ? What is the
general belief concerning nebulne ? What has spectrum analysis proved
some of the nebulae to be ? Where are they most abundant ? What
can you say about their distances? Into how many classes, for con-
venience, are they divided ? Describe and illustrate the elliptic nebu-
lae. What is said of the distance of the great nebula in Andromeda?

'ApKTOi' 9' rjv KoX aixa^av iTriK\ri<nv KoXfovatv,
'Ht' avTOv <TTpe<f)eTai, Kai t 'fipiwra SoKeiiii,
OtT) S' d/iifiopos e(7Ti Xoerpuiv 'fiKeoroio,

Iliad, xviii. 487—9. and Odys. v. 273—5.

*ApKTOt Kvav^ov Tre^uAa-yjuterot *n*cearoio.

Aratus. Ph.«nom. 48.
" Arctos oceani metuentes jequore tingui."

ViRG. Georg. L 246.
In order to understand the meaning of the expressions 7re<t)v\ayix€i'oi 'flKeat'oio, and
^^ aquoris expertes" as used by a Greek or Italian, we should remember that the
north polar distance ofr/ Ursse Majoris 1839° 56' 48" ; and since the latitude of Athens
is 37" 58', and that of Naples 40° 50', an inhabitant of the former city would see this
star descend below the northern horizon for a small portion of its course ; and an
inhabitant of Naples would see it sink within 3' of the horizon, so as just to move
along its northern edge.

310 QUESTIONS FOR CLASS USE.

The number of stars it contains ? Describe the annular nebulae. What
is said of the " ring universe " in Lyra? Its diameter? Describe the
spiral nebula in Canes Venatici. Describe the planetar>' nebulae.
What is said of the number and size of these "island universes"?
Describe the fantastic appearance of the irregular nebulae.

251. What are nebulous stars? What is their structure? What
are variable nebulae ? Give instances. What is said of double
nebulae ? Is an)-thing definite known with regard to them ? What
are the Magellanic clouds ?

253. Describe the Milky-way. What can you say of the number of
stars in the Galaxy? Are the stars uniformly distributed ?

254. What is Herschel's theory of the constitution of the universe?
If this theory be true, what is our sun ?

255. Give an account of the Nebular hypothesis. What is said of
Saturn's rings ? May they ultimately disappear?

259. What is spectrum analysis? Name the three kinds of spectra.
What colored rays will a flame absorb?* Describe the spectroscope.
What are Fraunhofer's lines? What is known of the constitution
of the sun ? What proof have we that iron exists in the sun ? What
elements have been found in the sun ? What proof have we that the

* The power which gases possess of cutting out the particular lines which belong
to the color that each emits has been beautifully illustrated by Prof. Newcomb. He
says: "■ Suppose nature should loan us an immense collection of many millions of
gold pieces, out of which we were to select those which would ser\'e us for money,
and leturn her the remainder. The English rummage through the pile, and pick out
all the pieces which are of the proper weight for sovereigns and half-sovereigns ; the
French pick out those which will make five, ten, twenty, or fifty-franc pieces; the
Americans the one, five, ten and twenty dollar pieces, and so on. After all the suit-
able pieces are thus selected, let the remaining mass be spread out on the ground
according to the respective weights of the pieces, the smallest pieces being placed in
a row, the next in weight in an adjoining row, and so on. We shall then find a num-
ber of rows missing: one which the French have taken out for five-franc pieces;
close to it another which the Americans have taken for dollars ; afterwards a row
which have gone for half-sovereigns, and so on. By thus arranging the pieces, one
would be able to tell what nations had culled over the pile, if he only knew of what
weight each one made its coins. The gaps in the places where the sovereigns and
half-sovereigns belonged would indicate the English, that in the dollars and eagles
the Americans, and so on. If, now, we reflect how utterly hopeless it would appear,
from the mere examination of the miscellaneous pile of pieces which had been left, to
ascertain what people had been selecting coins from it, and how easy the problem
would appear when once some genius should make the proposed arrangement of the
pieces in rows, we shall see in what the fundamental idea of spectrum analysis con-
sists. The formation of the spectrum is the separation and arrangement of the light
which comes from an object on the same system by which we have supposed the
gold pieces to be arranged. The gaps we see in the spectrum tell the tale of the at-
mosphere through which the light has passed us; in the case of the coins they would
tell what rutions had sorted over the 'p\\Qj" —Nrwcomb's Astronomy^ p. 228.

THE SIDEREAL SYSTEM. 311

stars are suns? What can you say of the similarity existing between
the stars and our earth ? What has been discovered with regard to
the constitution of the Nebulae ? Of their relative brightness? How
has the proper motion of the stars been shown?

263. Time. — What two methods of measuring time? What is a si-
dereal day? What are astronomical clocks? Tell how they are used.
Why do astronomers use sidereal time ? What is a solar day? What
causes the difference between a sidereal and a solar day ? To how much
time is a degree of space equal? Which is taken as the unit, the solar
or the sidereal day? How long is a solar day? A sidereal day? A
solar day equals how many sidereal hours ? A sidereal day equals how
many solar hours? Describe mean solar time. What is apparent
noon? Mean noon? The equation of time ? When is this greatest ?
When least ? When do mean and apparent time coincide ? Can a
watch keep apparent time? How may apparent time be kept? How
can it be changed into mean time ? Tell how to erect a sundial. When
will a sidereal and a mean time clock coincide ? A mean-time clock
and the sundial? How did the ancients measure time, before the
invention of clocks and watches?" State the two reasons wh}' the
solar days are of unequal length. What is the civil day ? Who invented
the present division ? Describe the customs of various nations. What

* " The ancients used clepsvdrje and siin-dials, to measure time. The clepsydrae,
in its simplest form, resembled the hour-glass, water being used instead of sand, and
the flow of time being measured by the flow of the water. After the era of Archi-
medes, clepsydrae of the most el.iborate construction were common ; but while they
were in use, the days, both winter and summer, were divided into twelve hours from
sunrise to sunset, and consequently the hours in winter were shorter than the hours
in summer; the ciepsydra, therefore, was almost useless except for measuring inter-
vals of time, unless different ones were employed at different seasons of the year.
The sun-dial was a great improvement upon the clepsydrae ; but at night and
in cloudy weather it could not be used, of course, and the rising, culmination, and
setting of the various constellations were the only means available for roughly telling
the lime during the night. Indeed, Euripides, who lived 480-407 b. c, makes the
Chorus in one of his tragedies ask the time in this form : —

' What is the star now passing ?'

and the answer is : —

' The Pleiades show themselves in the east ;
The Eagle soars in the summit of heaven.'

It is also on record that as late as a. d. 1108 the sacristan of the Abbey of Cluny con-
sulted the stars when he wished to know if the time had arrived to summon the
monks to their midnight prayers ; and in other cases, a monk remained awake, and
to measure the lapse of time repeated certain psalms, experience havmg taught him
in the day, by the aid of the sun-dial, how many psalms could be said in an hour.
When the proper number of psalms had been said, the monks were awakened."—
Lockyer.

312 QUESTIONS FOR CLASS USE.

is the origin of the names of the days?* What is the sidereal \-ear?
The mean solar year? What causes the difference? What is the
anomalistic year? How did the ancients find the length of the \'ear?
What error did they make ? What was the result ? Give an account
of the Julian calendar. The Gregorian calendar. What is the mean-
ing of the terms O. S. and N. S. ?f What country now uses O. S. ?
When was the change adopted in England ? X How was it received ?
How could a child be eight years old before a return of its birthday?
When do the Jews begin their year? Why does our year begin Jan-
uary 1st ? Show how the earth is our timepiece. What influence has
Jupiter's moons on the cotton trade?

Celestial Measlrements. — These problems are to be used
throughout the study. They require no questions but the formal state-
ment of the problem requiring solution.

* It is said that the Egyptians named the seven days from the seven celestial
bodies then known. The order was continued by the Romans Tuesday they called
Dies Martis ; Wednesday, Dies Mercurii : Thursday, Dies Jovis ; Friday, Dies
Veneris. In the Saxon mj-thology. Tius. Woden, Thor, and Friga are equivalent to
Mars, Mercury-. Jupiter, and Venus. Hence we see the origin of our English names.

+ " As an illustration of the effect of the change of style, we may instance the
case of Washington. He was bom February ii, 1732, before the change of style.
Inasmuch as 1752 began on the 25th of March and ended on the 31st of December, he
had no birth-day in that year ; hence, he was 20 years old on the 22nd of February,
1753, new style. Because anniversaries are always determined according to the
civil calendar, the birth-day of Washington is properly celebrated on the 22nd of
February, and not on the 23d, as some have contended, on account of the day drop-
ped in the year 1800." — Peck^s Astronomy, p. 216.

X " In England, from the 14th century till the change of style in 1752. the legal and
the ecclesiastical year began March 25. After the change was adopted in 1752, events
which had occurred in January, February, and before March of the old legal year,
would, according to the new arrangement, be reckoned in the next subsequent year.
Thus the revolution of 1688 occurred in February of that legal year, or. as we should
now say, in February, 1689 ; and it was, at one time, customary to write the date
thus: February, i68|." — Appleton's Cyclopaedia, article on Calendar.

GUIDE TO THE CONSTELLATIONS.

The following is a description of the appearance of the heavens on or about the
first da}' of each month in the year.

January. (7 p. m.) — In the North, Cassiopeia and Perseus are
above Polaris, Cepheus and Draco west, Ursa Minor fs below, and Ursa
Major below and to the east. /// the East, Cancer is just rising, Canis
Minor (Procyon) has just risen. Along the Ecliptic, Gemini is well up,
then Taurus, Aries — reaching to the meridian, next Pisces; Aquarius
(letter Y) and Capricornus are just setting. In the Southeast, Orion and
the Hare are well up. In the South, Cetus swims his huge bulk far to the
east and west. In the Southwest, is Piscis Austral is (Fomalhaut). North
of the Ecliptic, the Triangles are nearly in the zenith, Perseus is just
east, below is Auriga, Andromeda lies just west of the meridian, and
Pegasus is midway; Delphinus (the Dolphin, Job's Coffin), Aquila
(Altair), and Lyra (Vega) are fast sinking to the western horizon ; while,
along the Milky Way, blazes the brilliant cross of Cygnus.

February. (7 p. m.) — In the North, Ursa Major lies east of Polaris,
Ursa Minor and Draco are below, Cepheus is west, Cassiopeia above
and to the west. In the East, Regulus and Cor Hydrse are just rising.
Along the Ecliptic, Leo (Regulus, the sickle) just rising, Cancer well up,
Gemini midway, Taurus on the meridian, Aries (the scalene triangle)
past, Pisces next, and, lastly, Aquarius just setting. In the Southeast,
Canis Minor, Canis Major ^Sirius), and Orion are conspicuous. Iti the
Southivest, Cetus covers nearly the whole sky. North of the Ecliptic,
Perseus is on the meridian, while Auriga is a little east of it ; west of
Perseus is Andromeda, while the Great Square of Pegasus is fast
approaching the horizon. In the Northwest, Cygnus is setting.

March. (7 p. U.)—In the North, Ursa Major lies east of Polaris,
Draco and Ursa Minor are below, Cepheus is below and to the west, and
Cassiopeia west. In the East, Cor Caroli is well up, toward the north-
east, and Coma Berenices is rising. Along the Ecliptic. Leo is fully risen,
next Cancer, Gemini reaches to the meridian, Taurus is past, Aries
midway, and, lastly, Pisces is just beginning to set. In the Southeast, Cor
Hydrae, Canis Minor, and Canis Major are conspicuous. In tfu South,

314 GUIDE TO THE CONSTELLATIONS.

Orion blazes brilliantl}'. In tlie Southwest, Cetus is hiding below the
horizon. Xorth of tJu Ecliptic, Auriga is in the zenith ; west are Per-
seus and Andromeda, while Pegasus is just beginning to sink out of
sight.

April. (7 p. M.J — In the North, Ursa Major is above and to the
east of Polaris ; opposite and to the west is Perseus, Draco below and
to the east, Cepheus below and to the west, Cassiopeia west. Jit the
East, Bojtes ( Arcturus) is not quite fully r.sen. Along the Ecliptic, Virgo
(Spica is rising, Leo midway, Cancer reaches to the meridian, Gemini
is past, next Taurus, then Aries, and, lastly. Pisces just setting. In
the Southeast, is the Crater (the Cup) ; Hydra stretches its long neck to
the meridian. In tJu South. Canis Minor. In tJie Southioest, Sirius and
Orion ; the Eg^-ptian X ip. 229) can now be seen. A'orth of t/ie Ecliptic,
and in the northeast, are Coma Berenices and Cor Caroli ; above
Gemini and Taurus is Auriga, while Andromeda is just setting in the
northwest.

3Ijiy. (S p. M.) — In the Xorth. Ursa Major is above Polaris, Ursa
Minor and Draco are east, Cepheus and Cassiopeia below, and Perseus
is west. In the East, Lyra is rising, and Hercules is just up. Along
tlu Ecliptic, Libra is just rising, Virgo is midway. Leo is on the me-
ridian, Cancer is past, next Gemini, and lastly Taurus just setting. In
t/i€ South, stretching east and west of the meridian, is Hydra, with the
Crater and Corvus a little east. /;/ tJte South-u'cst, are Cor Hydrae, Canis
Major, and Canis Minor, while Orion is just setting in the west. Noi-th
of the Ecliptic, in the east, above Hercules, are Corona Borealis (The
Northern Crown), Bootes (Arcturus), Coma Berenices, and Cor Caroli,
which stretch nearly to the meridian. In tJie Xorthzvest, above Taurus
and Perseus, is Auriga.

Juue. (S p. .M.I — /« the Xorth, Ursa Major is above Polaris, Draco
and Ursa Minor are east, Cepheus is below and east, and Cassiopeia
directly below. /// the East, Cj-gnus (the Cross) and Aquila are rising,
Lyra and Taurus Poniatowskii are well up. Along the Ecliptic, Scoxp'xo
is rising. Libra is midway, Virgo on the meridian, Leo past, Cancer
midway, Gemini next, and Taurus just setting. In the South, are Cor
vus and the Crater a little past the meridian. In tJie Southwest, is Cor
Hydrae, and in the west Canis Minor is nearing the horizon. North
of the Ecliptic, in the east, above Scorpio, is Hercules ; then Corona
and Bootes, and, near the meridian. Cor Caroli. and Coma Berenices.
In the Northwest, is Auriga, just coming to the horizon.

July, (g p. M.) — In the Xorth, Draco and Ursa Minor are abo»e Pola
ris, Ursa Major is west, Cepheus east, and Cassiopeia below to the east.
In the East, the Dolphin (Job's Coffin) is now well up, Cygnus is almost

GtrlDE TO THE CONSTELLATIONS. 3l5

midway to the meridian, and Lyra is still higher. Along the Ecliptic,
Capricornus is rising, Sagittarius (the Archer) is next, Scorpio, with
its long tail swinging along the horizon, is directly south, Libra is past
the meridian, Virgo midway, and Leo has almost reached the horizon.
In the South'west, the Crater is setting, and Corvus is just above. A'orth
of the Ecliptic , above Scorpio and east of the meridian, are Serpentarius,
Hercules, and Taurus Poniatowskii ; Corona is almost on the meridian,
to the west of which lie Bootes, Cor Caroli, and Coma Berenices.

August. (9 P. M.) — In the North, Draco and Ursa Minor are above
Polaris, Cepheus is above and to the east, Cassiopeia east, and Ursa
Major west. In the Northeast, Perseus is just rising, while south of it,
Andromeda and Pegasus are fairly up. Along the Ecliptic, Aquarius is
risen, next Caprjcornus, Sagittarius reaches to the meridian. Scorpio is
just past, Libra next, and Virgo (Spica) just touches the horizon. North
of the Ecliptic, Taurus Poniatowskii is on and Lyra is just east of the
meridian ; the Swan and Dolphin are east of Lyra, Serpentarius and
Hercules are above Scorpio, and just west of the meridian ; thence west
are Corona and Bootes, while far in the northwest are Coma Berenices
and Cor Caroli.

September. (8 p. m.) — Draco is above and to the west of Polaris,
Cepheus above and to the east, Cassiopeia east, Ursa Major is below
and to the west. In the Northeast, Perseus is just rising. In the East,
Andromeda is fairly up, Pegasus is nearly midway to the meridian.
Along the Ecliptic, Pisces is just rising, next Aquarius, Capricornus in
the southwest, Sagittarius on the meridian in the south, next Scorpio in
the southwest. Libra, and, lastly, Virgo just setting. N^orth of the Ecliptic,
Lyra is on the meridian, Cygnus, the Dolphin, and Aquila are just to
the east ; while to the west, are Taurus Poniatowskii and Serpentarius ;
north of these latter are Hercules, Corona, Bootes, Cor Caroli, and
Coma Berenices.

October, (7 P. M.) — In the North, Cepheus and Draco are above
Polaris, Ursa Minor is west, Cassiopeia east, and Ursa Major below and
west. In the Northeast, Perseus is fairly risen. In the East, An^xom-
eda is nearly midway to the zenith. Along the Ecliptic, Aries is just
rising, Pisces well up, Aquarius and Capricornus are in the southeast,
Sagittarius is in the south, Scorpio far down in the southwest, and Libra
just setting. North of the Ecliptic, Cygnus and Aquila are on the me-
ridian ; the Dolphin is just east of it, and far south ; Lyra is west of the
meridian ; Taurus Poniatowskii is lower down and to the south ; Ser-
pentarius is just above Scorpio ; next, in line with Scorpio and Polaris,
is Hercules ; Corona and Bootes are toward the northwest, where
Coma Berenices is just setting.

31 & GUIDE TO THE CONSTELLATIONS.

November. (7 p. m.) — In the North, Ursa Major is below Polaris,
Ursa Minor and Draco are to the west, Cepheus is above, and Cassiopeia
above and to the east. In the Northecst, Auriga is just rising, and
Perseus is above, nearly midway to the meridian. Along the Ecliptic,
Taurus is just rising, next are Aries and Pisces ; Aquarius is on the me-
ridian, south ; then Capricornus, and lastly Sagittarius, in the southwest.
iVorth of the Ecliptic, Pegasus and Andromeda lie east of the meridian,
the Swan, Dolphin, Eagle, Taurus Poniatowskii, and Lyra west. In
the Northwest, are Hercules and Corona.

December. (7 p. m.) — In the North, Cassiopeia is above Polaris,
Cepheus above and to the west, Perseus above and to the east, Draco
west, and Ursa Major below. In tlie NoriJieast, below Perseus, is
Auriga. In the East, Orion is rising. Along the Ecliptic, Gemini is
just rising. Taurus is nearly midway, next Aries, Pisces is on the me-
ridian, then Aquarius, and lastly Capricornus, far in the southwest.
/;/ the South, east of the meridian, is Cetus, and west is Fomalhaut.
A'orth of the Ecliptic, Andromeda is nearly on the meridian, and Pega-
sus west of it ; Cvgnus, Delphinus, Lyra, and Aquila are about midway,
while Taurus Poniatowskii is just sinking 10 the horizon. In the North-
west, Hercules is just setting.

Note.— It should be borne in mind that a month makes a variation of about two
hours (30°) in the rise of a star ; hence, in the foregoinp; " Guide," the 'January Sky"
of 9 p. .M. would be about the same as the " February Sky " of 7 p. m. ; the "'January
Sky" of II p. M. would be about the same as the " March Sky" of 7 p. m.. &c. In
this way the "Guide" may be used for any hour in the night. The pupil will see
that in the " Guide " the prominent figures and stars in each constellation are given
in parentheses. Examples: the " Y" in Aquarius, the -'scalene triangle" in Aries,
"Job's Coffin " in the Dolphin, " Procyon " in Canis Minor, &c. These aid in iden-
tifying the constellation.

APPARATUS.

To ILLUSTRATE PRECESSION

OF THE Equinoxes,* make the
simple apparatus shown in the
cut. It represents Fig. 38,
and the explanation of that
figure and several subsequent
ones applies to it. The in-
genuity of pupil and teacher
will devise methods of explain-
ing by means of this instru-
ment many otherwise abstruse
points under this difficult sub-
ject. The following are sug-
gestive merely :

1. To show motion of earth' s
axis around pole of Ecliptic. —
Move P, axis of earth's plate,

around D F, whose circumference roughly represents the little circle
(ellipse) described by the pole of the earth. (Fig. 41.)

2. To sho'o change of Polar Star. — The pupil can readily see that the
north pole of the earth will, at different times, point to different stars
located around this circle. Now, Polaris; next, Lyra.

3. To show why present polar distance 7oill gradually diminish and then
increase {p. 217). — The polar star lies at a little distance from this circle
(edge of plate) and the pole is gradually approaching the star, but will
pass it and then recede further from it, until, finally, Lyra, lyings" from
this circle, will become the polar constellation.

4. To show Precession of Equinoxes. — Pass axis of earth around small

* The above apparatus was devised by Solomon Sias, A.M. M.D., Principal of
Schoharie Union School, N. Y. It can be made by any ingenious pupil. The plates
are cut out of tin ; the standard may be made with the knife or scroll-saw to suit
one's taste ; the earth is half of a little wooden ball balanced on the wire pin C ; and
the semicircle, poles, etc.. are of wire. The different parts may be soldered or
fastened together with tacks.

dl8 APPARATUS.

circle. Note the position of the equinoxes before moving, and theif
gradual change of position along the ecliptic.

5. To show cause of Precession. — Apply explanation of Fig. 39.

6. To show necessity f 01- new stellar maps occasionally, or careful reduc-
tions to previous standards. With the change of equinoxes, there is
also a change of the equinoctial system, p. 27.

7. To illustrate Fig. 40. — Remove the wire semicircle, and, inclining
the axis of the earth, spin the wire between the thumb and finger like
a top. The equinoxes will pass around the ecliptic as they did when
the axis was carried around in the previous experiments.

Letting G B E represent the plane of the ecliptic, and G E the line
of the equinoxes, we can use this apparatus to illustrate the seasons,
etc., (p 95). Also, by placing a lamp near S, the phenomenon of day
and night, long summer days, short winter days, etc. (pp. 97, etc.), can
be easily explained.

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INDEX.

PAGE

Aberration 117

Aerolites '. 175

Aldebaran ( Al d6b'-a-ran) . . 225

Algol 242

Altitude 27

Amplitude 27

Anaxagoras 8

Anaximander 7

Andromeda 233

Antinous and Eagle 236

Aphelion 18

Apogee 123

Apparent motion 85

Apsides Ill

Arcturus 805, 232

Argo 239

Aries 224

Asteroids 154

Astrology 13

Astronomy, Definition of 1

" Antiquity of 5

Auriga (Au rl'-ga) 225

Azimuth 27

Baily's Beads 141

Bella'trix 228

Berenice's Hair (Coma Berenices') 231

Betelgeuse (Be-tel'-je-uze) 238

Biela's Comet 1 4, ;i07

Bode'sLaw 154

Bootes (Bo-O'-tes) 232

Calendar, The 269

Canes Venatici 231

Canis Major, Canis Minor 229

Cancer 229

Capricornus 236

Cassiopeia (Kas'-se-o-pe'-e-S) 220

Castor and Pollux 227

Celestial Chemistry 258

" Longitude 30

" Measurements 271

" Pole 28

" Sphere 24

PAGE

Centaur 236

Cepheus 219

Ceres 155

Ceius 226

Chaldeans 6

Chinese 6

Chromosphere, The 53

Colures (€o-liire' ; plu. €o-lflres') 27

Comets 185

Conjunction 60, 64

" Inferior 64

" Superior 64

Constellations 209

Copernican System 14

Cor Caroli 231

Corona of Sun 53, 141, 143, 303

Cross, The Southern 239

Crystalline Spheres 8

Cygni,No 61 204,241

Cygnus 237

Day and Night 87

•' Change in Length of . 98

" Civil, The 267

" Sidereil 268

" Solar 264

Declination . 27, 29

Dipper 216

Diurnal Motion 87

Dolphin 237

Draco •. 218

Dry Moon 131

Earth 82

" Eccentricity of Orbit 91

" Rotation of 86

" Rotundityof 85

" Yearly Motion of 91

Earth-shine 127

Eccentricity 57

Eclipses 138

Ecliptic 29,94

" Obliquity of 29, 95

" Plane of 58

324

INDEX.

PAGE

Ecliptic, Poles of 30

Egvptiaus 8

Ellipse 17,57

Elongation -. . . . .- 64

Equinoxes 88, 30, 98

" Precession of 104

" Vernal 104

Equinoctial 27

Eudosus 8

Evening Star 65, 70

Faculae 50

Fixed Stars, The 204

Distance of 204

" Motion of 205, 261

I?ame# of 208

" Parallax of 204

Flames, Solar 262

Focus 17, 57

Galaxy 253

Galileo (Ga-i-lee-o) 19

Gemini 226

Geocentric 64

Gibbous 127

Golden Number 145

Granules 50

Gravitation 24

Grecians 6

Greek Alphabet 208

Gregorian Calendar 269

Hall, Prof. Asaph l?2-3

Hare 228

Harvest Moon 130

Heliocentric 64

Hercules 233

Herschel 170

Herschel's Theory 254

Hipparchus 8

Horizon 26

Horoscope 13

Hour Circles 27

Hyades 224

Hydra 231

Inferior Planets 63

Interior '' 56

Irradiation 123

Job's Coffin 237

Jupiter 157

Kepler 15

PAGE

Kepler's Laws 15

Kirchhoff's Theory (KirkTiof) 53

Latitude, To Calculate 218.281

Leo 229

Libra 235

Li'jrations 124

Lisht. Aberration of . . . 117

" Refraction of 118

•' Velocity of 162

Longitude, To Calculate 280

Lunarians, The 126

Lyra 238

Magellanic Clouds 253

Mars 150

Mean Day 264

Mercury 71

Meridian 26

Meteoroids 1S2

Meteors 175

Metonic Cycle 144

Milk Dipper 236

Milky Way 253

Minor Planets 154

Mira 242

Moon 122

•' Differential Effect of 148

•' Eclipse of 145

Morning Star 65, 70

Motion, Apparent 85

" Diurnal 87

" of Star 205,261

Nadir 26

Naos 239

Nebular Hypothesis 255

Nebulae 246

" Spectraof 262

Neptune 172

Newton 21

Nodes 58, 133

Noon-mark 265

North Potar Star 217

Nucleus 53, 186

Nutation 109

Occultation 132

Ophincus 834

Opposition 67

Orbit of Planets 57

" " Solar System 206

Orion (O-ri-on) 227

INDEX.

325

P&.GT:

Parallax 119

Annual 121

Change of Solar ST9

" Horizontal 121

" Lunar 2T3

" Solar 3G, 121,275

Pegasus (Peg'-a-sus) 223

Penumbra 51,139

Perigee ' 123

Perihelion 18

Perseus 221

Phases GO

Photosphere 51

Pisces 226

Planets 55

" Are they inhabited ? 61

" Definition of 3

" Size of 50

Pleiades (P16'-ya-dez) 235

Polaris 217

Polar Star 217

" distance 29

Precession of Equinoxes 104

Procyon (Pr6-9y-on) 229

Protuberances, Solar 53

Ptolemaic Theory 9

Ptolemy 9

Pythagoras 7

Quadrature 03

Refraction 112

Regulus 229

Retrograde motion 05

Right ascension 29

Rising and setting 86

Sagittarius 236

Saracens 11

Saros 6,144

Saturn 164

Scintillation 207

Scorpio 235

Seasons 95

Serpentarius 234

Shooting Stars 175

Sidereal Revolution 45, 69

Sidereal System 201

Signs, Zodiacal 31, lOfi, 210

Signs and Constellations not asreeiiiir 211

Sirius 204,229,308

Solar System 35

" Motion of 205

PAGE

Solar timo 264

Solstices 30, 98

Space 24

Spectra 258

Spectrum Analysis 258

Spectroscope 259

Spica 230

S:ais, The , 203

" Colored 241

" Distance of 204

" Diunial Orbits of 89

" Double 239

" Number of 200

" Proper motion of 205,261

" Size 207

" Temporary 243

" Variable 242

Sun 36

" Change in form and place of . ... 114

" Diurnal motion of 87

" Eclipse of 138

'• Heatof 54

" Path of 94

" Protuberances of o^i. 262

" Yearly Path of 94

" Spots 40

Superior Planets 63

Syzygies 149

SjTiodic Revolution 45, 69

Taurus 224

" Poniatowskii 234

Thales 6

Thuban 219

Tides 147

Time 263

Top 109

Transit 65

Transit of Venus 277

Triangles 224

Twilight 116

Twinkling 207

Tycho Brahe (Bra or Bra) 15

Umbra 138

Uranus (U'-ra-nns) 170

Ursa Major 315

" Minor 217

Vega 217,238

Venus 77

" Transit of 277

Vertical Circle 26

32»j

INDEX.

PAGE I

W-locit y of Light 168 } Year. Anomalistic —

Virgo 230 1 " of Astronomers

Vulcan "1 I '• Sidereal

Tropical

Wet Moon 131

Wilson's Theory 51

FA6B

.... 268
111,301

268

Year, The

Zeuith 26

Zodiac (Zo-di-ac) 31,210

2e8 i Zodiacal Light (Zo-dl'-ac-ai i 196

This book is DUE on the last date stamped below

3V7 19511

. - t, 19641

orm L-9-15m-7,'32

A 000 1 69 908 1

Mathematical

Sciences

Library

AUXILIARY

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M.72

OTIVEKSITY of CALIFORNIA

^ub ANGELKS
LIBR-ARY

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