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ELK ill: NTS

ASTRONOMY;

ADAPTED FOE USE IN SCHOOLS AND PRIVATE STUDY.

I^i' HUGO KEID,

LATH I-INCn-aL Of DA..M0II8IK OM.KCE, IIAUVAX, H.8.

Illustrator bg ^nk-m (Bmnhbp on Wioo)i.

Fourth Edition,

CAnEPUU.Y REVISED AND BKOUGll I DOWN TO THE PIIESKNT STATE OF

ASTKONOMICAL SCIENCE,

By Re\. ALEX. MACKAY, LL.D., FR.G.S.,

AUTHOR OF " MANUAL OF MODRRN OEOOnAl'Iiy," " FACTS AND 1>ATK«," ETC.

EDINBURGH :
OLIVER AND BOYD, TWEEDDALE COURT.

LONDON: SIJIPKIN, MARSHALL, AND CO.

PLAN OP THE SOLAR

>LAN OP THE SOLAR SYSTEM.

(As

6 2.0

4^ M

I>RINTKI> BY OUVKH ANIi n<n o, KDINPirnoif.

n,^Lx1>

PRRFACE TO TIIK FOUIlfll KDITION.

A MONO tho many nblo i.ftn.|.l,.)okii of phyHicftI ncfcnco that Imva
rocontly Apponrud, very few cnn bo c(»tiipftre.l xvitl. tlio work
now rc-JMucl, citlu r In nutunOncM of plan, torMcntw of ntylo
or occuracy of doHcriptiun *

Since tho publication, however, of the third edition, c or-
dinary progrcfn ImM been made in every department cf \»tro-
aomy, and dlMcovcries effected vying in importance with .he
brilliant nchiorom.nt*. of Newton, U. Place, and the el ' • Her-
iwhel. In piirticular, tho great physical problem of the t go—
the earth's mean distance from the ««/»— ha8 Leon well-nlgh
definitely and satigfactorily sg^lved. KquaUy important hare
lioen tho splendid disccvcri.^n reeulting from the researches of
Kirchhoff and others in Hpeetrum analyHis; inasir uch an they lay
open to UH, for tho first time, tho physieal constitution of the
great central orb of our syjtem, and conclusively dem.mBtrato
that tho sun, stars, earth, and all tho oth^^r niembcr« of tho
planetary systeia, aro mainly composed of tho same identical
materials; and they also throw special light on tho peculiar
ccntlition of the m bulaj.

Groat additions have been made to our knowledge of tho
individual members of our systen.. T» e existonco of a planet,
nearer the sun than Mercury, has been all but demonstrated'
The planet Mars, at the form of whoso orbit the illur.trioui
Keplor laboured A.r so many years— labours resulting in tho
tliree famous " laws ol motion " for which that astronomer will
be for ever distinguished— is now clearly shown to be enveloped
in an atmosphere; to have his surface variegated with conti-
ncnts, (KJeans, islands, and seas; and, in faet, to contain the
chief requisites esseuUal to animal and vegetable life. The
same researches, however, have also shown the great improba-
bility of Mars sharing these characteristics with any other
orb of the solar system; unless, indeed, we aro to except
tlie inner satellites of Jupiter, Saturn, and the other larger

* PnEFACE.

l)Ianctfl, which, 80 far ft.s our |irc8ent knotvlcdgc extcnda, mny
perhaps bo honioa of livuig beings. Jupiter and Saturn them-
selves can no longer be regarded as habitable worlds, notwith-
standing the eloquent pleadings of Sir David Brewster; but
rather as expirimi suns, no longer, indeed, shining with all their
wonted light, but still capable of yielding important supplies
of heat to the tiny orbs that revolve more immediately around
them.

Since the publication of the last edition, moreover, we have
made the acquaintdnce of many additional members of the great
family of asteroids, or smaller planets, which fill up the void
between Mars and Jupiter. The mysterious colossal rings of
Saturn have at length been shown to consist of myriads of small
opaque bodies, each moving independently, and in its own orbit,
around the planet, but forming collectively a series of luminous
streams, which, in all probability, have their analogues in those
streams of meteors and falling stars through which our planet
passes in October and November of each year. These, and
many more of the results of recent astronomical research, are em-
bodied in this new edition. At page 179 will be found elaborate
tables, prepared by Professor C. Piazzi Smyth, Astronomer-
Royal for Scotland, showing in detail all the more important
facts hitherto established by astronomers relative to the sun,
moon, and planets.

^ In preparing this new editirn, the Editor has freely availed
himself of the various admirable expositions of Astronomv by
Richard A. Proctor, B.A., including "The Sun— Ruler, Fire,
Light, and Life of the Planetary System," " Other Worlds than
Ours," "The Orbs Around Us," etc. i also of J. N. Lockyer's
exquisite little volume, entitled " Elementary Astronomy ;" of G.
F. Chambers's " Descriptive Astronomy," Oxford, 1867; " Popu^
Inr Astronomy," by Sir George B. Airy, Astronomer-Royal ; and
of the new edition of the late Sir John Herschel's " Outlines of
Astronomy," Longmans, 1871. A plan of the solar system, and
a good index, have also been added ; and, altogether, it is hoped
the vork will be found to be abreast of the present state of
astronomical science.

ALEX. MACKAY.

EDixnuROir, fanitari/ 1874.

CONTENTS.

Introduction P^Pfl

TAKT I.
The Sphere op tkl Heavens

1. Definitions. 2. Apparent Motion of thci'lleZ'onZNZh'^'7 ^
8. General Definitions. 4. IIow to dpfin„ « T "?"* ^"^«-
in the Heavens. 5. Noihern Itt'lttns ? z"o r"' ?r^^
stellations. 7. So,.them Constellations rEvteut"n??.Tr^ ^'"•
visible at any Place. "* °^ "'^ Heavens

TART II.

Leading Phenomena of the Earth, Sun, and Moov .«

Sect. I. Definitions °

Sect. II. Day and Night-ainKale-Seasons To

Day and Night, 49. Climate, 64. Seasons,67.

Sect. III. Trade- Winds and Tides

Trade-Winds, 69. Tides, 71. ^^

Sect. IV. Divisions of Time....

Sect. V. Moon'H Phases, Eclipses, etc

Moon's Pliases, 90. Eclipses and Occi.ltations 92 "'"

Refraction, 99. Koflection, 102. Twilight, 103. ^^

6 CONTENTS.

PART III.

Page

The Solau System 105

Sect, I. Definitions lOG

Sect. II. Forces acting throughout tl'o Solar System 114

Piojectil'i Force, 114, Attractive Force, 116,

Sect. III. Orbitual Motions of the Tlanots, Satellites, and

Comets 122

Sect. IV. Rotatory Motions and Forms of the Sun, Planets,

and Satellites 130

Sect. V. General Facts relating to the Solar System 135

Sect. VI. Of the Sun, Planets, Satellites, and Comets 139

The Sun, 139. Vulcan, 14(5. Mercury, 147. Venus, 148. Tlio
Earth, 150. The Moon 155. Mars, 15V. The Asteroids, 161.
Jupiter, 163. Saturn, 168. Uranus, 172. Neptune, 173. General
Illustrations, 176. Table of the Solar System, 179. Comets, ISO.
Zodiacal Light, 183. Meteoi-ic Systems, 184.

PART IV.

Parallax, Abkuration, and Precession 188

Parallax, 188. Aberration, 192. Precession, 196. Nutation, 199.

PART V.
Proofs 201

1. The Earth is Kound, 201. 2. The Karth Kotates, 205. 3. The
Earth and the other Planets Move Kound the Sun, 210.

PART VT.
The FiXED Stars 212

Their Proper Motion, 212. Distances, 214. Divisions of the Stars
according to their Brightness, 216. Ordinary Fixed Stars, 218.
Temporary Stars, 218. Variable Stars, 219. Binary Stars, 220.
Nebulae, 221. Clusters of Stars, 222.

PART VIT.

Sketch of the History of Astronomy 224

lNi>EX 235

ELEMENTS

ASTRONOMY.

Introduction.

1. Astronomy is the science which treats of the
heavenly bodies.

2. By the "heavenly bodies," we mean the Sun
the Moon, the Earth, and the Stars. '

3. The discoveries of astronomy have taught us to
class the world which we inhabit among the heavenly
bodies, having proved many resemblances between
them. We now know that our earth is a star, althouo-h
It does not appear to us to be one ; and that several ^of
the stars are large, solid, opaque bodies like the earth
under our feet.

4. The ancients would not admit any community of
nature between the earth and the stars. This idea of
tiie essentially opposite nature of the earth and the
brilliant lunnnanes which shine in the sky, for a long
tmie retarded the progress of astronomy.

5. Astronomy informs us of what is known regardin<
the forms of the heavenly bodies, their magnitude, dis-
tances, relative situations, apparent motions, real motions,
physical constitution, and actions on each other.

6. The term Astronomy is derived from the Greek

ELEMENTS OF ASTRONOMY.

words AtfTjj^ (aster), a star, and No/ao; (nomos), a law.
Its literal signification is, thorclore, the law of the
stars, or order of the stars.

7. A knowledge of astronomy is to \)q acquired
partly by the study of books, partly by observing the
appearances of the heavenly bodies and the changes going
on. amongst them. As the latter is very interesting in
itself, as well as essential to a full understanding of tho
phenomena of astronomy, and as it can easily be pur-
sued from the beginning, this work will open with a
description of the sphere of the heavens.

PART I.

THE SPHERE OF THE HEAVENS.
1. Definitions.

8. A Circle is a curved line, eveiy point of which is equidis-
tant from a point within it, called the Centre. Considered with
respect to the enclosed snriace, whicli it hoiuids, it is often called
the Circumference.

9. A Sphere is a round body (or a round space), every point
on the surface (or outside) of which \6 equidistant from a point
withm called the Centre.

10. A Diameter of a circle, or of a sphere, is a straight line
from any point in the circumference of the circle, or on the sur-
iace of the sphere, passing through the centre to the opposite
sivie. A Radius is that half of a diameter between the centre
and the circumference of the circle, or surface of the sphere ; or
a straight line from the centre to the circumference of the circle
or surface of the sphere.

11. The Horizon or Sensible Horizon of a place is
that circle all round where the earth and sky appear to
meet. _ We cannot see the earth beyond it, nor the sky
below it. It bounds or limits our view ; and takes its
name from a Greek word having this signification.

x_, A -ic ».v4«xv*A ii3 iKt pall 01 tiiu &A\ jiuai. uooYe

ELEMENTS OF ASTRONOMY, 9

tho head of the observer : and it means the same
whether we say that a heavenly body is in the zenith at

< f ^T'. '''*: *^'''^ '^ '' ^^^''^«^ «' '/'«^ place.
U. Rotation is the act of a body turn-'ng round on
self, without moving out of its ola/e ; as wlien a top
Bleeps in spinning. The body is t.en said to rotate^
The term is derived from the Latin verb rata, I whirl

a'wheel .'""" "' ' ''""^"' ^'"^'^^^ ^'"^ ^^^^ "«»^ ^^'^i

14. When a body rotates, there is an iman-inarv
s rmght line in it whieh keeps the same plae^-^every
other par describing a circle -oand some point in tha^
Ime.^ This line is called the Axis, or Axis of Rota-

15. A body may have a motion of translation, that
, be continually changing its place, at the same time

that It has one of rotation; as the wheel of a carria-e
m motion, or a ball rolling along the ground. But
each motion may be considered separately

16. Apparent Motion is the apparent change of a
body s position, arising from a change in the position of
the observer, not from a real motion of the body It
IS sometimes called relative motion.

17. Real Motion is when a body actually does
mS! ^''''^""- ^' '' sometimes called aloluTe

rJl^' ^P7?° ^«Y^"g ^^^^S ^ road in a carriage has
real or absolute motion : while the change of position
which he observes in the trees, houses, etc., is on y an
apparent or relative motion of these objects. ^

19. Motion is called Uniform, when^its rate remains
the same, that is, when the moving bo V passes over
equal spaces in equal times; Acceleratedrwhen the
rate of motion is continually increasing • Re arded
when the rate is continually diminishing'' ' retarded,

^^ ELEMENTS OF ASTRONOMY,

2. Apparent Motion of the Heavens-
North Pole.

20. When we turn our eyes towanla the nky, it
appears to us to bo the conc.ivo or inner surface of a
hollow sphere in the centre of which we arc placed.
It IS convenient to regard it as such, and to luia-ino
various lines drawn upon it, to enable us to define w'*h
precision the positions of the heavenly bodies. Thia
hollow sphere wo shall call "the heavens."

21. The heavens appear to be in continual motion
from east to west around us, carrying the sun, moon,
and stars along with them : for, if we observe the posi-
tion ol any heavenly body, in respect to the earth, at

*\"^,/^"";' ""'^ ^''^^ ^'''' ^^ •'^o''^"^ i" a» hour or two, wo
shall hnd it to the west of its former position.

22. Also, we shall find that there is a moVement of
the whole heavens, not merely of one body, as the sun
or any bright star we may watch ; for, 1. We find the
westward movement in every one whose course wo
watch, and, 2. The different celestial objects preserve
the same positions in relation to each other, showincr
that they all move together. Some ancient astrono°
mers, who believed that this was a real motion of the
heavens, were perplexed how to explain the various
bodies preserving the same relative positions in this
great westerly movement, and imagined that they were
all immovably connected to each other by being em-
bedded, like jewels when set, in a crijstal sphere, which
performed the revolution, and carried sun, moon, and
stars along with it.

23. The heavens are found to make one complete
revolution in 24 hours (correctly, 23 hours, 56 minutes,
4-09 seconds). This is known by noting carefully the
position of a star in reference to any fixed Icrrestrial
object, and observing the time tliat elapses before it
again comes into the same position.

ELEMENTS OF ASTRONOMY. H

24. But this is only an apparent motion: the real
motK.n winch causes it, is the rotation of the earth from
west to east m the same time. This is proved by a
variety of reasons, which will he alluded to snbse-
qnently; but tvvo thin.-s may bo stated at present,
wh.cM wdl satisfy us that the apparent revolution of
tlie sky mai/ ho caused by a real motion of the earth.
1. I> e may he in motion without perceiving if, as in tho
cabin of a^ ship, or of a canal boat, or in a railway
train, movm- gently, when we may be carried a con-
siderable way without knowing that we have moved
at all. We do not perceive motion when it is uniform
so hat there are no jarrings or joltings, and when the
bodies around us are moving at the same rate, so that
we retain the same relative position to them. 2. We
knoio also that our motion maj/ cause bodies to appear to
move wheh are really standing still ; as, when we are

appear to fl, quickly past us in a direction opposite to
tlij't^in which we ourselves are moving.

25. When the motions of the sta'is are observed
they all seem to move together from the east side of
the horizon towards the west. Some rise very far
south, ascend but a little way above the horizon, and
set far south on the west side of the horizon : some rise
in the cast ascen.l very high in the sky, and after de-
scribing a large curve in tht. heavens, set in the west •
others rise and set north of due east and west: others
o not set at all, but describe complete circles above
ih^ horizon round one point : others describe smaller

VP^I? I'/r^'- '"""'^ *^^'^* P^^"*5 ^"^^ «^^ stars

IJ ?I *^''^* "^""'"'i ''^W^^'^'"' t^ i'^^^ff^^ ^y the naked
eye, not to inove at all.

26. That point is the North Pole of the Heavens

v!Z'' 'Vlf'"l ''' P?^"l ^^PP^'^^" *^ '^ i" the southern

wg'i'^.ft'nf;^' v^"^ " '^' 'T -*^"- ^^^^^ '' *^-^

vw.j ii\, ,,Oxti} ol tuu equator, and whicli is the centre

LLEMENT8 OF AS-JRONOMV.

ronnd winch the j..onthcrn stars appear to move daily.
Iheso two points are the extreinitieH of tlie imaginary
hue or axin, ahout whicli the heavens appear to rotate
daily. 11,0, are vertical at tlie poles of the earth,
and in the horizon at its equator.

27. x\l any place on the eartii's surface, the pole of
the heavens, visible there, always appears in the same
position in relation to fixed olijeets at that place, while
every other point in the sky is continually chanLnn-
Its position in relation to them.

28. The poles of the heavens may also be defined as the

2 J. I here is a pretty bright star very near the north
pole of the heavi-ns, called the North Polar Star
winch may be easily found out.

30. Tiie ancients had the starry heavens mapped
out into constellations, each consisting of a collection
ot acjjoining stars, separated from th.e others by an
imaginary line, and included under one name, expres-
sive of some figure which the leading stars in the con-
stellation were sui>posed to resemble.

31. The stars in each constellation are named by
the letters of the Greek alphabet,— the brightest being
termed a (alpha) ; the next brightest /3 (beta), and so
on. When there are more stars in a constellation than
there ai^ Greek letters, the others are denoted by num-
bers. The leading stars in each constellation have
usually some name applied to each, as Dubhe, Capellr
Vega, Arcturus, Aldebaran. '

32. At the left side of Fig. 1, may be observed a
cluster ot stars disposed within the figure of a small
bear, and separated by a line from the adjoining
stars. 1 he stars within that line form a constellation^
termed Ursa Minor, or the Little Bear. In the same
hgure are seen parts of other constellations— the Great
Bear (Lisa Major), the Dragon (Draco), the hand of
Bootes, and the feet of Cepheus.

ELKMKNTS OF AHTKONOAIY.

33. The north pole-star is the Lri^'htest star in tho
coiistcllution r.ittlo Hear, at tho tip of itH tail. It is
marked V H in the fi^nirc. It is easily found out by
means of the well-known seven bri^dit stars commonly

FlK. I.

called the Bear, the Plough, Charles's Wain, the Butcher's
Cleaver. These stars are represented in Fig. 1, towards
the lower part of the right side. If, wnen these stars
are in any pobltlou. a straight line be imagined through

BLEMENTS OF A8TK0N0MY.

tbo two (6 un.l a) furthest from the tuil, niul i.ro.lii( ,.1
inad,roctH.n/n.. the limbs of the uni.nal i «

will pass cl,.so to the north ,H,hir star. Those two
Htarn an- hcnee culled "the l><,inters."-Tle sunll
circle " marked I> near the pole-star, nhows L trio
p(M ion of the north ^.^le <,f the heavens.

At one time th.-y are Reen betwe, ., the pole-star and

'^^tte zenith" ' "' ^'''"" ''"""^' ''-'' ^'^'"'^ "-"^^
35. If the directum of north be known, the pole-
ftar may easily be found. Looking north, n Hrita m
; w.ll be .3en a little higher than Indf- vay b wee n
the honzon iind the zenith.-The height ot" the ,,oh
above the horizon is always the same numl.r of dc™ 's
etc as the latitude of the place; and as Hrit ah ex-'
tends from about bif to GO' N. lat., the North l>ohr
Mar Will in that country be from 5(r to GO^ above the

^^'of V."''^^"^: ^*^ '^'' ^'^^''^' ^'*' t'le place.

36. Jhe hrst thing to be d,me, in studyin- the
heavens is to know the North Polar Star, whuh n y
be readily found out by the methods described, llavinfr
xound It, let some convenient fixed staticm be takeiP
such that when at that station the polar star appear^
n the same strai^d.t line ^vith the eye and some pmn.i-
nent object, as the top of a steeple, or tree, or the
cn^rner ot a house. We shall then find, that at aU
t nies of the n.ght, and at all times of the year, tlmt
star wdl always be m the very same position in relation
to our station and the object, and that every other
heaverJy body will appear to describe a daily circle
rouno it. Ihosc that are near it will describe small
circles near it, and, unless by very close observation,
will not appear to have moved at all ; while each will
describe a greater circle the farther it is from the p(,lar
Btar ; and the motion of those at a considerable distauce

Pill

ELKMENT8 OP A8TUONOMV. Ifi

fmni it will bo m ^nout, tlmt wo inny ol.sorvc them to
change tiioir [Hmluni in n-lution t.. m.y (ixcl ol.iVct on
tho earth (we remaining' still in the same place)" in the
course ot five or ten minui.H.

37. The pole-star and constellation fircat Hear boini:
known, the next thi.ig to be done m to louk for the ntani
ot the constellation Cassiopeia, an.l the very brijrht
Htars CapeUa and Vega. These stars never sink below
the horizon m liritain, so that they may almost alwava
be seen on clear ni-hts; and they are very distinct and
pronnnent, so that by their aid the other heavenly
bodies can easily be found out. See Fig. 2.

Fig. 2,

CyynvS

* *
*

CnstUpcia

**
***

•

* *

fey-.

*
^

■T- Crea.tSeaf

Ca,p«l/n

38. A line drawn from about i\\G middle of the tail
of the Great, I?eai throu^rh the pole-star, and cont'nued
nearly as tar on the other side of that star, will tcr-
mmate in the constellation Cassiopeia, or Lady in her
(hair. The prominent stars in this constellation are
-»- ni nutxi^/^i, a:i<.i iiiiuiij^ed so as to make u li'nire

■LEMIMTf or ASTROXOHr.

■omowlmt like tho letter W, hut Htra^jjlin^, and with
ono liinl) of tho \V Hliortcr than the other. ('iiHMioiM'irt
IH ono of tho cotiHtellrttiofiH in tlint hn.ml wlntinh belt
extcndinjjf acroHn tho Hky, cilh-d ••The M'lky Way."

3D. A Htrai;<ht lino from tlio poU^-Mtur, porpcndiciilnr
to the lino joining tho pointorH luA poh'-Htur, nnd or. tho
wimo sido of that lino us tho head of the iU-ar, paHm-g
close to a very hri^^ht Htar called Capella, a little far-
ther from tho polo-Htur than tho pointers. This is tho
brightest and most northern of tljo stars in tho constgl-
lation Aurigc, or ChariotcrT.

40. On tjje other side of tho polo-star from Capella,
but Ktill fartiier from tho pole, may bo seen a very
bright Htar called Vega, the principal slar in tho con-
stellation Lyra.

_ 41. These five— tho Oroat Bear, tho Pole-star, Cas-
siojM'ia, Capella, atul Vefra-_are almost always viniblo
in Great Britain, and t.:e student of ristronomy should,
as soon as possible, nnko himself ac(iuainted with their
appearance and relative positions, as well as with their
daily changes of position in relation to him. They aro
represented in tho preceding sketch, with ono or two
others that are prominent in that i)art of tho heavens
around the north polar star.

42. Tho student will do well also to observe carefully
tho ioilowing celestial phenomena, and make himself
practically accpiainted with the various details con-
nected with them.

43. The changes of tho moon; its monthly motion
completely round the heavens ; the -liM.t ai. I direction
of its daily motion through the sKy ; the stars near
which it passes in its course ; the position it occupies
in respect to the sun at different times ; the form of the
edge turned towards the sun, etc.

^ 44. With respect to the sun, he should observe tho
different points of the liorizon at which it rises at differ-
ent times, the ditlerences in the elevation it reaches at

ELEMENTS OF AiTRONOMy.

f'.o ViiiiouH scusoriH of the year; nnrl ho hIiouM tiiko
rirticiihir i„)tico of tin? «t.'irH near tl.o «iin, wliioh he
will diHcov.r by obsorvin^^ thono visible uonr whcr« thn
HUH has jiist set, or \,tmv ]w in al)oiu to rino. IJy the
Htier observatioriH, ho will soon fifKl that the sun
travels through the star«, aiul con.j)letely rotnid the sky
in one y.-ar; au.l that the eourse it takes throu-h the
sky iH nearly the same as that whi.h the n„.„n takes
m her monthly circuit. These interehting facts were
known to the ancient astroutmiers long before man's
i)ower of viHion was augmented by the telescope.

45. In this remarkable part of the sky, tiirouL'h
which the sun and moon pursue their course unceas-
in;;Iy, he will oe.-. sioiially find bright sitars .vhich do
not, like most of the stars, always retain il-j same rela-
tive positions to each other, but wl.icL move about
amongst tnem. Tiicse are the Planets. Tl.(>y do not
twi.ikle as the other stars doj and they chan-e their
positions in the sky, always however rem-ining near
that circle or belt of the heavens through which the
sun and moon run their conrscM. There are five planets
y».siblc to the nake<l eye. Mercury, Venus, Mars, .Turn,
ter, and feoturn The first of these, however, can be
seen but sehhjm, being very near the sun, amid the
hrilluincy of whoso rays Mercury's faint light is usuallv
lost to our view. "^

46. These phenomena, which have been just alluded
to, along with those of climate and seasons, the length
tlie day, trade-winds, tides, and eclipses, are the chief
Objects of astronomical investigation, and present nate-
rials lor observation and study open to every one. Thev
embrac(3 the grandest parts of the scenery of nature •
and a knowledge of the movements going on in the
heavenly bodies of their general nature, of the varieties
ot them and phenomena which they present, impart
peat additional interest to the view we enjov of the

lieavens on a starrv iiiLrht— t>nr1inn= +i,„ j^l. .

ficent object in creation: '' ~ '"' ^"'^ "'■"^"'-

ELEMENTS OF ASTRONOMY

3. General Definitions.*
47 A plane superficies, plane surface, or, as It 13

s lorly ('xi,resse(l, a plane, is a surface of such a nature,
tliat It a strai^rht hne be drawn between any two points in
It, every point in the straiglit line shall touch the surface,
wnl?" „^J.''?r'' '" ^;»"V"''" language, is cnlled ajlat surface. A
Znl'C ,7.',= '-';"'"'I^ '^'1 table a unn.y, in whatever positi.m it
n av be held, furnish exanu.lcs of a plane surface. Its nature
will be at once understood Lv cndeaviuring to apply the above
defnution to any curved surfi-e, as a sphere: thJ sfra Sit iin^
between two pon.ts will pass below thi curved surface

49. When two plane surfaces, on bein^ pvoduced so
as to meet would lorm one plane, the?/ are said to be in
trie same plane.

h\ Fig. 3, the planes m

A D n, K B C L, are in the
same plane, as, on being
produced, they form one
I)lane, m B C n.

50. 'J'wo tables, of the same
height above the floor, and n
perfectly horizontal, are in
the same plane.

51. When two planes, on being produced so as" to
mec, wonid cross each other, and then diverge, they
are sa^lto be rnclmed to each other, or, to cut each other.

In lug. 3, the planes E
K L H, A K L D, are in-
clined to each other. The
two plnnes in Fig. 4 are
also inclined to each other.
See also Fig. 5, in which a
number of planes arc shown
inclined to each other.

52. Planes which are

Fig. 4.

ELEMENTS OF ASTRONOMV.

every wliere at tlic same distance, are said to he parallel.

In Fig. 5, the planes Bbc

C, And U, are parallel ;

also the [danes Bba A, C

cdD. The opposite walls

of a room are parallel, also

the floor and ceiling.

53. If two planes cut
each other, the line where
they meet is a. straight line ; as K L, Fig. 3, E e, Fi"-.
4, or any of the straight lines in Fig. 5.

54. We speak of the plane of any figure, though the
interior of it be not filled up, if it be one every point of
which would touch a plane surface when laid upon it •
as a triangle, a square, a circle, an ellipse. Such a
figure is termed a plane figure. Thus, we speak of the
plane of the path of a planet, meaning the imaginary
plane surface which would touch every point of the
planet's course : and we may imagine it to be extended
on any side far beyond the actual path of the planet.
It is essential for the student of astronomy to understand
what is meant by "the i)l;ine of a figure."

55. An Angle is the
opening between two
straight lines, which meet,
but are not in the some
strai.o-ht line. In Fig. 6,

the opening between the ^ c y;

lines A B and C B is an angle, termed the angle B, or
the angle ABC: the letter at the point where the
lines meet is placed in the middle.

6Q. A Right Angle is the angle formed when one
straight line stands upon another in such a manner
that the angles on each side are equal to one another.
These angles are termed the adjacent angles. In
Fig. 7, A B C and A B D are right angles, for they
are equal to each other.

KVKMENTS OF ASTKONOMV.

said to bo perpendicular or ■A-

at right angles to the other. I

In Fig. 7, A B is prrpon- '

dicular to C 1) :~also D B
and C B are, cacli of them,
perpendicular to A B.

58. A straight line is said to be perpendicular to a
plane, when it is perpendioular to every straight lino
drawn on that plane which it meets.

59. An Obtuse Angle is one which is greater, or has a AvMor

aTr\ rcut/'A";:,;"^'^- ^". .*>. ^'^ ^ ^'^ is an obtt:
an^ic. An Acute Angle is one which s ess, or 1ms a narrnwpr

opening, than alright angle. In Fig. 6, A B'c\:a™cuto

60. The rnethjd of comparing angles exactly, with resnect to
their magiutiule, is described in Par. 75, page 22 ^

. 61. The Inclination of a Straight line to a Plane

IS the smallest angle which the line makes with any
straight lines drawn on the plane. ^

62. The IncUnation of a Plane to a Plane is the
acute angle contained by two straight lines, one in
each plane drawn from any point in the straight line

63. Two lines which are everywhere at the same

S:i finer' '^ '' ^''''''' '^ ^^^^ «^^-' -

64. Planes or lines which are parallel to the pl«ne
of the horizon, are caHed Horizontal; as the surface

cin:ta.^^^^^ '''' - ^-« -^ ^^^^ p-p-?;

65. Any straight line or plane that is perrondicular
to the plane of the horizon, is said to be Veriicai L
a plumb-line (a cord with a weight at its end freely
suspended), or the walls of buildings. ^

66. When the sun is right over head at any place,

:lements of astronomy.

l.is rays fuli upon it so as to Lo i)eri)endicular to the
plane of its horizon ; that is, vertically. He is then

said to be vertical at that place.

67. A Circle has been
nheady defined in Par. 8.
In Fig. 8, C is the centre,
»\nd the curved lino A B D
E H O, the circumference
of the circle; C A is a
radius; C B and C D are
also radii. A D is a
diameter.

68. It follows from the
definition of a circle (8.)
that all radii of the same
circle are equal in length,

69. The diameter is double
the radius, and it divides the
circle into two equal parts.

Ffp. 8.

70 An Arc of a circle is any portion of the circumference,
n Iig. 8, D E, E H, H 0, B E, E 0, «

A B H 0.

are arcs Oi the circle

71. A Semicircle is the part cut off by the diameter,
or half the area of the circle. In Fig. 8, A B D A and
A H D A are semicircles. A Quadrant is the half of
a semicircle. In Fig. 8, A B C A and D B C D are
quadrants.

72. The terms semicircle and quadrant are sometimes
applied to the portions of the circumference which
bound them, as well as to the area enclosed. In this
case, the semicircle is one-half, the quadrant one-fourth
of the circumference.

73. The magnitude of an arc of a circle is described
by stating what proportion of the whole circumference
It forms. For this purpose, the circumference of a
circle, wliatever its magnitude may be, is supposed to
be divided into 360 equal parts, called degrees, and
marked °. To express still smaller parts, each degree
IS divided into 60 c^timl nnrfo n^^^..A .v,,\,,.^.» _^j

Bqual part

s, called minutes, and

i-aia-eu , auu. cuni iiiinuLu la auuuiviaed into bU equal

ELEMENTS OP ASTilONOMY.

part^, tornied seconds, and marked - Tlins, an arc of
thirty - nine de-recs, forty minutes, and thirty -ono
seconds, is shortly expressed, 31)'' 40' 31"

74. A semicircle is an arc of 180 degrees; a quadrant
an are of !K) deirrecs.

75. An Angle is measured by makin- its sides radii
of a circle, the angular point being the centre, and
taking the lengtli of the arc on which it stands, in de-
grees, minutes, and seconds. The arc on which the
angle stands is the portion of the circumference be-
tween the extremities of its sides or radii. In Fiff 8
the angles ACE and D C E are measured by the
number of degrees in the respective arcs A H E and

^''Vr'V,'''^',' ^^'"^y stand,— the angle A C E by the
arc A n E, the angle D C E by the arc D E. We
thus speak of an angle, as of so many degrees and
minutes in magnitude. The angle BCD, whoso arc
H L) IS a quadrant or fourth-part of the circumference,

is therefore an angle of 00 di

rees. The angle D C E

an acute angle

must be considerably less, ab ut 60 degrees or 60°—
the angle A C E, about 120^ The arc on which an
angle stands is said to subtend that angle.

76. An angl^e of 90° is a right angle „,

is less than 90° ; an obtuse, angle greater than 90°.

77. The length of tlie radii, or magnitude of the circle, makes
no difference in the size of the angle, for the arc on which the

irr:-^^' 'T^' ^'"/ ^^^'^>^^ ^'^''^^^'^^ ^"'"^ proportfon to t e

vhole circuniference however dificrent ihat circumference may

be, or whatever the length of the radius. ^

78. The sphere has been defined in Par. 9 The
a(\joining figure will illustrate the nature of those lines
which -re imagined in or on a sphere with the view of
defining the relations of the different parts. In Fio-

9, straight lines from E to Q, from P to^, i from Z

to N, would be diameters.

79 A Great Circle of a sphere is a circle drawn
upon its surface, whos-^ plane passes through the centre

ELEMENTS OF ASTRONOMY.

of the sphere, "^n Fip^. 9, P f p r, the dotted lino, Z
A V N m, E f ' H -V. rf , and tlio outer circle,
are g-reat circ! -i.'

80. All grout circles of a si)here are equal to each
other; cross each other twice, divide each other into
two equal semicircles, and divide the surface of the
sphere into two equal parts called Hemispheres. In
Fig. 9, each of the great circles P T p, Q « T E,

Fig. 9.

divide each other into two equal parts or semicircles ,
the former into T P ^Os and ^ p r^^ the latter into
ai Q T and T E ^.

81. A Small Circle of a sphere is a circle drawn
upon its surface, whose plane does not cut the centre
of the sphere. In Fig. 9, Z 6? T, A a H, N v R, are

* This character, j, may be read, Aries— this mark, ^ct, Libra.

):lf,mrnts of astronomy.

small circles. Or, a sniiiU circle of u sphere divides
its surt'ace into two unequal parts.

82. Distances on a splierc, and the Lreadth or length
of figures on its surface, are expressed in degrees of
some of the circles drawn on its surface.

83. Wiien a sphere revolves, like a top Fpinning,
this is termed rotation on its axis. The Axis ab(.'Ut
which it rotates is usually a diameter: and the ex-
tremities of the axis are termed Poles of the sphere.

84. The extremities of a diameter at right angles to
the plane of a great circle are sometimes termed the
Poles of that great circle. In Fig. 9, P, p are the
poles of the great circle E T e Q ; Z, N, the poles of
the great circle 11 T sO..

85. The poles of a great circle are each of them 90"
distant from that circle. That is, if a great circle be
drawn through the poles of any great circle, cutting the
latter, the part of the former between each pole and

• the second great circle will be 90°, or l-4th of the
whole circumference. In Fig. 9, the arcs P a e, P cZ '"i"' ,
between the great circle E T Q and its pole P, are arcs
of 90° each.

86. It is impossible to represent a sphere accurately
on a flat surface. In Fig. 9, the outer circle is the
only one wliich retains its proper form — the others
appear as ovals. But each of these should represent
a true circle (8).

4. How to define the Positions of Objects
in the Heavens.

87. It is necessary, for astronomical purposes, to be
f.ble to define the exact position of the stars in the
heavens. This is done by taking certain fixed points
or lines, and marking the distance of each star from
these points or lines.

88. For this purpose, the surface of the heavens is

ELEMENTS OF ASTRONOMY. 25

ro^rrarfled as it appears to us, as a concave sphere and
an infinite number of lines, crossing each other' are
supposed to be drawn upon it. Thes? lines are di;ided
jnto degrees, numbered from certain fix "d points td
by observing the position of a star on these 1 'n , ^vh "re
two of them cross each other, its situation can bo de

Fi* To Th» fi , V '■.''P^o^^'"'"! I'y N and S in
«f ?;,: 1^ '" *"■' • ^' '^ *-''™ed the Nortliem Pole

the He^t^.If ''.'^ ""°"'^' ^' '^^ Southern Pole Jf
I'a^ 26 ^ "^ '"■'""'-'■'•''' "' described in

hJ'irJn o'J^'lt 'T' "fs^^t cireles is supposed to

teles Tlwl ''''"'■"'*" P"''''"S thresh the two
lelcs Ihese are represented in Fig. 10 by N e 8

;!;",-, -f ™- ^'"'^« "" t"™*^"! Honr-Circles o?

S aTs se^ "• ^""^ ^^"^^ "^ "- ™'- "tTi

91. A great circle round the heavens at ennil ^ic

anees fton, each pole, is termed The EquUocS It

EQin k" if r'^' "J"^ '^ ^^P™-"'^-! I'y 'he line

;, "P-^' "'g"*. because when the sun is on that fine

here IS equal day and night all over the world Tw'

IS on March 20 and September 23! ' ^^"

Ihe LSs eTif""""' Z "'%"'"' ^'>''='' "'^ ?'"»<= of
lue eaitn s equator would make round the <ikv if «,„

imagme that plane produced so as to cut 1 e sky.

. & <v-cu 11 aim eaCii pule, riiese are re-

ELFMrNTS OF ASTRONOMY.

proRcntcd in Fi<?. 10, l.y Z h, k o, etc, north of tho
Inninoctiul, and q 15, a 6, m v, etc ^i>"th of the oqm-
iKK-tial. These circles are terme.l Parallels of De-

clination. , , t • i i • *

94 All these circles are RUi)pose(l to he (hvi.led into
decrees, mhmtes, and seconds, as ck-serihed ui I ar 16.

95 In any hour-circle there will he arcs of 90 de-
rrrces hctween the points where it crosses the equi-
noctial and either pole, and these arcs will be crossed
hy every parallel on the same side of the equinoctial.

Fig. 10.

61. Z ','ft

7:> n

Accordhiglv, each parallel is named hy its distance
north or south from the equinoctial, measured by the
number of degrees along an liour-cuxle winch i i^^
distant from the equinoctial. Thus, from E by Z to IN
is 90° of the hour-circle N E S Q. The first pamllel
s lown north of E Q crosses N E S Q at 15° from E Q ;
that parallel (15 d 15) is therefore 15° north ; its dis-

ELEMENTS OF ASTRONOMY. £7

taneo nortli of E Q is tcrnioa its Nor*^ DecHnafinn

I' « ,^, r r p, hour-clrcloH. In that CilLo U ' '"""^"'^ ' '

north .Inclination of a; the arc r T t n nL ^l"-^ * -^ '« *'="
K n, the 8outh declinatiol, ofk ' " "''"'' declination of J;

t^es^eros.sitinspH;!^lS^^

I)omt of h,o sin^,, Aries: or the Vernal or Sprinff Equi

f,-., 1 " ?•• ■'^'^'^''*^ hour-circle crosses the eouino/
t al, and each is named according to the de^^ree Zto

St Asc^^^^^^^ '\^'\^^^ ^V'^^ox is termed its

xvi^at Ascension. 1 ^s, the honr-c rcle N r S Fio- in
crossmg the equinoctial at 45° east from tVn' ^'

™ position, as ever, paralldto^s' ::^;Z^

^^'^''<^n^,!^al^?rq\^^ Ho. of the

from . to a will ll th?dtlin!tTon t" h'o? t^ no'^^' '%''•''
right ascension will be the decrees from r rn ^iT^f ' ^"'^ ^*^''
Q, to e, r being the vernal en uhioVrJ- ^ "^^.^^^^ E^ =^ , and
from which the'degrees" c rTkS " CfJI^ ^ tSf'^t'
18 in right ascension 30°, nortl. dr-linatu ^n?* Ik' ^'^''.P?*"* «
n.,/.^ ascension 315^ nortli declinationll"; ami so on.^"'"' ^ '°
^ 100. R. A. is the contraption used for rio-bf „o
S'on; D. N. declination nnrfb n q "f V^^^^"'
south. ' " * ^'" ^' "<^ciuiation

KLKMKNTS OF ASTRONOMY.

101. The (listanct'H from the vornal equinox aro
Ronu'timc'8 reckoned in hours, each hour consisting of
IT) dej,'rees, and beinj,' divided into 60 minutes. In
Fi^'. 10, above the line E Q, the degrees aro marked
alternately in hours and degrees ; thus, commencing at
the equinox, we have first 15", next II hours (30°),
then 45°, then IV hours (GO"), 75°, VI hours (90°), etc.
To briiii? time to dej^'rees, v/c must multiply by 15.
Thus, 10"- 3"'- = 150° 45'.

102. The degrees of right ascension are reckoned
eastward from the eciuinox, all the v'ay round from 0°
to 300° : that is, in the direction from the head
towards the tail of the Great Bear, when that constel-
Iniion is hetweeri the pole-star and the horizon,

103. The places of the Bun and planets are expressed
in the almanacs by stating their right ascension and
decliiuition.

104. Observations of the sun have shown that he
appears to move round a great circle of the heavens in
a year. This ^-reat circle is called the Ecliptic; as
eclipses take placj only when the moon also is upon that
line. The ecliptic is represented by the line a o in
Fig. 10, and S P in Figs. 11 and 12. The points
where it crosses the equinoctial are called the equinoc-
tial points or equinoxes ; the first point of Aries, T,
where they cross in spring ; the first point of Libra, H,
where they cross in autumn.

105. The plane of the ecliptic makes an angle of
23i° with the i)lane of the equinoctial ; so that the far-

thest north point of the ecliptic is only 66^°, while the
farthest south point is 113^° (90° + 23f ) from the
north pole of the heavens.

106. The sun is in tlie north or highest point of
the ecliptic on o le 21, and he is then vertical at
the tropic of Cancer :— he is in the south or lowest
point on December 21, and is then vertical at the
tropic of Capricorn. When he crosses the equiuucLlal

ELEMENTS OP Ai^TRONOMY.

(March 20 and ScptcmbcT 23) bo

e«iiuit()r

is vertical at tho

in Ju

!ll.''-i' ••"?["! r' "f.",'" ™'il"i^. 'vhor.. ,1,.. „„„ in

»tN iH in that part of tho h

bright star called Capella

tho Great Hoar, but nt
polo. 8co Fig. 12

cavt'iiH a l-'ttic Kouth of
which lit'H to tho west of
a greater tlistanco from tiie north

heavens must move 15 cle^rees u,,n \n ™ o' T "

oJ^^•/^''^^f\Ml^c is divi<Io(l into 12 emial imrts of

there be.ng mostly figures of animals (from the Grrk

clu..ete,-s used f.,,- eaeh, a,,., one or two IV/itb!
11 1. As the sun enters the sign Cancer on th^ 9i .f

retraces his course, that paralleWdtW nn h"' '"•' ','"="
the heavens) is ^^'^^ ^"(^C ^::'^
'iSVea'onV""" "^ ''^P™"™ "-ives its .,amet;

112. As the sun appears to pause or stinrl »i;il „ i
or two before turnins-, the time is terme, Sols ilV'T
Iho sun, sto, I stanci) -Uocembur 92 f L • . ^'"'''
«tice; J„„e'21, the JnnmoTsokfc ' ""'""' '"'-

kLiMBNTS OF ABTRONOMY.

•s -

D. J2 a 3
-< .^ i-s "^

— «■ — •——

PI
a

B
O

o
fao

•BU«!S lu.niini)^

^ p s cl ^^ 4

B
O

n .2 I

o
u
to

■«-i

o
u

•c

c« e^i eo '^' c« O Ci

C^ CI CI C-l CI CI »-

•^ r* "^

•flUJpUMSV

•auivuooHoa

•ilujpuaoBv

c«

•3

113. The signs of the zodiac in which the sun appears
when he is north of tlie equinoctial, are callecl the nor-
tJicrn sigyis ; those in which he is when south of the
equinoctial, southern signs ; those in which he is i)assing
in a northerly direction are called ascending ; those in
which he is going south, descending.

-a
r-

KLKMKNT3 Of AiTUONOMY.

111. The terms lori^'itiido nnd lutilii.lo ftro uIbo
ployed for tho liouvons— l,iit with r.-lerci.co to i.
rr 7>//r ami its poles (85). Tims, tho iatitud, of
cidt'stial body va its distance

cm-

('(

IlDt

nortli or south from th

\tH lont/iUu/e is its distuiico (•u^t of the Imlt-
I circle troin tiio north to the sonth p.do of tho ecHptio

«^. passui- throu;,'h tht- lirst point of Aries. Tho north

pole ot the eoIii)tic is in the constellation Draco, ^U"
*rom tho north polo of tho heavens, and in U. A. 27(P
Ihe n )rth polo of tho heavens moves so as to describo
n circl,. ronnd tho pole of the ecliptic in 2r>,898 vears.
J he movement thus made is too sli-ht t<» be apparent
in a lifetime; but in time t!ie north p<de will be far re
moved from the present pole-star, and will return to it
a^'HWi at the end of the above-mentioned period

115. From this moti.m (which will be explained
afterwards-see '' Precession of tho K.ininoxes " par.
OOn the ecpnnoctial points move backwards upon tho
ecliptic ; and the si-ns of the zodiac, which were orb^i-
nally named fro- constellations in these si-ns, do not
now correspond with these constellations.

116. Tho following figure (11) represents tho con-
Rtel ations around tho first point of Aries, and the
ieading imaginary lines in that region. The line E Q
IS par^ ot the equinoctial ; S P represents the sun's
yearly path through the heavens, usually called the
ecliptic the sun moving in tho direction shown by tho
order of the letters, from S towards P. The point
where these t^vo great circle : cross in that quarter of
the .leavens, wla^-e the sun is on iiarch 20, is tho first
point of Aries. Ihe hour-circle passing through that
point IS the first hour-circle, from which degrees of
right ascension are reckoned. It is marked H C in
the figure ; and tho degrees are seen marked on the
equinoctial, increasing eastward from that hour-circle
or towards the left in the figure. The dotted lines are
parts ot par:uicis oi aeoll;...tioii and hour-circles. It

ELEMENTS OF ASTRONOMY.

vr*

It ""^''ll'i ^^'^^ '<^'i^^^ard is towards the right in

J^. In the latter, we see the stars as they appear to
us who view them from the intc-ior of the starry

we view the objects from the ouUide of the s.' ere on
which they are placed. ^

i,;.!/^i/^''' ^*"'^'''?* ^^ ''^''« fitrongly recommcndtd to make
h.n .elf acquamted with the right" Iscension and lecHimtion
of several leading stars, so that he knows thorough y and can
trace out for h.m.elf, on viewing the heavens, the pUt?)n of
the pru.c.rd hour-circles and parallels. He will tlfcm find it

IsTveU Ys anv':?r^'' f' '""^'"^ ^^'^'"^ ^^'^ Istdla'tfon;
knJws ^ '^'''* "'^ P^^"^* '^'^"^^ I^^- A. and Dec. ha

stcHat'ions wmlT'".^ ^r '"'T'^^ ''Z *^° P^""'^'P^^ ^^ars and con-

S;,L-^ learner. Ihe constellations are arranged in three
divisions, northern, southern, and zo<iiacal consteliftions The

Sre" rltlirr fT^ *^ .^^^^ "^ *'- heavens a Iw
aegress on each side of the ecliptic. The northern constel-
lations are those on the same side of the zodiaTas the north
JotielttloT *'^ "^^^'^^" ""'^ ^' ^^^ -^— thetuE^

6. Northern Constellations.

TtA^^' The Great Bear (/7r.aJ/a>r) is well known.
Its seven brightest 3tars are in the body and tail o^the

hf Fi; f r'f ' r^ ^^ ^'"' constellation is shown
in i^ ig. 1. Its brightest star, one of the pointers
marked a in Fig. I, and termed Dubhe, is in K. A
lO'^- 54-, or about 163°; and D. N. 62° 32^ The
star at the tip of the tail, called Benetnasch, is in R. 1
13h. 4im.^ or ai^Q^^ 205°; I). N. 50° 7'

120 Upon the other side of the Little Bear, but
nearer to it, is an irregular cluster, which have been
thrown into a male figure called Cepheus, the feet of
which are seen in Fig. 1. f , me leer ot

121. The constellation Cassioneia. nr l.^^^r\^ i,pr
Chair, and the stars Capella and Vega7have been already

B 2

i |H

ELEMENTS OF ASTRONOMY.

desc; ibod. Capella is in "R. A. 5^- 5'"-, or about 7G° ;
D. N. 45° 50'. This is the brif,'litest and most northt'in
of the leading: staio in the constelhition Auriga, or tlic
Charioteer. The principal stars in this constellation,
along with one of Taurus, form an elongated six-sided
figure, stretching from north to south, and very well
marked. Sec Fig. 12. Capella does not set in Great
Ikitain.

122. I'etwcen Capella and Cassiopeia, but further
south, is the constellation Perseus. Three of its leading
stars form a gentle curve. Tt is shown in Fig. 2, par.
37. The star above the letters vs in Perseus, is Algol,
a remarkable star, to be described al'i orwards.

123. A straight line from the pole-star, in the
direction nearly opposite to the line cutting Capella,
leads to another very bright star, Vega, the principal
star in the constellation Lyra. Vega is in li. A. 18''-
31'"-, or about 277° ; and D. N. 38° 38'. This star docs
not set north of liondon, just skirting the horizon.

124. East of Vega, in E. A. from 20»i- to 2^'-, and
D. N. about 33° to 45°, a^e seen four bright stars,
three nearly in a line, and one above the middle of the
three : these are the principal stars in the constellation
Cygnus, or Swan, which lies in the Milky Way, They
are shown in Fig. 2.

125. A straight line from the pole-star, passing near
the star in the tip of the tail of the Great Bear, and
twice the distance of the tail from the pole-star, cuts
Arcturus, a very bright star, of a distinct reddish
colour, the principal star in the constellation Bootes, or
the Huntsman. Arcturus is in R. A. 14^^- S""-, or about
212°; and D.N. 19° 56'.

126. A straight line south from Cassiopeia, and
nearly at right angles to the line joining Cassiopeia
and the Swan, will pass near a large square of four
stars; the furthest north and brightest of which is
is Alpherat, the principal star in the constellation

ELEMENTS OF ASTRONOMY.

Andromeda; while the other three of the square are
part of the constellation Pegasus. The star Alj.iierut
i« in K. A. Oi'- O'"- 47«-, about 0°, that is, on the hour-
circle crossnig the equinox; D. N. 28° 1(/. It is on
the eye of Andromeda, which constellation stretches
eastward across the heavens towards I'erseus, from
Alpiierat. '

^ 127. This prominent square, with several other prin-
cipal stars near it, are represented in Fig-. 11 The
constellations Aquarius, Pisces, and Arios of the zodiac
are seen extending along the ecliptic, S P. At the
south-east (left) are seen several stars of the great
constellation Cetus. In a line with the two most
westerly stars of the square, a and /3 of Pegasus, but
far south the brig]:^ star Fomalhaut may be seen It
IS in II. A. 342° 15' ; D. S. 30° 23^

128. In ancient times, when names were inven to
the signs and constellations, the ecliptic and equinoctial
crossed each other about 30° further east than now—
that IS, nearly where the sign of Taurus ( » ) is seen
on the ecleptic in the figure. Then the 30° east of
that point were in the constellation Aries, and these 30
were from their position termed the sign Aries. But
now the equinoctial has receded back or west along the
ecliptic, and the old names for both signs and constel-
lations being retained, they do not correspond. The
sign Aries is in the constellation Pisces ; the sign Pisces
in the constellation Aquarius, and so on.

129. The brightest star in the constellation Cassio-
peia has nearly the same R. A. as Alpherat; or rather
between 0^- and P- eastward. Between Arcturus and
Vega, but considerably nearer Arcturus, is a half-circle
of stars, ten.ied Corona Borealis.

TT.^rpli'li"''^ Ti ""'* ""^ ^°T^ ^^'^ *^^ ^'""^^ constellation
Hercules. Ihere are four stars forming a sort of

diamond or lozenge in Fig, 2, near Veo-a. The thr- o
nearest to the pole-star belong to thS constellation

ELEMENTS OF ASTRONOMY.

Drac(. the stars in wliich form a curve between tlie
Great and Little Bears, and between the latter and
Hercules. The other is in the foot of Hercules.

6. Zodiacal Constellations.

131. These are twelve in number, and encircle the
heavens like a bek. They are named Aries, the Ram ;
Taurus, the Bull; Gemini, the Twins; Cancer, the
Crab ; Leo, the Lion ; Virgo, the Virj^in ; Libra, the
Balance ; Scorpio, the Scorpion ; Sagittarius, the
Archer ; Capricomus, the Goat ; Aquarius, tlie Water-
man ; Pisces, the Fishes. The constellations of the
zodiac do not rise high above nor sink far below the
horizon in Great "Britain.

132. The sun, moon, and principal planets, are always
found in some of the constellations of the zodiac.

J33. The brightest star in Aries is in R. A. 1^- 58"™-,
about 29° , D. N. 22° 45'. See Fig. U.

134. Aldebaran, the brightest 3tar in Taurus, is in
R. A. 4»i- 27'»-, about 67°; and D. N. 16° 12'. The
Pleiades, or seven stars of Taurus, are in R. A. about
54° 30', and D. N. 23° 38'. See Fig. 12.

135. Castor and Pollux, the brightest stars in
Gemini, are very near each other; Castor, in R. A.
7h. 25">-, or about 111°; and D. N. 32° 12'; Pollux,
in R. A. 7h. 36'"-, or about 114°; and D. N. 28° 22'.

See Fig 12.

136. There arc no very prominent stars in the con-
stellation Cancer.

■ 137. Kegulus, the principal star in Leo, is on the
ecliptic in R. A. 10^>-, about 150°; D. N. 12° 41'.
The leading stars in this constellation form a figure
like a sickle, of which Regulus is the handle. This
great constellation is nearly due south of the Great Bear.
138. Spica, the brightest star in Virgo, is in R. A.
1.3i»- 17"i-, or about 199°; and D. S. (declination south)
10° 23'. It is a very little south of the ecliptic.

ELEMENTS OF ASTRONOMY. 87

13D. The constellations Libra, Scorpio, Sagittarius,
are seldom seen in Great Britain.

7. Southern Constellations.

140. The only southern constellations of interest that
are frcvpiently visible in Great Britain are, Orion, Canis
Minor, and Canis Major : these constellations lie due
south of Capella and Gemini, and are very prominent
in the heavens during our winter.

141. Orion is a large well-marked figure, a little
east of due^south from Capella. It is in the form o." a
four-sided i"gure, considerably elongated from north to
south. In the middle are three stars, lying in a south-
east and north-west direction, usually termed Orion's
Belt. Betelgeux, the brightest star in Orion, is in the
north-east angle. It is of a distinct reddish colour. It
is in R. A. 5h- 47i"-, about 88°; in D. N. 7° 22'.

142. Sirius, in the constellation Canis Major, the
Greater Dog, and the brightest of the fixed stars, is
south-east from Orion, in R. A. 6^- 38™-, about 100° •
and D. S. 16° 31'. '

143. Procyo^;, a star of the first magnitu.le, in the
constellation Canis Minor, or Lesser Dog, is nearly due
south from the Twins, and due east of Betelgeux Its
R. A. is 71^- 31™-, about 113°; its D. N. 5° 35'.

144. These stars and constellations are represented
in the fol lowing figure (12). At the upper part or north
is seen the constellation Auriga or Charioteer with the
bright star Capella, also shown in Fig. 2. At the east
we observe Gemini, Orion below, Taurus at the right'
with the clusters, Pleiades in the shoulder, and Hyades
in the head. At the south-east is bjen the very bright
star Sirius, a of the constellation Canis Major; and
north-east of ISirius, Procyon, a of the Lesser Dog, and
another prominent star jS near it. At tlie left toe of
Castor, the more northern of the Twins, is the first point

ELEMENTS or ASTRONOMV.

fl-' 12.

^^^-==*^%*f,\j 1

I ^...

r^'

)

of tlie sign Cancer, where the sun is on June 21, being
his greatest distance north from the equinoctial. This
is the summer Solstice.

l-i5. It will be observed that Sirius, Orion's Belt,
the llyades with Aldebaran (a), and Pleiades, are

ELEMENTS OP ASTRONOMY.

nearly in one lino; ami that Orion is nearly duo south
of Capc'lhi. 1 ho hinbs at the north-west corner of the
fi^Mire are those of Terseus, the leading, stars in whoso
body are shown in l\r, 2. Celow Orion is seen the
conste ation Lepus or Hare, and at the right the great
constellation Eridanus. Between Dec. S. 5G° and G2»
;t.Hl K. A mr and 190°, there is a brilliant constella-
tion, well known m southern latitudes as the Southern
Cross, once (about 4000 years sincej visible in the
south of liritain, now lost to our view by Precession

14G. The Milky Way, another prominent object" in
the heavens, lies between Procyon and Sirius, passes
north-west between Gemini and Orion, then throucrh
Auriga, south-west of Capella ; then passes through
several minor constellations and Cassio])eia, and south-
west, splitting into two divisions, south of the constel-
lation Cygnus or the Swan, not far from Vega.

8. Extent of the Heavens visible at any

Place.
147. With respect to the extent of the heavens
visible at any place, the celestial sphere may be divided
into three portions :--l. That part which never sets at
the place {i.e., never sinks below the horizon), and the
stai-s in which are always visible on clear ni-hts
^. Ihat part which is only occasionably visible bein^
sometimes above, sometimes below, the horizon of the
place. 3. That part which is always b'.dow the horizon
ot the place, and therefore can never be seen from thit
place. '*■*'

148. The Celestial Meridian of any place on earth
means the Hour-circle which passes through the zenith
ot the place. Ihe distance from the zenith to the hori
zon along that circle will be 90°.

149. At any place, the height of the pole of the
heavens above the horizon (called the elemfinn of^in
pole) IS always the same number of degrees, minutes"

ELEMENTS Or ASTHONOMV.

etc., as the latitude of the place. Tliat is, if wo
measure tlie number of defz^recs, etc., alou^ the celestial
meridian of r place from tlio horizon to the pole, there
will be exactly as many as in the latitude of the place.
The N. latitude of London is 51° 80', and there the
north pole (or north polar star, which is close to the
pole) is 5V 30' above the horizon. At Edinburgh,
the elevation of the pole is 55" 57', for that city is in
N. L. 55° 57'.

150. The distance in degrees, etc., of the zenith of a
place from the equinoctial is the same as the elevation
of the pole, or latitude of the place.

151. The distance of the zenith from the pole (called
the zenith distance of the pole) is equal to the difference
between the elevation of the pole and 90° ; at London,
38° 30' ; at Edinburgh, 34° 3'. And this is equal also
to the elevation of the equinoctial above the horizon on
one side, or its dejtression below the horizon on the
other side of the heavens.

152. Thus, at London, the terrestrial latitude, eleva-
tion of the pole, and zenith distance of the equinoctial,
are each 51° 30'. The zenith distance of the pole,
elevation of the equinoctial above the horizon, and its
depression below the horizon, are each 38° 30'.

153. That i)art of the heavens between the pole and
a parallel of declination the same distance from the
pole as its elevation at the place, nevet^ sets. Thus, at
London, the stars 51° 30' all round from the north pole
can always be seen on a clear night. A parallel 51°
30' from the pole is 38° 30' from the equinoctial, that
is, about 38° 1). N. If we look for that parallel on a
map of the stars, we ':nall find north of it all the stars
wUch may be seen at London. — Or, it may be said,
that the stars at a less distrjice from the pole than the
degrees in the latitude of the place, or than the north
point of the horizon, never set.

154. A like part of the heavens around the opposite

ELEMENTS OF ASTRONOMY.

p lo never rtses. Thus, at I.ondon, the stars 51" 30'
all romul from the south pole are .ever seen or all
those beyond 38° 30' D. S. ' '

holt^t J^'^^""'^ ""{ ^^'' '^y ^^"""^'"^ ^J'« intermediate
bolt IS sometimes above, sometimes below ^he horizon

of the place. That belt extends as many eVees o"

t lat line above the horizon. Thus, at London th«
«^ars in the belt of sky from 38° 30' D. N to 38° 30'

tto; tl^'^'Tf ^^° +)' ^^^' '^^''^'^ ^bove, some,
times below the horizon. '

%ife^*-^^"' ""'" ^' understood from the following

Fig. 13

the' eirth 1 f." T"" "'■"'"• '"^ ""^ °'''1'"« '•^P'-^^ont
t,,L f T / observer on its surface, about the lati-

" •' ^'^^'^'-'^n j then Z will be iiis zenith. Let

ELEMENTS OP ASTRONOMY.

N bo tl)(5 nortli polo of the Ijoavcns, S the south polo,
ftiid let K Q rojtruHent iho, [)l!ino of the efpiiuoclial ; the
imrt where it erosscH the cartli (e q) will represent the
e.'irth'H etpiator. From K to N will be 90^ and from
Q to N also [Hf. From S to E and to Q will be the
sutne nund)er of depfrees, niakiii'^' IM'Af all round.

158. The dotted Tum! a o will be the sensible hon'zon
of the observer at n ; the points a and o being the parts
of the sky below whieh ho could not see tho heavens
for the earth interposing. Let II K be a plane par-
allel to that of the sensible horizon, but passing through
the centre of the earth. It is i)lain th.it, if the inner
circle representing tlie earth were smaller, the place of
the observer, Wj^and also tho line a o, would be propor-
tionably nearer to II K; and that if the space in tho
figure occupied by the earth wore reduced to a mere
ponit, tho lines (or [)lanes) a o and II R would
coalesce. Now this is actually the case with respect to
tho horizon of any [)lace on the earth and tho starry
hcavena. Tho distance from the earth's surfMco to its
centre is as nothing^ — a more point — in relation to tho
distances of tho stars; and hence, in relation to them
there is no practical dilTf'erenco between tho sensib' '
horizon a o, and a plane parallel to it passing through
tho earth's centre, which is called the Rational Hori-
zon, and represented by the line H U in the figure.
We may therefore reason with respect to tho starry
heavens and tho positions of the earth in relation to
them, as if tho observer at n were at the earth's centre
0, and the distances a H, o R in the sky, and n
reduced to nothing.

159. H and R being tho points whore the horizon
meets the sky, the distances from Z to H and to R, will
bo 90° each.

160. From Z to R being 90°, and from E to N 90°,
taking away tho arc Z N, which is a part of each, there
will remain the arc N R, the elevation of the pole,

CtBIIENTS OP ASTRONOMY.

equal to tbo arc Z E, the zenith distanro of the eoui-
rjoctijil; w'ich i^ in manifost in the 8ani(3 mi in I. er of
de^frces in tho Cflcslinl nioridlan as n e on the terrestrial
meridian, tvhich is the. latitude of n.

IGl. Since 1[ PC, EN, and N Q are 90' each, by
t-^ -111^ EZ from each of tlio first two, and tho eniiul
a.c .i W from the last, there remain E H, the elevation
of tho equinoctial above tiio horizon, Z N, the zenith
distanco of the i)ole, and U Q, tho depression of tho
equinoctial below tho horizon, all equal to each other,
nnd equal to the diirerence between tho elevation of the
pole and 90°.

1G2. Now, in considering tho apparent daily rotation
ot tho sphere of tho heavens, so far as relates to the
remote fixed stars, we may re<(ard the observer at n as
It ho were at 0, and nis hor/'on H R as shutting out
from his view all below tho lino H K. Also, tho
points N and S, the poles of the heavens, maintaiii the
same places. Hence, in rotating, all the stars from N
by o, R, Q, and h, to S, will in 12 hours have come to
like distances from N and S on the other side of these
points along the lino N Z E « II S ; and stars on tho
latter line will be on the opposite line from N by Q to Z.
1G3. A star at r (the same distance from N as R)
will in 12 hours be at R, just on the horizon ; stars nt
K will have been elevated to r; and a]I north of these
points will have continued above the horizon lurino- the
whole rotation j that is, always to ihe observer at tiie
place n.

164. The stars from R by Q to h will in 12 hours
come to the position r E II, any star i.t h just appear-
ing upon the horizon at H ; and the stars from r to H
then sink below the horizon, as from R to h.

165 The stars, from A by S to H, in the'rotation of
tbe^ celestial sphere, evidently cannot rise above the
horizon at all. They are never seen at tlu latitude of n.

166. It mav easilv be

sli

«x tiJUL

the i

._._ CI 11 Q

aiUH (O XI, D il

'»

ELEMENTS OP ASTRONOMY.

nro each eqiml to E Z or N R ; and that the nrc Q h is
t'ciuul tocftcli of tho arcH K If, Q H, or Z N.

1 67. ThuH, at the latitude of n, the part of t.ie heavens
from r by N to H, never HetH; tho part from U to /i, or
r to H, iH Hoiiietimes above, HomctimeH below the horizon •
the part from H by 8 to A, is never above the horizon.'

1G8. At Londo!!, Vega just skirts the horizon when
at tho lowest point of its daily course; and Capella, in
the opposte (piarter of the heavens, at its lowest point
IS alK)ut 7° above tho horizon; so that these two very
bri^rht stars are almost always visible in Great Britain
at about from 50° to 45° from the north polar star. '

109. The motion of the earth round the sun, by which
wo undergo a change of place to the extent of no less
than 184 millions of miles, makes no sensible diflereneo
.i tho relative positions of tho earth and fixed stars.
Ihat enormous distance is but a mere point in com-
parison with the distance of the stars. At all times of
thv) year, the polo of tho heavens is in the same relative
position to every place upon earth.

170. It will be observed, that though tho stars in
their daily rotations preserve the same relative positions
at ench place, they arrive at these positions at dillerent
times of the day ; so that stars which are abovo the
horizon during night at one season, an; below the
horizon during night, and cannot bo seen at another
season. This arises from the time of one complete
daily rotation of the starry sphere being a little different
from the time occupied by the sun in his apparent daily
revolution round the earth, which is called a solar day
and by which tho periods of night and day, and our
divisions of time, are regulated.

171. Upon considering the relations of the various
parts of the earth's surface to the starrv 8»)hcre during
the apparent daily rotation of the latter,* it will be found
that at the poles, only one and the same half of the
btarry heavens will be above the horizon during the 24

ElBMENTS or A8TimKOMY.

J,onr«-tl)at bcmiNphcro north or 8ou*h of tho oquinoctinl-
jUHl thatut the equator, the Hpectntor will Imve a\mve
hi8 horizon in tho course of 24 hours tho whole of the
sphere or the l.ouvon£-tho poles of the heavens being
in his horizon. T he whole of the 8tarH are hroudit
muler h.e view witnin the night of 12 hours; one-half
at ho beginning of that period, whilo the opposite half
will be above his horiz.)n at tho end of that time. There
alone man may enjoy, during the ' ht of 12 hours, tho
magnificent spectacle of tho whole u tho sphere of the
heavens. '^

ELEMENTS OF ASTRONOMY.

PAUT II.

LEADING PHENOMENA OF THE EARTH, SUN,

AND MOON.

SECTION I.
1. Definitions.

172. The Poles are the extremities of the earth's axis (83).

173. The Equator is an imaginaiy great circle round the
earth, equidistant from the poles.

174. Parallels of Li\titude are small circles round the earth,
parallel to the equator. A parallel is called the parallel of any
place through which it passes. is the circle which each
place describes by the earth's dail_\ station.

175. Meridian" Circles arc sr»;at circles round the earth, pass-
ing through both p des. Each cuts the equator and every
parallel twice, and divides each of them into two equal semi-

Circles.

176. A Meridian is that half of a meridian circle hetween the
poles. It is called the meridian of any place through which it
passes. A meridian is so called from the Latin word meruhcs,
midda) ; because it is midday at a place when the sun is on its
meridian. ,

177. The sun is said to be on the meridian of a place when
he is in the plane of its meridian above the horizon ; that is, at
the greatest elevation in the sky, which he reaches at that place
at that time in his apparent d?ily motion.

178. The meridian continuous with or opposite to the meri-
dian of a place, is called the opposite, or inferior, or lower meri-
dian of that place. It is the other half of the meridian circio
passing through the place.

179. Those, who live on the same meridian have midday at
the same moment, midnight at the same moment, and their
time corresponds. ^ . ,. . xi

180. Thus, the earth is divided by imaginary lines ni the
same manner as the sky ; but the names applied to them are
different.

1
(
1
(
t
f
c
t
t

i
n
I

ELEMENTS OF ASTRONOMY.

The following table shows the

Heavens.
Poles.

Equinoctial.

Pamllels of Declination,
llour-circles.
Declination.
Kight Ascension,
lluur-circle through first point
of Aries.

corresponding terms : —

Earth.
Poles.
Equator.

Parallels of Latitude.
Meridian-circles.
Latitude.
Longitude.
Meridian of Greenwich.

181. The same fi^^jj, ^jji ggj^g ^^^ illustration. Let Fi?
lU, page 26, be now viewed as a representation of the earth
JN and b will be the north and south poles; E Q the equator-
all the lines from N to S meridians; the curved lines crossing
latiUuir'' *"* equator, as a b, k o, are parallels of

182. latitude is the distance of a place rorth or south from
tlie equator. It is measured in degrees, minutes, etc., along the
meru ban of the place— the line going north and south through
the place—and is marked on the parallels at the sides of the
map. In I ig. 10, the line c II is the N. latitude of C.

183. Longitude is the distance of a place cast or west of some
agreed-on meridian, as that of Greenwich for the 13ritish, of Paris
lor tlie brmch, etc. Longitude is measured in degrees, minutes,
etc, along the parallel of the place-the line going due east and
west from a place. It is marked on the meridians where thev
cross the equator, or at the top and bottom of the map Thus
m l<ig. 10, if the straight line N S represent the meridian of
Greenwich, the point r is in 45° E. L. (east longitude), as found
by tracing its meridian to the equator, under which, in that
l^gure, longitude is marked, e is in 60° W. L.

184 This gives us longitude and latitude in degrees, but does
not tel us the distance. To find the distance of a place north
or south from the equator— or east or west from the first meridian
we must multiply the number of miles in a degree of latitude
or longitude by the number of degrees. In a degree of latitude
there are 69 miles and 78 yards; but as the parallels decrease
from the equator to_ the poles, the degrees of longitude are
different at every latitude, and we must know the length of
t le degree of longitude at different parallels, to bo able to find
tlie distance between places expressed in degrees of hmgitude.
Ihis IS to be found m tables in must geographical works. At
tlie equator, a degree of longitude is 69 miles, 278 yards: 64
miles at 23*° N. L._43 m.ilcs at T-Hlon_38i mile/at Ldin-
burgh— 28 miles at 66^ N. or S. latitude; while at the poles

!l '

ELEMENTS OF ASTRONOMY.

where all the meridians meet, the degree of longitude is reduced
to nothing.

185. The earth is 7925 miles in diameter at the equator — the
widest part — 7899 miles from pole to ])ole. Hence, at the equa-
tor, it is nearly 4000 miles from the surface to tlie axis ; while
this distance diminishes towards the poles, where the axis
comes to the surface. This must be borne in mind in consider-
ing the effects of rotation.

186 The tropic of Cancer is the parallel 23i° north of the
equator. It is the furthest north parallel at which the sun is
vertical (05-G).

187. The tropic of Capricorn is the furthest south parallel at
which tho sun is ever vertical, and is at 23j^° S. latitude.

188. The part of the earth between the tropics is called the
torrid zone : it is 47" in breadth, and is the only part of the
earth's surface in which the sun is vertical. And the height of
the sun at any place is less, as the place is further from this
zone.

189. The parallel 23^° from the north pole, or G6J° N. lat.,
is called the arctic circle. The same parallel in respect to the
south polo is called the antarctic circle. These are sometimes
called the i^olar circles. See par. 207.

190. The parts of the earth's surface around the poles and
within these circles, are called tho polar or frozen regions, north
and south.

191. The part of tho earth's surface between the tropic of
Cancer and the arctic circle is called the north temperate zone.
The corresponding part in the southern hemisphere is the south
temperate zone.

Fig 14.

i IT

.M, s

192. In the preceding figure, if N and S be the north and
south poles, and A E the equator, then C n will be the tropic of
Cancer, c p the tropic of Capricorn, d e the arctic circle, and m, o

HLKMKNTS OF ASTRONOMV. 49

SECTION II.

DAY AND NIGIIT-CLIMATE-SEASONS.

1. Day and Night.

of h!?; ^I'"' ''Ti''"^ ^"'^ '^^"^^^^ alternation of a period

sun IS above the horizon of a place- nirZ'^ tZ r i
when tlip Qiin ia h.j ^1 1 ^ ' «?////?, the jjeriod
vvncn rne sun is bcluw the horizon of the nlifi nnrl
darkness prevails. ^ '^^'^' ^"^

11)5. The change from night to day and day to nio-ht
akes place in the following manner :_S sun fn
ghtens only that half of tife earth's surface wrdi is
turned towards him while the other half is in darWss
-and as the earth, by its rotation on its axis sue'
cessively presents different parts of its siirfLp S ^
turns them from the sun, th\.se pl'tf muSe^
nately light and darkness, or day and night.*

Proportions of Day and Night at Different

Places.

197. At the equator, th^ ^daygnd^^^ .^^e of

part of the earth's surlace is at som^ SSturSSj t'o wS^tt lun.' '^''^

ELEMENTS OP ASTRONOMY.

equnl length during tho whole of the year, and there
is a day and a night during each rotation of the e*irth
on its axis, i. e., in every '24 hours ; the day and the
night being 12 hours long each.

198. At the Poles, the day and the night are of
equal length during tho whole of the year; — hut there
is only (me day and one night during the year, each
being six months in duration (between March 20 and
September 23). The dreary winter of the polar regions
is relieved by reflected light or twilight, by auroras,
and by the full moon, which shines upon those parts of
the earth where the sun is below the horizon.

199. At other parts of the earth's surface the dura-
tion of day and night is different at different places,
and at the same place at different times. The parts
north of the equator {northern hemisphere) and those
south of the equator [southern hemisphere) are always
in exactly opposite conditions with respect to day and
night. At corresponding latitudes, north and south
(that is, at latitudes equally distant from the equator),
the one's day is equal to the other's night — the one has
short day and long nighty when the other has long day
and short night.

200. At two periods in each year, there is equal day
and night over all the world ; — and a day and a night
during each rotation of the earth. These two times are
termed Equinoxes (110). They are, March 20, the
spring or vernal equinox, and September 23, the autum-
nal equinox.

201. From the equator to latitude 66° 32' north, and
to the same distance south — that is, in the torrid and
temperate zones — the day and night are unequal at
different places (excepting at the Equinoxes), and at
the same place at different times : — and in this region
there is always a day and a night during each rotation
of the earth on its axis. These latitudes, it must be
observed, are -o -b distitnt iroin tuOi poles.

i
i

1
c
a

r(
fi

ELEMENTS OF ASTRONOMY.

202. From latltiulo GG' 32', north and Roiith, to each
pole—tliat IS, within tho polar circles— the day or
period of sunshine, durin- part of the year, continues
for several rotatin^, of the earth on its axis;* the time
the sun remains at ve the horizon varies from nothin-
to SIX months ; the day is extremely long when there
hrst comes to be a day and a ni-ht during each rota-
tion; and the proportions gradually change till there is
continual night during several rotations— and so on.

Manner in which these Differences occur.
203. The line between the dark part and the enlight-
ened part of the earth's surface is called the Terminator
It may be called the boundary line between night and
day. It forms a great circle (79), all round the globe,
and divides the earth's surface into two equal hemi-
spheres-one dark, where it is night ; the other receiv-
ing the sun s rays, and liaving day. In Fig. 14, par.
lyj, the line t v, bordering the shaded part, is the ter-
nnnator. The sun's rays are perpendicular to the plane
ot the terminator : as may be seen in the figure.

204. The terminator is on each spot of the earth
which has both day and night at two periods durino-
each rotation ; first, when it is sunrise, and, again, when
It is sunset at that place ; and the sun is on l^e merid-
ian of a place, or there is midday, when i.. place is
equally distant on both sides from the terminator. The
length of the day and of the night at diiferent places
depends upon the proportionate lengths of time which
are spent on each side of ihe terminator.

205. The sun is always perpendicular to some part
ot the earths surface, e.e.,is always in the zenith at
some place ; and the terminator will be 90° distant all
round,- from the spot on which he is vertical at anv
iime. ^

That is, for several dayi of 2i hours, the sun never sets.

ELEMENTS OP ASTRONOMY.

206. Diirinnr ono rotation of the earth on its axis
(disregarding the earth's motion in its orhit during the
time of one rotation), th'i ])arts successively brought
round to be perpendicuhirly under the sun will be
those in thn same parallel of latitude. Thus, il in Y\».
15, page 53, A N E S be the earth, N S a meridian, S
at the side the sun, and n the point at which the sun is
verticil, the line d o, 8U{)posed to represent the termi-
nator, will evidently be 90° distant from n. And as
the axis, about which the earth turns, extends from N
to S, the points that will be successively brought into
the position of n by the rotation, will be those on the
parallel C n ; and the sun is then said to be vertical or
perpendicular to that parallel : for, although he is per-
pendicular only to one point of it at a time, viz., where
it is midday, he is during the whole rotation perpendic-
ular to some part of it.

Long Day, etc., in the Northern Hemisphere.

207. The sun on the ^Ist of June is vertical at the
north tropic (tropic of Cancer), which is only 66° 32'
from the north pole ; accordingly the terminator will
then be 90° from that parallel, or 23° 28' beyond the
north pole ; that is, reaching to the arctic circle. In
fact, the polar circles are the parallels at the greatest
distances of the terminator from the poles. In revolving,
therefore, that pole and all within the arctic circle (the
parts around it for a distance of 23° 28'* south) will
have continual day ; for they will never cross the ter-
minator in their revolution. The sun will not set there,
but describe a circle near the horizon varying slightly
in Us elevation. The parts further south, from lat. 66°
32' to the eq^uator, will describe more than half of their
daily circle on the enlightened side of the terminator, and
have longer day than night. And the proportion on

-•■/WtaMtaMB;''

ELEMENTS OP ASTKONOMY. 53

«k,i!''!r'rM''"'" "^ '^ '«,™in«tor will dimhmh as tho
pluc. i« further south, till, at the equator, an equal

s^le o"f tt '° "'""."^ '"'f "" "'" '« "P""' <«• «'<-'h
be e,^,„l "•■'•"""'"<'>■' "-"l «'«= d'^y and the night will

tl.i'f, n '"■'"' r '" '"' ''''"" '""'"Stood by referring to

tiark and dlu.nnied jmrts when tho sun is on the tronio
of-0«„eer. Let A N K S represent the earth, Nt'ho

he r,Vht 'thf' ' "'T\'"' 4' "" « » --Wian, S a
tlie right, the sun, rf A o the part of the e^rth lot

pole C „ ,1 ,^'; "^"%T'"' '^° ^»' fr"-" ">« north

to, ' It wM ll '"^"^ ^''?'"' ^^° ^«' f™"' "'0 equa-
tor. It will be seen from the position of the sun that

Flf. IS.

tte north pole is turned towards it, and the south pole
A I hZr,t n,=^-"^ r'"'' '" "^^olving, will take twelve

fil tZdi^TT^r'r ""' '•"'o "f N S to the other, as

again and the middle of each twelve hours will be
where the point crosses N S. Now, the earth revolvin '
about a line from N to S, it will be at once evident that

I . 1 C tv" ow-''.'"'- °"' "^ ""^ ™"'^ "y^- ""<• ftere-

i.,ie ..avc vOiistauD any; tliat any point from d e to

ELEMENTS OP ASTRONOMY.

A E the equator, will bo more than half tho period of
rotation without the terminator ; that it will bo a less
proportion of its course without the terminator, us it is
further south from d e ; that any point in A E will
s{)end half its course within, and half without the ter-
minator, as that circle cuts A E, the circle in which
such a point revolves, exactly in the middle.

209. The point Z, for instance, revolving in 12 hours
from / to /j, will have h for its midday or noon, when
nearest to the sun ; tho point A', where the circle I h
cuts the terminator, for sunrise ; and I for midnight.
As the period from sunrise to noon (from k io h) is
longer than that from midnight to sunrise {I to k), so
likewise the time from noon to sunset is longer than
that from sunset to midnight : — and the whole day of
that point will be longer than its night.

Short Day in the Northern Hemisphere.

210. On the 22d of December, the sun is perpendic-
ular to the south tropic (tropic of Capricorn), which is
23° 2W south of the equator, or 1 i3" 28' (23° 28' + 90°)
from the north pole. Accordingly, the sun will tlieu
be 90° from the arctic circle, and the terminator will be
23° 28' on this side of the north pole. In revolving,
therefore, that pole, and all within the arctic circle
(the parts around it for a distance of 23° 28' north),
will wave continual night, for they will never cross the
terminator, but just skirt it in their revolution. The
sun will not rise there. The parts further south, from
66° 32' to the equator, will describe more than half their
daily circle on the dark side of the terminator, and have
longer night than day. And the proportion on the
dark side of the terminator will diminish as the place
is further south, till, at the equator, an equal portion of
the time of rotation will be spent on each side of the
terminator, and the dav and the niirht will be cnual.

EI.EMKNTS OF ASTRONOMY.

• ?/^r '^1'" *''"''''' "^ Capricorn, latitude 2.'^ 28' south
IS the urthcHt Houth parallel at which the sun }>econ es
ver ncal. At places farther south, he never reaches to
/enith — never appears perpendicuUir — and appears
lower as the ph-iee is farther south. ^^

212. This vyill be better understood by reference to
tlie ad,<„nn,g hgure ; which illustrates tlu) relation of

Fig. 16.

the dark and illun.ined parts when the sun is on the
ropic of Capricorn. The same letters indicate t le
arts as in the former figure (page 53), excepting those
utthe terminator, w])ich is now from e to n., complete y
enveloping in the shade the north pole and the loZZ
around i for 23° 28' south. Within the arctic circle
lere will be continual night. From that paral to
the equator A E, any point, in its daily rotation, will
pass from midday to the terminator before it has gone
through half ot Its circle, and will therefore have onger
night than day. It will be enveloped in the dLk ImTf
sooner after noon, and have longer night, in ^rinor io
as It IS nearer to the pole. And as the tWmfnaTor I"
stili cuts the equator in two equal parts, there w^Il be

point /'r "^' r^'"' f ^ /^" ^^'"''^- ^^1-MhV ame
-ii-i.n„ior, 1,. a^uTv uuiii six nours, and to be less than

£6

ELEMENTS OP A8TR0N0MV

Hix lioura in revolvinjj from k to A, its middny. From
midday to Runso twill also bo less thuii nix hours ; and
its whole ni'^'ht must be greater than its day.

213. Thus, the noithern homiH[)here, with respect to
the «lurk part of the earth's surface, is placed on Decem-
ber 22 (in Fi^. !(>) in exactly the same position for-
merly illustratotl with respect to the illumined part.

State of the Southern Hemisphere.

214. As already mentioned, the two hemispheres aro
always in exactly opposite conditions in regard to day
and ui;j:ht, except at the time of the cipiinoxes. This
is at once seen by inspection of Fi^nires 15 and 1(5.
When the sun is on the north tropic (Fig. !'>), and Iho
north polo is entirely in the illiiinined part, the south
pole is entirely in the shade. When the snn is on tho
south tropic (Fig. Ifi), the north pole is out of the
reach of his rays, and tho south pole is never out of
them, etc. At tlie e(|uinoxes, when the sun is vertical
at the equator, and tho terminator passes through both
poles, each hemisphere is situated in tho same position
with respect to the sun's rays, and they are therefore
in similar conditions as to day and U'ght.

Equal Day and Night over all the Earth.

215. At the c(pilnoxes, tho sun is vertical at tho
equator, or appears in the zenith there, ayid the termi-
nator^ always 90° distant from the parallel at which the
sun is vertical, passes through both poles ; and its plane
passes thiough the earth's axis. Then, there is equal
day and night over all the earth ; for the terminator
will cut every parallel equally, and every part will
spend half of its rotation without and half within the
terminator.

216. This is illustrated by the following figure, whicli
exhibits tho state of the world with respect to dav and

, I

, 1

ILEMENT8 OP ASTRONOMY.

night when tho mn is or. tlic cquntor. The biiu la soct,
|.crpc.,Khcuhtr to the earth ut the equator, the e „ nnt

wlucli the sun 18 vertical. *

FIf. 17.

^K-

c1..k iJtlf N r"s^ '' '\ '- '"^^'"^ '^^'-^^ '-^ P«'"t in the
noon h, vvill cross tho terminator at /L- exactly half way

8.imc w 1 take place m passm^^ from h back to I; and
ti.e ni^ht and day must therefore bo equal on every
part ot the earth's surface. ^ ^

218. At the poles at this period the sun will appear
p move round the horizon ; and, as he thus nei her
rises nor sets, there will be continual day there 1 or

ltV:7 ""'W' ''^^-'- -fraction and ^^.flection 'nZ
long he day, there will be continual day there even
though he be actually a little below the horizon. '

Of the Change in the Length of the Day.

iJl^\ ^- "^ '^'''"^' ""^ *^^' ^^""Sth of the day occurs in
he following manner :-When it is said that the sun

invl%f"\ '■ '' '"^ r^'^^"^^' ^' i« meant t at he
290 wT ''.1^ ^''*^'-"^ '" ^*' '' ''' ^^^"^ ^^''tical to it.

-_-, 1.. ... a. tliu laitbctii north paiuilei which he reaches

c 2

ELEMKNT8 OF ASTRONOMY.

(Fig. 16), and from that trav.'ln south from n to E ar.<l
P ; — l>y this, tjjo termi'intor Hh)wly wht'cls round from
do (Fiij. 15) to N 8 (Fig. 17), atid then onwards in
the same direction till it reaches e m (Fig. IG). This
brings more and more of the northern hemisphere into
the hhadc, so tlint each piirt in revolving is more of 'ts
time on the dark side of the terminator, and its day be-
C\*mes sliorter till the terminator comes to e m. Thero
the sun ceaees its motion south, and begins to retrace
his steps from p by E to n, which causes the ternjinator
also to retrace its course, and move back from c m by
N S to </ o. This gradiuilly brings more and more of
the northern hcmisplu're into the illumined half, so that
each pai, in i-evcdving is less of its time on the dark
side of the terminator, and its day lengthens till the
terminator comes to d o, when it begins to move back,
and the same series of changes occur again.

221. The exactly opposite series of changes goes on
in the southern hemisphere.

222. This is partly illustrated by Fig. 14, i)ago 48,
whe»-e the sun is represented on its way south, now per-
pendicular to a point farther south than the tropic of
Cancer, and the terminator has advanced to ? v, towar Is
the poles, and receded from the polar cireles. Thus,
the day and night will not differ so very much as when
the sun is on the tropic, and a smaller circle round each
pole will have constjvnt night or constant day.

The folhowing is a summary of these changes.

223. The daily circle through which each person
passes in consequence of the earth's rotation, is his
parallel of latitude ; and the proportion of his night to
his day at any time depends upon the manner in which
that parallel lies as regards the Teriainator (see par.
203).

224. If, in rotating, the terminator docs not cross his
parallel at all, he will then have no daij^ or no nighty
according as he is on the dark or illumiued side of the

'*JIMeNT8 OF ABTUONOMY.

par.

torminator; ifthc teniiliiatnr cuts liis parallel unequally,
he will Imvc his day and ni^rht lUHMiual at that time , hut
if It cutH his parallel into two equal parts he will thon
have equal day and night.

225. As every ^reat circle on a sphere cuts every
other ^reat cirele into two equrl parts, the terminator
must alwjiys cut the equator into two semicircles, one
dark, the other illumined, so that day and nif/ht at the
equator arc nlwat/s equal, or, each is of 12 hours' duiation.

22(). At 2()th March and 23d September, the sun is
vertical at the ccpiator ; so that the terminator passes
through hv)th poles and cuts everi/ parallel into two
equal parts. Hence, there is e(pial day and night all
over the world at these periods, called the Equinoxes.

227. At other times, the sun is vertical at some
point north or south of the equator; the terminator
then extends beyond one pole, and falls sh/yt of tho
other pole. Some parallels aro not cut by it at ail;—
at these there is no day or no night •— tlu other par-
allels are cut unequally, and at these, day and night are
unequal.

228. On Juno 21, the sun is on tho north tropic,
the north hemisphere is most turned towards the sun,
and it is iherefore longest day north of tho equator, and
shortest day south of it. The day gradually shortens
in the northern hemisphere and lengthens in the south-
ern till the 22d December, when the sun is on the south
tropic, and the southern hemisphere is most turned to-
wards the sun, pnd it is longest day in the south
hemispheie, and shortest day in the north hemisphere.
From this position the day gradually shortens in the
southern hemisphere, and lengthens in the northern
hemisphere, till the 2 1st of June, when the same series
of changes recommence.

229. Day and night are more nearly equal in pro-
portion as the time is nearer to an equinox^ or the plc^.e

, ELEMENTS OF ASTRONOMY.

i f

t 1'

230. From the arctic to the ant.irctic circle, that is, in
the torrid and temperate zones, there is always some
day and some night during each rotation (every 24
hours), liovvever unequal tl)ey may be.

231. Within tlie polar circles, at one time there is
both day and night in each rotation,— at anotiier, no
day, the snn remaining below the horiz':^n for several
rotations together,— at another time no night, the sun
remaining above the horizon for several rotitions to-
gether.

232. At the poles, there is six months day, and six
months night.

233. The northern and southern hemispheres are
always in exactly opposite states, at corresponding lati-
tudes north and south, in respect to day and night.

234. Thus, the sun oscillate s backwards and forwards
between the tropics, and the terminator oscillates be-
tween the opposite sides of the polar c-x-cles. When on
the tropic of Cancer, the sun is highest above the hori-
zon to those north of the equator; on the tropic of
Capricorn, highest to those south of the equator. When
on the equator, the sun appears at the same elevation,
at corresponding latitudes north and south.

Causes of these Differences and Changes.

235. Tliree causes unite to rjroduce these differences •
in the le ,^th o± day and night at different places and
different times.— 1. The earth's annual motion round
the sun. 2. The earth's axis being inclined, and not
perpendicular, to theplane of its orbit. 3. The earth's
axis remaining always parallel to itself in all parts of
its orbit. These causes place the earth and sun in suc-
cessive relative positions, which give rise to these
changes.

236. From the axis of the earth being inclined to
the plane of its orbit, one pole leans towards the sun
at one period, Vvhile the other is tamed from the sun ;

ELEMENTS OF ASTRONOMY.

when the earth has moved fropi that point round one
quarter of its orbit, the axis will be placed sideways
with respect to the sun, and en.ch pole will be equally
turned towards tlie sun. A.s the earth advances and
completes another quarter, the poles now reverse their
relative positions ; the pole formerly turned towards the
sun is now turned from it; and the other leans towards
the sun. On completing another quarter the axis will
be again placed 'ewr>s towards the sun. As the earth
proceeds onwar it gradually gets into the position
which it occupied at first.

237. This will be better illustrated by the following
figure. Let the twelve circles represent the earth in

A Fijj. 18.

tTT,l»lxrp rl 'flV.r.rt'J-'f ■•-.T.io. r^f^i^ -«,,,.'--^ V-" ' Jl- T

ELKMENTS OF ASTRONOMY.

the lino n s represent the axis of the eartli or a meridian
lino, n beinn; the north pole, s tht sontii polo. The
strai^'ht edge of the shaded part will represent the termi-
nator, or the plane of the -minator, seen always perpen-
dicuJar to the sun's rays, hut varying its position with
respeet to the axisns, which points in all the positions in
tue same direction. At tlie top is seen the position on
Decemher 22, the north pole within the dark half, and
turned from the sun, the south polo in the illumined
halt, and leaning towards the sun. At the left and
right, the positions on March 20 and September 23 arc
represented, the terminator passing through the north
and south poles, coinciding with a meridian line, and
the axis lying sideways towards the sun, so that each
pole IS equally under the sun's influence. At the bot-
tom IS seen the position on June 21, the north pole in
the sun s rays ; the south pole in the shade.*

238. The following figure will also illustrate the state of the
dhlcrcnt parts of the earth at difRrent seasons in regard to light
and shade. The terminator is distinctly seen, the gently curved
line between the dark and light parts. The north pole is shown
at the upper part m each of the four positions, with all tlie
meridian lines radiating fro.n it. At the top it is seen entirely
enveloped in darkness, and so tliat the earth's rotation about its
axis does not bring it all out of the shade. At the sides the
terminator is seen passing through both poles, the axis lyinj?
sideways towards the sun. At the bottonf. the north pole and
regions around it are seen entirely in the illumined part, havin-
continual day. J he upper position represents December 22!
the lower June 21, the solstices; ~ih^ two sides, March 20
and fceptember 2;], the equinoxes. '

239. Tho date at each position in Fig. 18 shows the
period when the earth gets into each position ; and the
accompanying character is the astronomical mark for

nvi«1r?H I ? . • ^'l^^ °^M^'' prece.hns fi.^'ure, nrnl of tlio inclination of the
HMs to the te nnin.i or, will convey a precise idea of tlie various cl.an-es
J I.e h^n.re .s irregular as a dra>viug, being a mixture of plan, sc^H, , an'i
persp<.ct.ve but gives a c ear view of the cflTect of the ear h's notion romd
tiie suii, on the relation ot the terminator to the diffcreul henu"iheres

ELKMKNTS OF AHTKONOMY.

tlie sign, OT part of tlie zodiac in which the sun appears
at the time of tl)c earth entering into each position.
Of course, the earth, as seen from tlie sun, wouhl appear
iu the opposite sign.

Fig. 13.

240. Thus, the effect of the earth's motion round the
sun is to make the latter appear to oscillate backwards
and forwards between the highest and lowest positions,
shown in Figs. 15 and 16— that is, from being perpen-
dicular to the tropic of Capricorn, c p, 23° 28' south ot
the equator, on December 22,— to being perpendicular
to the tropic of Cancer Cn, 23° 28' north of the equator,

ELEMENTS OF A3TnONOMY.

on June 21, crossing the oqnator twice during these
osculations, at March 20 and September 23.*

241. The sun is about 7 days, 16 hours longer in
the northern than in the southern half of the ecliptic ;
being about 187 days in the northern signs, 179 davs
among the southern signs. *'

242. The following table shows the length of the
longest days in different latitudes, from the equator to
the poles : —

0°

(E(

Hours.

Hours

. 12

65"

48'

. 22

. 13

. 23

. 1A

. 24

. 15

Months

. 16

. 17

. 2

. 18

. 3

. 19

. 4

. 20

. 5

. 21

(Pole)

. 6

2. Climate.

243. The Climate of a place signifies the prevailing
character of the weather at that place ; that is, the
temperature, moisture, atmospheric pressijre, winds,
electric condition of the air, etc. These, as is well
known, are very different at different places.

244. The leading circumstances which determine the
character of the climate are,— 1. The latitude of the
place ; that is, its distance from the equator : 2. The
height of the place above the level of the sea : 3. Its
position with respect to large tracts of land and water :
4. Oceanic currents: 5. The character of the prevailing
winds.— -Only the first of these can be regarded as aa
astronomical cause ; and it alone requires consideration
in this work.

From o to o, Fig. 10, page 2G ; and then back from o to

ELEMENTS OF ASTRONOMY.

245. The climate is wvrmost about the equator, and
becomes gradually colder as tlic place is fartlier north
or south from the equator, that is, as the latitude
increases.

246. This rule is only true generally^ and of largo
changes in latitude. Considerable deviations from it
are produced by the other causes.

247. The position of the different latitudes, in respect
to the sun's rays, is the cause of these differences in
climate.

248. The heat at any place is in proportion to the
number of the sun's rays which fall upon it ; and the
number of rays which it receives (other things being
equal) depends upon the direction in which they fall.
Any surface receives more rays tb*^ more perpendicu-
larly they strike upon it, and less, in proportion as the
rays fall more obliquely ; that is, as the angle they
form with it is further from a right angle.

249. This may be illustrated by the following figure.

Let R and R^ be rays proceeding in the direction from
S towards T, and falling upon the equal surfaces A B,
A C, A D, A E, A F, etc., all lying in different direc-
tions. It is plain that more rays fall upon tlie surface
A B, which is perpendicular to the direction of the rays,
than on any other; that A C, which is nearest to the
perpendicular, receives more than A D, A D

tiian A E, and so on ; more rays being received in

ELEMENTS OF ASTHONOMY.

})roportion as the surfuce is nearer to being pcrpeu-
tlicular to ll'.e mys.

250. As the Klin osoillatos between the tropics, con-
tinually vei tieal to some parallel in the torrid zone, liis
rays always fall [jerpendicularly at some parallel between
the tropics, and less so as the parallel is further north
of the north tropic, or south of the south tropic.
Accordingly, more rays are received in a given space at
the torrid 2one, than in an equal space north or south ;
so that the temperature is always higher there than
anywhere else. And as fewer rays are received in
proportion as the place is further to the north or south,
the heat must diminish in these directions.

251. This is illustrated by the following figure. Let
S represent the sun, E the earth's equator, c, d the

Fig. 21.

tropics, «, h the polar circles, N the n.irth pole. It is
seen that the earth's surface at the equator is perpen-
dicular to the sun's rays, the zone between the tropics
more nearly perpendicular to the sun's rays than any
parts north or south, while all the other parts are much
inclined ; that they are more inclined as they are
furAer from the equator, the rays towards the poles
only skirting the ground.

252. The air absorbs part of the sun's rays, — little in
the upper strata, but a considerable portion in the dense
lower strata loaded with vapour. Hence, from this

ELEMENTS OF ASTRONOMV

cause also, loss of the sun's rays strike upon a place in
proportion to the quantity of atniosphero t]irou«,^h which
they pass, and in proportion to the density of that
atmosphere. Perpondicular rays pass throu^di least of
the air before coming to ihe ground. Oblique rays not
only pass throng i more air, but tiirough a larger pro-
portion of the dense parts, so that a much greater por-
tion is absorbed before tlu>y strike the soil.

253 Th(^ diminution of the mean temperature in passing from
the equator to the poles, so far as this is regulated by the solar
influcii ;, la in proportion to the square of the cosine of the
latitude. The change is, therefore, slight from the equator
towards either tropic, greatest about lat. 46", and slight from
the polar circles to the poles.

3. The Seasons.

254. Tliat regular alternation of different kinds of
weather which takes place at any place during the year
is termed change in season.

255. The same causes which give rise to the change
in the length of the day and to diflerences in climate,
produce the change in the seasons. The sun imparts
more heat in proportion ; 1. as he is higher above the
horizon of a place, and his rays fall more perpendicu-
larly ; 2. as he is longer in the day above the horizon
of a place. In the northern hemisphere, the sun rises
higher and ^ remains longer above the liorizon, from
IVIarch to September : we have warm weather, or
summer, then. From September to March the sun's
rays fall in a more slanting direction, and he is a
shorter time daily above the horizon : there is cold
weather,^ or winter, then, in the northern hemisphere.

256. The southern hemisphere is in exactly the
reverse state then — summer during our winter, winter
during our summer.

257. This is illustrated by Figs. 15 and 16. In
I ig. 15 the sun's rays fall more perpendicularly on the

ELEMENTS OF AbTKONOMY.

northern humiaplifirp, and sljintingly on tlio Routbern
hcmiRphero. In Fi^. IG the reverse is seen. In Fig. 17
and Fifjf. 21, the relative position of the snn and earth at
the eqninoxt'S is shown — the snn vertical at the eqnator,
the rays more and more slanting as the place is further
north or south.

258. It is evident that if the axis of the earth wero
perpendicular to the plane of its orbit, each parallel
would always be turned in the same degree towards the
sun, and we would therefore have no chamje in its
seasons. It is the inclination of the axis that causes
the same parallel to lie differently towards the sun in
different parts of the orbit. Hence, in a phinet such as
Jupiter or the moon, where the axis is perpendicular to
the plane of the orbit, there can be no change in seasons;
while in Venus, where the axis is very much inclined,
the change is very great, — so great that a marked
difference prevails between the state of the equatorial
regions at the equinoxes and solstices. From this, the
equatorial regions have two winters at the solstices, and
two summers at the equinoxes. The same prevails on
the earth in a slight degree — but it is very marked at
Venus, from her tropics being so far from ner equator.

259. The sun being nearer to the earth in our winter
than in our summer, it might be supposed that the
weather should be warmer over all the world then.
But thi3 makes no di*^.rence ; for, as much heat is lost
by our more rapid motion in winter as is gained by our
greater proximity to the sun then ; and in summer,
while there is less heat from our greater distance, this
is compensated for by our slower motion.

260. The warmest part of the season is not when the sun is
highest and longest above the horizon; nor the coldest, when
the sua is h)west and the day shortest — but some weeks after
these periods. The reasons of this are, that the heat must
increase, so long as the earth receives more heat during the day
than it parts with during the niglit ; and this is tlie case for a
considerable time after the longest day : and that the tempera-

FLEMENTS OP ASTRONOMY.

turc muHt (h'creaac, m long an mor ^ heat is lost during tlio niglit
tlian iH gained during tlio day; which goes on for nevcral weeks
after the shortest day.

2G1. In like manner, the warmest part of the day is not at
noon, when the sun in highest, and his rays fall most perpen-
dicularly, but some little time after, about* 2 p.m. The reason
is, that till that period the heat received is greater than what is
lost,* so that the temperature must rise till that time, although
less new heat is received then than at no<m. And tlie coldest
time during the night is about an h* ur before Kunrise, up to
which time the earth has continually been losing heat, without
receiving any.

SECTION III.

Trade-Winds and the Tides,

2G2. There are certain uniform and continual motions of the
atmosphere and the ocean, which are produced by astronomical
causes, and are therefore proper subjects of consideration in a
treatise on Astronomy. — These are the Trade- Winds and the
Tides.

1. Trade-Winds.

203. In certain districts between the tropics, the
winds blow regularly in a north-east and south-east
direction, — north-east north of the equator, south-east
south of the equator. These steady winds nre called
the Trade-winds.

264. The trade-winds are caused by the great cur-
rents which are continually rushing from the polar and
temperate regions towards the equator, modified by the
earth's rotation from west to east.

2G5. Owing tc the great heat which prevails in the

* It must be observed that heat is at all times passing out from bodies, and
that a body's temperature depends upon the proportion between what is
received and what is given out; rising in temperature if heat is absorbed
luuie rapidly tijan Uuowii off, and faiiirig in temperature If more heat in
given out than is received.

PLEMENTS OP ASTRONOMY.

oquflt^rlkl rrifiris, the air there is ex|)nii(l(»(], and thero-
fon Kpecitically lin:htcr tlian the air further north or
Boi.th ; wliich, being colder and heavier, rnwhes towards
and dis{)hic(!S tlio warm air of the torrid zone. This
j)r()duce8 continual currents towards the equator from
the north and soul! latituch-s. Thene arc the source of
the tnide-windH, which, were there no other cause
iidlucncing them, wiiuld bo in u duo north and duo
south direction.

206. Now, the air from any place partakes of the
motion of the earth at that jdacc ; and the parts have
most rapid motion in proportion as they are nearer the
ecpnitor (being there furthest from the axis — 185).
Accordingly, any current of air which proceeds towards
the equator, will have a slower rotatory motion than the
parts to which it is tending. It will therefore cause an
apparent current in a direction opposite to that in which
the earth is moving — that is, appear to blow in an easterly
direction, as the earth revolves in a westerly direction.
The easterly direction which the wind thus acquires,
combined with its northerly direction, gives the invari-
able north-easterly wind in certain regions of the north-
ern hemisphere — combined with its S(mtherly direction
in the southern hemisphere, gives the south-east trade-
wind which is found south of the equator.

267. If we 8up[)0se the air at any place to be at rest while
the earth continues its rotation, an individual at that placo
would experience a wind in the direction opposite to its motion,
as tiie effect is the same whether the air move and the individual
stand still — or the air stand still and the individual rush against
it. Now, it will be tlie same sort of effect, though less in degree,
when the air is moving in the same direction as any part of the
earth's surface, hut xinth a slower motion — the place will over-
take it, rush against it, and cause a wind in an ojjpusite direc-
tion. If this air have, besides the slow motion in the same di-
rection, another in a different direction, the two vill bo com-
pounded into a middle course, according to the law of the
composition of forces. Thus it is with the current which pro
duces the trade- winds It has a sloio westeTli' motion, whii'l?
produces an easterly wind in a place having a rapid westerly

ELEMENTS OF ASTRONOMY.

motion ; which eaHterly wiiul, comhincd with itH north or tjouth
motion, gives the north-eaMt or Bouth-cuitt trHdc-wind.

208. Ah the distance from the iixi.s increnses very
little in the viciiiify of the equator, the rotatory inotiou
will iiiereuHe very hli^Mitly there, and there will bo little
Rfhlitioii to tile easterly «lireetion of tlie eiirreiit created
about thee<iuntor; and as the air <,'radually tte<inires
the inotiiiii of tiie partH it is over, it will have Required
the inereased motion of the equatorial parts by the time
it roaches them; the easterly direction will diminish as
they come near the equator, and the north and south
currents meeting there, the two will neutralize each
other; accordingly there is a zone of comparative calm,
or irregidar breezes, in the immediate vicinity of the
equator— ut about 3° to 10' north latitude (see par.

2G9. The heated air vvliich ascends from the equa-
torial Fi'gions gradually descends towards the earth's
surface ; and, bringing to the parts where it descends a
higher velocity than they possess, moves in advance of
them, in the same direction, and constitutes a sort of
trade-wind ; which, compounded of the westerly motion
of rotation and the motion from the equator, gives a
south-westerly direction in the temperate regions of the
northern hemisphere : a north-westerly direction in the
temperate regions south of the equator.

270. These are the leading causes which give rise to
the trade-winds. They are modified in various ways by
nuiny local circumstances, the consideration of which
belongs to physical geography.

2. The Tides.

271. By the tides we mean that regular succcl^sion
of rise and fall of the surfiice of the waters of the globe,
which is observed in all the great oceans, and in the
seas and rivers which freely communicate with the
oceans.

CLEMENTH OF ASTRONOMY.

272. ♦• It i« a v«ry rcmnrkublo operation of imtnrn, t)m' wo
obsiu-vo on tlio ghorcn of tho ocean, when, in tlio ciilii'tiKt
wrnther ftiul tnoBt gerunc sky, tho vnst body of waters that bat.io
onr confltt* udvanccM on our Hhoros, inunilathigall the flat nanda,
rising to a conHidcrablc height, and then an gradnally retiring
again to the IkuI of tlie ocean ; and nil tliiM without the appear-
ance of any cauHo to Impel tho wateru to our HhorcH, and again
to draw them ofl". Twice every day is thin repeated. In many
placcH, this motion of tho waterH in trcmendouH, the sea advan-
cing, oven in tho calmest weathei, with a hij^h siirgo, rolling
along tlio flatH with resiHtless violence, and rining to tlio height
of many fathoms." — livbiiuion't Mechanical Philoaophy.

273. When tho wators riHo to the hipfhcst point which
they roach in tho conrso of tho day, it is said to ho
hign water, or flood, and the risin*^ is called the flood-
tiae ; when at tho lowj.'st, low water, o • ebb, an<' tho
fill is termed ebb-tide. The hif,dieHt or lullcst jlood
is called a spring-tide ; the lowest flood is terined
neap-tide. The tide on tlie side of tho earth next tho
moon, is called the superior tide ; that on the opposite
side, the inferior tide.

274. The phenomena of tho tides are prodnced hy the
joint action of the moon and tho s m npon tho waters of
the ocean ; chiefly by the moon. The attractive fi)rco of
those bodies is snfliciently strong; to draw slightly to-
wards them those parts of the eartli which are movable,
and whose particles can easily be made to slide over
each other ;— that is, the waters.

Thy following are the loading phenomena of tho
tides.

275. (1.) Tho waters when at their highest, grad-
ually sink for about 6''- 12'"- when it is low water ^ and
from this gradually rises again for 6*'- 12™- when it is
high water, then sink again, rise again, and so on unre-
mittingly. Upon an average, the tide ebbs in about
10 minutes less than the time it takes to rise, and re-
mains stationary for a little at ebb and at flood. The
whole period between two successive floods, or between

IVVO SUCCSiioiVe €OUS» io iliiJUUi* Xm llUUlo «ic» lUiiiULCS. s-av

1.1. .'ENTfl OP ASTRONOMY.

10,

the

intervfti 'u>t'n\n two wiccessivo floods \h 12 hours 19
nimutcs V ind ut full moon ; 12 ho\iTn 30 minutes

«.t the qnurtt

270. Thtif 'uring every 24 hours and 50 minutes
there is I ] .iter tvv.ce and low water twice nt every
olaee, and tlie flood ia al)u..i threi-(iuurt(!rH of an hour
atcr every day Iler.ce, therefore, as tlie earth turns
halt round m that time, 12 hours, there muKt be hiirh
water at the opponite parts of the earUi's surface at the
same time— that is, at the two pl-iees having the same
m.iruiia!. circle (or on the opposite meridians, reckoned
hy tlieir nii.nbers).

277. (2.) The height of high water, as well as that
ot low water, varies* very considerably, but regularly
But, when the tide risos highest, it falls lowest, ami
when It rises least, falls least. At Plymouth, there is
sometimes a difference of 21 feet between high and low
water; sometimes only 12 feet.

278. The highest or sprmg^iide occurs once every
fortnight, and is usually about the third or fourth
high tide after new moon, and the third after full moon
—about a day and a half to two days and a half after
these periods.

279. The lowest tide,* or neap-tide, also occurs once
every fortnight, being the third or fourth high ti(1e after
the moon is in her quarters-from about a day and a
hall to two days and a half after these periods.

280. The tides gradually decrease from about new
moon to the first quarter, increase from tlie first quarter
to lull moon, decrease from full moon to the third (,uar-
ter, and again increase from the third quarter to new
moon.

• ?^^*i ^?''. "^^^^^^ ^^ ^^^*' ^ monthly period of change
m tlie height of the tides: the highest spring-tide is
that which occurs when the moon is \n perigee (nearest

* Thftt is, when tUo water falls least and rises Ic

ast.

'iki-

ELF.MENTS OF ASTIIONOMY.

to the earth) ; and the next spnn-.tidc ih the smnnest,
occurrin- when the moon is in apogee (iarthest from
the eartli). Tlie force of the moon's attraction l)em.£f
the main cause of the tides, it is to be expected that
they will vary somewhat as her dintance vanes.

28'> 4 ) The height of the tides is also aftected l)>
the foilowin- causes: the san's distance ;* the ele-
vation of the sun and moon; the latitude of the place;
the local circumstances, such as banks in the ocean,
and the form and elevati(m of the shores, channels, cur-
rents of rivers, winds, etc.

High Water at the Part nearest to the Moon.

283. The chief cause of the phenoruena of the tides
is the /ore. of the moon's attraction. The waters under
the moon (/. e, at the place where the moon is on the
meridian) are attracted towards her, while the waters
at the side^, where the moon appears in the horizon,
are also drawn towards that part, forming high water
there. As, by the earth's rotatiim and moon s motion,
the moon comes on the meridian of each place once in
24 hours 50 minutes, there N.ill be high water every-
where once in every period of 24 hours 50 minutes

284. The period of high water will not be exactly
when the moon is on the meridian and her action
strongest ; for, the impetus the waters ^^^^ received,
and the continuance of the moon's action (still strong,
thouo-h decreasing), cause them to continue rising lor
some°time after. ^The period of high water, therefore,
is usually (local causes being disregardedj Irom two to
fiiree hours after the moon is on the meridian. At new
moon, the sun and moon cross the meridian together,
at noon. At fall moon she crosses the meridian at

!lSt Idea abolu the cciuator -the least within the polar circles.

ELEMENTS OP ASTRONOMY.

niillest,
^t from

ed tliiit

cteil by
he ek-
i place ;
I ocean,
els, cur-

Moon.

he tides
rs under
5 on the
e waters
horizon,
jh water
J mc>tion,
; once in
sr every-
Qutes
} exactly
3r action
received,
11 strong,
rising lor
therefore,
)m two to
, At new
together,
eridian at

1 liemisphere,
lighest in the

midnight — when in her quarters, at G o'clock, a.m.
or P.M. '

Low Water where the Moon is in the

Horizon.

2S5. As the moon tends to draw the waters in straight
hnes towards her, she will evidently draw off the sur-
lace, or depress the waters on which she acts sideways;
i.e., those parts at which she appears in the horizon, 90°
on each side from the meridian she is on ; this will
cause low vmter at these two places once every 12 hours
25 minutes.

High Water at the Part farthest from the

Moon.

286. As the moon attracts the earth, as well af the
loose waters on its surface, she will tend to draw the
earth from the waters which are on the side of the
earth most distant from the moon. As she attracts the
earth more forcibly, being nearer, thj^n those distant
waters, she will dra?,' the earth further towards her
than those waters. This will cause the earth to recede
from under these waters, which causes them to rise
relatively to the land at those parts, and thus there is
high water there also. As the moon come.s into this
position every 24 hours 50 minutes, there will be high
'vater at the part farthest from the moon once in every
period of 24 hours 50 minutes.

287. This accounts for the two tides daily; each
part, by the earth's daily rotation, being brought once
near to and once remote from the moon during each 24
hours 50 minutes.

288. The following diagram will illustrate the action
of the moon upon the waters.

\li

ELEMENTS OP ASTRONOMY,
Fig. 22.

Let A B C D E F represent the earth, M the moon,
C the pomt nearest to the moon, F the meridian far-
thest from the moon, A and E the points at which the
moon appears in the horizon, 90° east and west from C
and F ; then there will be high water at C and F ; low
water at A and E.

289. The moon's attractive force at C evidently
tends to raise the waters towards c, and to draw them
from A to B, E to D, and from B and D towards c ; as
shown by the arrows in Fig. 22. Also, as the moon's
force draws the earth from F with more force than it
draws the waters at F, the earth must recede from
these loose waters and cause them to be proportionally
elevated. Hence, there is high water at C, where the
moon is on the meridian, and also at the opposite meri-
dian F.

290. But the moon's action evidently draws the
waters from. A and E, and tends to make thorn low
at these points : while these waters tend also to rush
towards F, where, from the earth's recession, the
waters are lighter. These two causes depress the
waters at A and E, and there is therefore low water

there.

291. The action will be readily understood, if we reflect du
the simple law of the diminution of the force of attraction as the
distance increases; and boar in mind that the earth is less

ELEMENTS OF ASTRONOMY.

attracted by the moon tlmn the waters near hor, but more ♦^an
tlio waters more remote.

Sun's Action upon the Tides.

292. The sun also by his action influences the
waters of the ocean, and is the main cause of those
regular changes in the height of the tides which are
everywhere observed.

293. His action raises the tides higher than usual
when It unites with that of the moon,— renders them
lower when his action is opposed to that of the moon
At new moon and full moon, the action of the sun and
moon coincide. At the quarters, they are opposed in
their action. Therefore there are spring-tides at new
and full moon, neap-tides at the quarters.

_ 294. This will be readily understood from the follow-
ing figures.

Fig. 23.

If S represent the sun, m the moon, and /A c E the
earth, then, as the sun and moon act in the same man-
ner a^ -: the same direction, it is evident that ti^•
ettect upou the waters will be increased, or there Wiil
be aspnng.tide at c and/ and very low water at A and
/' ..^^^ ^'^^ ""^^ ""^^^ *^"^ *^ ^^^s« c and/ to depre:-

V^;l^oA ?i"* '^*^^ '^''''^ ^"^ ^" ^"^ of ^er quarters, as in
i? ig. ^4, then they act in opposition to each other. The
sun tends to draw the waters towards n from A ,ind E •
and thus prevents the tide at A and E from being so

5 .J

ELEMENTS OF ASTRONOMY.

hi^h, and that at c from bcini^ so low, — or forms neap-
tides. The moon tends to raise A and E, to depress c
and/— the sun exactly the reverse.

296. The ratio of the sun's action on the tides to
that of the moon is as 38 to 100, while the ratio of
spring-tides to neap-tides is as 138 to 62.

297. 'Y\\Q Baltic Sea has no perceptible videri ; and that in the
Mediterranean Sea is very slight. These seas have no tides in
themselves, because, being of comparatively small extent, the
moon's action is equal at every part , and they do not receive
the influence of the Atlantic tide, because their entrances art-
narrow, and do not lie in the direction of the curreni produced
by that great tidal wave.

SECTION IV.

Dh ions of Time.
298. The leading divisions of time are, the Day,

tLEMKNTS OF ASTUONOMY,

the Month, and the Year.* Tlio civil standiinl, In tho
reckoning of time, is the Mean Solar Day of twenty-
four hours ; that is, the mean or average time which
the earth takes in revolvin , from the moment when tho
sun is on the meridian of a place till he returns to that
meridian again. The most perfect measure of time is
the sidereal day, or the time which the earth takes in
revolving from the moment when any star is on the
meridian of a place till it returns to the same meridian.

1. The Day.

299. There are four different kinds of days. 1. The
sidereal day. 2. The solar day. 3. The mean solar
day, 4. The lunar day.

1. Sidereal Day.

300. The sidereal day is the true time of one com-
plete rotation of the earth on its axis, — and its length
is 23 hours, 56 minutes, 4*09 seconds. It is called
" Kidercal " from sidus, a star, because it is determined
by the interval between the two successive appulses ot
any star to the same meridian.

301. The true time of the earth's rotation on its
axis is judged of from the return of a star to the meri-
dian, because the distance from the earth to the fixed
stars is so great, that its position in the most distant
parts of its orbit may he considered always the same
with respect to any fixed star. Hence, in reference to
the fixed stars, the earth may be looked on as not
moving round the sun at all, but ever remaining in the
same spot, rotating at a uniform rate upon its axis,
and therefore ever returning to any star in the same
time.

2. Solar Day.

302. The soltir day is the time from the sun's being
on the meridian of a place till he returns to that meri-

ELEMENTS OF ASTRONOMY.

dian, — in other words, the intcrvul "between two suc-
cessive apijulses of the sun to the s.aine meridian.

303. The solar day is lon^ifei than tlie sidereal day, and
its duration is different at different periods of the year.

304. The solar day is longer than the sidereal day,
because, while the earth's motion onwards in its orbit
makes no sensible change in its position in relation to the
fixed starsj it makes a material difference in its position
in relation to the sun. This affects the solar day in the
following manner : After the earth has made one com-
plete turn on its axis, and brought any meridian on
which the sua and any star were at the commencement
of that rotation to the same star, that meridian loill not
have reached the sun at the close of the rotation ; for, the
earth has during that period moved onward in its orbit
and, having in a manner moved past the sun, must turn
further round than the complete rotation to bring that
meridian to the sun again.

Fig. 25.

1 2 3

305. This may be illustrated by the above figure.

ELEMENTS OP ASTRONOMY.

Let S represent the sun, E A F, a section of the earth
through the equator, A the meridian on which the sun
is at any given time. If the earth revolve on its axis in
the direction F A E, as indicated by the arrows, and move
in its orbit from 1 to 2 in the time in which it turns on its
axis, it will have moved to 2 when the point A or A' has
returned to the same star; but as it has moved so as to
place the sun in a manner behind its new position, it will
not have brought that point to the sun, but require to
move round to a before that meridian comes again to tho
sun. Hence, the solar day is longer than the sidereal day
by the time the point A' takes in revolving from A' to a.

306. The length of the solar day is different at
different periods of the year from two causes : — 1. The
inequality in the rate of the earth's motion round the
sun ; — 2. The inclination of the earth's u is to tho
plane of its orbit.

307. As the earth does not alv.dys move at the same
rate round the sun, it will, at different times, have
moved different distances during one rotation, and
therefore the excess over the complete rotation neces-
sary to bring a point back to the sun will be more at
certain times than at others.

308. Thus, if the earth, instead of moving from 1 to
2 (Fig. 25), during one rotation, had moved from 1 to
3, the point A, or A", at the end of the rotation, would
be still further from the sun, and have to move further
besides the complete rotation to bring A to have the
sun on its meridian — namely to a\

309. Perhaps the best illustration of the difference between
the solar and sidereal days is afforded by the motion of the
hands of a watch or clock. If both hands be at twelve o'clock,
and set out together from that point, the long or minute hand,
when it has made a complete revolution, will have returned
again to 12 o'clock, but will not have reached the hour hand,
because it also has been moving, though more slowly, in the
same direction; and the long hand will have to go more ihau
the complete circle before it overtakes the short hand. Now,
the long hand resembles any terrestrial meridian, the short hand

— c\

ELEMENTS OF A8TKUN0MY.

the «un, and the dial-plato and figures the «ta>-ry Rpl.cro tt d
Btars If the hour haud he HuppoHid to move at different DiteJi
in different parts of its circuit, the rninuto hand must take
different periods to come up to it.

310. The inclination of the earth's axis to the plane
of its orhit affects the Icn^Hh of the day by causing the
earth to move in a plane inclined to that of its equa-
tor ; that is, inclined to the direction of the earth 8
rotatory motion, which is parallel to its eopiator. ^ ^

311 This will he best explained by the supposition
that (as it reallv appears) the sun moves round the
earth in the ecliptic, while the earth turns daily on its
axis- and that while the real sun moves in the ecliptic,
another, which marks uniform time, moves in the equi-
noctial. With these suppositions, and the aid ot tlie
following figure, the inequality caused by the sun being

Fig. 26.

in the ecliptic may be comprehended. Let the figure

KI.KMENTS OF ASTRONOMY.

represent the sphere of the heavens,* Ten the
('(liiinoctial, and T ^ JQ. the ecliptic. Let the real sun
be supposed to move from T hy o' &' c' lY ef S\, g h I
m, roturnin<]f to T in th<' year, wliile the other, startin**
nt the same time from T, moves along the equinoctial
by a 6 c </ e n, returning again in the ojiposito direc-
tion in the year. The ecliptic and equinoctial are
great circles of the same sphere, and hence are equal
to each other. Let each be divided into the same
number of equal parts in «, a\ etc. ; then, as the whole
circles are equal, the parts in one will be equal to those
in the other, and therefore the part T a' of the ecliptic
is equal to the part T « of the equinoctial, a' h' to a b,
h' c to h c, and so on.

312. Now, let the meridian of any place and the two
suns be on T at any given moment — then all set out
together in th^ same direction, i.e.^ towards ^. When
the place has completed its daily revolution and re-
turned to T, the two suns will have advanced in their
course, let us suppose to a' and a, equal distances from
T, — in continuing its revolution, it is plain that the
meridian of the place will come sooner to the sun a'
than to «, as the latter is furthest, f in the direction of
rotation^ from the meridian line of the [)lace. Hence,
the time, as marked by the sun in the ecliptic, will be
before that as marked by the snn in the equinoctial ;
and this goes on so long as the sun is increasing his
distance from the equinoctial — that is, from T to c.
When he comes to the solstice at c\ the reverse takes
place ; the meridian, from c' to zCiz^ comes to the sun d
on the e(ininoctial before d' in tlie ecli[)tic. — The same
takes place again when the sun moves from ^ by g, h,
etc., to T.
313. Tiie sun-dial exhibits time as indicated by the

* T i« Aries; gg Cnnci-r; ^rv Libra; and Vy Capricorn.
t Tlip distance of a point from a line ia tlic pirpcudiciilar on (he iiuii
drawn from tl»e point.

KI.KMENT8 OF ASTRONOMY.

real tiinc of tlic stm heitiff on tlio inoridlim,— the actunl
Boliir (lay, soiiutiiiu's li.ii^'iT, Hoiiu'timeH Kliorter. Tho
clock exhibits time tt(ljust<'(i to tho mean sohir (hiy.

314. The coitinrehcuHion of the variuti.)n in the day iMMtiff
crtUBcd by tho mu'n motion In the ecUptic, will Ih! ^rt'iitly iiued
l)V nmrkiiiL' points on the t(ininoclial und cclintic at cqiml diH-
tiincfH from tho enuinox, on a tcm»tinl or celestial k1..Ih', and
„hHervinK in what onK r they come to tho brazen niendian,
uecordinff n« they aro approaching to or receding from tho equi-
noctial. It may Ih! rudely illustrated by making a (JRurc such
an the following (Kip. 27) -fixing by a pin one end ol a thread
nt l\ and causinK it to pasH from V H to T C I he thread, m
pasHinR from 1» M to P 3', will always come sooner to any ponit
in H 1) than to a cornspondinpj (me in H .'J', and tho reverse ui
nassinc fiotn V 3' to VC. Now P represents tho pole, the thread
the meridian of a place, H 1) C the ecliptic, U C tho equmoctml.

Fig. 27.

3. Mean Solar Day.

315. The mean time occupied by the earth in re-
volving from the sun's being on the meridian of a phice
till it returns to the same meridian, is called the 7nean
solar day. It is this to which the clocks are adjusted,
eo that they may give equal time at all periods of the year.

316. Four times a year, the mean solar day and

ELEMENTS OF A8TRONOM /.

adual solar day nre tho same. Thon, the clock (monn
Pftlnrday) luid Hun-diul (actual solar day) arc coincident.
These four times are, December 24, April IT), Juno 15,
Septcmher 1,

317. ThcRo pcrioflB mo ncnr the cqttinoxcd, Imt do not coin-
cid»5 with them. Did tho varintion in the Holar day de|wiul
nololy on tho inclinntion of tho earth's axIh to tho cchotic, tho
perioils of tlio clock and Hnn being tho Hnnio wouhl Iks tiio cqui-
noxefl and HolsticcH; hut tho other element which givcH rino to
variation in tho H(tlar day — tho inequality in tho rate of the earth'H
in'itii)n round the fiun — causes the times of coincitUuiee (if clock
and sun to deviate a little from the equinoxes and solstices.

318. From December 24 to April 15, and from June
15 to September 1, tho mean solar day is shorter than
tho actual solar day, — tho sun takes longer than 24
hours between two successive appulses to the same meri-
dian, — ai)parent time is behind mean time,— or, the clock
is before the sun.

That is, from nhont c' to rfi., and h to T in Fip. 20. In
these quarters of its course, tho meridian of a place comes to tho
sun in the equinoctial before it reaches that in the ecliptic.

319. From April 15 to June 16, and from Septeml)cr
1 to December 21, the actual Folar day is shorter than
the mean solar day, — the sun occupies less than 24 hours
between two successive appulscs to the rame meridian, —
apparent time is before mean time, — or, the clock is
behind the sun.

That is, from about T to c', and ^ to «, Fig. 26, a place in
its daily rcvoluti(m comes sooner to the real sun in tho ecliptic
than to the imaginary sun we have supposed to rovolvo in tho
equinoctial.

320. See column " Equation of Time " in the alma-
nacs; which shows how much the clock is before or
behind the sun every day of the year, so that when wo
take the true time by an observation of the sun's alti-
tude, we may be able to add or subtract the necessary
time to set the clock by mean time.

321. Generally speaking-, it may be said that from
about the time of the equinoxes, the clock is behind the

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ELKMENTS OP ASTRONOMY.

sun, — frona about the time of the solstices, the clock is
before the sun. The days vary so much, that sometimes
apparent noon is 16^"'- before mean noon, — sometimes
14j^™- after mean noon.

322. Althoup;h the sidoroal day never varies, it cannot be
adopted as the standard of time for ordinary occasions of life;
because it would not conform with those natural divisions into
day and night (periods of sunshine and periods of darkness)
from which, for the sike of convenience, our arrangement of
time cannot depart far. As the stars always come to the meri-
dian at tlie same successive intervals, and the sun does not, —
if the hours were regulated by the sidereal day, the same hour
would now be at sunrise, now at noon, now at sunset, continually
changing its relation to tl.o occurrence of those changes of the
day and night, by which it Is most convenient to divide our

time.

4. Lunar Day.

323. The lunar flay is the interval between two suc-
cessive appulses of the moon to the same meridian.
This is different from the true day, as the moon has a
motion throug'h the heavens, so that the earth has to
make more than a compl(;te rotation before she brings
any meridian again round to the moon. As the moon
moves daily about 13° through the sky, to overtake
which the earth requires about 50 minutes, this time
must be added to the common day to constitute a lunar
day ; which is theaefore 24'i' 50"'- in duration.

2. The Month.

324. The month is of three kinds, — the sidereal^ or
periodical months of 27 days, 7 hours, 43 minutes ; the
si/nodical, or lunar month, of 29 days, 12 hours, 44
minutes, being the interval from one new moon to the
next (335) ; and the calendar, or common month,
January, February, March, etc., 31 or 30 days, except-
ing February, which is 28 or 29 days. In each year
there are 12 common or calendar months; a little less

ELBMENTS OF ASTRONOMY.

than 12J synodical raonlbs; and a little less than ISA
sidereal months.

3. Tho Year.

325. The year is of five kinds, — 1. The equinoctial,
or tropical year;— 2. The sidereal year;— 3. The
anomalistic year;— 4. The common year, of 365
days ;— 5. The leap year, of 3G6 days.

1. The Tropical Year.

326. The period of time adopted for the astronomical
year is ;he interval between two returns of the sun to
the same equinox; called, therefore, the equinoctial
year. Us duration is 365 days, 5 hours, 48 minutes,
and 49"7 seconds.

327. The calendar or common year contains 365
days. The odd hours, S''- 48'n- 49-7''-, would soon
accumulate to a serious amount of error : and, by a
chfinge of about a day in four years, would gradually
throw the seasons forward, so as to arrive later each
year, till they would no longer occur at the same months
as at present; and this change would be continually
going on, the fixed relation between the months and the
seasons being destroyed. To prevent this inconvenience,
the odd hours are disposed of as follows : They amount
to nearly a quarter of a day, and are allowed to accumu-
late till every fourth year^ when they amount to a day,
and are got rid of by making that year one day longer,
or 366 days. That additional day is added in February,
which has then 29 days : and that year is called leap
year, or bissextile. This and other important improve-
ments were introduced by Julius Caesar, according to
the plans of the astronomer Sosigenes.

328. But the excess of the tropical year over 365
days is not quite a quarter of a day, being 1 1 minutes
!0 seconds less; hence one day every four years is too
much to add, and causes an excess of one day in about

ELEMENTS Ok' A8TU0N0MY.

135 years. This iii compensated for, within a very
trifling quiintity, bi/ making every hundredth year a
common year, except the fourth, which is to be a leap
year. Thus in every four hundred years (3 limes 135
= 405), three years, which wo\..d otherwise be leap
years, have only 365 days, which takes off the excess
of the day added in each leap year over four times
5h. 48m. 49-78. The remaining error amounts only to
one day of excess in 3866 years.

The principle of arrangement may be thus shortly
stated. Every common year which leaves no remainder
when divided by 4 (as the year 1872), and every hun-
dredth year (years ending in 00), which leaves no
remainder when divided by 400 (as the year 2000),
are leap years, having 366 days. All the others are
years of 365 days. Thus, 1600, 2000, 2400, ure leap
years; but 1700, 1800, 1900, 2100, 2200, 2300 are
ordinary years.

329. This arrangement was introduced by Pope
Gregory XVI. in 1582, and has been adopted in all
civilized countries, except Russia. It was introduced
into Great Britain in 1752,— being termed the new
style. The error had then accumulated to 11 days, and
was rectified by advancing the days of the month 11
days : the 3d of September to be the 14tli. The differ-
ence between the new and old styles now amounts to
12 days.

2. The Sidereal Year.

330. The true time of the earth's revolution in its
orbit, is the period of its return to the same star. After
the sun has returned to the same equinox, as that has
receded 50*1" (by the precession of the equinoxes, —
which see), he has still 50*1'' of his orbit to complete
the real revolution, which requires 20™- 19-9«- of time,
which must therefore be added to the tropical year to

ELEMENTS OF ASTttONOMY.

5 are
leap
' are

make the sidereal year. The duration of the latter is
therefore, 365'>- G*'- 9°" 9-6«-

3. The AnomaUatio Year.

331. The line of the earth's aphelion and perihelion
—that is, the longer axis of the orbit— undergoes a
gradual change, which shifts the perihelion ll"-8
annually. The earth, therefore, must describe this
small arc in addition to its real revolution to bring
it round to the perihelion again. It requires 4">- 39-78-
to complete this small arc, and therefore the anomalistic
year— as the time required to bring the earth to the
same relative position to the major axis of its orbit is
called— is 4'"- 39-7«- more than the sidereal year, cr
'SarA 6h- IS™- 49 3«-

332. The following table exhibits the leading divi-
sions of time : —

Standards of Time.

Hours. Mirutes. Seconds.

Rotation of earth on axis ... 23 56
Vibration of pendulum 3913 inches long

at the lat. of London, 51° 30' N. .

Civil day, 86400 of the above seconds
(divided into 24 hours of 3600 such
seconds each ; and each nour divided
into 60 minutes, composed of 60 such
seconds) 24

4-09

Days.

Sidereal day (as above)
Solar day, more or less than .
Mean solar day (civil day) .
Lunar day

Months.

Sidereal month ,

Lunar month

Calendar month, 31, 30, 29, or 28 days.

409

Days.

Hours.

Minutes

LLtMliNTS OF ASTHONOMy.

Years.

Hidcrral ycir
Equinoctial year
Calendar year
Leap year ,

Days.

lIoiirH. I

klinutu

H. SfCH.

365

(♦

3r,.5

4'J

365

306

SECTION V.
MOON'S PHASES.— ECLIPSES, ETC.

1. The Moon's Phases,

333. The changes in tlie moon's appearance, from a
small illumined crescent to tliat of a full enlightened
orb, and from that to a crescent again till she disappears

^entirely, are termed phases.

334. The moon's phases are owing to the following
causes. She is a dark body in herself, and shines only
by reflecting the sun's light; so that only ony half of
her surface — that which is next the sun — is illumined
at a time: and the only parts that can be seen are those
parts of the illumined half which are turned toWt,.ds
the observer. But, owing to the moon's motion round
the earth, different amounts of the bright half are
turned towards the earth at different times, so that she
appeal of every different magnitude, from full moon
till she disappears altogether.

335. The following figure will illustrate the moon's
phases, and also those of Mercury and Venus.

Let S represent the sun, E the earth, and A, B, C,
D, F, G, H, K, in the inner circle, the moon, revolving
round the earth in the direction of the order in which
the letters have just been named. Then at A, when
the sun and moon are in conjunction, the unenlightened
half of the moon is turned towards the earth, as shown
at A' in the outer circle, and the moon is not seen ; or,
it is new moon ; or, as sometimes expressed, the moon

ELKMENTS OK ASTICONUMV.

changes. At B, as is scon by the points wlioro the
inner circle cits tlic inooji, a small part of the enlight-

Fig. 29,

moon

ened side comes into view to the earth ; but the greater
part of the side turned towards the earth is dark, and
the moon appears an illumined crescent, as shown at
13 , in the outer circle. At C, half of the enlightened
part iS turned towards the earth, the moon appears like
a semicircle, as at C in the outer circle. At F, the
enlightened side is turned towards the earth, and the
moon appears full, as at F', being then in opposition.
As she continues her course, she gradually presents less
and less of her bright side to the earth, successively
appearing as at G', H', K', in the out r circle, until
she again comes into conjunction, and entirely dis-
appears. By tracing the figure, the phases will be

f.li:mi:nt8 of astronomy.

11!

roftdily tindcrstoofl. The inner circle sliows the illu-
mined and dark parts — the outer circle the appearnnro
presented to tlio earth.

336. When the moon is in conjunction or opposition
fas at A and F, Fig. 28), she is said to he in her
Syzigies — when midway between these points (as at C
and II, Fig. 28), in her ftuadratr'^es (or quarters).
The points in her course between the syzigies and
quadratures are called Octants.

337. The earth appears to the moon as she does to
us, but of about tliirtoen times the size, and affords her
a very considerable degree of light. The light which
the earth yields the moon renders the dark parts of l". i
latter faintly visible, a little before and after new moou.
At these periods the moon's light, as it appears at he
earth, is weakest, and the illumined half of the eaitii
most fully turned towards her. See A, K, and D, Fl,(.
28. This enlightens the dark part of the moon, and this
light reflected back to us, renders these dark parts
visible with a dull, grayish light — forming tho appear-
ance popularly termed *' the old moon in the new moon's

arms.'

Eclipses and Occultations.

338. When the sun or moon is, in whole or in part,
obscured by a shadow which gradually comes over the
disc and then glides off, this phenomenon is termed an
eclipse. Eclij)ses arc of two kinds — eclipses of the sun
and eclipses of the moon.

339. When any fixed star or planet is obscured by
the moon, or a planet passing between it and the earth,
this phenomenon is termed an "occultation."

340. Eclipses occur only when the moon is in or near
to her nodes (that is, when she is crossing the plane of
the ecliptic) ; because then only the moon, earth, and
Bun can be in the same straight line, or so nearly so that
^ part of one can obscure part of another. They also

ELEMENTS OP ASTRONOMY.

occur only at new and at A.ll moon ; that is, when the
moon iH in conjunction or in opjwsition.

1. Eclipse of the Moon.

341. An eclipse of the moon is caused by the inter-
position of the earth between the sun and moon, so that
the earth prevents the moon from receiving the sun's
light, and she is therefore obscured.

342 Hence, an eclipse of the moon takes place only
when she is in opposition. ^

343. The following figure will illustrate an eclii)so
ot the moon. ^

Fig. 29.

Let A S. B represent the surface of the sun, H the
earth, m th cmoon, dnip the moon's orbit. Then, the
sun's rayp from A and B, skirting the earth at H and
will project a conical shadow H C 0, within any part
of which no rays will be received from the sun. But
the parts A and B of the sun also send rays cross-
ing in K, A a, and B U b ; and between the' line of
these rays, and the perfectly dark cone H C 0, any
object will receive part, but not be illumined by the
whole of the sun's rays, part being cut off by the inter-
posed earth. Thus, at n, no rays from the sun's surface
between S and B will be received, and therefore a leso
perfect light will be shed at n.

ELEMENTS OF ASTRONOMY.

844. Tlio perfectly dark rnnce 11 C 0, is called tl)0
umbra or sluulo ; tlu^ surroniKllrvc^ parts 6 H (!, a (',
the penumbra or " aiiuostshado " (/)^'nf, almost; umbra^

Bliade). Ml t r 11

345, Now, when tho moon is at d hIjo will be fally
illuminated, receivinj; rays from the whole of tho sun's
surface A S B; but whenever she enters the penumbra,
by crossing tho line b H, the earth will cut off a por-
ti<m of the sun's li^dit, and she will receive less and less
as she passes onwards towards v: after crossing tho lino
H C she is totally eclipsed, and remains so from v to /;
on crossing C she emerges from the umbra into the
])enumbra"c a, in wliich she gradually receives light
from more of the sun's surface, and becomes nioro
luminous till siie passes out of it, and again receives
the full light of the sun.

346 Tho apex of tho cono of tho umbra extends to a distance
(.f about 77i,0()0 miles beyond the earth; its length, of course,
varying according to the earth's distance from the sun.

347. The moon does not disappear from our view
completely, even in a total eclipse ; this term indicating
only that the shadow extends over tho whole of tho
moon's surface next to us. She appears of an earthy
brown colour, receiving a few rays by lefraction which
still enable her to transmit a faint light to us, by which
she is rendered visible.

2. Eclipse of the Sun.

348. An eclipse of the sun is produced by tho moon
passing between tho earth and sun, and thus, accord-
ing to circumstances, cutting off a part or whole of his
surface from our view. Hence an eclipse of the sun
happens only when the moon is in conjunction, i.e., at
new moon. The eclipse may bo total, when the
whole of the sun's disc is obscureJ ; partial, when only
a part of his surface is obscured ; annular ^ when the

elrmknth of astronomy.

moon cutH ofT an n.nor circle, leaving a luminous riuff
nnMuul the purl ohscurc.l ; an.l central, wlic, tl.o plrice
o ho observer is in tl.c sa,,,,. strai^rl.t Ij,,,, vvitii centres
ul the 8IUI und moon.

849. The (rustanceH of the earth, 8un, and moon, from
ouch other are the circumstana 8 which determine the
nature of the eclipne. The extremity of the cone wiiich
formH the moon « umbra fail.s always near the earth,
but soraetimoH tails short of the earth, sometiuu's just
touches It, sometimes is so long that it could reach u
pomt within the surface, and then there is a spot on the
earth where the sun is entirely obscured.

a.-iO. As the sun is lar^rer and the moon less than tho
earth, tne earth can never bo entirely immersed in tho
iuo<m 8 shadow ; bo that an eclipse of the sun is visible
^"1/.;^^ V'^''^ "^^''^' hemisphere turned towards him,

Ain. Ihe following drawing will illustrate total and
partial eclipses of the sun.

%^l

Fig. SO.

b Iv it f. \r n'^^^."^ ^^'^ """' H the moon,
Ann 1 Tn',^^ ^ ^ ^^" "™^^^ ^' fJ'-^rk shadow
y 1 L and « C the penumbra, within which only part

wifMn '""? "'f'"' ir^^^"^^'^- Jt is evident that
hin V and i there will be a total eclipse ; from h to v

w r ' \u ^ ^""'^'"'^ ''^'^'''- ^^^"« ^t « the moon
will obscure that part of the sun's surface which lies

or,

RLRMr.NTf or ASTRONOMY.

Ih'Iow tho Hnr» n 11 B, no tliat nn obiwrvcr ft* n will
|»';rct'ivf only ♦ho \uv't hIkjvo that; line, or S A, nnd the
extent of tho hiui'h disc cclipHcd will bo grout*;*' accord-
ing as tho place is nearer to r and i, where tho umbra
connnonccf.

362. An aur.nlar odipso of the sun occurs when tho
apex of tho cono of tho moon's shallow or unihra falls
short of tho earth ; then there is a margin of the sun's
surface loft bright all roun(l, whih) tho moon darkens
tho middh; part.

353. This will bo illustrated by Fig. 31.

Fig. 31.

Let A S 5 B, as before, be the sur- I'''k s^.
face of tho sun, the apex of the conical
sbadow reaching only to c. It is plain
that an observer at v will have part [
only of the sun's surface cut off from \^ V
his view — namely, tho part within the
letters S s— the parts A S and B s will
be seen forming a luminous ring all roand as shown

in Fig. 32. ^ ,

354. There would be an eclipse of the sun every
new moon, and of the moon every full moon, if the
planes of the moon's orbit and the ecliptic were the
aame. But as the moon's orbit is inclined upwards of

KttMllfTS OF A8TIIONOMV. JJ

wlijwo. ' ''"'' '" '"••«'»«"T l<> cttiiBo ail

int!?™l«"whl!l"i ""'"r" ''" '•«^'" »' <="t»in rcK,.lar

Sun eclipsed,

Moon eclipsed,
Moon

((

1828.
April 1 J.
Oct. i).

1829.

Mftr. 20.

Aj.ril 3.

iscpt. 13.

Sept. 28.

Sun eclipsed,

Moon eclipHed,

Moon
Sun

1846.
April 2.5.
Oct 20.

1847.
Mar. 81.
April l.^i.
^ept 24.
«- -t. 9.

<lesoribu<lintlienrtici..»oifKL.i, • ?u ''?," """"bcr, lias boon

IP>

Full Moon,
Nuw ...
Full ...
Now ...
Full ...
Now ...
Full ...
etc.

ELEMENT? O" ASTRONOMY

1828.
January 2.
January 17.
Fcbn.ary 1.
February 15.
March 1 .
March 15.
March 31.

etc.

1847.
.January 1.
January 17.
January 31.
February 15.
March 2.
March 16.
March 31.
etc."

A. De Mouoan, in Companion to the Almanac for 1847.

357. A remarkable plienomenon, hitherto unex-
plained, was seen during the total eclipse of the sun
on the 8th of July 1842. " The moon was like a
black patch on the sky, surrounded by a faint whitish
light about the eighth of the moon's diameter in
breadth, in which three red flames appeared in form
like the teeth of a saw."— ifrs Somerville.

SECTION VI.

Influence of the Atmosphere on Astronomical

Phenoroena.

358. The heavenly bodies are rendered visible to
us by rays of light which emanate from them and pro-
duce impressions of their forms on the eyes of the
observer. These rays are somewhat modified in their
course before they reach us by passing through the
atmosphere.

359. The atmosphere is an aerial fluid, which surrounds the
earth on every side. It extends above the surface to a height*
of about fortj-five or fifty miles. it is heavy and dense m
tlie lower regions, but becomes gradually 1; ^'hter^ and more
rare or expanded as it is higher up ; as represented m l^ig. 66,
where the particles are shown densely crowded b-low, becom-
inff moie open as they are iunucr irom v^v- ^^i..,?.,.. -_ '*",. I,
figure the extent ot the atmosphere in relation to the size ot the

ELEMF.NTS OF ASTRONOMY.

fru V^ greatly exapprerated.
I he depth of the atmosphere
18 about one hundredth of the
distance from the surface of
the earth to the centre.

360. The atmosphcro
is concerned in astronom-
ical phenomena by its
power of refracting and
reflecting the rays of

F itf. rw.

'•,'>>■;(!

•ht.

36 ] . Refraction is tliat " ■••••■/■^: :!::-•?• ••::■• '

bending of the rays of

light which takes place when they pass obliquely from
?ro n atT ''r't'' '' ' ^^/ohght pal obl'ueT^
s ace to onV7 ''' f""^ ""'''' *« ^^^' fi-om surrounding
densifio. If f ""'P^^^^^' ^' between strata of different
densities of the same medium, it does not continue in

les^ inTo '''7^'}}^^ -^ before, but is bent, more or
Is ; '""^Z' \'\^'^^'^oUon, which it preserves'so long

ir.f 4"^'" '" *^^ ^""''^ medium.
T oi r ^'^'■'^^*^^" ^s illustrated by the following figure,
i^ct L D represent a

ray of light passing at
D from a rare into a
dense medium, to the
surface of which it is
not perpendicular. It

will not continue in the
direction of C D, but
will be bent into the
direction D E. If the
ray had come from the

tZS';?D i? '^t'^'r ^ ^' ^^*^^^"^ *be rare
medium at D, it would be bent into the direction D C.

Fig. 34.

s-3

i" reicreuce to that influence, as air to light

<^o is trariiiniitted is called "t

100

ELEMENTS OP ASTRONOMY.

I i

It^ ^i

Let the line G D F be perpendicular to ilio surface
betwf^«n the two media; then, it will be evident
from the figure that the following is the rule of
refraction :

" When the ray passes into a denser medium, it k
refracted so as to be nearer to the perpendicular than
before ; when it passes into a rarer medium, it is re-
fracted so as to pursue its course further from the per-
pendicular than before."

363. If the ray entered perpendicularly, as in the
direction G D or F D, it would not be refracted,
but would continue in the same course, G D F, or
FDG.

364. Owing to Refraction, no heavenly body is
seen in its true place unless it be in the zenith. Every-
where else, refraction causes bodies to appear to be
higher above the horizon than they really are.

365. This takes place in the following manner:
When a ray of light enters the atmosphere obliquely,
it is bent down towards the surface of t) a earth, and as
it approaches the ground, it becomes more and more
bent in passing from the rarer strata above to the
denser medium below. Now the object from which the
ray comes appears in the direction which the ray has at
the moment when it strikes the eye. Accordingly, as
refraction in a denser medium bends the ray towards
the perpendicular direction, the object will be seen in
a direction more perpendicular to the surface than its
real one— that is, nearer the zenith, or more elevated
above the horizon than it should be.

366. Refraction is very slight on the first ray entering the
atmosphere, owing to the extreme tenuity of the air in the upper
regions; but gradually increases as the ray approaches^ the
earth, the strata of air becoming more and more dense. It is
affected by the air's temperature and pressure, which must be
taken into account by the astronomer.

367. As the rays from a celestial object in the zenith
enter the atmosphere perpendicularly, there can be no

ELEMENTS OF ASTRONOMY.

101

refraction of its rays, and it will bo seen in its true
place But, from every other position, the rays will
enter the atmosphere obliquely, be refracted, and there-
tore represent it tor elevated.

368. The following figure illustrates atmospheric ra-
traction. Let o represent any point on the earth's

Fig. 36.

surface, and A any star or heavenly body. It is seen
at o by means of rays of light from it which reach he

more den.7 Z. "^u'i ^ ^^' ^^^' ^' ^^' ^^' ^^'^^^^
more dense, to v o, which is the direction they have on

ftwefe tth"'r- /-T^-^ly^ the star I s'n ns
but aTT ' ''"' ""^''^ represents it not at A,

Lt:^!?lril^-? '-^-od the obji^t visible al o had 1"
" '•-' ^-"«^""", VIZ., A ii, is refracted down to e.

102

el^:ment8 or astronomy.

370. Refraction increases as the object is nearer tlio
horizon, as the rays then pass through more of the dense
strata of air. At the zenth it is 0° 0' 0". At 45° ahovo
the horizon it elevates the apparent ahove the true
position of a celestial object about V — more correctly
ST''. At the horizon it elevates the object so much as
33', or about half a (Jegree — that is, about as much as
the sun's apparent diameter.

371. Refraction, by elevating the position of celestial
objects, brings into view bodies actually below the hori-
zon, and which, therefore, could not otherwise be seen.
Thus, in Fig. 35, the body S, which is below the hori-
zon H o of a spectator at o, and would therefore be
invisible to him, is brought into view and made to
appear above his horizon at s.

372. Thus, refraction raises the sun above the horizon
at both suncet and sunrise, and by causing him to rise
earlier and set later than he otherwise would, lengthens
the day. As refraction at the horizon is 33', and the
sun's diametpr is about 32', the sun will have sunk
below the horizon when his lower margin appears to us
resting on it, just about to dip beneath it.

?73. Refraction also distorts the figures of the sun and moon
T/hen they are near the horizon, rendering them of an oval form,
and flattened at the lower part. This is caused by the very
rapid increase of refraction near the horizon, so that the lower
margins of these orbs are much more elevated than the upper,
which shortens the perpendicular diameter, and gives the figure
a somewhat oval shape.

374. The Reflection of light signifies the bounding
off of rays of light from bodies on which they strike ;
this takes place in the same manner in which a ball
rebounds from any hard surface on which it is thrown,
or in which sound and beat are reflected.

375. The atmosphere reflects and disperses in all
directions the rays which it receives from the sun.
Were there no atmosphere, those bodies only would

ELEMENTS OF ASTRONOMY.

103

be visible to us which are in 'he direct rays of the
sun, and thus receive light, whic they would transmit
to us and render us sensible of their presence. But, by
the reflective power of the atmosphere, bodies have light
thrown upon them, though out of the direct course of
the sun's rays ; and thus, as tL°) atmojphere is every-
where present^ they receive light in whatever positioUj
which they in turn reflect to us, and thus render them-
selves visible.

Twilight.

376. Twilight is the faint and gradually diminishing
light which we enjoy for a considerable time after the
sun has fdrly sunk below the horizon ; and we are
indebted for twilight to the reflective power of the air.
Those portions of air which are a little nearer to the
sun than any place at which the sun has just set, will
reflect down to that p»lace (as well as to the parts which
have had the sun still longer below the horizon) a part
of the light which they receive ; accordingly that place
will, for a little after sunset, receive an inferior degree
of light reflected from the air — or twilight. And, as it
will receive reflected light from a less body of air as the
sun sinks lower below the horizon of a place, its twilight
will diminish gradually till total darkness supervenes.

377, This is illustrated by the lower part of the above
figure, 35. Let the sun, P, be on the horizon of the
place m, having completely set to n and t. These
places, n and <, would be completely dark were there no
atmosphere. But though the sun is far below the hori-
zon of ^, it will receive a small portion of light reflected
from the upper air at k, and from all the air higher than
the shaded p'vrt and beyond the line k t. The earth at
n, not so far out of the sun's rays, will receive reflected
light from a much larger portion of the atmosphere,
viz., from b, lower down, and from all the air without
the shaded part and beyond thrj line 6 n.

104

ELEMENTS OF ASTHONOMY.

■\

378. Twilight continues wLilc the sun is less than
18° below the horizon. Hence, some parts of the earth
have continual twilight at certain periods of the year,
as at London, from May 22 to July 21. The real, or
astronomical twilight, is of much longer duration than
what is popularly regarded as twilight; for it com-
mences immediately after the sun is below the horizon,
when there is still good daylight, and continues for
some time after it is apparently dark. There is long
twilight around the polos, which illumines these desolate
regions when the day is short, or the sun below the
horizon for days or months together.

379. There is shorter twilight the nearer the place is
to the equator — there the astronomical twilight con-
tinues for about 1 hour 12 minutes. The rapid rotation
of the parts about the equator, and the great distance
from the axis, bring very soon a considerable convexity
between the sun and the spectator, so that the period of
reflection is cut short. Hence the diminishing duration
of twilight as we approach the equator, where it is
often said that there is no twilight. This is incorrect ;
the twilight is merely very short.

380. The daivn, or light before the sun has actually
risen above the horizon, is due to the same cause and
subject to the same general rules as the twilight in the
evening. Owing to the ditferent condition of the atmos-
phere as to vapours, which abound after sunset, twilight
is a little longer than dawn.

ELEMENTS OP ASTRONOMY,

105

PART III.

THE SOLAR SYSTEAf.

381. The solar system (or our system) consists of
certain of the heavenly bodies, which are connected
with the sun, and form a system by themselves, apart
from the others. Tlie word " solar " is derived from
" Sol," the Latin word for " the Sun."

382. The solar system is composed of the Sun, the
Planets with their Satellites, the Comets, Aerolites
(or meteoric asteroids), and, probably, a thin fluid or
Ether occupying the intermediate spaces, and spreading
out into space far beyond the limits of our system.

383. The Planets are those stars which do not*
remain in one spot, but are found to change their posi-
tions in the heavens. They are therefore termed
planets, from the Greek word wXavi^Trie, signifying a
wanderer. They are so named in contradistinction to
the fixed stars, which preserve their relative positions
comparatively unchanged. Five are visible to the
naked eye.— See par. 45. The earth also is a planet,
and must appear as a star to. those who live on the
neighbouring planets, as Venus and Mars. A Comet
is a kind of irregular and less substantial planet.

384. The connexion between these bodies is this:
The planets and comets revolve round the sun, receive
light and heat from him, and are preserved, mainly by
his action, in their proper paths around him. The
Satellites are lesser or secondary planets, which revolve
round some of the larger planets, are carried with them
round the sun, and also receive light and heat from
that luminary. The moon is a satellite to the earib,

385. The heavenly bodies which compose the solar

E 2

lOG

ELEMENTS OP A8TK0NOMY.

Bystcm have each two principal motions, ono round tlio
sun, tormcd its revolution or orbitual motim ; anotlior,
turninf? on itst'lf, round an ima«,Mnary lino called the
axis, paKsin/^ throu^di it, termed its rotation. Some, as
the earth, have other motions; see Precession. In
addition to these, the sun himself, with all the planets
in his train, is rapidly advancing throu^'h space. — Sco
pars. 502 and 707.

SECTION I.
Definitions.

386. An Ellipse, or Oval, is a curved line, such,
that the sum of two straight lines, drawn from two
points within to any i)oint in the curve, shall always
be the same. These two points are termed the Foci of

'the ellipse.

Fi^. 36 represents
an ellipse. F and E
are its foci, and if G,
K, L, be any points
in its circumference,
then G F and G E
together will be of the
same length as K F
and K E together, or
L F and L E together.

387. An ellipse may easily bo drawn in the foUowinpf man-
ner : — Fix two pins iu tlio paper at the points selected for the
foci, as at F and E; and pass a thread having its ends tied
togetlier (an endless thread) round them ; the thread being
longer or shorter, according to the size and form of ellipse
desired. Then stretch the tlircad out by the pencil, giving the
thread the form F G E F. or F K E F, as may bo, and then
carry the pencil round the fi>cx, pressing it gently but firmly on
the paper, and keeping the thread equally stretched till the
pencil has completed its circuit. The pencil will then have
loiiued &i> oval or eiiipse upon the paper.

elkmi;nt8 of astronomy.

107

388. The eltipite in ono of tho nffun-s called rnnlc »e tmni,,
formed by tho section or cutting of a c.no by a idano. If tho
plane cut the cono parallel t» the bane, tho section will Imj a
oircle: oblu/udf/ throiujh both aiden, the Huction will Jh! an
ellipM parallel to the aide, tho section will bo a parabola;
Hud when tho cutting piano maken a greater amjle with the ba«e
than the side of the cone, tho section is a hyperbola. If tho
plane pass through tho vertex, tho section will bo a triangle.

^ 389. The Major Axis of an cllipso is the Rtruight
line druwn throui^'h tho foci, and terminated both ways
by tho circiimfen-ncc, as A 0. T\ Middle point of
this line, C, in the Centre of tho clHj^se. The minor
axis of the ellipse is tiie Htrai«,dit line through iht centre
at right angles to the Major Axis, as J5 D.

390. A Tangent (or touciiing line) to a curve is a
straight line which touches the curve, and being pro-
duced both ways, does not cut it, that is, does not go
mto it. In Fig. 8, D K and P H Q are tangents. (A
tangent of a circle is at riglit angles to the diameter
drawn through tho point of contact.— There may bo
tangents^to other curve lines as well as to circles.)

391. The path or course in which any of the heavenly
bodies moves is termed its orbit; from the Latin orbita.
The orbits of the planets, satellites, and many of tho
comets, are ellipses, the sun, or planet round which a
satellite revolves, being in one of tho foci ; as at S ir.
Fig. 37 below.

392. Tiie Linear Eccentricity (from ex, out of, and
centrum, the centre) of
a planet's orbit is the
distance from the centre
of the ellipse in which
it revolves to the body
round which it revolves.
fn the adjoining figure,
if A B i) l<] represent
tho orbit of a planet or
satelJite, if C I

Fig. 37.

U\J ibS

lOS

BLEMENTB OF ABlRONOMY.

centre, and S the sun for primary planet), tlien tho
dhtanco C S will bo the linear eccentricity.

393. The eccentrldUj in moro usunUy taken to nicnn tlio pro-
portion which the lineal eccentricity bears to tlie Hoini-uxii*
major, t.*-., the proportion of C 8 to A C. 'I'liuH, if C 8 were \ of
A C, wo should say that the eccentricity Ih \ or 'jr).

391. As the minor axis (B E, Fi«,'. 37) of nn oUipso
increases, or the mtijor axis decreases, the foci (S and V)
draw nearer to the centre and to each other; the eccen-
tricity (C S or C F) lessens ; and the figure approaches
to that of the circle. When the major and minor axc8
are equal, the foci and centre coincide, and the figure is
Ok circle.

39.*). In the case of the planets and Batellites the two
axes are nearly equal, the eccentricity Is small, and the
orbits, therefore, are nearly circular. In the orbits of
the comets, the minor axis is considerably less than the
major axis, the eccentricity great, and the ellipse
elongated.

396. A planet's greatest distance from the sun is
when it is at the extremity of the major a.ds farthest
from that luminary — its least distance when at the other
end, that nearest the sun. And the difference between
the least and greatest distances will be twice the eccen-
tricity. In Fig. 37 above, if S represent the sun, a
planet is farther from the sun at D than at A by S F,
i.e., C S and C F, or twice the eccentricity. The mean
distance of the planet from the sun is the half of the
major axis, or, the semi-axis major, as C A or C D.
Observe the sun, S, is not in the centre of the ellipse.

397. These two points in a planet's orbit, where it is
at its greatest and' least distances from the sun, are
termed its Apsides, from the Greek word a-^ig (apsis),
the curvature or bend of an arch. The point fur^V.est
from the sun (D, Fig. 37) is termed the Aphelion: the
point nearest the sun, Perihelion (A, Fig. 37) -.—from

,1 r^..-.i_ l„ .^^ .-- /l-.r.1i<\t<N 4lio Biin r/rrh ^nnni.

lUe UTC'Uii. V.UiUS rjr.s-Ja -^ ii •- ii '_•!-• y, v..^^ ,.ui.j ^.-^_^,

from, and mo, (peri), around or near.

ELEMENTS OP A8TB0N0MV.

109

3( 8. Apogee is tho point in tho nicwn's orhit whom
it is farthcHt from tho earth ; Perigee, the point whoro
it \n nearcHt to the earth :— from yjj (go), tho earth, Airh
(upo), and mpi fperi).

•99. The Ecliptic \h that ^'reat circle of the fitarry
siilicre in which tho Kun's centre appears to movo during
the year ; or, thr pat'i through the, heavens which the earth
would appear to describe^ if seen from the sun (pa/. 104).

400. The Nodes of a planet are tho two points whoro
its orbit cuts tiie piano pjg gg

of the ecliptic. In Fig.

38, if K \\ e k represent

tho plane of tho ecliptic,

and E D c C tho orbit

of any planet, the points a

E and e are tho nodes.

Tho lino , joining tho

nodes is called the line

of the nodes ; a line from

E to e would be the line of the nodes.

401. The point at which a planet crosses to the north
of the plane of the ecliptic is termed its ascending node ;
the point at which it crosses to tho south of the plane
of the ecliptic, its descending node.

The term node is fiuxU the Latin nodus, applied to
signify an intersection.

402. It must be observed of tlie above figure that E B <? A is
meant to represent thej;/awe of the ecliptic, not the ecliptic itself,
wliich niny bo far beyond, or vnthin E li e A. If the curves in
the figures above represented both tlie actual orbits of planets,
there wouUl be danger of collision, as was apprehended of tho
earth and the comet of 1832, which actually did pass through a
point in tho earth's orbit, fortunately, however, a month hefor".
the earth in its annual revolution reached that point in space,
the two bodies b.^iur then about fift)'^five millions of miles from
each other.

403. The inferior planets are those which are nearei
to the sun iiian the earth, aa Venus and Mercury. — Tho

BLEMBNTS OF A^TKONOMY.

rig. 99.
a

■e-

•aperior ilanotH nro tlioKo which arc furtlior from tho
HUH thim tho cnrth, m Mars, .hipltcr, Hatuin, etc.

^^^i. When two c('luHti..l (il»)«>rtH are on tho fiune
meridian *uoy nro mu\ to bo in oonjunotion ; when on the
oppoHite nifridiiiuH, thoy are suid to be in opposition.

405. For cxainple, wlien a snpciior planet, aa Mars,
Ib on the same meridian aa the
Hun, the mu being between tho
earth and the planet, which ap-
pears in tho «anio part of tho
iiettvens &h the sun, and on the
Banie celestial meridian, the sun
and planet are said to be in con-
junction. Thus, in Fig. 39, if E
be tho earth, and a Mars, ihe sun
and Mars would bo said to be in
conjunction. When the earth is
between the sun and planet, so
that tiiey appear on tho opposite
parts of tho heavens, that planet
and the sun are said to be in op-
position, as at b in tho same figure.

400. In tho case of the inferior planets, when the
planet is between tho earth and
sun, it is said to bo in infc ior*-
conjunction; when the fiun is be-
tweon tho earth and planet, it is
said to bo in superior conjunc-
tion fn I'ig. 40, if E bo the
ea' .. '"d '., b, different positions
of an inferior planet, it is in hi-
ferior conjunction at 6, hi superior
conjunction at a.

407. Hence, when a planet is
in conjunction, it rises and sets about the same time as
tho sun. When it is in opposition, it sets when the sun

rlKPS. nnd risPR vvlion ihn snn atita

Fig. 40.

■t'ifENTS OF AgTIlONnMY.

408. Tlio disc of u liiuvoiily b<Mly ih tho fncc, or
apparently brjiui flut surfuco which it prcsciitH to tho
eye.

400. The phaiett of tlio moon or a planet nre tho
different appearances it presents, according om more or
less of its illuminated surfato is turned towanlt tho
earth, from the (ireck word (patti^ (phaKJs), tho appear-
ance presented by a body.

410. The term traniic »o -'sually applied to the passing
of Mercury or Venus betwe^ u the earth and sun, appear-
ing like a black si)ot on his disc. It i^ from tho Latin
iransi'tis, 8i;;nifying a passage ( " iroing over,

4 11. Occultation (disappearing, or being iiid) is tho
eclipse ot v. star or planot by the i iterposilion of tho
moou or some planet, which intercejjts our view of it.

412. Motion is said to be uniform when its rate
remains the same; accelerated, when it becomes vjuicker
every moment; retarded, when it becomes every moment
8k)wer. The mean motion of a [)lanfct is the rate at
whicii it would go if it moved uniformly, still describing
tho same distance in the same time.

413. A spheroid is a figure like a sphere, but having
its surface flattened at the two extremities of one of its
diameters, like an orange. That diameter is the shortest,
and the diameter at right angles to it is the longest
diameter of the spheroid. It is sometimes termed an
oblate spheroid, in contradistinction to a figure like an
egg, called a prolate spheroid.

414. A pendulum is any body suspended at a fixed
point, about which it swings backwards and forwards.
It performs its oscillations (vibrations) in equal ii^es
however difterentin length they may be, so long as the
pendnlum continues of th same length, or the force
which causes it to move remains the lame. But if
the pendulum be made shortc, or the moving force bo
greater, it will move m.ore quickly, raid vice versa. As
it io tiie force of gravity which causes its oscillations,

112

ELEMENTS OF ASTRONOMY.

these arc more rapid the strotif^cr tlic action of lliat force
is; and accordiii^dy it 1ms been used as a measure of
tlie strength of gravity. The vibrations of the pendu-
lum are more rapid as the rod is shorter (the time of
each vibration bcnng in proportion to the square root of
the length of the rod). At the latitude of London, 51"
30' 47-59'' north, the length of the pendulum which
vibrates in one second is 39-138 (about 39 J) inchcB.
From the isochronism (equal times) of its vibrations it
is used for measuring time.

415. The term force is applied to cxprcFS anything that pro-
duces, or prcvnts, or changes motion, or tends to produce, pre-
vent, or change motion.

416. A body actuated by a single force would, if that
force were sufficient to impart motion to it, move on for
ever in a straight line in tha direction of the force.

417. When two forces act upon a body at the same
moment, it moves in a certain direction, which is found
as follows: — Let the forces be represented by two

straight lines

meeting

in a point, whose directions

Fig. 41.

represent the directions in which the forces act, and
whose lengths represent the comparative intensities of
the forces : complete a parallelogram having these two
lines for sides. The diagonal drawn from the point
where the lines meet will show the direction in which
the body will move, and the force with which it will
be impelled. If, in the following figure, the lines A B
and A C repre-
sent the directions
and comparative
strengths of two
forces acting on a
body at A, so that
the force A C, act-
ing alono, would
carry it to C in the same time in which the force A
B, acting alone, would carry it to B, the body A will

^Xi —

ELEMENTS OF ASTUOXOMY.

113

nnivc in tliiit ♦.ime at D, tho otlici cxtrcmily of the
diap^onal, drawn from A, of the parallelogram A C D B,
of which A 13 and A C are sides.

418. The finding tho direction in which two or more forces
impel a body i.s termed the " Cvmpoaition of Motion."

419. The direction which the body takes always inclines
towards the direction of the greater force. Thus, in the above
figure, the angle J> A H Ih less than 1) A C, A H being greater
than A C, and A U evidently lies nearer to A B than to A C.
A single force which produces the same effect as two or more
other forces is called their resultant.

420. Centre of Gravity,— There is a certain point in
every body which bears such a relation to it, that the
same effects would ensue from its gravity (weight) if
a single force, equal to its weight ancl in tlie same direc-
tion (vertically downwards), acted upon the bod)' at
that point, the rest of it being then sup{)oscd to be
without weight. Such a point in a body is called its
centre of gravity. It is in the middle of a straight rod
or bar, in the centre of a circular plate of any mate-
rial, and in the centre of a sphere or spheroid, suppos-
ing these to be of uniform density at every part. As
any number of forces acting on a body may be replaced
by a single force called their resultant^ so a body may
be considered as an assemblage of separate particles
rigidly connected, the gravity or weight of each particle
is a force acting upon it, and the centre of gravity of
the body is the point through which, in whatever posi-
tion^ it might be placed, the resultant of the forces of
all its separate particles would pass. Hence, all the
parts exactly balance each other about that point.

421. Two or more bodies taken together may have
a common centre of gravity. The centre of gravity of
two bodies is found by joining their centres of gravity,
and dividing the joining line so that the weight of the
first is to the weight of the second, as the distance of
the centre of gravity of the second from the point of
division of the joining line, is to the distance of the

'il»

\i I

(

M II
II

114

ELEMENTS OF ASTRONOMY.

centre of gravity of the first from that point. Then,
by ' simihir method, the centre common to that centre
of ^^ravity, and the centre of gravity of a third body
may be found, and so on.

SECTION II.

Forces Acting Throughout the Solar
System.

422. The heavenly bodies are found to move in
curved lines: they must, therefore, be acted upon by
more than one force. (410.)

423. The orbitual moiions of the planets, satellites,
and comets, are caused by the concurrent action of at
least two forces ; — irst, A projectile, tangential, or
centrifugal force; second^ An attractive, central, or
centripetal force.

1. Projectile Force.

424. Any body thrown or projected forward, is called
a projectile^ as a stone from the hand or a sling, an
arrow, a musket or cannon ball ; and any force which
tends to impel a body in such a manner, is called a
projectile force. It is considered that, at some time,
the planets, satellites, and comets must have received
some such impulse, vvhich set them ^'n motion, and
which, combined with the attractive force, preserves
them in motioi in that course which they now pursue.

425. As, by the inertia of matter, a body tends to continue in
the state in which it is, whether that be one of rest or of motion,
the heavenly bodies do not require new impulses to preserve
their motion. That motion, once imparted, continues uninter-
ruptedly till it is weakened or destroyed by the oppusinj^ action
of some other force. — " The force by which the body is projected

ELEMENTS OP ASTRONOMY.

115

II-

18 one which we suppose to ho necessary at some past timo to
account for the planet's motion, but which acts no more. The

Elanets are in motion, and it is of no consequence to our inquiry
ow they received this motion; but it is convenient, for the pur-
poses of'calculation. to suppose that at some time they received
an impulse of the same kind as that which a stone receives
when thrown from the hand ; and this is the whole meaning of
the term 'projectile force.' " — Gravitation, by Amy.

426. The projectile force, acting alone, would throw
the revolving body out of its orbit, and cause it to
move on for ever in a straight line. The direction of
this line would be a tangent (see par. 390, and Fig. 8,
p. 21) to the orbit at that point where the attractive
force ceased and the projectile force alone acted on the
body : or, would bo in the direction which the planet
had at the moment of quitting the orbit.

427. Thus, in the adjoining figure, let the circle A B C D
represent the course of a body moving round in the direction
from A towards B, B towards C, and so on, and at the same
time drawn by a central force towards F. If, when at A, the
central force were to cease, the projectile force would cause the
body to break off from its course and proceed on in the straight
line A G, v/hich would be a tangent to
the circle at A. At B, C, or D were the
projectile force alone acting, the body
would proceed in the lines drawn from
t' ese points in the figure. And in the
whole course of Its revolution, the body
has a tendency to break off" in this man-
ner — in a tangent to the curve at the
point where it is when the central force
ceases.

428. This force, therefore, is termed projectile^ as it
tends to throw the body out of its orbit, and resembles
the force with which any projectile is impelled from
the surface of the earth. It is termed tangential, as it
tends to throw the body off in a tangential direction ,
and centrifugal, as it tends to impel it from the centre
round which it has been revolvinsr.

42y. The Gxistence of the projectile force is iaiferred from the

116

ELEMENTS OF A8T110N0MY,

orbitiial motions of tho plnncts; but no pnrticularB nn to Un
Hourco or nature have been nHccrtainod. It lias bocu caleulatod
that if tho earth received its motions of rotation and round the
sun from a ninglo impulse, that impulse must have passer
through a point alK)Ut twenty-five miles from its centre. An
impulse through tiio centre of gravity of a sphere would cause
it to move forwards in a straight line — through any other point,
tho impulse would cause both rotation and motion in space.

430. This cciitrifu«^al force, or tendency of a revolv-
ing body to fly off from tlie circle in which it moves, in
the tangential direction, is well illustrated by the pro-
jection of the mild which adheres to a carriage wheel,
— by tho water being thrown out of a mop when it is
rapidly whirled, — by the necessity which compels the
eipiestrian galloping in a circular arena to lean inwards
when the speed is great, to ovtirconie the tendency of
his rapid motion to throw him outwards, — in the manu-
facture ot crown or window glass, in wliich a knob of
glass is made to become a bowl of many forms, gradu-
ally spreading out, until it suddenly expands ''nto a
broad flat sheet, — in a railway train running off the
rails at a curve when the speed is not slackened, — and
very strikingly, in the centrifugal railway.

2. Attractive Force.

431. This is the force exerted by the central body,
on a planet, comet, or satellite revolving round it, by
which the latter is preserved in its orbit, and prevented
receding from the centre into free space, as it otherwise
would do from the action of the projectile force.

432. This power is termed attractive^ as it tends to
draw the planets towards the sun, and bodies genera^y
towards each other, etc. ; central or centripetal (centre-
seeking), as it urges them towards the centre round
which they rr-volve; and radial^ as it acts in the direc-
tion of the .ins.

433. The attractive force, acting alone, would draw
the revolving bodies inwards from their brbits in tho

ELEMENTS OP ASTRONOMY.

117

(lircction of tlio rad'uia, and procipitato tlio planots and
comets on tlio surfaco of the 8un, and satellites en the
primary planet round which they revolve.

Thus, if A, Fi^'. 42, be a body revolvin^^ in the orbit
A B C D, and the projectile force were suspended at A,
the planet would move to the central body in the direc-
tion A F.

434. It WftB dcmonatratod by Sn* Isaac Newton that a particle
of matter placed on the outer wurfaco of a hollow sphere in
attracted by it in the same manner as if the whole niattor of the
hollow sphere were collected into one particle in its centre. As
a solid sphere may bo rej^ardedas made npof an infinite numhc^r
of concentric hollow spheres, of uach of which the ahove would
be true; the same must bo the case with the solid sphere: it
must attract a particle at its surface in the same manner as if
its whole nmss wore in its centre. The heavenly hodies are bo
nearly spheres, and so distant, that they act upon each other as
if each were gathered into one very dense particle at its centre
of gravity, possessing the aggregate force of the mass. And the
true centre of the planetary motions is not the sun's centre, hut
the centre of gravity of the solar system (421). This point,
however, owing to the enormous mass of the sun comi)ared with
that of all the planets, etc., is only twenty-five miics distant from
his actual centre. In like manner, the earth and moon movo
round their common centre of gravity, and that is to bo regarded
as the point which is attracted by and moves round the sun.

435. Thefofxe of attraction is measured by the space
through which it draws a body in one second of time
after the body ij set at liberty. If the attractive force
be great, it will cause the body to move thiough a
greater space than a smaller attractive force would do ;
and this is the best measure of gravity, as it applies
equally to all bodies, irrespective of their size or density.
For, it has been found (first by the celebrated experi-
ments of Galileo) that, disregarding the resistance of
the air, large and small bodies, dense and light bodies,
f;ill through the same space in the same time. And if
the planets, satellites, and comets were at equal distances
from the sun, and left solely to his action, they would all
reach his surface in the same time. The force of the

113

r.r.EMRNTS OF ASTRONOMY.

earth 'h attraction, or force of pfravity, is such as to cause
a body witliiti any moderate distance of the earth's sur-
face to fall IG feet (correctly, IGi'.r feet) in a second, im-
I)arting to it at the end of the second a velocity of 32
feet per second, and continuinp; its action at the same
rate ; while the effect of the j)revi()us inij)ulse8 remain,
and thus the rate of descnit becomes rapidly accelerated.
The pendulum, when drawn out of the vertical line and
left to itself, is, in a manner, a falling body; and the
time in which it descends affords a measure of the
earth's attraction— the force which causes its oscillation
(a descent and ascent).

436. The attractive force prevailing through the
solar system seems to be the same as that well-known
power, distinguished on our i)lanet as the force of
gravity, which causes bodies to fall to the ground when
l«ft unsupported in the air, and which makes them exert
on bodies beneath them the pressure which we term
weight. Newton first showed that the force which
retains the moon in her orbit is the same which causes
heavy bodies to fall to the ground. The sun in like
manner attracts the planets and comets ; the moon in
her turn attracts the earth, and thus moves the waters ;
and each body is the source of an attractive energy
spreading through space, and more or less acting on
every other.

437. When spoken of with respect to its action
throughout the solar system, it is termed attraction of
gra"itation, or, simply, gravitation.

438. Three things may bo noted with respect to
gravitation. 1. That it acts in all directions, spread-
ing its influence out from a body like rays of light from
a luminous object. 2. That its force is in direct* pro-
portion to the quantity of matter {i. e., to the mass) in
the attracting body. 3. That its force is in inverse*
proportion to the square of the distance.

* When one thing alters in a certain way in the same proportion in

ELEMENTS OP ASTRONOMY.

119

439. Tliat prmvitation nets in all (Urcctions is shown
l)y a plmnmet Busponded near tlie top of a lii^'li preci-
pice loaning towards the rock, — hy bodioH tending
towardB the earth on every side,— hy the action of iho
moon in raising the waters of the ocean, and forming
the tides, — by tlio mutnal action of the sun, planets,
and satellites,— -and by Mr Cavendish's celebrated ex-
periments, in which small leaden balls were supported
on the ends of a rod which was suspended at tlio
middle by a slender wire ; and when large leaden balls
were brought near to them, it was found that the wire
was immediately twisted by the motion of the balls.

440. Oil tho eartli'8 surface gravitation acts in one predomi-
nating direction— namely, towards tho centre of tlio earth-
giving bodies that strong nnd invariable tendency downwards
called Gravity. This is not owing to any difference in nature
between the mass of tho earth and bodies upon it— but to tho
cncu instance of that mass being so very great compared with
that of any body on its surface, that all lotcral attractions aro
overpowered by the overwhelming force of the immense mass
under our feet. Also, lateral attractions neutralize each other,
while the force of the earth's attraction is not neutralized by
any opposite force equally near.

4^1. That the force of gravitation is in inverse pro-
portion to the square of the distance, is a mathematical
deduction from the elliptic form of the orbits of the
planets, with the mn in one focus ; following, there-
fore, from the second law of Kepler (see par. 454).

442. That gravitation has force in inverse proportion
to the square of the distance, signifies, that tho attraction
of one body for another, when placed at dillerent dis-
tances (other things remaining tlie same), is as much
greater as the square of the distance is less, and as

which another alters in the same way, this is torrnod direct proportion
IhuB, in tlie above instance, if the quantity of mutter diminish>s, the force
ot attraction diminishes as much. When one thinK alters in a certain way
in tho same proportion in which another altcrn in the opposite way this ia
invarse proportion. Thus, in tlie above instance, if the square of the dis-
tance between two bodii-.s dtnii/iis/ii».T, thn forpo rtf ntfronti,,,, »>,.* iU ,

tucreases as much as the square of tho dibtance has diminished.

L^i

, JK :

120

ELKMENT8 OF AbTKONOMY.

I ■

much less as the eqiuiro of the distatico is greater.
This is iihistrated by the following table, wlicrc the
first two columns reprcKont diftcront clibtanct'H,— tho
next two, the proportionate forces of attraction at these
distances, in sciuares, — and the last two, the value of
these squares expressed in numbers.

Attraction Attract

lot?

at 1 is to at 2

as 2*^

as 4 :

Ji •

... O

2-'

... 25 :

m •*. ... X

... 1^

2'J

... 1

. 4

•«• ... i

... X'

... 49

: 9

Thus, if a body be first at a distance of 1, and then
of 2 from another bjdy, the proportionate forces of
attraction exerted by it on the latter body will be 1 at
the distance of 2, 4 at the distance of 1. If they be at
distances of 3 and 7, the attraction at 3 will be 49,
while that at 7 will be 9.

443. The diminution in the above proportion of an influence
radiating from a central point, may bo iUustratod by the follow-
ing figure. Let G represent any luminous body, A \\ C D,

Fig. 43.

and E F, boards at the same successive distances as A B from
G, A B being at 1, C D at 2, E F at 3. The same quantity of
light which spreads over A B will, at C D, innce the distance,
spread over /bwr times the surface ; at E F, ilirice, the distance,
spread over nine times the surface. But the same amount of
light, when diffused over four times the space, will only have
oue-fouTtli the intensity — over ■^me times the space, one-ninth
of the intensity. Hence tiie light at 1 i» to that at 3 as 3^ : 1^,
as 9 : 1, or as 1 : i ;— that is, inversely as the squares of the
distances.

ELEMENTS OP ASTRONOMY.

121

444. It IS known tlmt tins universally diffused power
extend^ to the utmost limits cf the solar system. We
have reason to believe, from the phenomena of binary
stars, that it also prevails among the fixed stars. And
there 18 reason to suppose, that its various forms, as
exhibited on our globe,-gravity, cohesion, chemical
attraction, electric and magnetic attraction,— are merely
varieties of one fundamental power.

445. Few things are more striking than that invisihle and
S foT' '"""^T" ^'^•^^ «ubHiHtsl.etween the epa a e par
eacn other— acting with such enormous power, and at such itn-
thosunhfnllT ";•'" '*'« ««"tral force Vhich, Bpreading from

^1Z« of th""'"-'"?"' ^r'^ir' ^^'^ P'°"«*« '" t»'^''r 'orbits, at
distances of thousands ot millions of miles, and perhaps pre-

in'SZf ?* jn their proper positions, at distanc^JsSu^^^^^
m millions of millions of miles-exhibited also between the plan-

clot« °^^* •'^'"^."'''°" ^^'^^ P^'^"«« °" ^^^'h other or oHho
comets-drawmg a stone or drop of rain to the ground-causing
the ram or dew drops to form into globules like tuns and planet!

hBtr«'"in^'"^l^^ ^''° r''''^'' ^^ ^r'^" t« each other with

i nlod^f^rn rf? '' • ^^''. '^ *r '"■* '""'• prodigious strains-aiding

m producing the singular phenomena of electricity and matmet-

r«";:f?h' ^T^' '" ''' ^''r l^'^^^^^" dlfferentUies g^lnl
S J5' phenomena of chemistry, and creating the irinlmer?

eviry slde.'"''"'"'''''^'"^ combinations which surround us on

♦l,A InV^* ''f ^^^" inferred from astronomical phenomena, that

lvTsZfj'ST''7- '' V'^''' instantaneous%r that its'velo-
c ty IS at le„st//<i/ miUion tmiea greater than that of light; and

acts'nn tnkP'"f ;"*"' ^T^^ *^'^«"gh the densest bo^die and
acts on another body, without any diminution of its energy.

447. Besides gravitation, the force of Heat spreads tlifough-
an 1 t^^t: '^f 'T ' *^"'^- ^^ ^""^ *'"^*' "" our planet at leas ,
Iht i«rnl.« ™? •' t^ '^''*".'" *™'?' ^* S'^«" ^'^^ to motions among
DhenomPnf iui. ''T'^^'' ™''"' important and interesting
phenomena. Although we can ascertain little of its operation
in other parts of the solar system, it is not probable that an in-
^rTpn^;?;!'^ ^V^^ '^^^ ^^y^'. ^"^ ^^P^hfe of effects so great
nL.d fofi '/'l^ De without action on other bodies equallf ex-
& fi, r^'^*.' of whatever emanates from the sun! Accord-
l"?i^:f'l5^'_f J^^.^,'^t'„^^^?^•ohahle element in astronomical
^....^i.gv^, :iiu=fc iu;i uu OvcnooKeu la enumerating the forces of
the universe.-Heat, Light, and Gravitation link us with far

129

EI.EMF.NTfl or A8TRONOMA-

l\ I

ilintftnt woHdn ; nnd nerli,ip« Ohto arn utill other Influonccn. io
fine and iimnprcciahhi um to h.ivi! Iiitliurto uHcnpt-d our notice,
oiwratitiK nihintly upon uh, and binding toR«3tlior in one con-
nected chain tho moHt . jtnoto part« of creation.

448. Tho solar influences which ^vo rise to heat
and li^'ht, obey tlu^ same hiw v;ith respect to their
strength at difTercnt diKtanccs, »i.s gravitation, i.e.,
inverse proportion to the scjuare of tho distance. Thus,
the distance of Mercury being 36 millions of miles, and
that of tho Earth 92 millions of miles, the following
proportion will hold : —

Sun'H Influ.nce SunV Influence ^ ^

at Earth : at Mercury, tnverseii/, as n^- : 6b%
directly/, as, 362 . 922 ; as 1250 : 8481, or as 1 : 6J.

Thus, the sun's influence at Mercury is nearly seven
times what it is ot the earth.

4 19. But it must not be inferred from this, that tho
temperature at the dilTfn-ent planets is in exact propor-
tion to the sun's influence ; for the temperature is de-
pendent not only on the amount of tho heating agent,
but on many circumstances connected with the body-
acted on. This is illustrated on our own planet, where,
owing to the increasing rarity of the air, as theeleva-
tion is greater, the solar rays produce less effect in pro-
portion as tho height above the level of tho sea increases :
so that, at any latitude, the low grounds are warmer
than the elevated districts and mountains, which are
actually nearer to the sun.

SECTION III.

Orbitual Motions of the Planets, Satellites,

and Comets.

A r.A p^ ^Vc«,.Trn+;nr> nf tho rhnricres in Dosition of the
planets, satellites, and comets, and by computation

ELRMKNTP OV AHTKONOMV.

12.1

from tluar (.bsema i ov<.ment«, it lm« been ascertaind
orbL ♦{ "T ^^^"«*'^?^'y '^'"l '"K"larly in certain

satelliteH roun.i some plur.ot. And tlieno niotionn aro
expiaiiuMi by the supposition of a prcyectile mvulse
orifrumily imparted to each body, and a central or at-
tractive force continually acting on it.

451. Suppose that a body is projected in a direction
transverse to, or crossing the direction in which tho
force ot attraction draws it, how will it move ?
J. ♦!!..» T'^'P'r* instAuco of this motloa that -ve can Imneine
in the inotum of a stoi.o when it Ih thrown from the hand w u
hor,^„ntal (hrcction, or ir» a directio,, nearly .or ionta We
all know that the Htonc «cM.n falls to the ground ; amlif wo o^,-
Bcrve .t8 n.otion with tluj least ftttentio„,^e «", ha t d<^H no
move iu a HtraiK'ht lino It I^KinH to „ ovc Tn tl e d irec LnTn
winch .t .s thrown; b„t this direction iH speedily chanK^^^^^^ ft
.Tlkrthe ::.o un'i'''" '?'--'"""y/"d.c--tal.tly, Ld the «tono'
S^A fi *^ •"•'' TV""^ .*' ''"** ♦''"" '" a direction much in-
chncd to tlie onguial direction. The most powerful eflrot that
wo can nmko. even when we uhc artidcial meannVaJ in pnduc

forp «. n r^yfr'"^ falling at last. This experiment, there-
if a'lx^dv 'aP .^nl '' n "y!"f' '^tcly to judge w'hat will heco,.to
01 a body ^ap a planet) which is put in motion at a creat dis-
ancofrorn another body, which' attracts it (as the S- Im
It w. 1 assist „H much in judging generally what is en" nature

^fersrr^S.'" ^^^::t^^^ ^~^-

th;'b^^:;:£^L ,,,;S'-»-t«re of the motion is this:
path, of which the first part has
the same direction as the line
in which it is projected. Tho
circumstances of the motion of
a strjne may be calculated with
the utmost accuracy from tho
following ru'e, called the second
law of motion the accuracy of
which has been established by
many simple experiments, and
n^any inferences from compli-
cated motion ^. If A V;.^ AA :^^\^. ' ■ e -

Fig. 44.

194

rXEMKMTi or A8TR0NCMY.

If wo wliih to know whor.? tho ntom will Iw at tl«o on.l of tnr
nirticulfir turn (Mupp'»H«, for luHtuicc, tliro.o nocomlii), and If
tho vol.Kjitv with whii^h It W.IH thrown wouia, In throe •econ.N,
hare oarruHl it to H, nupiMHinR gravity not to havo ftctoil
on 't' and if f^rAvUy would have inadn it frtll from A tot.,.
*upp,)^inK it to hiivc JMJon n»«r«ly droppctl from th" hand ; then,
at tho on.l of thrco second**. t!u! «U)no i-m'.ly wdl ho at tho p<Hnt
1). which in dotermined hy drawing B U parallel and ''qtwl.to
A C- and it will havo reached it hy a curved ;»th A U, «>/, which
differont point* can Imj determined in the aamo way for differont
initants of time."— .4 tr^/ on (huinlution.

452. In giviii{? tho planets tlieir orbitual motions,
those two forces act on tlio prujciple of the composition
of motion (par. -117). Any curved lino maybe con-
sidored as made up of a iminler of infinitely Hmall
straiglit lines, which will be tho diagonals of a series
of parallelograms, whoso sides will bo lines m the
directions of tho projectile and attractive forces at each
point, and of lengths proportionate to tho intensities of
these forces. As the directions of the tangent and
radius change at every step, the body enters every
moment upon a new diagonal, the series of which will
form the curve which it describes.

4.'}3. " It is demonstrated that if a body (a planet^, for
instance) is by sor o torce projected from A, Fig. 45, m
the direction A B, and if the at-
traction of the sun, situated at S,
begins immediately to act on it,
and continues to act on it accord-
ing to the law we have mentioned
(that is, being inversely propor-
tional to the square of its distance
from S, and always directed to S) ;
and if no other force whatever but
this attraction acts upon the body ;
then the body will mov in one of
the following curves — a circle, an

^llittco o nnrjiVinln.. or ft hvDHrholft.
•-•"i J •- i < -- " ■-.• t -- " -=---

" In every case the curve will, at the point A, have

BLtirani or astkonouy.

125

the wimp direction ns Iho lino A II ; or (to tine the lan-
gnagu of niftthointtticians) A H will be u tungent to tlio
curve at A.

^ "The curve cannot be a circle unless the line A B
w iwrpendiculur to S A, and, moreover, unlcsii the
velocity with which the planet in projec^-d i» neither
greater nor Ichh than one particular velocity determined
by the k-n^'th of 8 A and the nuihH of the IxKly S. If
it diflcFH little from this particular velocity (either
greater or less), the body will move in an eliipHo; but
if It is much greater, the botly will move in a parabola
or hyperbola.

^ •* If A B is oblique to S A, afid the velocity of pro-
jection is small, the body will move in an ellipHr ; but
if the velocity is great, it may move in a parabola or
hyperbola, >>ut not in a circle.

" If the body describcH a circle, the sun is the centre
of the circle.

" If the body dcRcribeR an rllipse, the sun is not in
the centre of the ellipse, but in one focus."— ^/ry.

454. The following general laws, developed by
Kepler, are found to prevail tliroiighout the solar
system : —

I. The planets move round the sun in such a manner
that the Ik.? drawn from a planet to the sun passes over
areas proportional to the times of the motions.

II. Each planet descr'hes an ellipse, having the sun
in one of the foci.

III. The squares of the periodic times* of the planets
nre in the same proportion as the cubes of their mean
distances from the sun.

455. The first of Kepler's Laws is shortly expressed
as follows : — " The radius vector of a planet describes
areas proportional to the times."

456. This will be illustrated by the following figure.

tlon^u i'^'orbit:^""'''' ^'"'^ "^^"P^'^^ ■»>• -"3^ '^> i" conipieting one revoliT.

126

liLEMENTS OF ASTUONOAIY.

The radius vector of a planet is an iniai^'inary straight
lino passing from tlio sun to the planet, supposed to
emain fixed at the formor, but to follow the planet m
its course round that orb, expanding or contracting
according as the planet is further from or nearer to the
Bun.

Fig. 46.

457. In the above figure let S be the sun, and A, E,
G, H, a, e, successive positions of a planet ; then S A,
S E, S G, S H, i- , S e, will be the radius vector in
these several positions. Now, let it be supp^aed that
the planet moves from A to E, in the same time in
which it moves from a to e : it would then be found that
the radius vector, in passing from S A to S E, has tra-
versed the same extent of space as in passing from S a
to Se; that is, that the shaded area S A E is equal
to the shaded area S ae; or, as expressed above, that
the area S A E bears the same proportion to the area
Sac, as the time of the motion from A to E does to
the time of the motion between a and e; i.e., that
the areas are proportional to the times — (equal in the
instance just given, since the times were supposed equal).
458. Conversely, if the area S E G be equal to
ft a TT iho Tilunet will move from E to G

the

?a ?5

ELEMENTS OF ASTRONOMY.

127

in tho sarno time .is from G to H. And any area,
S G H, will bear tho same proportion to any other
area, S 11 K, as the time in [)assin^ from G to H
docs to the time in passing from H to K.

459. Hence, then, a planet does not move round the
Bnn at a uniform rate ; its motion is at one time acceler-
ated, at another retarded.

4G0, For, as the planet is at different distances from the sun,
and its radius vector describes equal areas in equal times, any
area, when the planet is near the sun, must be broader than an
equal area when the planet is remote : tho part of the orbit which
bounds the broad area must be longer than that which bounds
the narrow one ; find as they are both described in tlio same
time, the planet must move faster in that nearest the sun.

461. The velocity of a planet is least when furthest
from the sun, becomes accelerated as it comes nearer, is
at its highest when the planet is nearest to the sun, and
becomes retarded as its distance from the sun increases.

462. The velocity of a planet in different parts of its orbit is
in inverse proportion to the square of its distance from tho sun.

463. From this law, that the areas described by the
radius vector are proportional to the times, tho conclu-
sion was drawn by Newton that the powers by wh'ch
the tangential force of the planets is neutralized is
directed towards the sun.

464. The second of Kepler's Laws is, that " the
orbits of the planets are ellipses, with the sun in one
focus."

465. The planets and comets do not fall to the sun
when they approach nearer to him, as might be ex-
pected, because the tangential force becomes stronger at
the same time ; and they do not fly from the sun alto-
gether when they recede from him, because while they
recede from him the tangential force at the same time
diminishes in energy.

466. That this is the case will be illustrated by the
folic wing figure. Let this figure represent the orbit uf

128

ELEMKNTS OF A8TU0N0MY.

1 !i

IB I

a planet, A ; S being the sun. Let the phanet be in its
aphelion at A. It is there under the influence of tho
attractive and pro-
jectile forces, whoso
united operation
brings it to B. Be-
ing there nearer to
the sun, it is more
powerfully attracted
and drawn still nearer
to him ; and, as the at-
tractive and projectile
forces are operating ?
V arly in the same \
direction^ the velo-
city is increased, the
planet proceeding
from B to C, a greater
distance, in the same
time in which it
passed over the short-
er distance from A to B. At C, being nearer than
at B, the attractive energy is further increased, and as
this still concurs in direction with the tangential force,
the velocity is augmented. This goes on till it comes
to its perihelion at E, where its velocity is greatest.
The great projectile force thus acquired prevents it
going still nearer, overcomes the incrvrased attractive
force, and causes it, at E, to begin to increase its dis-
tance, which it does, step ^ step, from E towards A,
in the reverse order to that by which it had lessened
its distance. The attractive force decreases rapidly as
the distance increases; but the projectile force also
diminishes, as the sun's attraction is now acting nearly
directly against the projectile force^ and it thus lessens
its impetus at every step. By this, in progressing
from E to Fj Gj H, the projectile force is so much

ELEMRNT3 OF ASTKONOMY.

129

•;"t

weakened that the attraction of the sun overcomes
it 111 turn, and bends the phinet's course towards A,
where, when it arrives, the same series of action? com-
mence again.

467. That the attractive and projectile forces act mostly to-
gether as the planet passes from its aphelion to perihelion, and
mostly of/ainst each other when it is proceeding trom perihelion
to aphelion, is shown hy the directions of the arrows.

468. Were the planets, satellites, and comets under
the influence only of their own projectile force, and the
attractive force of the central body round which each
revolves, they would obey strictly the laws which have
just been developed, of motion in ellipses with their radii
vectores describing areas proportional to the times. But
each is a source of attractive force which more or less
influences every other one, and causes disturb-^nces and
irregularities ir^ their movements, by which tht^ deviate,
often considerably, from truly elliptical orbits. These
disturbing forces render the motions of the heavenly
bodies exceedingly complicated. The deviation of the
planet Uranus from his calculated course led astrono-
mers to suspect some disturbing force, previously un-
known ; which idea, followed out, led to the great
discovery of the planet Neptune, the body whose action
on Uranus caused the irregularities in the movements
of the latter.

469. The third law of Kepler establishes a relation
between the distances of the planets from the sun and
the periods in which they complete their revolutions
round him— namely, that " the squares* of the periods
are proportional to the cubes* of the distances."

470. That is, the square of the number of days any

The square of a number is tlie number produced by mnltiplyinp the
number by itself: the cube of a number is the product obtained by multi-
plyinff it twice bv itself. Thus. 9 is the Rfinarp nf a 97 tha /.i.Ko of 3- a the
square of ii, 8 the cube of 2 ; 25 the square of 5, 125 tlie cube of 5.' ' "*

F 2

ELEMENTS OP ASTRONOMY.

planet takes to go once round the sun, bears the same
proportion to the square of the number of days any other
planet tp-kes to complete its revolution round the sun,
ks the cube of the distance of the first planet from the
sun b'^ars to the cube of the distance of the second planet
from the sun.

471. Or, in the case of Venus and the earth,

cube of cube 0/

: 66-6 : 92

Bqnare of
224-7

sqtiare of
365-25

The first number expresses the number of days occupied by
Venus fn her revohition round the sun ; the second the number
of days the earth takes to go round the sun; the ^ird number
Z the distance of Venus from the sun, expressed m milhons of
miles ; the fourth, the distance of the earth from the sua m
niillions of miles.

SECTION IV.

Rotatory Motions and Forms of the Sun,
Planets, and Satellites.

472 The sun, planets, and satellites, rotate, or turn
^ipon themselves, in regular periods. The time in
which this rotation is completed is called the day ot
the revolving body ; the imaginary line about jliich it
turns the axis ; and the two extremities of this line,
the poles. They are known to have this rotatory
motion by the motion of spots upon tbeir discs ; and,
by observing the tiiue a spot takes to move through
aiiv arc, the time of a complete rotation is ascertained.
This motion goes on simultaneously with their motion
in ' space, iust as the wheel of a carriage, or a ball in
rolling along the ground, rotates while movmg on-

^ItS The sun and planets are of a globular form,
but not perfect spheres. They are oblate spneroius

ELEMENTS OF ASTRONOMY.

131

(413). The flattening is at the poles, or opposite ex-
tremities of the axis, and is somotinies termed the
" polar compression. " This f\k. 4s.

flattening is most remarkable
iu Jupiter, in which it is so
great as to give to that planet
a distinctly oval shape. The
spheroidal form is shown, con-
siderably exaggerated as re-
gards the r>un and planets, in
the adjoining figure, where N
and S are the poles, and E Q
the equatorial diameter.

474. The spheroidal form of the sun and planets is
most probably canned by their rotatory motion ; which
has a tendency to produce a flattening at the poles and
bulging out at the equatorial regions, even though they
had. at first been formed perfectly spherical : and their
having this form affords a presumption in favour of
their having: a iotatorv motion. A fluid mass of uni-
form density, the particles of which mutually attract
each other, will take the form of a sphere if at rest, but
will become spheroidal if it rotates.

475. That the earth is of a spheroidal form is proved
bv two cirouiiiStances ; the slower vibration of the pen-
dulum as the place is nearer the equator, and the in-
crease in length of the degree of latitude af the place
is farther from the equator.

476. The rapidity of descent of a falling body is the
measure of the force of gravity : the pendulum is such
a body ; and as it moves {i. e., falls) more slowly the
nearer it is to the equator, we infer that the force of
gravity diminishes towards the equator; and as the
diminution in the rate of movement of the pendulum
is greater than can be accounted for by other causes
(heat and centrifugal force), part is attributed to the
Bpheroidal formj which must lessen the force of gravity

■

132

ELEMENTS OF ASTRONOMY.

fl I

1 :''

of the parts near the equator, hy phicing them at a
greater distance from the centre.*

477. But the spheroidal form of our eartli is still
more clearly proved by the length of the degree of lat-
itude, which is not everywhere the same, but increases
slightly towards the poles. Our means of measuring
the length of a degree of latitude consist in measuring
a degree of an hour-circle in the starry sphere, and
finding the distance we must go north or south on the
earth, to cause any star to rise or sink one degree in
the heavens. It is found that a greater distance is
required to effect this the nearer wo are to the poles ;
this is exactly what would take place on a spheroid ;
and the degree of difference affords a means of estimat-
ing the degree of spheroidicity. This may be illus-
trated by the adjoin-
ing figure. Let P Q
represent the earth's
surface from onr pole
to the equator, and p
q the corresponding
arc of the heavens, p
being the pole of the
heavens and q the
equinoctial, 90° dis-
tant from the pole.
Let the arc p q ha
divided by the points
1,2,3,4,5 into six ^
equal arcs, -"^^"^^

Fig. 49.

may represent degrees of declination (being in fact arcs
of 15° each). Then, the places on the earth at which
these points (1,2, 3, 4, 5) are vertical, would be the
corresponding degrees of latitu de on the earth. But it

• As mentioned in par. 414, the pendulum vibrates more slowly as It is
lonecr. Now, heat expands or lengthens a pendulum rod; so that, ftora

, ." 1 - - J-1-- ~,.,ot »Y,n„n Tnni-n cinwlv in tlip. wiirm rn "iiiiia

tbis caubu aiouc, £1 ^iciiuuniui mi.~. TT.n ,. - .<,-

of the torrid zone than in higher latitudes.

ELEMENTS OF ASTRONOMY.

133

IS evident from the fignro, in which the lines a 1, J 2,
c 3, rf 4, etc., are perpendicular to the earth's surface,
that the distance l)etvveen the two adjoining points in-
creases as we pass from the equator to the poles ; or
that a degree of latitude is longer in proportion as the
distance from the equator increases. Thus, one degree
of latitude is not exactly one 360th part of a meridian
circle. If the earth were a true sphere, arcs in the
celestial meridian would correspond with arcs of like
number of degrees in the terrestrial meridian ; that is,
to refer to the above figure, straigiit lines from the
earth's centre, C, to the points q, 1,2, 3, 4, 5, 6, in the
heavens would divide the arc P Q on the earth into six
parts exactly equal to each other ; and these points in
the heavens would be vertical at the points where these
lines would cut the earth's surface.

478. In consequence of the rotatory motion, the parts
at a distance from the axis have a considerable centri-
fugal force, or tendency to fly off in the tangential
direction, and they would do so if they were not held
together by a firm attractive force ; or if the centrifugal
force were sufficiently strong. And they have the
greater tendency to fly off, the nearer they are to the
equator, for the parts at the surface of a rotating body
move with different degrees of rapidity, and conse-
quently different degrees of force. The polar points do
not move out of their places, but simply turn round
during the whole rotation ; and each point describes a
larger circle of rotation as it is nearer to the equator.
Thus, while a person at the equator is carried 24,897
miles by rotation in the 24 hours, one tit London moves
only 15,500 miles; at the arctic circle, about 10,000
miles in the same time. But, by the planet's attraction,
the parts have also a tendency towards the centre, in
the direction of the radius ; and as the tendency to fly
off acts in some degree against the tendency to the
centre, it neutralizes a part of that tendency, and thuB

131

ELEMENTS OF ASTRONOMY.

diminishes the gravitating' force sensibly where the
centrifugal force is great; that is, in ti»o equatorial

regions. .11

479. As the centrifugal force of a rotating boMy
thus lessens gravity towards the eciuatorial regions, the
parts there are urged outwards by this centrifugal
force, until an equilibrium is induced by the increased
quantity of matter between the centre and the equa-
torial regions.

480 Ry thcso forccH it is at onco evident that in a rolatinj?
body tho crcatcr centrifugal force in the equatorial regions
would cauMO a bulging out of these regions, if in th" fluid state.
IJut even were a planet, «uch m the earth, with large portions
of its surface covered with water, mainly in the solid state ana
perfectly spherical at first, a rotatory motion would cause a polar
co.npressi.m. For, the parts at the surface m the liamd form
would be thrown towards the etiuatorial nigions, and heaped up
there, while the polar regions would bo left dry. Again, as tho
parts at the surface are elevated by volcanic heat, and tluis to a
certain extent movable, and the earth is continually worn dovyu
by the action of disintegrating agents, dilTused through the
waters and thus rendered loose and movable, subsiding after-
wards and filling up the lower parts of tho deep seas,— the ex-
cess of land near the poles might be in time reinoved, spread out
over the equatorial regions, and thus an equal distribution of
land and sea might take place over the whole.

481. In the case of the earth it is probable, from
geological considerations, that the spheroidal form was
assumed while it was mainly or entirely in the fluid
state; the opinion being held that the eaHh was atone
time wholly, and that it is still partially, in a molten,

fluid state. n 1 i i

482. Next, the spheroidal form of the planet also
lessens the force of gravity about the equatorial regions ;
the parts there being further from the centre. Hence,
even though the planet did not rotate, if it had the
spheroidal form, the force of gravity would be somewhat
greater at the flattened than at the projecting parts.
Thus, directlfj^ by giving the great centrifugal ibrce to

ELEMENTS OP ASTUONOMY.

135

tlie parts in tlio equatorial ro^ons; and indiredly^ in
causing the spheroidal form, tlic rotatory motion is the
Bource of the diflerences of the force of gravity ut dif-
ferent latitudes.

483. Accordinfjiy, it is actually found that a body
weighs less, or produces less downward pressure, in
proportion as it is nearer to the earth's equator ; that
its gravity increases as we approach either pole. This
difference in the force of gravity at the poles and middle
regions cannot be manifested by a c(mjmon balance, as
the weights used would bo as much affected as tlie body
to be weiglicd. But it is at once detected by a spring
balance, or by the pendulum. The spring is less
stretched by the same body in proportion as the distance
from the pole is greater: and the pendulum vibrates
slower. Both of these circumstances indicate a diminu-
tion in the force of gravity. For particulars, see the
description of the eai'th in ISect. VI.

SECTION V.

General Facts relating to the Solar System.

'?4. The solar system, as presently known to astro-
nomers, consists of IGO distinct bodies, viz., the Sun;
9 large planets revolving around him in nearly circular
orbits ; 132 asteroids, or small planetary bodies, between
the orbits of Mars and Jupiter; 18 moons or satellites,
one of which belongs to the earth, and all the others to
the four most distant planets; together with a host of
comets and countless myriads of meteorites. The
large planets, in the order of their distance from the
sun, are: — Vulcan (whose existence is somewhat doubt-
ful), Mercury, Venus, the Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune. The asteroids have
all been discovered during the present century, the
first of them (Ceres) huviiig been detected on Junuaiy

130

ELEMENTS OP ASTUONOMY.

1, 1801 (roo par. r>71). Owinj? to its proximity to

1 (r.

1, V

tlio Hun, Vulcuii ia never seen by the nsiked cyo, and
Mercury very soldom, nor can Uranus or Neptune,
o\vin«? to their vast distance from tlie sun and earth,
while the asteroids are invisible on account of the small-
ness of their dimensions. Hence, with the exception of
Mercury, all these bodies were wholly unkno ^ .: to the
ancients.

485. All the p'.mets move round the sun in the same
direction as the carih— west by south to east ,- and their
rotations on their axis are in the same direction— /row
west to east,

486. La Place calculated that the probaT)ility is as
four millions to one that all the motions of the planets,
both of rotation and revolution, were imparted at tho
same time by on-j original cause, common to them all.

487. The [)lanes of the orbits of the principal planets
are not much inclined to that of the earth's orbit ; but
all are inclined to it a little, so that one half of a planet's
course lies north of the plane of the ecliptic— the other
half south of it. From the orbits of the leadin^-j planets
being a little above or below the plane of the ecliptic,
they°always appear i.:ar the ecliptic. Few lie beyond
the zodiac.

488. If a spectator were to view the solar system
from a point on the north side of the planes of their
orbits, the planets and satellites would appear to move
in a direction opposite to that of the hands of a watch,
as in Fig. 47 and Fig. 18 ; and this is the manner in
which the courses of the planets, etc., are usually repre-
sented in diagrams.

489. From the earth, as well as each of the planets,
being in motion round the sun, the planets appear at
times to be actually stationary in the heavens, or even
to move back, i.e., in a retrograde direction. But these
apparent irregularities can be explained and calculated :
the real motion of all is from west to east. The reUo-

I.I.HMENT8 or ASTRONOMY.

137

Fig. 60.

grade niovoinont of a planet may 1)0 undiTstcMHl from the
nfljoinin^^ ri«;uro. If a planet, moving in the orbit ro-

t)resente(l in tho
ower curve, move
through tho dis-
tances between
the anocessive
numbers in tho
same lime as the
planet in tho up-
per curve moves
between the cor-
responding' num-
bers, both starting
together from tho
positions num-
bered 1, then tho
planet in the
upper curve will
appear to those
who view it from
tho other planet
to move from I
to 2 in the sky —
represented by the
dotted line at the
top; then from 2
to 3, a retrograde
movement; then
froir. 3 to 4, also retrograde; and then will resume
its original direction, and pass ftom 4 to 5.

490. Though the planets appear as mere points to
the naked eye, they present discs of considerable breadth
when viewed through telescope, wen of very moderate
magnifying power.

491. This affords a very striking illustration of the remote-
ness of t- fixed RtArs: an ordinary telescope magnifies a planet

138

ELEMENT! OY ABTUONOMY.

into a tK'.rccptil.lo i\Uc, or br«R<UIi <.f «urf.iCO ; l.ut tho (Ixcl iiUrii,
vu!Wo.l by the iiumt p..wuilul tclc^ooptif, ilill appear a« luww
luininouM jmi'mU.

492. The part of my plunct whicli is turned towardu
tljc I'urtli will ulwayHbu one-half of its spherical surface,
and it will uppi-ar rh a tha circular Hiirlaco whou tlio
whole (.f the part next uh in visible, jiiKt as the sun and
full moon appear; while, if le.ss than the whole of the
half next UK he visible, the idanet's disc will be propor-
tionately less, and not of ft circular form.

493. The planets and satellites do not shine by their
own inherent li-ht, but by reflecting the li-ht which
they receive from the sun. This is known by the
phases wliieh they present, for a planet varies m the
m».'nitude of the illuminated surface which is turned
lowm-dsus; aiHl it is found that, ./ Me «;^« «'^"f ."
next us, thai part only appears luminous which is at the
same time turned towards the sun, so as to receive his
light. The adjoining figure represents the phases ot

Fig. 61.

Venus for the year 1851, as they appeared in the tele-

scope.

494 The nine leading planets may be arranged m
two characteristic groups— the interior, those nearer to
the sun than the asteroids; and the exterior, those
beyond the asteroids. The interior are Vulcan, Mer
..,:„ Vonna fhft Rjifth, juid Mais j the exterior an

Ul-viV. T - "••■•7 ) '

are

UUIJ,

KLCMENTS OF ASTRONOMY.

189

Jupiter, Saturn, Urfttms, nnd Noptuno. The followinj^
(liflferenccs uro found between theHe twogroupH: — I. The
nvcm^'i' eciuatorial diameter of the exterior planets 13
about yj tiineH that of tlie interior phuieth. '?. The
ttvera^'e den«ity of the interior phmetH (l*')3) is . eaHy
«lx timert that of tho outer planets fO-Hi). 3. i he
average day (time of rotation^ of tlio inner planets

24^h. jra.) ig jnore than <l()ul)lc that of tho outer planets

gh. 45m.), The exterior planets have each seveial

«!.-■ lites, and one has a ring, while, excepting tho

eavlli, which has one satellite, no such appendages are

found attending tho interior planets.

SECTION VI.
Of the Sun, Planets, Satellites, and Comets.

The Sun {Sol), C

495. Tho sun is the centre of tho solar system, and
is a globular body of immense magnitude.

496. Its mean distance from tho earth has within
the last ten years been ascertamed to be about ninety-
two millions of miles (92,093,000); or about 23,:U5
times the length of the earth's polar radius. But the
earth is nearly three millions of miles nearer to the sun
in our winter than in our summer. — See Earth.

497. The sun is not perfectly spherical ; but, like
all the planets, is flattened at the two opposite points,
termed poles. Its form is, therefore, a spheroid, like
an orange. Tlie sun's polar diameter is about eight
hundred iuid fifty-two thousand miles (852,389': — or
108 times the length of ihe polar diameter of the
earth.

140

ELEMENTS OF A8TR0N0MV.

408. The mapnitu(^«' (or volume) of the sun is to that of our
earth as 1,245,130 to 1.* That is, the sun is extended through
1,245,130 times the space occupied by tlie earth. The gravi-
tating force, or mass, of the sun is to that of the earth as
314,760 to 1 ; and is more than 745 times that of all the planets
taken together. As the sun v eeds the earth so much more in
bulk than in weight, the dens y f of the sun must be less than
that of the earth : the sun's Icnsity is little more than one-
fourth of that of the earth, or as 0"25 to 1.

Hence, owing to the comparatively small gravitating force of
the sun's matter, and the distance from the surface to the
centre, the force of gravity at the sun's surface is only 27 "2
times that at the earth's surface. A body, therefore, which at
the earth's surface would compress a spring to an extent indi-
cating a weight of one pound, would, at the surface of the sun,
compress that spring as much as 27-2 pounds would at the
earth's surface. — The equatorial circumference of the sun is
about 2,680,000 miles.

499. The sun rotates upon its axis in a little more
than twenty-five days (25^- 7^- 48™) in a direction from
west to east. This is ascertained by observation of the
motion of the spots on his surface. These are found to
disappear on one side, while others appear on the
opposite side, move round in the same direction, and
disappear in their turn on the same side as the former.
The uniform progressive motion of the spots, and in the
same direction, can only be explained by a rotatory
motion of the body of the sun in that direction.

500. " Tt has been foiind, however, that the spots, caused by
their being carried round by the sun in its rotation, have a
motion of their own. This proper motion, is distinguished fi'om
their apparent motion, has recently been investigated in the
most complete manner by Mr Carrington. What he has dis-
covered snows that there need be no wonder that different
observers have varied so greatly in the time they have assigned
to the sun's rotation. He shows that all sun-ppots have a

* The magnitudes of spheres are as the cubes of their diameters, — that is,
in the present instance,

Magnitude Magnitude
of earth : of sun : : 1^ . 10£3 : : 1 : 1,245,130

t Specific gravity, or comparative quantity of gravitating matter in the
same volume.

ELEMENTS OP ASTRONOMY.

141

movement of their own, and tliat the rapidity of this movement
varies regularly with their distance from the solar equator.
The spots near the equator travel faster than those away from
it, so that if we take an equatorial spot we shall say that the
sun rotates in about twenty-five days ; but if we take a spot
situated half-way between the equator and the poles, in either
hemisphere, wo shall say that it rotates in about twenty-eight
days." — Lockyer'8 Elementary Astronomy.

501. The sun's axis is not perpendicular to the plane
of the earth's orbit; it leaiis 7" 15' from the perpen-
dicular ; forming therefore an angle of 82° 45' with the
plane of the ecliptic.

502. It may now be regarded as certain that the sun is not
fixed in one spot, but that it has a proper motion, as it is termed,
throucrh space, carrying the planets, etc., along with it. See
par. 707.

i>03. The sun has two apparent motions^ one daily,
through the sky, giving rise to the alternations of night
and day; another yearly through the constellations of
the zodiac, causing its different degrees of elevation
above the horizon at different periods of the year.
These apparent motions of the sun are caused, the first
by the rotation of the earth on its axis, the latter by
the earth's annual revolution round the sun.

504. The sun is considered to be opaque in its body,
but to be surrounded by a highly luminous atmosphere,
from which emanate the rays that cause light and heat
when they strike upon bodies.

505. Viewed through a telescope, the sun presents a
somewhat mottled appearance, with minute shady spots
scattered through the luminous matter. Large dark
spots, which are not permanent, and which change
both in size and form, are se:n upon its surface. These
are termed maculae : they consist of a dark or black
part in the centre, called nucleus, with a surrounding
part not so dark, termed penumbra. In the vicinity of
the spot, brilliant and highly luminous strej^.ks are seen ;

^^r|g« 1

142

ELEMENTS OF ASTRONOMY.

Ill

506. The maculae are found only about tlie equa-
torial regions of the sun. Their magnitude is very
various — from a few hundreds to upwards of 45,000
miles. A circle of 461 miles in diameter, corresponding
to a single second of angular measure on the sun's disc,
is the least space which can be distinctly discerned on
the sun as a visible area. " Herr Schwabe, of Dessau,
has very recently made a most interesting discovery
regarding these sun-spots, viz., that ihey increase and
decrease in frequency periodically. He has shown that
in the course of about ten uiid a half years they pass
through a complete cycle of changes. They become
grfidually more and more humerous up to a certain
maximum, and then as gradually diminish. At length
tluo sun becomes not only clear of spots, but a certain
well-marked darkening around the border of his disc
disappears altogether for a brief season. At this time
the sun presents a perfectly uniform disc. Then gradu-
ally the spots return, become more and more numerous,
and so the cycle of changes is run through again. There
seems every reason for believing that these periodic
changes are due to the influence of the planets, especi-
ally of Jupiter, upon the solar photosphere ; though in
what way that influence is exerted is not at present
perfectly clear. Some have thought that the mere
attraction of the planets tends to yn'oduce tides of some
sort in the solar envelopes. Another equally interesting
discovery is closely allied to that now mentioned. It
appears that the periodic changes in the solar spots are
associated with the periodic changes in the character of
the earth's magnetism. The sun-spots vary in frequency
within a period of ten and a half yeai-s, and the mag-
netic diurnal vibrations vary within a period of the
same duration. They agree most perfectly, not merely
in length, but maximum for maximum and minimum
for minimum. When the sun-spots are most numerous,
then the daily vibration of the magnet is most extensive,

ELEMENTS OP ASTRONOMY.

143

while, when the sun's disc is clear of spots, the needle
vibrates over its smallest diurnal arc. In short, such is
the bond of sympathy between the earth and the sun,
that no disturbance can affect the solar photosphere
without affecting the earth, and doubtless all the other
planets, in a greater or less degree." — Other Worlds than
Ours, p. 26, et seq.

507. It is but very recently that astronomers arrived
at any definite knowledge of the physical constitution
of the Sun, and this knowledge has resulted form a
skilfully contrived instrument called the Spectroscope.
Sir Isaac Newton showed that if a pencil o^' solar light
be introduced into a dark room through a small round
aperture in the window-shutter, and be then made to
pass through a glass prism with its refracting angle
downward, an oblong rainbow-coloured picture of the
sun, called the solar spectrum, will be formed on the
opposite wall, or on a white vertical screen placed in a
line with the glass prism and the aperture. The prism
has analyzed or decomposed the pencil of colourless
light, and portrayed its seven constituent colours (vio-
let, indigo, blue, green, yellow, orange, red), according
to their various degrees of refrangibility, on the wall or
screen. By employing another prism he re-combined
these seven coloured rays, and reproduced the clear,
transparent, solar ray ; but here his beautiful discovery
ended. In 1802, Wollaston discovered that there were
dark lines crossing the spectrum in different places.
Subsequently, a celebrated German optician, named
Fraunhofer, mapped down the principal of these lines
with great exactitude. He also discovered similar lines
in the spectra of the stars. Such lines are now known
as the Fraunhofer lines, and they have led to the most
brilliant discoveries of modern times. Kirchhoif, an-
other celebrated German, has greatly distinguished
himself by aflfording a satisfactory explanation of the

d--l_ i: I-:!- cj-.I-cc T>,^^er-.■,^ Cf^.^To^.^- T ^^I'.T/^.,
Uik iilit'H ; WililC QLUr..Cb, OJllliUlil i^lcvvail, xjv^r.yti,

\0.

144

ELEMENTS OF ASTRONOMY.

Frankland, and others, in our own country, have power-
fully contributed still further to develop the woudertul
science of spectroscopy. " They examined the spectra
of the light from incandescent substances (white-hot
metals and the like), and found that in these spectra
there are no dark lines. They examined the spectra of
the light from the stars, and found that these spectra
are crossed by dark lines resembling those m the solar
spectrum, but differenthj arranged. They next tried
the spectra of glowing vapours, and they obtained a
perplexing result: instead of a number of dark lines
across a rainbow-tinted streak, they found bright lines
of various colour; some gases gave a few such lines,
others many, some only one or two. Then they tried
the spectrum of the electric spark, and they found hero
also a series of bright lines, but not always the same
series— the spectrum varied according to the substances
between which the spark was taken, and the medium
through which it passed. Lastly, they found that the
light from an incandescent solid or liquid, when shimng
thr(»igh various vapours, no longer gives a spectrum
without dark lines, but that the dark lines which then
appear vary in position, according to the nature of the
vapour through which the light has passed. Here
were a number of strange facts, seemingly too discor-
dant to admit of being interpreted. One discovery only
was wanting to bring them all into unison. In l»5y,
Kirchhoff, while engaged in observing the solar spec-
trum, lighted on the discovery that a certain double
dark line, which had already been found to correspond
exactly in position with the double bright line forming
the spectrum of the glowing vapour of sodium, was
intensified when the light of the sun was allowed to
pass through that vapour. This at once suggested the
idea that the presence of this dark line (or rather, pair
of linfia^ in the spectrum of the sun is due to the exist-
ence of the vapour of sodium in the solar atmosphere,

ELEMENTS OF ASTRONOMY.

145

was

and that this vapour has the power of ahsorhing the
same order of light-waves as it emits. It would of
course follow from this, that the other dark lines in the
solar spectrnm are due to the presence of other absor-
bent vapours in its atmosphere." — Other Worlds than
Ours.

508. It is now found that besides sodium (first dis-
covered), the sun's atmosphere contains the vapours of
iron, magnesium, barium, copper, zinc, calcium, chro-
mium, nickel, titanium, and probably gold, together
with the non-metallic element hydrogen, which is largely
present. As yet it has not been proved that gold, sil-
ver, mercury, tin, lead, arsenic, antimony, or aluminium
exist in the sun, though it by no means follows that
these, and indeed all the sixty-five elements which are
known to exist in our planet, are really absent.

509. Now, if the vapours of these metallic bodies
exist in such marked quantity in the sOlar atmosphere,
such vapours must arise from molten oceans of these
substances covering the sun's surface. This will enable
us to form some idea, however inadequate, of the intense
heat characterizing the great central luminary. Indeed,
it has been calculated that the heat thrown out by
every square yard of the sun's surface is as great as that
which would be produced by burning six tons of coals
on it each ^lour. Now, the surface of the sun amounts
to about 2 4,000,000,000 square miles, and there are
3,097,600 square yards in each square mile. Then,
with regard to its brightness. Sir John Herschel has
found that the sun's light exceeds in brilliancy the
brightest light we can form, viz., the lime-light (pro-
duced by a flame of oxygen and hydrogen playing on a
ball of lime made intensely hot), at least 146 times.
Now, as the earth is so far away from the sun, and is,
moreover, so very small as compared with the sun's
volume, it is clear that we receive but a very small
portion of the total amount of light and heat which ho

116

ELEMENTS OF ASTIlONOMY.

is constantly radiatin-. If wo suppose al the I gl t
and heat which the sun discharges to be divided into
227 million parts, our earth only receives orie of them.

510 As to the transmission of light and heat from
the sun to the earth and the other planetary worlds,
two leading theories have prevailed. Sir I^'-^^^ NewV^J
lield that the sun is constantly discharging from his
surface inconceivable multitudes of extrenriely mmuto
luminiferous particles of matter capable ot producmg,
when they strike against bodies, the well-known pheno-
mena of light and heat; but this theory, besides the
improbability of a continual loss of substance without
any diminution cf brilliancy, is not considered to expLam
satisfactorily the phenomena of light and heat Tne
opinion now most generally entertained is, that light
and heat are to be attributed to vibrations or undulations
in a thin fluid called ether diffused throughout space
a lluid supposed to be excited by the presence of
luminous and hot bodies into unduhiticms capable of
causing impressions of light and heat on bodies which
they meet that the sun causes these undulations in
this ethereal fluid, which being propagated through
space in waves, cause heat and light on the surface of
the planets.

The Planet Vulcan.
511 The existence of this planet is somewhat doubt-
ful, mainly owing to conflicting evidence _ Its supposed
discovery was made on 26th March 1859, by M. Les-
cXultf a French physician, who o^-^-f^^^/^c
dark object, like a planet, crossmg the sun s disc
Having given publicity to his discovery, Lev^^"^;"'/^^^
ominent astronomer, hastened to the residence of M.
I escarbault, whom he very closely interrogated retrard-
ingTl the alleged facts. The result was that Leverrier
l"i,... .pvfpr..tlv satisfied that an intra-Mercurialplanet
ad beenVeally'observed. On the other hand, M. Liais

ELEMENTS OF ASTRONOMY.

147

asserts that he was watching the sun in Brazil at the
very time when Lescarbault professes to have seen the
dark object crossing the sun's disc, and that he is per-
fectly certain nothing of the kind was visible. Still,
this is merely negative evidence, and it does not demon-
strate the non-existence of the planet. On 20th March
1862, Mr Lummis, of Manchester, while examining
the sun's disc, between the hours of eight and nine a.m.,
was struck by the appearance of a spot possessed of a
rapid proper motion, and of circular form. After fol-
lowing it for about twenty minutes, he was unfortu-
nately called away to other duties ; but he has no doubt
upon the subject. As the results of Mr Lunimis's
observations, it is calculated thai Vulcan is 13,082,000
miles distant from the sun ; that his periodic time of
revolution is 19-70 days; his velocity in orbit per hour,
174,000 miles ; and his polar diameter 785 miles.

■^,:

The Planet Mercury, 5

512. With the exception of Vulcan, this is the
nearest of all the planets to the sun, so fur as is yet
known. His mean distance from the sun is about
thirty-six millions of miles (35,649,000). His orbit is
rather more elongated than is usual among the planets,
his eccentricity (par. 393) being more than a fifth of his
mean distance from the sun. He will thus be at one
time 2-5ths of his mean solar distance nearer to the sun
than at another.

513. Mercury is the smallest of the planets, excepting
Vulcan and the asteroids. His diameter is a little less
than 3000 miles — correctly, 2962 miles. Mercury
rotates upon his axis in 24 hours, 5 minutes, and a few
seconds. He completes his course round the sun in
nearly 88 days — correctly, 87 days, 23 hours, 15
minutes; moving in his orbit at the amazing rate of
105,330 miles in an hour, or 29|^ miles per second.

148

ELEMF.NT8 OF ASTRONOMY.

614. The orbit of Mercury is inclined about seven
dep^rees to that of the earth : that is, there is an anp^le
of 7° at the intersection of the planes of their orbits (62).

515. This planet can be seen by the naked eye but
very seldom, and only for a short time. Being so near
to the sun, he is always in that part of the sky close
around the sun, and his inferior light is lost amid the
sun's rays. He nev.r departs above 29° from the sun,
and when ho is visible, can only be seen for a httle
before sunrise and a little after sunset.

516 Mercury occasionally passes directly between the earth
andfiun; appearing then as a black spot traversing the sun s
Burfaco. This is tcnned a transit of Mercury over the sunt
iligc.

517. Mercury, as seen through a telescope, does not
always appear of the same size and form. He has
phases, like the moon, being sometimes horned like the
new moon, sometimes full like the full moon. The
cause of these changes is this:— We can see only that
part which is illuniinated by the sun ; and as different
ouniititios of thnt part are turned towards us succes-
Bively, we see different amounts of the illuminated halt
at ditferent times.

518. At Mercury, the sun will present a diameter
about two and a half times greater than at the earth.
And he will receive nearly seven times as much of the
influence which, emanating from the sun, gives rise to
the phenomena of heat and light.

The Planet Venus, $
519. This planet is the third in order from the sun—
her orbit lying between those of Mercury and the earth.
Her mean distance from the sun is a little less than
sixty-seven millions of miles (66,614,000), Her dis-
tance from the sun does not vary much, her eccen-
+nr.;tv "hfMnP- onlv l-147th of her mean distance from

tv h
the sun.

ELEMENTS OF ASTRONOMY.

149

520. The polar diameter of Venus is about 7510
miles. »She is very little less than tiio earth, the polar
diameter of the latter being only 389 miles more.
Venus rotates upon her axis in 23 hours, 21 minutes;
and completes her course round the sun in 224 days,
16 hours, and 49 minutes: — moving at the rate of
77,050 miles per hour.

521. The orbit of Venus is inclined about three de-
grees, twenty-three minutes to the ecliptic (3° 23') : so
that the plane of tlie earth's orbit and that of this planet
are nearly coincident.

522. Venus is visible frequently. She is the most
bcf'iutiful of the planets (whence her name), and, being
near to us, she appears as bright and large as Jupiter,
although that planet exceeds her very much in magni-
tude. Venus is seen only about the times of sunriso
and sunset ; but is visible for a mucli longer time
before sunrise and after sunset than Mercury, departing
much further from the sun than that planet can do, —
namely, to a distance of forty-seven degrees (47°) from
that luminary. When seen before fcunrise, Venus is
well known as Phosphorus, Lucifer, or the morning
star; when she appears after sunset, she is termed
Hesperus, Vesi^er, or the evening star.

523. The transit of Venus over the ami's disc is a rare
occurrence, for the same reasons assigned above for the rare
occurrence of the transit of Mercury. The transit of Venus
takes place alternately at intervals of 8, 122, 8, 105, 8, 122, etc.
years. The last was in 1769, the next will be in 1874, and
there will be another in 1882. — This phenomenon is of great
use in practical astronomy: — it has been taken fmvantage of to
aid us in determining exactly the sun's distance.

524. Venus exhibits phases, as Mercury and the
moon do, and for similar reasons. She appears to us
of very different degrees of magnitude and brilliancy ;
being only 25,479,000 miles distant when nearest to
us, and then receding till she is about six times that
distance from the earth (see Fig. 61).

■i

i m'

150

ELEMENTS OP A8TUONOMY.

525; At Venus, the dianieter of the sun appears about
one third greater tlian at the eartli; anti liis anparent
Burfaco dimensions, on which, of course, his heating
and illuminating powers depend, are greater in the pro-
portion of sixteen to nine.

526. The axis of this planet leans very much towards
the plane of her orbit, forming with it an angle of only
15 degrees; that is, inclining 75 degrees from the per-
pendicular. Her tropics are therefore ordy 15 degrees
from her poles, and her polar circles only 15 degrees
from her equator. This gives rise to some striking
peculiarities in the constitution of Venus; namely,
that there is much greater diversity of seasons than
prevails on the earth,— that the days are much longer
where it is summer, and much shorter where it is win-
ter, — that a larger proportion of the regions about the
poles have day or night for several rotations, — and that
the middle or equatorial regions have two summers
and two winters in each of her years.

527. This planet is believed to be surrounded by an
atmosphere : and it has been conjectured that she may
have a satellite, though that idea is now discredited.

The Planet Earth [Tellus\ ©

528. The next planet aftor Venus, in order from the
sun, is that which we inhabit ; having its orbit situated
between those of Venus and Mars.

529. The mean distance of the earth from the sun is
about ninety-two millions of miles (92,093,000). Her
eccentricity is about A, or 0-0167918. The least dis-
tance of the earth from the sun is about ninety and a
half millions (90,562,000) miles ; the greatest distance
about ninety-three and a half millions (93,624,00 )
miles. The earth is in its aphelion on the 1st of July ;
in its perihelion on the 31st of December. The sun,
therefore, appears larger on December 31 than on July
1 , in the proportion of 32 J to 31 4.

ELEMENTS UF A8TUONOMY.

151

fiSO. If tho mean dintancc of the cnrth from the mm bu
1-00000, iu diHtauco on July I is 101679; on December 31,
0*98321.

631. Tho mean diamoter of tho earth is 7912 niileg;
tho shorter or palar diameter is 7899 miles ; the longer
or e-j-'alorial diameter, 7925 miles. Tho diflference
betw( n the polar and equiitorial diameters is therefore
26 miles. The ecjuator, or circtimference of tho earth
at the widest part, is 2 1,907 miles in len^Hh,*— ahoiit
2,'),000 miles. A degree of longitudo at the equator is
305,144 feet, or 09 British miles and 824 feet.

632. Tho earth's surface is marked by lines in tho
same manner as the spliero of the heavens. — See
Parallels, Meridians, Latitude, Longitude, in the Urtii
section of Part III.

633. From the Bphcroidal form of the earth, tho (IcgrecB of
Irttitude arc not all of tho Raino maKnitiulo, hut incnsaMe from
tho cnuator towardfl either pole. Tho following table ahows tho
Icnpjth at every 30° of latitude : —

Latitude.

0° (Kquator^ . . * i

80^

GO"

90° (Poles) ....

Degrees of longitude, of course, graduall.
greatest, at the equator, to nothing at the poles. The following
tibles represent the length of a degree of longitude at every b"
latitude, and at four leading latitudes :—

iv (

LenRth of degree
in JlnKllHli ttt't.

302,734
. 3(i3,»;4l

365,454
. 306,361
[liminish from their

Liitittiilo.
0"

10"
15°
20»
25°
80°
35"
4u°
45°

£n,{liHh miles. Latitude.

69-07
68-81
67-95
60-65
64-84
6253
59-75
6651
52-81
48-78

50°
55°
60°
65°
70°
75°
80°
85°
90°

English miles.
. 44-35

39-58
. 34-53

2915
. 23-60

17-86
. 11-98
6-00
.

' 'I'lic circumference of a circle Is obtained by multii»lyinj,' its diameter
byJUioi).

I.V2

ELEMKNTS OP ASTRONOMY.

2.1" 28' (Cancer) nearly 63 miltm.

6r32'(Lotul..n) — 43 ^

55'57'(K.lmburgli) . . . . — 8H§ --
66" 32' (Arctic Circle) . . . . — 28 —

534. Tho enrth turiiH upon Uh axis in 23 hotirH, 50
minutoB, 4'09 Hoconds. This Ih a truo or tidere&l
day (300).

535, Tho equatorial parts of tho oarth'Hclrcumferonco rorolve
ftt the rate of 17'3 milcH per mimite, or 1038 miles an hour.

586. Tlio interval between two successive apj/ulses
of tho 8itn to the meridian of a place is termed a lolar
day; that is, the time from the sun being on the
meridian of a place till tho earth's rotation brings it
round again to the sun.

537. The earth completes her revolution round tho
sun in 3(15 days, 5 hours, 48 minutes, 49*7 seconds;
which period is termed a tropical year. The earth's
orbit is 578,000,000 miles in length; and her daily
motion in her orbit 1,572,892 miles, or 65,533 miles
an hour ; that is, at the mean rate of 18^ miles in a sec-
ond, or 1092 miles every minute.

638. The auhrcal year (see Procession of tho Equinoxes) is
longer than tlie tropical year by 20 minutes, 199 seconds ; being
805 days, G hours, 9 minutes, 9't) seconds.

5^9. The mean diameter of the sun, as seen from-^the ea*th,
is about half a degree, or thirty-two minutes (82'). Its apparent
diameter on the Ist of July, when furthest from us, is 31' 32"
—on the 31st Decemb r, when nearest to us, 32' 36".

640. That is, supposing a great circle of the heavens to bo
divided into 360 equal parts, the sun's diameter would be equal
in length to one-half of one of these parts or degrees.

641. The mean daily motion of the earth is 59' 8"33"; mo-
tion on 31st December, 1° 1' 9'9"; on Ist July, 57' 11*5". Tho
mean velocity being I'OOOOO, the velocity on 3l8t December Is
1'03386; on Ist July, 0-96671.

542. The axis of tho earth is considerably inclined
to the plane of its orbit. It makes an angle of 66° 32'
36" with the ecliptic, thus leaning 23° 27' 24" from
the pcfpcudiculuf to tiiC orbit. IIcncG arise the ebangcs

■LBMENTS or ASTRONOMY.

153

in tho seasons und in tho longth of tho day. — SetSea-
sons.

643. ITenco, tho trnpien, roprpm-ntlnpf tho furtho«t north «in«l
Bnuth parnlloU nt which the «un i« vcrticnl, an^ 2W- 1' 24"
fr«)m h«r eqimtor; and her arctic circlet, the paralloU within
which tho dun doe* not net or risu for r rX rotationn, are 23"
27' 24" from her \w\ca. Tho extent of I .o inclination nmy Iks
■een in Vig. 10, pnge 20. L<Jt tho lino ao reprefient the ecliptic;
then N H will nhow the direction " ' ' axis, forming an angle
of 23° 27' 24" with Z n, the porpenu.cular— or, an angle ot 60"
82' 36 " with a o.

544. As the din Motion of tho earth's axis, at any
time, is always parallel to its diiection at any other
time, and tho greatest distance of any two ixmitions of
the earth (from its aphelion to its perihol'on) shrinks to
a point in comparison with the distance to tho fixed
stars, the earth's axis during: the year always point.*- to
the same phi^c in the starry heavens, close to thj north
pole-star (06) : tlongh its direction does change in a
very long 1, erics of years ; — see " Precession."

545. Thv' mean density of the earth has not yet
been absolutely ascertained, though, as the result of
numerous elaborate experiments conductea by eminent
astronomers, during the last hundred years, there cor be
no doubt that the mean specific gravity of our plane< '>
at least five and a half times that of distilled watCi ,.1
the temperature of 68° Fahr. The famous Schehallitii
experiment, conducted by Maskelyne in I'l 74, indicated
a density of only 4-713. By certahi pendulum experi-
ments, conducted on Alont Cenis by Carlini, at a mnch
later date, the mean density is 4950. In 1793. the
celebrated Mr Henry Cavendish, by an ingeniously
executed experiment, arrived at a result of 5 "480.
More recently, the late F. Daily carefully repeated the
Cavendish experiment with consummate skill and
patience, and obtained as his result 5-GGO. The
present Astronomc-Royal (Mr Airy) has also con-
ducted au elaborate pendulum experiment, m the

154

ELEMENTS OF ASTRONOMY.

Ilarton coal-pit, near South Shields, by which he
estimates the density of the earth as 6 "565, a result
which we have no doubt the progress of science will
show to be greatly too large. The accomplished
author of " Life and Work at the Great Pyramid in
1865," conclusively shows that the architect of that
stupendous structure kn3w the specific gravity of our
planet upwards of 4000 years ago, and gave it as 5*7
times that of pure water at 68° Fahr.* Seeing that all
the other ph) sical revelations treasured up in that pro-
foundly mysterious pile have been found to be in exact
harmony with the latest results of modern science, we
entertain very little doubt that tlie specific gravity
which it assigns to the earth will be found altogether
reliable. The immortal Newton predicted, some two
hundred years ago, that the density of the earth would
be found to lie between five and six times that of
water.

546 Experiment shows that the f^ ace of gravity at
the earth's surface causes a body to fall 16-rV f«et in the
first second, three times that amount in the next second^
Jive times 16t*t in the third second, etc. The force of
gravity at the earth's equator is diminished about
l-289th by centrifugal force: from this cause alono,
therefore, a body will weigh 1 -289th less than at the
poles. From the spheroidal form of the earth, the force
of gravity is l-590th less at tbe equator than at the
poles. The total difference in cbe force of gravity at
the poles and equator is equal to the sum of these
quantities, or 1-1 94th, for ^i^ and -^io make ^K-
Accordingly, a body which weighs any given quantity
at the poles, as indicated by a spring balance, m.ust be
increased in weight by 1- 194th ^jart, to produce the
same effect on the spring at the equator.-j-

• See Life and Work at the Great Pyramid in 1866, by C. Piazzi Smyth..
Astronomer-Royal for Scotland (Edinburgh, Edmonston and Douglas).

t The increase of the force of gravity from the equator towaicds either
polo is in the proportion of the sq-iare oj the suie of the latitude'.

ELEMENTS OP ASTRONOMY.

155

647. The earth is found, in every part where it has vet been
tried, to become sensibly and regularly warmer in propm-tion as
the distance below the surface is greater — the temperature
increasing about one degree Fahrenheit for every de-cent of 55
o^fnAo i^a'culating at this rate of increase, a temperature of
J400 l-ahr. would be reached at a depth of twenty-five miles
sufficient to keep in fusion such rocks as basalt, greenstone, and
r''S7.7 ' at a depth of thirty-six miles, the temperature would
be 3272 sufficient to melt iron ; and at a depth of fifty-four
miles, a heat of 4892° would prevail— a temperature at which ail
known substances would pass into the liquid or molten form
1 he phenomena of hot springs, volcanoes, and earthquakes!
attord other and independent evidence of the intense heat pre-
vailing m the interior of our planet, t rom an apparent coiiicU
dence between the positions of the moon in relation to the earth
and the penods at which many earthquakes have occurred, it has
been conjectured by M. Perrey of Dijon, that earthquakes may
be caused by the attraction exercised b^ the moon on the fluid
mass m the interior of the earth.

The Moon (Luna), j

548. The moon is a satellite or secondary planet to
the earth, round which it revolves, and with which it
is carried annually round the sun.

549. The mean distance of the moon from the earth
is about two hundred and forty thousand miles (238,793)
or 60-2734 times ^ le equatorial radius of the earth!
Her distance from he earth does not varv much. Her
eccentricity is about l-20th of her mean" distance from
the earth, or about 13,000 miles.

^v^^^v'^'^^® ®*'*^'^ ^'" appear at the moon about 13 times larger
than the moon does to the earth, and supply that satellite with
a proportionately more brilliant light than she affords to us.

551. The diameter of the moon is 2158 miles, a little
more than l-4th of that of the earth. The bulk of the
moon is about ^^Vth of „hat of our earth, or as -02012 to
1 : her mass is about ^V (-0125) of tha of the earth •
her density 0-6'', that of the earth being 1. '

552. The moon performs her revolution round the
earth in 29 days, 12 hours, 44 mmutes. This is the

156

ELEMENTS OF ASTRONOMY.

period from one new moon to the next, — from the time
of the moon being in conjunction with the sun till she
comet to the the same position again, — and it is termed
a lunar month, a si/nodical month, or her si/nodical
revolution. She moves in her orbit at the rate of 2-3d8
of a mile each second, 37*9 miles in a minute, or 2277
miles per hour.

553. The period of the moon's rotation on her axis
is the same as that of her revolution round the earth
(see 554) ; and this is the reason why she always pre-
sents the same side to the earth. And that side is never
totally dark, having one fortnight of sunlight, and
being illuminated by the earth the other fortnight.
The other side has alternately a fortnight of sunlight
and a fortnight of darkness. So far as yet ascertained,
all the other satellites belonging to our system follow
the same law — that is, they rotate on their axes in the
same time as they revolve i: round their primaries.

554. Viewed, not with respect to the sun, but to the
stars, the moon is found to return to the same star in
27 days, 7 hours, 43 minutes. This is termed a sidereal
or periodical month; and is the true period of the moon's
revolution round the earth and on her axis.

555. The plane of the moon's orbit forms an angle
of 5° 8' 40'' with the plane of the earth's orbit ; so that
the two orbits are not very far from being in the same
plane. The moon's axis scarcely 1 ans towards the
earth's orbit, forming an angle of 88° 30' with the plane
of the ecliptic, — or leaning only 1° 30' to the ecliptic.
Being so nearly perpendicular to the plaae of the
ecliptic— the path in which the moon moves round the
sun — the moon can have little or no change in her
seasons, or in the length of her day.

556. When the moon is viewed by a telescope, its
surface appears irregularly illuminated, with dark and
bright parts, now considered to be mountains and
valleys; the former very high in proportion to the

ELEMENTS OP ASTRONOMY.

157

magnitude of the moon, some being about five mile . in
height above the general level of the moon's suriace.
These mountains have much of a volcanic aspect, with
craters of great breadth — some upwards of 100 miles.
It was at one time supposed that the darker spots were
seas or lakes ; but it is now believed that there is no
water on the surface of thj moon. These mountains
have been measured, as to altitude, and names have
been assigned to them, taken from the names of cele-
brated astronomers and other philosophers: as Plato,
Tycho, Newton, Flamsteed, Herschel, Huyghens. There
is no appearance of clouds, or any indications of an
atmosphere on the moon. When the moon presents
less than her full enlightened surface to us, the edge
next the sun, which is fully illuminated, appears smooth
and rounded, while the other appears rough and broken
— probably from the hills being enlightened by the
sun's rays, while the low grounds are dark. The same
side is always turned towards us ; but from her motion
in her orbit not being uniform, we sometimes see a
little of her surface beyond that half on each- side; and,
from her axis being inclined to the plane of her orbit,
we see at times a little beyond her poles. ^ These shift-
ings of the face next us are termed llbrations.

The Planet Mars, ^

557. Mars is the next planet beycmd the eaith, the
orbit of this planet lying between those of the earth and
Flora, or that Minor Planet which is nearest the sun.
It is the first of the superior planets.

558. The mean distance of Mars from the sun is one
hundred and forty millions three hundred thousand miles
(140,322,000). His distance from the sun varies con-^
siderably ; his eccentricity is a little less than 1-lOth of
}i\a TYic.Q.n flistance from that luminary.

559. The diameter of the sun, seen from Mars, is to

I J--

1 <ii

15S

ELEMENTS OF ASTRONOMY.

Ill

his apparent diameter at the earth as 21 to 32, nearly
as 2 to 3. The sun's influence at Mars is less than one-
half of what it is at the earth — correctly, as 92- : 140^*,
or nearly as 84 : 196.

560. The polar diameter of Mars is about 4036 miles
— a little more than one-half of the earth's diameter.
He is sensibly flattened at the poles, his equatorial dia-
meter being 4920.

561. Mars rotates on his axis In 24 hours 37 minutes,
and performs his revolution round the sun in in 686 days
23 hours, moving in his orbit at the rate of 14| miles
in a second, or 53,090 miles per hour.

662. Tlie planes of the orbits of Mars and the earth are nearly
coinciilent, tlie angle between them being one of only 1° 61' 6".
—See Fig. 52.

563. The axis of Mars is considerably inclined to the
plane of his orbit, forming \vith it an angle of 61° 9',
about two- thirds of a right angle, leaning from the per-
pendicular about 28° 51'. Therefore there must be a
considerable variety in the seasons at Mars — more, pro-
portionally, than at the earth, thoughless than at Venus.

564. This planet is frequently pretty near the earth,
and therefore, though small, appears tolerably large,
but much less than Venus or Jupiter.

665. Owing to the great length of the diameter of Mars's
orbit, there is a very gri-at difference between his distance from
lis in opposition and in conjunction; so much so that, while his
diameter appears equal to 18" in opposition, it is only 4" in
conjunction.

665. When viewed through a telescope. Mars is found to
exhibit phase; ; his apparent magnitude varying according to
the amount of the illuminated part which is turned towards us.
This shows that Mars is not self-luminous, but shines by reflect-
ing the sun's light. But he never appears horned, like the new
moon, nor even like th ^ half moon. The enlightened portion of
his disc is never less ihan seven-eighths of the whole, being
then gibbous, or like the moon three or four days before full
mnnn. This F.h.ows that Mars's orbit is cxtcviov to ours* thrf
horned phase can only be presented by a body between us and

ELEUENTS OF ASTRONOMY.

159

the sun. Jupltor, Saturn, and Uranufl do ot exhibit any pha8i!«
at all, a pioot' at onco that tlieir orbits are exterior to ours, and
that their distances from the sun are so great compared with
ours that we are never very far from the centre of their orbits.

667. The regions about the poles of Mars are observed to bo
more bright than the other parts. This is doubtless caused by
an accumulation of snow and ice around his poles, similar to
what prevails in the polar regions of the earth. 8now and ice
reflect light brilliantly ; and if his poles are covered by these, as
ours are, this certainly would give rise to a brilliancy in the
appearance of these regions. The conjecture that this white-
ness is caused by snow or ice is confirmed by the circumstance
that it disappears after long exposure to the sun, and is largest
and brightest after the winter of the planet.

' 568. Mars is believed to possess a considerable at-
mosphere (to the density of which his ruddy colour has
been ascribed), and his surface is variegated in a manner
wb'^h has been attributed to parts of it being covered
with water. '' At a meeting of the Astronomical Society
two or three years ago, a globe was exhibited by Mr
Browning, on which lands and seas were pictured as
upon an ordinary terrestrial globe. By far the larger
part of these lands and seas were laid down as well-
known entities, respecting which no more doubt is felt
among astronomers than is felt by geographers respect-
ing the oceans and continents of our own earth. Yet
the world which is represented by this globe is one
which is never less than 120 times farther from us
than our own moon. It is rather singular that the
planet Mars — the orb represented by Mr Browning's
globe — is the only object in the heavens which is known
to exhibit features resembling those of our earth. As-
tronomers have examined the moon in vain for such
features: she presents an arid waste of extinct vol-
canoes, dreary mountain-scenery surrounding lifeless
plains (the seas of the old astronomers) ; an airless
hemisphere of desolation which has no counterpart
on the tftrrfistTifll c-lobe. The olanets Juniter and
Saturn, orbs which far transcend our earth in mass

;

160

ELEMENTS OF ASTRONOMY.

and volume, which are adorned with magnificent sys-
tems of subsidiary bodies, and which seem in every
respect worthy to be the abodes of nobler races than
those which subsist upon our earth, afford no indications
which justify us in asserting that they resemble the
earth in any of those points which we are accustomed
to regard as essential to the wants of living creatures.
Nearly the whole of the light which we recei"o from
these splendid orbs is reflected, not from their real
surface, but from vaporous masses suspended in their
atmospheres. It is, indeed, doubtful whether anything
has ever been seen of the real surface of either .ilanet,
save, perhaps, that a small spot has here and there been
faintly visible through the dense overhanging mantle
of vapour. And, strangely enough, the two small
planets, Venus and Mercury, which present in other
respects the most marked contrast to the giant members
of our system, resemble them in this point. Both Venus
and Mercury seem to be protected from the intense heat
to which they would otherwise be exposed by densely
vaporous envelopes, which only permit the true surface
of the planets to be faintly seen, even under the most
favourable circumstances. The planet Mars, however,
discloses to us his real surface, and this surface presents
indications which cannot reasonably be doubted to result
from the existence of continents and oceans, resembling
those of our own earth in all essential features. More-
over, that wonderfully delicate instrument of research,
the spectroscope, has confirmed these indications in a
manner which hardly suffers any further dubiety to rest
upon their meaning." *

569. The presence of an atmosphere around Mars is
inferred from the appearance presented when he ap-
proaches any star. The star is dimmed, changed in

* See the admirable work, entitled "The Orbs Around Us," by R. A. Proc-
tor, B.A., p. 105. London : Longmans, Green, & Co., 1872. In this work, as
well as in the companion volume, •* Other Worlds tiian Uurs," the student
'.vill find beautifully executed charts of Mars, from drawings by Dawes.

' I

KLEMENTS OF ASTRONUMY.

161

colour, aii J disappears before it reaclu h the body of the
planet.

The Asteroids.

570. The asteroids, as presently known, consist of 1 32
small bodies, careering round the sun between the orbits
of Mars and Jupiter. They are sometimes denominated
planetoids, as they resemble planets in everything save
size ; sometimes telescopic planets, as they are not visible
to the naked eye, but can only be seen by the aid of
the telescope. Ceres, the first of them, was discovered
by the astronomer Piazzi on the first day of the present
century; the three following — I^allas, Juno, and Vesta
— were observed by Gibers and Harding during the
following six yearp ; after which no more were detected
until 1 845, when Hencke discovered Astrea. Since then,
scarcely a year has passed without adding one or more
to the catalogue of these bodies, which, in the autumn
of 1S73, numbered no fewer than 132.

571. It would be out of place here to give all the
details ascertained regarding these members of the
solar system. The following will be found to comprise
the principal facts. Their distances from the sun vary
from 201 millions of miles, the distance of Flora, to 313
milli(ms of miles, the distance of Maximiliana; and
their periods are from 3J years, the period of Flora, to
6^ years, that of Maximiliana.

572. These planets are very small ; none of them can
be seen by the naked eye, except occasionally Ceres and
Vesta. Pallas, the largest of them, has a diameter of
only 670 miles, while many of the smaller ones are less
than 50 miles in diameter. "Those last discovered
shine as stars of the tenth or eleventh magnitude, and
the only way in which they can be detected is to com-
pare the star-charts of different parts of the heavens
with the heavens themselves, night after night." *

* Lockyer's " Elementary Lessons iu Astronomy," p. 121, London: Muc-
mUlan&Co, 1871.

162

ELEMENTS OP ASTRONOMY.

■>

573. Their eccentricities are consideruLle, and they
are remarkable for the great angle which the planes of
their orbits form with the ecliptic, the inclinations being
as follows -.—Vesta, 7° T 50" ; Juno, 13° V 9"; Ceres,
10° 36'; Ilebe, 14° 47'; Pallas, 34° 4.Y. See Fig. 52,
in which their great departure from tiie plane of the
ecliptic is shown. From their orbits being so much out
of the plane of the ecliptic they are seldom seen in the
zodiac, being generally above or below it, while all the
other planets constantly appear in that zone of the
heavens. Hence, these planets are sometimes termed
ultra- zodiacal^ i.e.j out of or beyond the zodiac.

574. The asteroids present several peculiarities, in
•which they differ considerably from the other planets.
They are extremely small, while, generally speaking,
the planets rather increase in size as they are more
distant from the sun. They are comparatively near to
each other, whereas a very diflferent law prevails with
respect to the other planets. The distance between
two planets increaics in a very high proportion as they
are further from the sun, as follows : — Mercury, 36 ;
Venus, 67; Earth, 92; Mars, 140; Asteroids, 259;
Jupit'jr, 479; Saturn, 878; Uranus, 1766; Neptune,
2766. Also, their orbits are very far out of the plane
of the ecliptic, whereas the orbits of the other planets
are nearly coincident with that plane. The orbits of
Venus, Mars, Jupiter, Saturn, Uranus, form angles with
the plane of the ecliptic of from 3° 23' to 0° 46', and
Mercury of 7° ; but the angles which several of the
asteroids form with that plane are 5°, 7°, 10°, 13°, 14°,
and 34°.— See Fig. 52.

575. The following singular relation has been observed regard-
ing the distances of the planets, and it led to the conjecture of
the existence of another planet between Mars and Jupiter, before
the discovery of the asteroids.

If the numbers 0, 3, 6, V?.. 24, 48, 96, 192, be taken, and the
number 4 added to each, the sum will express the proportionate
distances of th© planets in order from the sun :

».e

ELEMENTS OP ASTRONOMY.

1C3

8
6
18
24
48
98

192

= 4,

Distance of Mercury

= 7,

•(•

> enuM,

= 10,

•••

Earth.

= 16,

•••

Mars.

= 28,

••1

= 62,

••t

Jupiter.

= 100,

••«

Huturn.

added afterward

»t

= 196,

• • •

Uranus.

A void being observed between the numbers 10 and 52, Pro-
fessor Hodo conjectured that a planet filling up the vacant
number might exist, which was confirmed by the discovery of
the asteroids, in the situation in the solar system indicated by
the vacant place. This law has failed, however, in the case of
Neptune.

576. Those peculiarities led to the singular conjecture, that
these planets originally formed one planet ; that that planet has
been ruptured by some great convulsion, which has divided the
one into a number of separate parts, and thrown the fragments
out of the former orbit into orbits deviating considerably from
the general order. But this is a mere speculation at present,
though strengthened by the ascertained intersection of many of
their orbits ; as, if a planet were so ruptured, the fragments
would return periodically to the spot where the explosion had
taken place.

The Planet Jupiter, 7f

577. Jupiter is the next planet beyond the asteroids,
his orbit lying between them and Saturn. He is the
largest of the planets, and, though so remote from the
earth, owing to his great magnitude often appears as
bright and large as Venus.

578. The mean distance of Jupiter from the sun is
about four hundred and seventy-nine millions of miles
(479,141,098). His distance from the sun does not
vary much, his eccentricity being less than l-20th of
his mean distance.

579. The sun's diameter as seen from Jupiter is only
l-5th of its apparent magnitude at the earth. — cor-
rectly, as 6 to 32. The relative proportion of the sun's
influence at Jupiter and at the earth is as 1 to 27^ (as

1C4

ELEMENTS OP A8TR0N0MV.

92' : 479^). Gravity at Ins surface is about 2 J timci.
as great as on our earth's; bo that such creatures as
exist around us would find their weight much more
than doubled if they were removed to Jupiter.

580. The diameter of Jupiter is eighty-eight thousand
miles (88,400), more than eleven times that of the earth.
This is the equatorial diameter. Ho is about 1387 times
larger than the earth. The polar or shorter diameter
of Jupiter is about l-18th, or 5000 miles, less than tho
equatorial diameter ; or, as 83 to 88. Tho great differ-
ence between the polar and equatorial diameters of this
planet is attributed to the very great centrifugal force
generated by his rapid rotation on his axis. When
viewed through a telescope, Jupiter appears of a dis-
tinctly oval shape, from the extreme polar flattening.

581. Jupiter turns on his axis in a little less than 10
hours, — correctly, 9 hours, 55 minutes, 28 seconds.
His equatorial parts, therefore, revolve at the amazing
rate of 7*5 miles in a second, or 453 miles per minute.

582. Jupiter completes his revolution round the sun
in 4332 J days, nearly 12 of our years (more correctly,
11 years 314 days) : — moving in his orbit at tlie rate
of 8 miles in a second, 480 miles in a minute, or 28,744
miles per hour.

583. The planes of tlie orbits of Jupiter and the earth are
nearly coincident, the angle between them being only 1° 18' 61".

584. The axis of Jupiter does not lean more than
3° 4' towards the plane of its orbit, being nearly perpen-
dicular to that plane. From this, Jupiter can have
little or no variety in his seasons, and little or no
change in the length of the day. This planet, there-
fore, will have a perpetual winter around his poles, and
continual summer in his equatorial regions; and the
weather c mparatively uniform.

585. The density or specific gravity of Jupiter, in
common with all the remoter orbs of our sysLem, is

Ij timcb
tares m
ch more

bousand
le cnrth.
87 times
liameter
ban tho
it (liffcr-
'8 of tbis
;al force
When
f a dis-
ening.
than 10
seconds,
imazing
minute,
the sun
orrectly,
the rate
r 28,744

earth are
°18'61".

)re than
perpen-
an have
e or no
t, there -
)les, and
and the

piter, in
^sLem, is

ILBMENTS OF ASTRONOMY.

16!

very small as compared with the density of tlio oarth,
and of all tho planets revolving inside of Jupiter's orbit.
Thus, regarding tho density of our planet as unity, and
that of Mercury, Venus, and Mars as respectively
1-24, '92, and -96, wo find the density of Jupiter as
only -22, of Saturn -12, of Uranus -18, and of Neptune
•17. Be it now remembered that the density of tho sun
is '25, or a very little more than the density of Jupiter.
Wo cannot regard this vast difference of density be-
tween tho superior and inferior members of the solar
system as arising from any great diflerenco in tho
nature of their constituent materials, for tho spectro-
scope has conclusively shown that all the members of
the system are in this respect identical with the great
central orb. In all probability tho cause of the great
difference will be found, not in their constituents, but
in their condition with respect to their internal heat.
Doubtless all tho planets had at one time the same
temperature as tho sun, and were real stars, shining
with intrinsic splendour. In the course of ages the
smaller planets have gradually cooled down, lost their
luminosity, but increased in density. The existence of
volcanoes, boiling springs, and increase of temperature
as we descend beneath the surface, afford ample evi-
dence that the only member of the solar system which
we can fully examine, once possessed a much higher
temperature than now ; while there appears no reason
why the other worlds, which in other respects so closely
resemble it, should not resemble it in this also. Jupiter
and the other more distant planets have, in like man-
ner, cooled down, but owing to their vastly greater
dimensions the progress has been much less rapid, and
accordingly their density continues pretty closely to
correspond with that of the great central orb. They
have not wholly lost their luminosity even, for Jupiter
and Saturn exhibit several phenomena which can be
accounted for only on the supposition that they are still

IGG

EI.EMKNTS OF A8TR0NOMV.

faintly flowing* The larper plnnots, therefore, of ur
syBtem cannot, by r. 'ly poHHibility, Im) inhabited hy • v-
inpf creatiircH, thoiigi it seems not unlikely that t'
SAtcllites may.

Satellites of Jupiter.

586. This planet is attended by four BatellitcH or
moons, named, respectively, To, Europa, (lanymedo,
and (.allisto. These cannot bo scon by the naked eye,
and hence they were not known till after the invention
of the telescope. In IGIO, within a very siiort timo
after the discovery of tiiis powerful instrument of as-
tronomical observation, the satellites of Jupiter were
discovered by Oalileo.

587. The distance wom Jupiter of luH noaroHt Batclllte is
267 .UUO miles ; its diameter is 2252 miles ; and it revolves round
its primary planet in 1 day, 18 hours. 27 minutes. The distance
from the planet of t' e next satellite is 42.''»,000 miles; its
diameter is 2099 miles; audi', co npletes its revolution round
Jupiter in .3 days, 13 hours, 14 minutes. Jupiter's third satel-
lite is at a distance of 678,000 miles ; its diameter is 3436 miles ;
and it revolves round Jupiter in 7 ''"y." 3 hours, and 43 minutes.
TliJ fourth and most remote of oupiter's satellites is distant
from him 1,192,000 miles;— its diameter is 2929 miles;— and it
occupies 16 days, 16 hours, 32 minutes in its revolution round
Jupiter. The satellites of this nhinet are rather larger in gene-
ral than our moon.

588. Jupiter's satellites re\olveroun«i him /rom west
to east, as the moon does rcund the earth, and the
planets round the sun. Tho pi^ruds of rotation on
their axis are the same as their })criod8 of revolution
round taeir primary planet ;— obeying, in this respect,
the same law as our satellite, the il on.

589. When the body of Jupiter interposes between

* " We are Unis led to the conclusion that Jupiter Ih still a glowing mass
fluid probably tlironghout, still bubblinf? and Heething with the intensity
of the primeval tiros, sending up continually enormous inaases of cloud
to be gathered Into bands under the influence of tlit< swift rotntiuu
giant planet."— C<A«r Worlds t/uin Oura, ' 12.

■tf tu

,i I.-

Ef.EMi:NTg or ABTnONOUY.

1C7

tho siin and any of his satollites, tlmt fltttellito will dia-
apiMJttr from our view, or be eclipsed.

590. The cclipHCH of Jupiter'M natillitoB are phononiRna of
CotiHidurabiu iinportanco in practical OHtronumy. I'hoy attbrdcd
tho first accurate method ordotcrmining tho hmpritudo of places
on tho earth's surfocu ; this nuKle, however, is now si'peraedod
in a groat measure hy Inn tr observations.

Velocity of Lierht.

591. Tho eclipses of tho sate Hi tea of .Tupitcr have
boon the means of Unulirjfj to tho ffrcat discovery that
the passage of light from one point to another is not
instantaneous, but requires a certain time ; and tiicy have
also enabled tlie rate of its motion to be calcujuted. Thig
great discovery was made by Roemer, a Danish astrono-
nier, in the year 1675. By observing the ecl'pso of
one of tho satellites, he calculated the velocity of light
at 192.000 miles per second, inasmuch as it traversed
the diameter of the earth's orbit (at that time held to
be 190,000,000 of miles) in 16 minutes, 36 seconds.
The distance of the earth from the sun, however, can
no longer be regarded as 95,000,000 miles, but
92,000,000. This makes the velocity of light, as de-
termined by Roemer, to be 186,000 miles per sec-
ond. In 1862, M. Foucault, a French philosopher,
made bis celebrated experiments on the velocity of ligbt
by means of a rotating mirror, and announced it as
185,170 miles per second, and all astronomer/', now re-
gard this as the real rate of its velocity.

592. Owing to the great distance of Jupiter from the
sun, there must be a considerable difference between his
distance from the earth when he is nearest us, or in
opposition, and his distance when he is furthest from
us, or in conjunction. Novv, lioemer found that the
eclipses of Jupiter's satellites took place sooner than
might be expected when he was nearest the earth ; and
later than might be expected when he was most distant
from the earth, the total difference amounting to IG

i-\

x-./

IGd

ELEMENTS OF ASTHONOMV.

*it

irinutes, 36 seconds. This be explained by the sup-
position tliat light does not pass instantaneously from
one point to another, bMt requires time for its trans-
mission, and that therefore the rays which intimate to
us the eclipse of one of Jupiter's satellites must be
longer in coming to us when he is remote than when
near, an eclipse in the former case appearing later than
in the flatter. That the variation from the computed
time of an eclipse of a satellite of Jupiter is owing to
light occupying time in its transmission, was afterwards
confirmed by Bradley' b great discovery of the Aber-
ration of Light. Thut^, we do not see distant pheno-
mena at the actual moment of their occurrence, but
some time after, sooner or later according to the dis-
tance : a circumstance which tlie penetration of the
illustrious Bacon led him to conjecture as being possi-
ble, long before there were any means of proving it.

The Planet Saturn, Tj .

593. This is the most remote of the planets known
to the ancients and visible to the naked eye. Its orbit
lies between those of Jupiter and Uranus. In apparent
magnitude Saturn is between Mars and Jupiter, and
shines with a rather dull light.

594. The mean distance of Saturn from the sun is
eight hundred and seventy-eight millions of miles
(878,461,000). His eccentricity is more than l-18th
of his mean distance from the sun. At Saturn, the sun
will present a diameter about 1-lOth of that seen at the
earth. The proportion of the sun's influence which
reaches Saturn is about l-90th of that enjoyed at the
earth— as 92^ : 8781

595. The equatorial diameter of Saturn is about
72,000 miles, and he is about 747 times larger than the
earth. The polar diameter of this planet is stated to be
64,714 miles, or l-9th less than the equatorial. Having
a very rapid rotation on its axis, 't is to be expected that

ELEMENTS OP ASTRONOMY.

IGO

Siitiirn, hko Jupiter, will be very much flattened at his
poles. He rotates on his axis in 10 hours, 29 mi»;ates
His specific gravity is only -12, that of our eart' oein^
unity and he is therefore by far the least dense of all
the planets.

^ 596. Saturn completes h^s revolution round the sim
in 10,759 days, or about .9^^ years j—moving in his
orbit at the rate of about 6 miles in a second, or 21,221
miles per hour.

697. The planes of the earth's orbit and of Saturn's are
nearly coincident, the angle between them boing only 2° 29' 35".

598. The axis of Saturn is not at right angles to tho
plane of his orbit ; but makes an angle of about 63°
with that plane, leaning 26° 49' from the perpendicular.
There must be therefore considerable variety in the
seasons at Saturn. When viewed through a telescope
stripes or belts are observed on Saturn's surface, resem-
bling those seen on Jupiter, but more faint (see 585).

599. The most remarkable features observed about
this planet are the enormous Rings by which it is sur-
rounded. These very singular appendages (see the
representation of Saturn in Fig. 52, page 178) lono-
seemed to be of 3olid matter; for they throw a shadow
on the body of the planet on the side nearest to the
sun, while on the other side the body of the planet
throws a shadow on the ring ; and they are thin, flat-
tish, broad, and generally opaque. They are at a con-
siderable distance from the body of the planet ; and
consist of two principal rings, one within the other,
and both in the same plane, which is nearly the same
as the plane of the planet's equator. The rings rotate
in their own plane, in about 10 hours and 32 minutes.
Thus, the axis of rotation of the planet and of its ring
are nearly the same. A third ring, interior to the
others, has been discovered lately. It is dark, and
appears in the telescope of a duirpurplish hue ; while

170

ELEMENTS OF ASTRONOMY.

Miles.

166,920

147,670

144,310

109,100

71,904

9,700

1,680

100

the luminous rings are bright and whitish yellow, like
the body of the planet.

600. The following table exhibits the magnitudes and dis-
tances of the exterior luminous rings : —

Exterior diameter of exterior ring, .

Interior ditto, . • ■

Exterior diameter of interior ring, .

Interior ditto, . . • •• •

Equatorial diameter of the body, . • •

Interval between the planet and interior ring, .

Interval of the rings, . •

Thickness of the rings not exceeding, .

601. Galileo, in 1610-12, observed several remarkable pecu-
liarities in the appearance of Saturn ; the exact nature of which
he was unable to ascertain. The planet appeared to him to bo
triple, or to have appendages at the sides like two smaller
planets joined to it. In 1655, Huyghens, provided with better
telescopes, discovered these peculiarities to be caused by a nng
surrounding the planet. Towards the close of last century,
about 1790, the ring was discovered by Sir William Herschel to
be double, consisting of one ring within another, both in the
same plane. An inner dark ring has been discovered withm
these few years by Galle of Beriin, Bond of Cambridge, U.S.,
and others; and appearances of lines of division along the
various rings have lately led to the supposition that they are
composed of innumerable narrow rings.

602. *' Of what, then, are these rings composed ?
There is great reason for believing that they are neither
solid nor liquid ; and the idea now generally accepted
is that they are composed of myriads of satellites or
little bodies, moving independently, each in its own
orbit, round the planet ; giving rise to the appearance
of a bright ring when they are closely packed together^
and a very dim one when they are most scattt. t ' In
this way we may account for the varying brightness of
the different part?, and for the haziness on both sides
of the ring near the planet, which is supp -d to be due
to the bodies being drawn out of the ring l>y the attrac-
tion of the planet." *

* Lockycr's Elementary Lessons in Astronomy, p. 117.

ELEMENTS OP ASTRONOMY.

171

Satellites of Saturn.
^ 603. This planet is accompanied by no less than
eight satellites. "" \e six which aro nearest to the
planet have their orbits nearly in the same plane as the
ring. The satellites of Saturn are supposed to revolve
on their axes in the same periods in which they com-
plete their revolutions round the planet: which has
been ascertained of the eighth. They are believed to
vary in size from 500 to 3300 miles in diameter.
"The eight satellites, taken in their order from the
planet, cover spaces on the Saturnian heavens which
bear to the space covered by our moon the resr>ertive
proportions of 2, 1, U, f, |, I ^i^, ^^^. In all, then,
they cover an area about six limes that of our moon.
But as, owing to their great distance from the sun, they
are illumined by only t^^ c^ the light which illuminates
our moon, they can only send back to the planet about
rVth part of the light we receive from the full moon,
even if it were possible for them tc ')e all full at once."
— Other Worlds than Ours.

604. The first satellite is at a distance of about 120,000
miles from Saturn, and revolves round it in 22 hours, 37 min-
utes : — the spcond is about 155,000 iniles from the planet, and
completes its revrolution in 1 day, 8 hours, 52 minutes :— the
third is about 191,000 miles frv,m Saturn, and Its period is 1 day,
21 hours, 18 minutes: — the distance of the fourth is about
246,000 miles ; its period 2 dp ys, 17 hours, 41 minutes .-—the
jftfth is about 343,000 miles from Saturn, and revolves round him
in 4 days, 12 hours, 25 minute's: — the sixth is about 796,000
miles from the planet, and resolves round him in 15 days. 22
hours, 41 minutes : — the di"* mce of the seventh is about
1,007,000 miles, and its "^nou 21 days, 7 hours - :he eighth
is about 2,314,000 mile^ , ,ai Saturn; and its period, 79 days,
7 hours, 55 m'nutes.

605. The satelUtes of Saturn were discovered by Huyghens,
Cassini, Herschel, Lassel, and Bond;— one by Huyghens in
1 655,— four by Cassini m , 6V1 and subsequent years, — two by Sir
W. Herschel m 1 <"89, — and one (the seventh) in 1848, on the same
day, by Lassel in Liverpool, and Bond in Cambridge, United
States. The satellites of Saturn have received the names Mimas,
Enoeladus, Tethys, Dione, Rhea, Titan, Hyperion, -Japetus.

„i

172

ii.i:

ELEMENTS OF ASTRONOMY.

The Planet Uranus, I^I

606. Though Uranus is of considerable magnitude,
it is rarely and with difficulty seen by the naked eye,
owing to its great distance. This planet was discovered
by the celebrated astronomer Sir William Herschel,
on the 13th of March 1781. It was called by him
"Georgium Sidus," in honour of George III., and by
some astronomers, "Herschel," in honour of the dis-
coverer. The name Uranus, however, from one of the
characters in the ancient mythology, is preferred, as
being more in harmony with the appellations of the
other planets.

607. The mean distance of Uranus from the sun is
seventeen hundred and sixty-six millions of miles
(1,766,565,000), a little more than IS times the dis-
tance of the earth from the sun. The listance of
Uranus from the sun does not vary much, Ins eccen-
tricity being less than l-20th of his mean distance.

608. The sun's diameter appears at Uranus of l-19th
his apparent magnitude at the earth, or as If to 32 :
and the proportion of the sun's influence enjoyed by
this planet is only l-368th ot that experienced at the
earth: as 92^ to*J766^

609. The polar diameter of this planet is 29,722
miles; his equatorial diameter, 33,024 miles; his
volume, 72 times that of the earth ; but his density is
so small (-18) that his weig^st or mass is only 12-64:
times that of our planet, and the force of gravity at his
surface only ^'^tb greater f.han at the earth's.

610. Uranus completes his revolution round the sun
in 30,686 days, about 84 years ; — moving in his orbit
at the rate of 4 miles in a second, or 14,963 miles per
hour.

611. The plane of the orbit of Uranus is more nearly coinci-
dent with that of the ecliptic than in the case of any other
planet, the angle between them being only 0° 46' 28*4". The
inclination of the planet's equator to the plane of his orbit is

ELEMENTS OF ASTRONOMY.

173

believed to bo ftljout 7G°. From this, it follows that tli'i
Uranian sun, which has an ajjparent magnitude of only i,4flth
part his size as seen from the earth, has a range of about 70° on
either side of the celestial equator, and that he will continue 23j
of our years above the horizon, while in winter ho will continue
for the same period of time under the horizon. These facts
have a most important bearing on the question of the habita-
bility of the planet. It seems certain that none of the animals
or plants with which we are acquainted could possibly live
either on Uran-is or Neptune. Indeed, it is far more probable
that, like Jupiter and iSaturn, these planets perform the part of
suns to the systems of satellites which respectively revolve
around them, aflbrding to the latter a large amount of heat, to-
gether with, possibly, an appreciable amount of light also (585).

Satellites of Uranus.

612. This planet is known to be attended by at least four
satellites, known as Ariel, Umbriel, Titania, and Oberon. The
latter two were discovered by Sir W. Herschel in 1787. Umbriel
was discovered by Otto Struve in 1S47, and Ariel in the same
year by Lassel of Liverpool, Ariel, the nearest to the planet,
IS 123,000 miles distant; Umbriel, 171,000; Titania, 281,000;
and Oberon, 376,000 miles. The first revolves round his
primary in .' days 12 hours, and the last in 18 days 11 hours.
Hitherto their magnitudes have not been determined.

613. These satellites present some remarkable pecu-
liarities and departures from the usual order in the solar
system. The planes of their orbits are nearly perpen-
dicular to the plane of Uranus's orbit, forming an angle
of 78° 58' with the plane of the ecliptic (which is nearly
the plane of Uranus's orbit) ; and they do not move in
the same direction which prevails everywhere else in
the solar system, viz., from west to east; but in a retro-
grade direction, 2.e., from east to west.

The Planet Neptune,

614. This recently-discovered planet is the most
distant member of the planetary system, so far as we
know at present. Neptune is a planet of considerable
magnitude, being 36,620 miles in diameter; and he
revolves round the sun in about 164*6 years (60126'7

174

ELEMENTS OP ASTRONOMY.

days), at a mean distance of 2766 millions of miles.
Tlic incliii'ition of his orbit to the ecliptic is V 47';
and one satellite has been observed near him, which
revolves round him in 5^- 21"™-, at a distance of about
220,000 miles. It is probable, however, from the
analogy of Jupiter, Saturn, and Uranus, that he has a
greater number of moons attendant upon him.

G15. Neptune was discovered in an interesting and
very remarkable manner. In the year 1846, his exist-
ence and position were predicted simultaneously by two
astronomers, M. Leverrier of Paris, and Mr Adams of
Cambridge. On the 23d of September in that year,
"a day," says Sir John Herschel, " for ever memorable
in the annals of Astronomy," Dr Galle, of the Royal
Observatory at Berlin, received a letter from M.
Leverrier, requesting him to look for the predicted
planet about the place in which he had calculated it
should be found. Dr Galle did so on that very night,
and found the new planet within one degree of the
place assigned to it by M. Leverrier. Next night it
was found to have moved from its place, and repeated
subsequent observations have fully confirmed the exist-
ence of this new planet, and enabled its orbit, period,
and distance to be laid down correctly.

616. As already mentioned, the mutual actions of the planets
on each other cause disturbances in their movements ; that is,
deviations from the course which each would pursue if influenced
only by the sun's attractive force and its own projectile energy.
As might be expected, these disturbing forces produce consider-
able effects among the more remote planets where the sun's in-
fluence is comparatively weak. Soon after the discoverer of
Uranus in 1781, it was found, on searching the astronomical
records, that he had been seen often by previous astronomers,
once so far back as 1 690, though not even conjectured to be a
planet. But it was observed, on calculating what his position
should have been at former times, to judge from the ascertained
elements of his orbit, that, making every allowance for disturb-
ances by known planets, the recorded positions were far from
coinciding with those assigned by computation. Moreover, his
actual course after discovery did not coincide with the theory

ELEMENTS OP ASTRONOMY.

175

deduced fiom tho elomcnta of his orliit as found by the first ob-
nervations. Up to 1830-1, his real position was continually in
advance of his computed position; about that time they corre-
sponded, subsequently he fell behind his calculated positions.
The true cause, the disturbing influence of a more remote planet,
h.'xd been conjectured by several astronomers. Simultaneously,
Leverrier and Adams, from the observed deviations, deduced
the mass of the previously unknown disturbing body ; and
assigned its position within 3° of each other's calculations.

17G

ELEMENTS OP ASTUONOMV,

General Illustrations of the Solar System.

Havinpf now concluded m account of the pInnetH in detail,
wo Bhall endeavour to illuHtrato tJioin as parts of one great
PVHtom, and give some idoa of thoir relative niagnitudcH, diH-
tance«, etc.

617. "Chooflo any well-levelled field or bowling-grncn. On
it place a globe, two feet in diameter; tliis will represent the
Bun; Mercury will be represented by a grain of mustard seed,
on tbo circumference of a circle 1(51 feet in diameter for its
orbi* ; Venus, a pea, on a circle 284 fe(!t in diameter ; the Earth,
also a pea on a circle of 430 feet; Mars, a rather large pin's
head, on a circle of <)5t feet; Jjino, Ceres, Vesta, and I^illas.
grains of sand, in orbits of from 1000 to 1200 feet ; Jupiter, a
moderate-sized orange, in a circle nearly half a mile across ;
Saturn, a small ()rangc, on a circle of four-fifths of a njile ;
Uranus, a full-sized cherry, or small j)lum, upon the circum-
ference of a circle more than a mile and a half; and Neptune, a
good-sized plum, on a circle about two miles and a half in
diameter. As to getting correct notions on this subject by
drawing circles on paper, or, still worse, from those very childish
toys called orreries, it is out of the question. To imitate the
motions of tlu! jilnriets, in the above-mentioned orbits. Mercury
must de 'M'ibo its own diameter in 41 seconds; Venus, in
4'"14»-; :he Karth, in 7 minutes ; Mars, in 4'"- 4S''- ; Jupiter,
in 2^- SG-"-; Saturn, in S^- la-"-; Uranus, in 2^- IG'"-; and Nep-
tune, in Sh- .3U'n- "Sir John Ilc.rfichel.

To this it may be added, that owing to the inclinations of their
orbits to the plane of the ecliptic, the objects representing the
planets, in the above illustration, ought to be some a little
above, some a little below the level of the ground, and some just
on it, or with one half above, the other half below the surface of
the ground— the planet being then in its nodes. The greatest
elevations above or deptha below the f/round attained in the
above example would be, 10 feet for Mercury ; 8 feet for Venus;
10 feet for Mars; 128 feet for Hebe (14" 47'); 284 feet for
Pallas (34° 37') ; 23 feet for Jupiter ; 87 feet for Saturn ; 52 feet
for Uranus; and 183 feet for Neptune. Thus, all the planets,
excepting Juno, Hebe, and Pallas, would circulate near the
level of the ground; 87 and 183 feet, the greatest distances of
Saturn and Neptune from the level of the ground, being little in
comparison with their distances from the sun — two-fifths of a
mile, and a mile and a quarter.

618. The following woodcut (Fig. 52) will give an idea of
several important particulars regarding the planets. The white
line at the right (looking at the figure in the way in which the
book is held for reading) is intended to represent the sun's radius

ELEMENTS OF ASTRONOMY.

177

nilc ;

or «emi(1i*nnictor ; nnd the fif^iirofi next it, roprcBent tlio mftgni-
tudcs of the planets, in proper pruportinn to tlio bum'k sciiii-

Fi'. .-.

diameter and to each other, in the tbllowinoj order: Saturn,
Jupiter, Uranus, the Earth, Venus, Mars, Mercury, Neptune,
with their signs. The circular outlines in the middle of th'/

H '^

^^:

178

BLEUENTa OF ASTRONOMY.

fijruro n-prt'scmt tlic reliitivo magnitude* of tlio cnrtli nnd tTHt<»n.
The lin.!H luul charurtcrH at tho lefi «i<lo of tho flKuro nro do-
iigned to nhow tho relative dintnnceH of the ninnctu from tho «nn,
and the inclinationH of their orhitH to the pl.mo of tite ecliptic.
Tht! Bun i« repicHcnted at the left-hand lower comer. The doti
at tho ends of tho white lincH noarcHt t<) the Run, rcpreuent tho
plnncts at their proportionate dlstancea from the Hun, the sips
of the planetH being at the other ends of the white linea. Vho
di8tance of Neptune from the nun would be rcprenented nearly
by a line from tho Bun to tho opnoHito corner of tho figure-
being about half the distance of llranuH further from the nun.
'J\) obtain a proper idea from the figure of tho inclinations of the
orbits of the planets to the plane of tho ecliptic, the b(M)k should
1x5 turned Hidowavs Ro as to place the line representing the sun's
radius at tho top l)f the figure. Then, the lowest line, stretching
out to the left of the sun will represent the plane of the ecliptic,
and the other lines the planes of orbits of the various nlanets.
It will bo observed how near tho orbits of the planets Uranus,
Jupiter, Mars, Saturn, and Venus, are to the plane of tl.o ecliptic ;
and Neptune's orbit nhould be represented between the hues of
the orbits of Mars and Jupiter. Thus, the orbits of none of the
largo phnetH are inclined to tho Earth's orbit more than 3° 23',
the inclination of Venus's orbit. The inclination of Mercury's
orbit is seen to bo considerable (7°) ; but the furthest removed
from the general level (so to speak) are tho orbits of tho aster-
olds, some of which have an inclination of from 24 to nearly 35
degrees; as for example, Pallas (34° 420, Euphrosyne (2G° 2J'),
Niobo (23° 19'), Phocea (21° 34').

619. The following tables — originally constructed
by C. Piazzi Smyth, Astronomer-Royal for Scotland,
and recently revised by him for the new edition of
Mackay's " Manual of Modern Geogi-aphy " (Edin-
burgh, W, Blackwood ^ Sons, 1873)— give in detail
all the most important discoveries hitherto made by
astronomers relative to the sun, moon, and planets.
On comparing them with the corresponding tables in
former editions of this work, it will be seen what im-
mense progress our science has made during the last
fifteen years. In particular, the great physical problem
of the age — the true mean distance of the sun from the
earth — has made rapid progress towards solution.
That distance, it may now be confidently affirmed, is
not 95,000,000 miles, but 92,093,000 ; for that is at

ELKMnNTH OK ASTRONOMV.

179

once the grand moan of nil recent reKciarclicH, ab also
the diHtanco clearly irulicated by the Great I'yrauud of
Jeezah, which, as if planned by supernatural wiwdom, an-
ticipates so many of the cosmolo^ncal discoveries of mod-
ern science, though erected upward?; of 4000 yean ago.

TIIK 8()LAH 8YSTI:M.

N^nit of D«l}.

Sum

Vulcan

Mercury

Voniis

Earth

Mnrii

The AHturoiUo..

Jupiter

Saturn

Uranua

Neptune

Mnn Ditlanr*

from Sun In

Mil**.

TlBW of

Havolutixii
In M**))

Tdnriijr
in Orhll
|ttr hour
In Milct.

Tim* of
Hoiaiiun
on AlU.

il.

h>i. ni.

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...

I7,6aj

7 48

...

18,0fl2,00O

1970

174,000

...

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85,649,000

87ii7

105,3;JO

6m]

6fl,614,0(Xt

22470

77,050

23 21

1932

t)2,093,(X)(»

305-25

65,633

I'OOO

140,322,0()0

686 9H

63,000

•' 37

•436

259,00<),(X)0

1,884-7 1

39,882

...

•130

479,141,098

i,33•i■(i^^

28,744

9 65

•030

878,461,000

10,759-30

21,221

10 2U

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1,766,565,000

30,686-82

14,96.1

9 30

•003

2,766,133,000

60,126-71

11,958

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-001

No. of
Mooni,

4
8
4
1

Name of Iludy,

SUW

Vulc^vn

Mercury

Venus

Earth

Mars

The Asteroids

Jupiter

Saturn

Uranus

Neptune

Moon

852,38(1

785?

2,902

7,510

7,809

4,030

670'

83,151

64,714

29,722

36,620

2,ir)S

862,584

786 y

2,9G2

7,510

7,925

4,920

670'

88,400

71,904

33,024

.W.fi'JO

Volume or

tizc.
Earth = 1.

Mau or

Welghl,

Eartb = 1.

1,245,130 000

•052

•861

1-000

•139

1,387-431

746-898

72-369

98-664

•021

314,760000

•070

•790

l-OOO

•120

300-860

90-030

] 2-640

16-760

-01:(

^fell

•25

1-24
-92

1-00
-9(i

•12
•18
•17
•6.'!

27- 2

1-15
-91

1-00
•60

245
1-09
1-05
1-20
•17

Inclina-
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Plain't'i
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of Uibil.

0° O'O*

49 58

23 27 24

28 61

3 4

26 49 C

26? C

Pallas, the largest of them.

IMAGE EVALUATION
TEST TARGET (MT-3)

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HiotDgraphic

Sciences
Corporation

23 WEST MAIN STREET

WEBSTER, N.Y. 14580

(716) 872-4503

:«

.^fc*

c.^

130

ELEMENTS OP ASTRONOMY.

Comets.

620. The comets are those stars which appear at
times in various parts of the heavens, describing an
apparently irregular course when compared with the plan-
ets ; approaching very near to the sun, and again reced-
ing to a great distance from him and disappearing.

621. Comets appear under very various aspects.
Usually, there is a brilliant luminous point called the nu-
cleus ; and a more diffuse light surrounding the nucleus,
called the coma or hair. These two constitute the
head; and there is oiten present, thor.gh not always, a
long luminous appendage, called the tail. The tail is
generally turned in a direction from the sun : and
frequently is bifurcated, that is, divides into two
branches, sometimes into more. Occasionally it is bent
into a gentle curve. Some conicts have several tails,
that is, streams of light diverging from them ; and
many small comets are without any such appendage.

_ 622. The comets are considered to be masses of
highly-heated, self-luminous vaporous matter, or solid
nuclei surrounded by much gaseous matter, revolving
round the sun in very elongated ellipses, so that at one
time they are very near the earth and sun, and at an-
other time very remote from these orbs. But the exist-
ence of a solid nucleus is by no means established.

623. The tails of the comets vary considerably in
magnitude. The tail is often scarcely perceptible at
first, enlarges as the comet approaches to the sun, is
most developed just after it has passed its perihelion ;
and gradually diminishes as the distance from the sun
increases, and the influence of that body lessens. The
tails of some comets have been estimated at upwards of
100 millions of miles in length— that of the comet of
1843 at^ 200 millions of miles. Some have had the
extremities of their tails in the zenith, while they them-
selves were in the horizon. From the above phenomena,

ELEMENTS OP ASTRONOMY.

181

it has been conjectured that the tail of a comet is formed
of matter ejected from its body by the sun's heat.

624. The comets revolve in extremely eccentric
orbits. The groat comet of 1680 was calculated to
have approached within about 150,000 miles of the
sun, — about ono-sixiii of his diameter.

625. The periods in which several of the comets
revolve round the sun have been computed, and the
correctness of the calculation proved by the return of
the comet several times. A very bright comet was
recorded to have been seen in the years 1531 and 1607.
In 1682, a comet appeared, which was observed by the
celebrated astronomer Halley, and calculated by him
to be the same which had been seen in 1531 and 1607
(and which might be traced back to the year 11 b.c,
and is supposed identical with the comets of 1305 and
1456). He accordingly predicted its return in about
1758, computing it to have a period of between seventy-
five and seventy-s'x years. It did appear, to the greac
delight of the astronomical world, in 1759, nearly about
the assigned period ; its delay being caused, as pre-
dicted by Halley and correctly calculated by Clairault,
by the action of Jupiter and Saturn upon it. This
comet again, in 1835, returned at the calculated time
(28,105 days), and followed very nearly the course
among the stars predicted for it. It remained visible,
by the aid of the telescope, from August 1835 to May
1836 ; and was very bright when crossing among the
stars of the Great Bear. In 1682, the tail of this comet
stretched over a space of 30°. It is expected again
about the year 1910. The great comet of 1680 has
been conjectured to be identical with the comets of 43
B.C., 575 A.D., and 1105 a.d., having a period of about
575 years. A brilliant comet with a very long tail,
which appeared in the year 1 264, is supposed to have
been the same which was seen in 1556. If so, it must
have a period of about 300 years ; while the great comet

182

RLEMENTS OP ASTRONOMY.

-of 1811 is supposed to have a period of about 3000
years.

62G. The comet of Biela performs its revolution in
the short period of 6J years (2413 days). It does not
pass much beyond the orbit of Jupiter. The c. bit of
this comet crosses that of the earth, and we were within
a month of encountering it in the year ] 832. -A re-
markable phenomenon recently took place in this comet.
Shortly after its second last appearance, between
November 1845 and January 1846, it separated into
two distinct comets, each with a nucleus and a short
tail, which moved on together, as if independent, and
still continued separate when the comet had run
through one complete revolution in its orbit, and re-
appeared in 1852. The distance between the nuclei
had then considerably increased.

627. A comet, with a period of about 7} years, was
discovered in 1843 by M. Faye. The comet of Encke
has a still shorter period, 3 J years, or 1210 days.
There are two other comets, De Vice's and Brorsen's,
whose periods have been calculated — 1993 and 2042
days.

G28. The comet of Encke has led to some singular specu-
lations regarding the existence of a fluid called the eiher, sup-
posed to be spread out through space. Its period of revolution
round the sun is found to be diminishing. This is attributed
to a resistance opposed to its progress by some material fluid
through which it passes, which weakens its centrifugal force,
gives the sun's attractive force greater proportionate power,
enables that body to draw it into a smaller orbit, in which it
moves more rapidly, and therefore runs through its course iu a
shorter time.

629. Some comets have been so exceedingly bright
as to be visible in daylight. This was the case with
the great comets of 43 u.c, 1402, 1532, and 1843,

A.D.

630. The number of comets which circulate in the
Bolar system is supposed to be very great, — perhaps

ELEMENTS OF ASTRONOMY.

183

thousands. Some 800 have been recorded ; and it is
believed that the proportion, of comets whicli are visi-
ble to us must be greatly under the number which really
exist.

631. Comets appear in all parts of the heavens, move
in all directions, and with very different degrees of
velocity. They are not, like the principal planets,
confined to the zodiacal belt.

632. Comets are considered to be mostly, if not
entirely, in the aerial state, for the following reasons.
The stars, even those of very small magnitude, can be
seen through their substance. They have been found
to cause no sensible derangement in the motions of the
satellites of Jupiter near to which they have passed,
while they themselves have been considerably influenced
and divci A from their course by the latter — indica-
tions that their mass is small, and therefore, as their
bulk is considerable, that they are in the aerial state.
Also, they present no phases^ which seems to show that
light is reflected from every part of the comet, and
hence, that the sun's light penetrates their substance,
which indicates an aerial state.

633. In ancient times, comets were supposed to resemble
planets, and, like them, to go through certain revolutions in
regular periods. But from the commencement of the Christian,
era to the time of Tyoho Brahe, they were generally regarded
by astronomers merely as meteors, existing in the atmosphere.
He found that their distances were beyond that of the moon ;
and the idea that comets are at considerable distances, and re-
volve round the sun in regular periods, was confirmed by Kep-
ler, Hevelius, Dorfel, Newton, Halley, and others.

The Zodiacal Light.

634. This is a faint luminosity in the sky, visible in
the west, immediately after twilight in spring ; and in
the east, towards the close of autumn, just before sun-
rise. It is very distinct in tropical regions, and is par-
ticularly descriJbcd by Humboldt, who speaks of " the

184

ELEMENTS OP ABTRONOMV.

mud radiance with which the zodiacal light, shooting
pyramidally upwards, illuraineb a part of the uniform
length of tropical nights." He states that in his voy-
age from Spain to South America, " the strength of
the light — it might almost be called illumination —
increased surprisingly the more I approached the equa-
tor in South America and the South Sea. In the
continually dry, clear air of Cumana, in the grass
steppes of Caraccjts, upon the elevated plains of Quito
and the Mexican seas, especially at heights from eight
to twelve thousand feet, the brightness sometimes ex-
ceeded that of the most beautiful sparks of the Milky
Way." — Cosmos, vol. iv. It has been supposed to be a
vast nebulous ring revolving between the orbits of
Venus and Mars ; but, within the last few years, some
curious particulars have transpired, which would seem to
show that that theory must now be abandoned. In 1855,
the Rev. G. Jones, chaplain of the U. S. steam-frigate
" Mississippi," narrates the following : " I was fortunate
enough to be twice near the latitude of 23" 28' N., when
the sun was at the opposite solstice, in which position
the observer has the ecliptic at midnight at right
angles with his horizon, and bearing east and west.
There I had the extraordinary spectacle of the zodiacal
light simultaneously ?>*^ both east and west horizons,
from 11 to 1 o'clock, i . several nights in succession.
It seems to me that this can be explained only on the
supposition that the zodiacal light is a nebulous
ring having the earth for its centre, and lying within
the orbit of the moon." *

The Meteoric Systems.

635. Meteors have been noticed traversing the sky
in all ages, and at all parts of the earth's surface.
Many of them have been attended by the actual fall of

* " Descriptive Astronomy," by G. F. Chambers. Oxford, 1867.

ELEMENTS OF ASTRONOMY.

185

^Rome 8ul)«tanco or body, to which the names of aerolites
and meteoric stones have been applied. When the body
explodes into small fragments before reaching the
earth, the fragments are termed fire-balls^ and some-
times bolides ; and when the fragments are apparently
consumed whilst traversing the upper regions of the
atmosphere, they are known ns shooting-stars. Many
ingenious theories have been devised to account for
these remarkable phenomena. But it is now generally
fiuppooed that they consist of small fragmentary masses,
revolving in extremely eccentric orbits round the sun,
some of which, coming within the sphere of the earth's
attraction, are precipitated on its surface ; the intense
heat and explosion being caused by the action of the
atmosphere on bodies passing through it at a very high
velocity. Other planets of the solar system, especial'/
the remoter ones, pass through similar rings of meteors,
and, like the earth, attract multitudes of them to their
surfaces ; while, doubtless, the sun himself, owing to
his superior gravitating power, attracts inconceivable
numbers of them, precipitating them with tremendous
momentum on his own surface ; thus providing inex-
haustible materials for maintaining his incessant light
and heat, with which he vivifies and blesses our
luiiverse.

636. Two remarkable circumstances in the history of
shooting stars render it almost certain that they are of
a planetary nature — their periodical recurrence at cer-
tain times of the year, and their divergence from certain
fixed points in the sky. A brilliant display of falling-
stars has been observed in many years to occur between
the 12th and 14th of November ; and a less numerous
shower, very often about the 10th of August. The
meteors of November appear to diverge from a star in
the constellation Leo, — those of August from a star in

the Camelopard, or the vicinity of Algol, in Perseus,

whatever may be the elevation of these stars above the

%^ .

■■■

186

ELEMENTS OF ASTRONOMY".

horizon. " During tljc culobrated fall of Hbooting-stnrs,
on the night bt'twecn the 12th and 13th of November
1833, the fire-balls and shooting-stars all (^'merged from
one and the same quarter of the heavens, namely, in the
vicinity of the star y in Leo, and did not deviate from
this point although tlie star changed its apparent height
and azimuth during the time of the observation. S..ch
an independence of the earth's rotation shows that tho
luminous body must have reached our atmosphere from
without." — Cosmos. Astronomers have determined
that the orbit of each member of tho November star-
shower is an elongated ellipse, having its perihelion
lying on the earth's orbit, and its aphelion beyond the
orbit of Uranus ; that the time of revolution of each
member of the ring is 33J years ; that the inclination
to the earth's ecliptic is 17°; and that the motion is
not direct, as in tho case of the planets, but retrograde.
The August meteors, again, travel in a path so eccentric,
that near the earth's orbit it is almost parabolic. Its
period, like that of the comet of 1862, with which it
seems closely associated, is 145 years; while its aphelion
distance is about twice the distance of Neptune.

637. The periodical occurrence of meteors arises
from their circulating round the sun in broken
rings, in orbits near which tho eartn is at tho
periods when the meteors are seen. There may be
several planetary masses having the same orbit, follow-
ing each other at certain intervfils, and crossing near
the earth's orbit, so that they become visible at certain
times, — " a stream of meteors in their progress of cir-
culation round the sun." This supposition would also
explain their appearance of diverging from one point in
the heavens.

638. Upon an average, about seven shooting-stars
of every description may be seen at any one spot every
hour on clear nights. In the August shower of 1842,
one observer saw 34 in ten minutes. In 1852, on the

ELEMENTS OF ASTP.ONOMY.

Ift7

10th of August, the writer saw 27 in h.ilf-an-hour. It
has been roughly calcuhitod that tlio average number
of meteors which traverse our atmosphere daily, and
that are visible to the naked eye, does not fall short of
7,500,000. So far as observation has yet gone, these
are divisible into 56 distinct grotips, pursuing their
courses in 56 different orbits, and having as many
radiant points in the celestial sphere. Having highly
eccentric orbits, their velocities are very different in
different parts of their paths. When in their perihelia,
somfj of .licm travel at the rate of 200 miles per second,
b' t.evr average velocity does not exceed 34 miles per
.4 j'liJ, being nec^ily tvice the average velocity of the
earf'a ii« bcr orbit. ShooLing-stars usually become
vir'bie to the 7>aked eye at a height of 72 miles, and
aga-ii diriapprar at a h^'if^ht of 52 miles. Some are
cynriosed riodtly of netidlic iron always alloyed with
nickel, and smaii quantities of other metals, as cobalt,
coppev, tin, and chromium. In the main, they art
composed of metallic iron and various compounds of
silica, the iron forming in some cases as much as 95
per cent. Among the silicates may be mentioned
olivine, a min ral found abundantly in volcanic rockf-',
and augito.

188

LLEMENTS OF ASTRONOMY.

PART IV.

PARALLAX, ARKKRATIOX, AND PRECESSION.

BIXTION I.

Parallax.

639. The word Pamllnx is used in Astronomy in two
senses ; a special and a general sense. In the special
sense, parallax signifies the difference in tho fxltitudo of
a celestial body as seen from a point on the surface of the
earth and from the centre, if it could be seen from that
place. This is sometimes termed the diurnal parallax,

640. This will be illustrated by the following figure
(53) r Let the dotted circle A E B bo the earth ; p^ p\
and p"^ ai y heavenly bodies ; and the dotted arc at the
right, G c', the imaginary surface of the heavens to
which we refer the positions of celestial objects. Let
E be the position of an observer on the earth's surface.
Now, if the heavenly body p"^ be viewed from E, it will
appear at e^ on the surface of the heavens, i.3 shown by
the dotted line E p'^ e'^; but if viewed from C, the
earth's centre, it would appear at c'^, as shown by the
line C p^ c^. The difference of these two positions is
the arc c^ c'^, which is therefore termed the parallax at
E of the body p'^ ; or, instead of the arc c'^ e'\ the angle
c^ p^ e'\ which the arc c^ e^ subtends, and which is of
the same number of degrees (73-5), and may be called
the parallax of p"^ at E. Or, the angle E p^ C, which
is equal to the angle c^p"^ e^y may be called the parallax
of/)' at E. The latter is what is usually stated as the
parallax of the object. Thus the parallax of a celestial
body is the angle at it formed by two straight lines,
one drawn to it from the earth's centre, the other from
the observer's position on the earth's surface.

ELEMENTS OF AflTRONOMY.

180

Fig. 5S.

#0.*,

* * # • VA*a.

<?** *''it*

'-e-'

641. The effect of parallax Is to depress the body,
always making it appear nearer to the horizon, as seen

^S^lP^'l'*"

190

PLRMKNTI or ABTRONOMr.

from tl>o Riirfuce, tlmn when W!cn from the centre.
Thin iH evident, an c\ the |)OHitum of »• on seen from the
centre, is nearer the zenith of E tlmn e\ its poHition m
won from E.

642. Puralhix \h ulw.iyM groaleht wlit-n the b<xly iw »Vi
the horizon, and preater an llie body {h neartr to the
horizon of the ohNervet. This -8 evident from the preced-
ing fignre, in which the parallax, c* eU^t p\ is less than
c' e\ the paraUax of p\ which is ncRror to the hori/on
of E than p'K

^\X There is no parallax of a body in the zenith.
This is evident from the pn ceding figure, in wliich the
body p, which Is in the zenith of in observer at B,
appears in the same position in the heavens, c, whether
viewed from H or from t' o centre.

CM. As bidies mni^t appear in diftcront positions
when viewed from different points of the earth's surface,
it is desirable, for astrmomical and nautical purposes,'
to calculate their apparent positions in reference to some
fixed point; this being known, a correction can be
applied for the difference in the apparent position as
seen from the surface, and that given as the position
seen from the fixed point. The point selected for this
purpose is the earth's centre, anu the ordinary meaning
of parallax, therel.re, is the difference in the positions
of a body as seen from the earth's surface and from the
earth's centre.

^ 645. Parallax, in the wider acceptation of the term,
signifies the apparent change of position in an object

arising from a change in the position of the observer

sometimes termed its parallactic motion. As in the
above case, it may be expressed by the angle formed at
the object hy two straight lines drawn from the object to
the two points fr or I which it is observed.

646. When an object is viewed from two different
points, it will appear in different directions at these
points. And the exact amount of difference will bo

El,r.MP.NTf OF A«TRON)MY.

lUi

espraand l»y tho ftnglo at tbc ohu .-t f«»rmc»<l by th« two
lines of view [GiO). ThuH, in Fig. 63, if A and B in
tho (lottuil circle ImjIow ropre»»ent two iKwitionn from
which tho object S^ is seen, that object will be '.mm in
tho direction A S^ from A, fin<l H <S' from H. Tho an^^k-
A o' }^ will be the anjount of (lifferenco of the direc-
tiona A S- and ii S\ or tho parallax of SK

647. Tho parallax of any object diminishes at its
distance increahcs ; and a bo<ly may ' ? so remote that
its parallax will become ho umall a:i bo inscnHible.

648. ThiH is illnHtrated by V'lg. ;... Ut A and B
be two points from which tiie btviics *S'', *S^, Si'\ S\ 8'\
arc viewed. As the distance incrcses tlie angle at the
body diminishes, A S'^ B beir>g lets than A »S'' B, and
A <S^ B being less than A ^'^ B, and bo on. It is evi-
dent that if the distance from the lino A B weie very
much incicased, the angle at the body wouKl disappear
altogether, the lines to it fr.m A und B would co'ucide,
and tho distance fro!n A to B would be as nothing
compared with their distance from the object.

G49. Now, it is by means of the p<\rdllax of a heavenly
bo<ly that its distance is calculated. The distance be-
tween the two points beinj^ known, and the angle of its
parallax, trigonometry furnishes the distance from the
position to tho object.

650, The parallax of any body increases as the dis-
tance between the points of view increases. This is
evident ; for if tho observer at B were to shift his posi-
tion to the right till he came to c, then tho angle A S^
B would bo very much increase ..\ as the line from ^S"^ to
B would now be in the direction from *S-' to c ; and thus
a body too far distant to give a perceptible parallax at
two points might give a very sensible parallav if tho
distance between the two points of view were 7ery
much increased.

651. Mow, none of the fixed stars give any parallax
with the radius of the earth; it is therefore known that

192

ELEMENTS OF ASTRONOMY.

their distance is infinite when compared with that
radius, and that with it alone we have no means of
measuring the distance of a fixed star. Therefore a
much longer basis has been tried — the longest which
we can command — the radius of the earth's orbit. But
few of the fixed stars give any parallax even with this
extended basis : the most of them have no annual
parallax as it is termed : the earth's orbit sinks to
nothing compared with the enormous distances of these
glittering points; and with respect to the greater num-
ber of the fixed stars we have no means of ascertaining
their distance. See Part VI., on the Fixed Stars.

052. That distant objects give no parallax with points near
each other is very obvious in the case of any short distances on
the earth with the moon. For example, looking at one limb of
the moon along two parallel streets, it will be found to appear in
the very same direction along eitlier street. The ^>ngle of paral-
lax ig too small to be discernible ; the two lines from the street
to the moon coalesce ; the distance between the two points of
observation is a mere point compared with their distance from
the moon; and we could not, with that distance as a basis,
measure the moon's distance. The same may be observed of an
object on earth. If an individual walk along a road for any
given distance, and take some very remote object, such as the
stars or a far-off tower or mountain, whereby to judge of the
apparent motions of bodies between his line of motion and the
fixed objects by which he judges of their apparent motions, he
will find that these various bodies will have shifted their appa-
rent position less and less according as they are further from
him — that is, their parallax will be less. In proportion as they
are more remote, he will find the angle at the body, formed by
two straight lines drawr. to the two points of view, become less
and less till it disappears find the two lines coalesce ; then there
is no parallax, and ^'rt'iih. that basis nc means of measuring the
distance of the object.

SECTION n.
Aberration.

653. Aberration is an apparent displacement of the

true pORitiuii, arising from

ceiesiiai

bod

lus irom

theix

KLEMENTa OP ASTRONOMY.

193

the motion of the earth in its orbit, and the time
required by light to traverse space.

6o4. Every object is seen in the direction which the
rays from it have when they strike the eye, if the eye he

^'t\x^ .. '^ ^^^ "^y^ ^^ ^^ "lotion, the direction in
which the object is seen will be one compounded of the
direction of the eye's motion and that of the rays fVom
the object. The difference between the real direction

aberration ''^ ^""^ ^^""^ '" '^^'''^ '* "'^P^'''^ '' '^'''"''^ ^^®
655. Let A, Fig. 54, be the position of an observer,
and fe the position of a star. Let B be the situation of
tlie earth, when that ray emanates from S, by which
t^je star is seen at A,— the earth and ray coming to A
at the same instant. Then, by the composition of motion,
t^e star, when the earth comes to A, will appear at 5, in
advance of its true position. The angle S A s is the aber-
ration. As S A, which repre-
sents the motion of light, is ^ ^
very great compared with B A |4 ^
or A C, which represents the
earth's motion in the same
time (as 185,000 to 18), the
aberration is very small.

656. The aberration is great-
est when (as in Fig. 54) the
direction of the ray is perpen-
Qicular to the direction of the

earth's motion : in this position there is a displacement,
by abberration, to the extent of 20-5'' (twenty seconds).
It diminishes from this till the directions of the two
motions are parallel, when it ceases altogether. There is
also some aberration from the motion of the parts by the
earth's rotation ; but this is insensible.

657. Had the earth been stationary at A, or had
light come instantaneously from S to A, the star would

appear in its true position at S. The'ph

194

ELEMENTS OF ASTRONOMY.

aberration is one of the most convincing proofs of tl»e
earth's motion roun^l the sun ; and it confirms vory
satisfactorily Roemer's discovery of the progressive
motion of light, and also the rate of its »riotion as
inferred from the eclipses of Jupiter's satellit-cs, and de-
termined by M. Foucault in 1862 by a rotating mirror.

658. Aberration may bo illustrated by the manner in which
drops of rain strike upon an individual, according as ho is in
motion or at rest. If the drops fall perpendicularly, and he bo
at rest, they will bo f5lt only on his head — that is, they iv'dl
strike in the direction of their oion motion. But if he bo mov-
ing quickly, they will strike upon his face, and appear to he
coming in a slanting direction towards him, as if they fell from
a point not only above him but in advance of him. It is evident
that the direction in which they would appear to come must
depend upon the real directions and comparative velocities of
the two motions.

659. Or, if we conceive a ball to be let fall in the direction
S A, Fig. 55, and to enter the tube S B, which has a motion

Fig. 55.

... V

in the direction B A sufficient to carry it to the position A s
in the same time in whieh the ball would fall from S to A ;
then the ball would, while passing from S to A, move in the
axis of the tube (not touching the sides), and to an observer at
the bottom of the tube would necessarily appear to hfive come
from «, not from S, and to move in the direction s A, not S A.
The ball ia analogous to the ray of light, the bottom of the tube

ELEMENTS OP ASTRONOMY. 195

to the earth and the direction in which the ball appears to move
to the directum m which the star is seen.

^ 660. Besides the aberration of the fixed stars, there
18 also an aberration of the planets and comets, arising
from their motion. When a ray of light from a plant^
arrives at the earth, its direction does not show the
true position oi the planet; as the latter has made a
certain progress in its course smce that ray left it—
and Its true position must be in advance of its apparent
position. For the discovery of the aberration tVlwht
and determination of its amount, science is indebted
to the distinguished English astronomer Dr Bradley

SECTION III.

Precession of the Equinoxes— Nutation of the

Earth's Axis.
661. Besides its rotation on its axis, the earth has two aihor
TZZnIi t-^rf^-'^'"^''^" ^^^^^ parts wi?h^^faL'rtg

except bvvP Tn 'f , ^'\ r'y '^"^' '^"^ ^'^ »«t discoverable
except by ve./ caretul and long-continued observation.

1. Precession of the Equinoxes.

n u^a' .V'^ ^'^^'.P^''' ''"*' *^'^ equinoctial in two points,
called the equinoxes, during one revolution of the
earth round the sun. But these two great circles cut
each other in different points each year; that h ?he

points (or stars) in the starry heavens where they inter-^
sec are different each year. This change in the posi-

663. The line of the equir. xe s moves backwards*

6ac-A«;a4: ^" """'' "' ''S^'"'*^ '^« ''"^''' «f »^« signs, retrograde or

196

ELEMENTS OP ASTRONOMY.

Upon the ecliptic, tlisit is, from east to tvcsf, or in a
direction contrary to the sun's apparent annual course
through the ecliptic, which is from west to east; so
tlutt each yi-ar the s\m crosses the equinoctial in a point
west of that in which they last met. The amount of
this retrocession is 50-21" yearly.

664. As the sun moves eastward through tie ecliptic,
and the equinox moves westward, the sun vfiV. come
sooner to the eqiiinox in each revolution, for they move
round towards each other; hence the period of each
e(piinox will come a little earlier evevy year ; from
which the expression precession of the equinoxes is
derived. Tlie time which the equinox precedes each
year is 20'"- 20"* , that being the time in which the sun
goes through an arc of 50*21''.

665. From this circumstance, the positions of the
signs of the zodiac among the stars change regularly
backwards (in the opposite direction to the sun's motion).
The rate at which the equinox recedes is such, that it
makes a complete revolution round the ecliptic in
25,898 years. This is equal to l'' in 71*6 years, or 30°
(one sign) in about 2000 years. And accordingly it is
found that the position of the equinox is now about 30°
behind what it was 2000 years since : then, the signs
and constellations of the same name were the same, —
but each sign, which still retains the name it had then,
is now in the preceding constellation. — See Par. 1 14.

6G6. From this recession of the equinox, the true year, or the
period of the earth's return to the same star, is a little longer
than the equinoctial or tropical year, which is the iuterval be-
tween two returns to the same equinox. — See Par. 330.

667. This precession of the equinoxes is caused by
a conical motion of the earth's axis^ by which, while
the middle point remains fixed, the poles describe a
small circle, as in the following figure. Let A a,
B 6, C c, D rf, represent the earth's axis, being the
middle point. It does not always remain in the

ELEMENTS OF ASTRONOMY.

197

Fig. 66.

■#

same direction, pointing to
ibe star a', — but shifts on
its centre, passing from A
a to B i, etc., the poles do- c ,-■'
scribing tbe circles A B C 41
1), a 6 c c?, and tuccessively \
pointing to the stars a\ b\ *'■*- ^°

c\ d\ Thus, each radius
describes a cone — com-
pleting it in the long period
of 25,898 years. The ra-
dius A describes the
cone A B C D,— the
radius a, the cone B
ah c d.

668. The inotion of each ra-
dius exactly resembles that of
a top spinning. It is often
observed, besides its rotatory
inotion, to have a swinging
motion, inclining to one side,
then gradually shifting, and inclining as much all round, so that
Its axis has a compound motion, turning like any revolving
body, and at the same time describing a cone of which the
apex IS at the ground. In this swinging motion, 'j always
keeps the same inclination to the horizon, as the earth's axis
does to the ecliptic.

669. From this, the pole of the heavens (the point in
the heavens towards which the earth's pole is directed)
describes a circle round the pole of the ecliptic ; this
circle is always 23" 28' from that pole.

670. Hence the earth has not always the same star
for its pole-star.* The precent pole-star is 1° 24' from
the pole ; at the time of Hipparchus (about 140 b.c.) it
was about 12° from the pole. It will be nearer to the

• The brightest star near the pole of the heavens is the pole-star at the
time. At present this star is Polaris in the constellation Ursa Minor At
the time of the erection of the Great Pyramid (b.c. 2170), it was the' star
ace i' tttia arm i/aios y^. i2a;, BiacKwooa «& jous, 1870.

198

ELEMENTS OP ASTRONOMY.

pol(! still for a little, .and then will recede from it apain ;
and in about 12,000 years the pole of the heavens will be
on the opposite side of the pole of the ecliptic, 46° 56'
from its present position, and very near Vega, the
principal star in the constellation Lyra, which star will
then servo for a pole-star. See Fig. 56, in which the
axis is seen pointing successively to the diflercnt stars
a', b\ c', cT in the sphere of the heavens.

671. But the angle of inclination of the axis to the
ecliptic always remains unchanged ; so that the angl5
between the planes of the ecliptic and equinoctial still
continues the same — 23° 28'. And the whole earth
parta^'es of this motion, so that the axis and poles still
bear the same relative j)osition to the other parts of ilie
earth's surface ; the latitudes remain the same, and the
waters are unaffected.

672. From this, then, the points where the ecliptic
and equinoctial cut each other must be continually
shifting. This will be illustrated by the following
figure. Let P represent the pole of the ecliptic, the
sphere representing the sphere of the heavens. Let B
C be the ecliptic, and the small circle h h' h" the circle
in the heavens marked out by the earth's pole round
the pole of the ecliptic, and the points h hf h" on that
circle different positions of the pole of the heavens.
"When h is the pole, the equinoctial will be E r Q,
intersecting the ecliptic in r. When h' is the pole,
E' r Q' will be the equinoctial, cutting the ecliptic in
r' — and when the pole is at A", tlie position of the equi-
noctial will be E" r" Q", having r for the fequinox

673. The cause of this conical motion of the earth's
axis is the action of the sun and moon upon the pro-
tuberant matter at the earth's equator. As they move
in the ecliptic, and the projecting matter at the equator
is out of the plane of the ecliptic, their action tends to
draw this towards the plane of the ecliptic and to make
tho planes of tiic ecliptic and equator coiiicidc. But

ELKMiiNTS OP ASTRONOMY.

199

the rotation of the earth on its axis prevents any
change in the inclination of the equator and ecliptic ;

Fig. 67.

and causes the earth to have tlic gyratory motion in its
axis which gives rise to precession.

674. The amount of precession caused by the action
of the sun is about 15*21''; that produced by the
moon 35", or nearly as 2 to 5.

2. Nutation.

G75. The circle which the earth's
pole describes round the pole of the
ecliptic is not a true circle, but waved
or undulating, as represented in the
adjacent figure. This oscillatory mo-
tion of the pole backwards and for-
wards is termed nutation, being a sort
of nodding motion of the earth's axis.

Fig. 68.

676. Nutation will be best understood bv supposing

■ i

200

KLRMENTS OP ASTRONOMY.

the point rcprcsontinp; tlie mean place of the pole to
describo the uniform circle round the pole of the
ecliptic, while the real position of the pole describes a
biiiall circle (or rather ellipse) round the mean place.

677. This small ellipse is completed in a little less
than nineteen years, at a distance from the mean place
of the pole of about 9"— the lonpfcr axis of the ellipse,
which points towards the polo of the ecliptic, Leinff
about 18-5^ ^

C78. Nutation is caused by the action of the mooii
on the protuberant parts at the earth's equator, which, as
tlie moon's orbit is inclined 5° 8' 40'' to the ecliptic, and
its nodes complete their revolution round the ecliptic in
eight<3en years seven months, causes ♦he above described
motion, accompanying that of precession.

;

ELEMENTS OP ASTRONOMY.

202

PART V.

PROOFS.

679. Having now described the leading phenomena
of the sphere of the heavens, of the earth, sun, and
moon, and of the soiar wystem ; and having given tlio
generally received explanations of these phenomena. •*
yet remains to render some account of the reasons b^
which it is provfed that these are the correct explana-
tions. There are three principal points to bo i)roved :
that the earth is round; that it rotates; and that it
moves round the sun.

SECTION I.
The Earth is Eound.

680. There is no scientific fact which can be estab-
lished by a greater number of irresistible arguments
than that the world we live on is round; and the
greater number of these arguments are quite intelligible
even to those who do not possess any mathematical
knowledge.

681. (1.) Men have sailed round the world. 4 ship,
starting from one place and sailing onwards, without
ever turning back, merely moving a little to right or
left to avoid running upon the land, has come back to
the place from which it set out. This could only happen
on a round body. It was first done by the e .pedition
of Magellan (or Magalhaens) in the years 1.518-21 ;
afterwards by Drake, Anson, Cooke, and by great
numbers of navigators recently. ^ Magellan did not live
to coiiipleto his voyage : lie vvas killed in the PLiiippino

202

ELEMENTS OP ASTRONOMY.

ill

Iwlftnds, and liiH Hlnps were broiiglit back by one of bis
oflicers. Hut the 8trait8 of Mat^i'llati, at tliu Hoiith of
tho American continent, and the Hingiilar ncbulw near
the southern polo of tiie heavenH, called the Magellanic
(^h)udH, will proHCi've to distant ages the nanie of the
leader ot the first expedition that elreuninaviguted tho
globe. It was found, on their return to Spain, that the
navigators had lost une entire day during their long,
circuitous voyage. Had they circumnavigated tho
globe by sailing in tho oppoHite direction, tliey would
have gained a day. No more striking proof of tho
earth's rotundity could possibly be afTonlcd.

GH2. It has also been well established that the world
can bo sailed round in shorter time, the further south
the voyage is performed — as in the South Atlantic
Ocean, beyond Capes Horn and Agulhas — which shows
that its circumference narrows as we pass from tho
equator in a southerly direction ; and, though we can-
not sail round the world in the northern hemisphere,
on account of the land and state of the north seas, tho
distance round can be very well ascertained by other
means, and proves the same fact — the regular diminu-
tion of the thickness, or of the circumference of the
earth, as we pass north or south from the equator.

683. (2.) When a ship sails from us, the lower part,
the broad massive hull, first disappears; the slender
top-masts and rigging go last out cf view. And, if we
look at it through a telescope, when it is too small and
indistinct to be seen by the naked eye, we shall find
that the highest parts of the masts are the last to dis-
appear. Similarly, when a ship approaches, the upper
parts come first into view. These things prove that
the sea is not a flat plain, but that it bulges out between
an observer and distant objects ; that is, that it is con-
vex. As similar appearances are observed at every
part of the wide ocean, and on land too, where we can
got an extensive flat piece or olain, it followB that tiie

ELKMKNTS OF AHTRONOMV.

203

world must bo ovurywhuro more <?r Icms rouriil. See
Fi^. 59, ill which u Mhip iH icprewiitcMl sailini^ on a
round Hurfiico, and ut diffureut distttnces from the
observer,

Pl«. 00.

684. At a diiitunco of eight niilus, two pointH, each
ten feet above the 8ea or any extenwivo plain, are
found to bo inviKiblo from each otiicr, owin^ to tho
bulf^ing out of tlio interniediato surface. Tho lino
joinin*( thcni would be a tan^'cnt to the curved sirfacj-,
just skirting its iiighost point. From this, by biniple
mathematical calculationH, the diameler of tho earth
may be found ; and it dilTcrH little from tho diameter
us found by nictluids capable of much greater accuracy.

<iH5. (.'i.) When the sini rises, he does not give light
to all the world at once : he shines on part only. Ho
rises and gives light at any place sooner than at any
other place further west. This may be easily ascertaincl
even in Britain, which is so narrow from east to west ;
for the sun rises in that island about one minute later for
every ten miles farther west. Aberystwith and Lowe-
stoft are nearly east and west of each other, and about
210 mileu distant; and the sun therefore rises about 24
minutes earlier at Lowestoft than at Aberystwith, which
is 24 times ten miles west of it. Of this fact every one
may easily convince himself by comparison of the timcH
of sunrise, noon, or sunset, at places east or west of each
other ; and as the sun would evidently give light .at
onco to the whole of any Jlat plain above the level of
which he had risen, we have here a most convincing

WTi

20i

ILKMENTt or AHTRONOMY.

pn)of tlmt tim worM Ih nioro or Icsh round in the direc-
tion from woHt to cast.

6H(>. (4.) Ifwo iiduiit tho Bun •»nd moon to see from
day to <lay durinff tho wlio'.) courBc of (Mir livt'B, to bo
Always tho Kiunu Hun niid moon, it foUoWH from tho
phenomena of tlieir riHing and Hotting daily, that they
have A way rourul tho earth by puHsing ut. ler it; in
fact, that they take many ways nnder it, as they riKO
and 8ct at many diiTcrent poitits of tlio iiorizon during
tiio course of the yer»r. From this it is ,phun cither
that tho earth rotaten daily, or that the Hun, m()on, and
Btarn go daily round the earth— in either of which cases,
tlio earth mupt bo a body of limited extent towards tho
east and west, and more or less round in that direction.

687. •'5.) As wo pass further south, the pole-star
and si'irs round it sink, till at the equator the former
appears >•• tho horizon, and disappears altogether if wo
go south b'^yond that lino ; while new stars come into
view foi every step wo take south, till at tho equator
we commanci during one night a view of the whole of
tho stars of the heavens. As wo can ascertain by simple
measurements that the stars are at a very great distance
from us, this regular change of elevation in any star as
we pass north or south, and appearance of now stars
and total disappearance of others, as we go soui.i, ulmw
that wo avo moving along a surface that is more or less
round from north to south. It is to be observed that
the stars in sinking p^aI tben disappearing do not fade
gradually in lustre, a? if vo were gral r.lly increasing
our distance from th , i, \,at icmain equally bright to
the last, showing that they are lost to our view from tho
interposition of something opaque between the star and
the observer.

fi88. (fi.) In eclipses of the moon, the earth's shadow
always has a circular edge, and none but a round body
can give a circular shadow in whatever position it may

bfi r»l np.pH.

Wo know that eclit>sc'S of tho moon aro

ELRMF.NT8 Ol' A*" lONOMY.

205

CAUsod by the intorvontion of iho oartli Ijctwcon the nun
and moon ; an, from tiino iinmctnorial, thewj phenom«iia
have nevor tiiUen place except when the oarth, sun,
and intMHi aro in one straight line — whc-n the ni'n and
moon aro in exjictly oi)|)08ito parts of the sky ; that ut^
in op|K)sition.

689. (7.) The horizon, or oJJ^nf/, ai it is soiretinieH
caUed, is always circular, from whatever point we take
it; itrf extent is greater, and the angle l)ei,."een the ob-.
server and ts opposite points less, the higbui the point
from wiiich we view it ; and the dip of the horizon (the
angle between a honzont.d line as given by m s[tirit
level and a line from the spectator to the aetnal h i /, "u)
's greater, the greater our elevation. All these, .)b-
fc^rved everywhere, givo certain indications of a surface
more or Ichh round

690. (8.) Tile lessening duration of twilight, as wo
pass towards tlio equator, gives distinct evidence of a
spherical form in the earth, and of the variour parts at
tlie surface being further from the axis of rotation the
nearer they are to the e(piator — whether we regard the
earth, or the heavens, as performing the daily rotation
by which the phenomena of sunrise and sun.set are
caused.

SECTION II.

The Earth Rotates.

691. That the world turns daily on an axis, from
west to east, makm.^ a complete rotation in about 24
hours, is well established, by the following consider
ations : —

692. (1.) The whole sphere of the heavens appears
to rot^ite from east to v/est in 24 hours ; and we know

206

ELCMENT8 OF ASTRONOMY.

that this apparent motion can be perfectly explained by
a rotatory movement of the earth in the opposite di-
rection in the same time. Thus, in the adjoining figure,
let the circle A B „, «„

J , Fig. 60.

a b, represent any
parallel of latitude
on the earth (the
sign of the earth at
A and of the sun in
the centre being dis- ;'• :
regarded) ; and let :■'.*
it be supposed that •'•.'
the earth rotates so -y.
as to bring A to B, a
the stary sphere B '•
S A S remaining
fixed. It is mani-
fest that any one
who was at A will,

when at B, have come to a very different position as
regards any object in the part oS.the starry sphere
within his view, as B S. If he is not sensible of his
motion, finding that any objects at B S are now in the
position formerly occupied by those of A S, the former
must have appeared to him to have moved from B S to
A S, in a direction opposite to that in which he has
moved,* and the whole starry sphere must appear to be
moving round in the same direction. See par. 24,
page 11.

693. (2.) The supposition of the daily revolution of
the heavens is liable to this almost insuperable objection
that, as the stars always, and the sun, moou, and plan-
ets, for the most part, preserve the same reLtive
positions to each other during this great movement, a

* And opposite to tiiat shown by tlie arrow at the top of the figure, wLkli
it) tLuru ilir u diil'urcut pur|ju»c, vlh will prjbuully be iicuii.

ELEMENTS OP ASTRONOMY.

207

vast number of bodies, some of them much larger than
the earth, very distant from us, and far distant and de-
tatched from each other, must have one circular motion
in common, in the same direction, each completing its
circle in the same time, and the circles described by all
being perfectly parallel to each other ; an almost im-
possible series of coincidences, and the extreme im-
pobability of which led to the absurd theory of crystal
spheres. (See par. 22, page 10). This simple and
single act of the earth's rotation on its axis afibrds an
easy and natural explanation of these otherwise extra-
ordinary and unaccountable coincidences in rate and
direction of motion in so many different bodies.

604. (3.) The slower oscillation of the pendulum as
we pass to\vards the equator — this diminution of the
force of gravity being more than can be accounted for
by the increased length of the pendulum from warmth,
and by the spheroidal form — affords a strong presump-
tion in favour of a rotatory motion of the earth, which,
by the great centrifugal force it would generate at the
equatorial parts, must lessen the force of gravity.

69.'). (4.) The spheroidal form itself affords an argu-
ment in favour of rotation, as we know that rotation
tends to produce such a form, (5.) The analogy of
the sun, moon, and planets, which are known by obser-
vation to rotate, affords a presumption that the earth
also, which resembles them in so many points, has this
motion

696. (6.) The easterly direction acquired by the
great currents of wind that are constantly rushing from
the cold regions of the earth towards the heated parts
on each side of the equator, afford a strong argument
in favour of a rotatory motion of the earth from west to
east, which, by the great velocity of the equatorial
parts compared with that of parts at a distance from
the equator, imparts this easterly direction to the north-
ern and southern currents. See Trade-winds.

208

ELEMENTS OF ASTRONOMY.

697. (7.^ A remarkable proof of the rotation of the
earth from west to . ast is derived from the experiment
of letting a pebble fall from the east side of the top of
a very high tower; when it does not fall exactly at the
bottom, as it would if it descended truly in the plumb
line, but a little out from the bottom of the tower.
This is inexplicable, except upon the supposition that the
earth rotates; in which case, the pebble when at the
top of the tower, being further from the axis of rotation
than the bottom, moves in a larger circle, and therefore
more rapidly, as the rotations of all the different parti
are performed in the same time. Hence, while the
pebble descends by the force of gravity, it also moves
a little in advance of the lower parts of the tower by
its greater velocity of rotation, and falls a ^ittle beyond
the base. In short, it has a greater eastv/ard tendency
than the bottom of the tower, and must therefore, in
falling, take a position a little in advance of it.

698. (8.) A very clear and satisfactory proof of the
earth's rotation is afforded by the celebrated pendulum
experiment^ devised by M. Foucault. If a heavy
body be suspended by a wire and made to vibrate
(to oscillate backwards and forwards), its vibrations will
taxe place in the same plane, even though the points
from whi' h it is suspended have a slow rotatory motion
imparted to them. M. Foucault's pendulum was sus-
pended from the dome of the Pantheon in Paris, and a
fine point at the bottom of the weight was made to
leave a mark in sand at each swing. The murks suc-
cessively made in the sand showed that the plane oi"
oscillation varied with regard to the building. Here,
then, was a proof that the building, and therefore the
earth, moved. If, at the pole of the earth, such a
pendulum were suspended and set in motion, and a
circle divided into degrees were placed beneath it, as
the earth rotates in 24 hours, while the pendulum con-
tinues to vibrate in the same plane, the circle (attached

ELEMENTS OP ASTRONOMY.

209

to the e.arth) would rotate, and bring the pendulum
opposite to every successive degree till a complete revo-
lution had been accomplished ;— and as tlio earth's
motion is not sensible to us, it would appear as if the
pendulum had been continually changing its piano of
motion in the opposite direction, making a complete
revolution and returning to the original plane in 24
hours (or rather 23»'- 5G'"- etc.). If made to vibrate in
the plane of the meridian at other places, the same ap-
pearances would be caused by the earth's rotation ; but
the rate of apparent movement of the plane of vibraticm
would gradually lessen as the place is nearer the equa-
tor, at which it would cease, as the circle would there
cease to have any motion of rotation, and have no
motion except what would be common to itself and the
pendulum. It has been computed that the apj)arent
motion of the plane of vibration would be about 1 5°
hourly at the pole, 12 7° at Aberdeen, 11'5° at Paris,
9-7° at New York, 1-8° at Ceylon, and 0° at the equator ;
and as experiments made at many places coincide very
closely with the calculated effects, we derive hence a
very convincing proof that the earth rotates, — a means
of ascertaining the period of its rotation (found to coin-
cide as closely a^ can be expected with the time as
ascertained by other methods)^ — and a means of ascer-
taining also the latitude of the place.

699. That the earth is of a spheroidal form, and not
a perfect sphere, is proved by the diminishing force of
gravity towards the equator, by the diminution of the
degree of latitude the nearer the place is to the equator,
by analogy from the other planets, as Jupiter and
Saturn, by the tendency of rotation to produce such a
form, and by the phenomena of precession and nutation,
in wliich we observe effects such as would arise from a
protuberance towards the equator.

Ii['

210

ELEMENTS OF ASTRONOMY.

11'

I!!

SECTION III.

The Earth and the other Planets move Round

the Sun.

700. (1.) When tlic sun is observed from day to d;iy
tlirouf^^liout the year, and his position in relation to tlio
constellations watelicd, it is found that every 8()5 days
ho makes a comi)lete tour of the heavens; that, setting
out from the vicinity of any star, he moves to the east
of it, daily increases his distance from it, till he is 180°
from it in the opposite quarter of the heavens, then
approaches towards it, and returns to the same position
near it at the end of one year. This may be ascertained
by ob!?erving his successive distances from any promi-
nent star that is visible, or by observing from time to
time what stars rise just before or set just after him, by
which we can ascertain his position among the stars
with tolerab] exactness. Now this apparent yearly
motion of the un can be perfectly explained on the sup-
position that lie sun is a fixed centre round wl ich the
earth moves in the same direction in the same time.
This will be understood by reference to Fig. 60. Let
A re})resent the earth, moving in the ovhh A B a 6 in
the direction indicated by the arrows, t'le sun being
represented in the; centre of the figure. It is manifest,
that if the earth move from A to B, the sun, seen from
A in the position a s among the stars, will from B ap-
pear at b s, and thus must appear to have moved from
a s to b s through the starry heavens. As the earth
passes from B eastwards, the sun must appear to pass
westwards from b s ; and when the earth arrives at the
opposite part of its orbit, a b, on the other side of the
sun, the sun must be seen in +he opposite quarter of the
heavens A S B S, and wL the earth moves in her
orbit from a to 6, the sun appears to move among the
stari:, from A S to B S.

y.i

ELEMENTS OF ASTRONOMY.

211

701. (2.) The theory of the fixity of tlio sun and
..-»nular motion of tlie eartli round that luminary is so

Mch simpler, and avoids so m.'vny improhabilities, that
it lias been adopted as the true one since about the times
of Co[)ernicus and Galileo. It seems very unlikely that
a large body like the sun should be revolving round
one so much smaller, and is quite contrary to what we
observe in the other paits of the solar system, as the
lesser systems of Jupiter, Saturn, Uranus, and our own
earth, with their attendant satellites. This improba-
bility is greatly increased when we consider the number
of large planets circulating round the sun as their
undoubted centre, at vast distances from him. We
cannot realize the idea cf so vast a system being depen-
dent upon and revolving round our comparatively puny
earth. Also the motions of the planets (particularly
their retrograde movements) are infinitely complex and
almost unintelligible on the theory of the earth's fixity
as the centre of the universe ; while all their movements
become plain, simple, and uniform on the other theory.

702. (3.) The other planets are known to move round
the sun by observing their movements through the
starry sphere, the different positions they occupy in
respect to the earth and sun at different times, and par-
ticularly in the cases of Mercury, Venus, and Mars, by
the magnitude and position of the phases which they
exhibit at different times.

703. (4.) The phenomenon of the aberration of light
(653) affords the most convincing proof cf the earth's
annual motion.

704. (5.) By the laws of mechanics, any force which
would impart a motion of .otation to a body free to
move, must also impart to it a motion of translation,
which could then be stopped only by a force in an oppo-
site direction through its centre of gravity ; from which
there is necessarily a stronGf i^resumDtion in favour of
an outward motion in any body rotating in free space.

fell

rrl

212

ELEMENTS OF ASTRONOMY.

PART VI.

THE FIXED STARS.

705. Those stars which preserve the same positions
in relation to each other, without material change, are
called " fixed stars." Such are the stars in the con-
stellation "Great Bear," which appear to the "oldest
inhabitant " to be clustered in the same form in which
he saw them in his childhood. It is known from good
records that form has not materially altered for hundreds
of years. And we have reason to believe that the stars
of the leading constellations appear to us now just as
they did to the astronomers who flourished long before
the Christian eia, and who arranged the stars in con-
stellations, and gave them the names they still bear —
names derived from the heroes and heroines of antiquity,
and which have stamped on the heavens in indelible
characters the heroic deeds and elegant fables of ancient
times.

706. The fixed stars, however, are not absolutely
fixed. Many of them do change their positions in rela-
tion to each other. But this change, called their proper
motion, is very slight; so much so that it must go on
for thousands of years before it causes a change in posi-
tion appreciable by 'he naked eye. The stars Sirius,
Arcturus, and Aldebaran have moved southward, re-
spectively, 37', 42', and 33', since the time of Hippar-
chus — more than half a degree. The star 61 Cygni
has been ascertained to have a proper motion to the
extent of 5'3'' yearly, about a degree in 700 years; and
some are believed to have a proper motion of so much
as 7*7''' annually.

707. It was believed by Sir William Herschel, and

ELEMENTS OP ASTRONOMY.

213

are

and

has been confirraed by succeeding astronomers, that there
was a tendency to an opening out or spreading of the stars
in a particular region of the heavens, such as would bo
caused by our approach towards that quarter of tlio
lieavens, and a crowding together of the stars in the
opposite part of the sky, such as would be caused by our
receding from these stars. Hence has arisen the bold
conjecture, otherwise not improbable, since every ana-
logy is in favour of motions both of rotation and trans-
lation in the heavenly bodies, that our sun has a proper
motion through space, carrying all the planets, satellites,
and comets along with it. It is supposed that +hi8
motion is towards the constellation Hercules (to a point
in R. A. 261', N. P. D. 37°) ; and that it is at the rate
of 422,000 miles (or nearly the sun's radius) daily, or
154,185,000 miles in a year. Speculations have even
been entered into as to the centre round which our sun
moves; oae astronomer (Miidler) having conjectured
that that remarkable point is the star Alcyone, in the
constellation Pleiades ; and this is now the general belief
of astronomers.

708. All the stars usually seen in the heavens by
the naked eye are fixed stars, excepting five, and an
occasional comet.— See par. 45. They may be distin-
guished from the planets and comets by their property
of twinkling, that is, alternately expanding and con-
tracting their rays. The planets shine with a steady
equal light.

709. Until recently very little was known regarding
the fixed stars. We could judge of their relative bright-
ness, we could ascertain the direction and velocitv of
their apparent daily motion round the earth, and any
other motions they exhibit, and we could determine that
they are at not less than a certain distance from us.
But we did not know their actual magnitude, nor, except
in the case of a few, their distance from the earth.

710. In the first place, all attempts to measure the

214

ELEMENTS OP ASTRONOMY.

distance of any of tlio fixed stars Imd fi'ilcd. None of
them ffave any parallax with the loni^est known base-
lino within onv reach — the radins of the earth's orbit,
92 millions of miles. In the year 1888, however, the
parallax was nieasnred in the case of three of them.
The parallax of a Centanri was ascertained by Profes-
sor Henderson, at the Royal Observatory of the (^ape of
Good Hope, to be 091 28" or about 4?ths of a second;
that of Gl Cy<,'ni, by Professor liessel of Koni;,^sberg, to
be 0'3483''; and that of a Lyra?, by Otto Struve, to
be 0'2.>". The major diameter of the earth's orbit being
about 185,000,000 miles, a parallax of owe second wifi
give a distance of 20,000,000,000,000 (twenty billion)
miles, which is, therefore, the i)robable distance of «
Centauri from the sun — a distance so great that light,
tiavelling at the rate of 185,000 miles per second, would
require three and a half years to traverse it. The dis-
tance of the star 61 Cygni, its parallax being only J
of a aecond, will be thrice this number ; and of a Lyraj,
four times twenty billi(ms. The distance of about
twenty fixe 1 stars (including Sirius, Arcturus, Polaria,
and Capella) is now approximately determined.

711. We have no certain knowledge of their size, for,
even when viewed through the telescope, they present
no disc or surface whose breadth can be measured.
Even by the aid of this instrument they appear, as to
the naked eye, brilliant shining points, only more bright
and luminous.

712. The different maj^nitudes which the fixed stars present
to us may arise from their different distances, different degrees
of brightness, or from actual differences in magnitude : being, in
most cases, ignorant of the two former, we have no sure dataVor
judging of the latter. For the present we must be satisfied with
approximate results. The star a Centauri is ascertained to be
three times brigliter than our sun. "Sirius is mo'-c than four
times brighter than a Centauri, but yet shows aa annual change
of position among the stars of not more than one-fourth of that
star's. It is therufurc suppoacd to bo four times farther away

ELEMENTS OF ASTRONOMY.

215

from ns tlmn o rentftiiri ; aihI, did k emit no greater nmntmt of
light, woi\l(l jippear tosliino with but ono-sixtceiith at' tiiat Htar*8
luHtro. r.tit «H in rciilit,, it is four times i\h bri;-'.it, the real
amount of li^rht it omita must cxcuiod that of a Centauri no Ichs
than Hixty-f(»ur times, and that of our sun no lesH than 192 times.
So th.it, judged from tliis indicatidii alone, the diamet»!r of tViriu!*
may ho lield to exceed that of our huu in the proportiosi of about
14 to 1— an estimate wliich assigns to Hiriusadiameter of nearly
12,000,000 miles, and a v«)lumo of 2688 times as lar^o as the
sun's."— Proctor.

713. It has been calculated t'lat if any of the more remote
stars which the telescope lirings into view bo equally bright
with those near us, the light of such stars must have orcuuied
more tlian 2000 vears in coining to us; and that the rays which
render them visil)le to us do not indicate their existenco noWf but
their existence 200(j years ago.

714. The fixed stars are supposed to be suns, having
planets revolving round them, wliich they preserve in
tlieir orbits, supply with light and heat, and thus render
fit to be places of abode for living beings.

715. It is considered that they are independent systems, not
subservient to our universe, muci. less to our earth ; because
th' ^ shine from their own light, because, from their great dis-
tance, their influence on the solar system must be very slight,
and because it is improbable that bodies of the magnitude wh ch
their distance shows the fixed stars to possess, are subsidiary to
our comparatively small system. That the stars shine by their
own inherent light, not by reflecting the sun's rays, is shown
by their enormous distance from that luminary.

716. "The stars of our firmament, instead of being scattered
in all directions indifferently through space, form a stratum, of
which its thickness is small, in comparison with its length and
breadth ; and in which the earth occupies a place somewhero
about the middle of its thickness, and near the point where it
subdivides into two principal lamina) inclined at a small angle
to each other. For it is certain that, to an eye so situated, the
apparent density of the stars, supposing them pretty equally
scattered through the space they occupy, would be least in a
direction of the vi al ray (as S A), perpendicular to the lamina,
and greatest in that of its breadth, as S B, S C, S D; increasing
rapidly in passing from one to the other direction, just as we setj
a slight ^aze in the atmosphere thickening into a decided fog-
bank near the horizon, by the rapid increase of the mere length

216

ELEMENTS OP ASTRONOMY.

of the v\mn\ my. Snch Ir tho \iovr of tlu! rniiHtniction <.f tlio
Blurry finmiuu'nt tnkiii by hir VVilliuin ll.iMchi!!, wliose |)<.w«r-
ful tolcHcoiHis iltHt efllctcJ a complete a,ia\y»h of thin womlorful

Fig. 61.

fnno, nnd (lomonstrntcd the fact of its entirely consisting of 8tarn.
N) crowded are tliey in Homo parts of it tlint, by countln-' tlio
Btars in a single field of his telescope, he was led to conclude that
r)0,()()0 had passed under his review in a zone two degrees In
breadth, during a single hour's observation. In that part of the
Milky Way which is situated in 10»» 30«»- H. A,, and between the
147th and ir)Oth degree of N. P. I)., upwards of nooO stars have
been reckoned to exist in a square degree. The immenso dis-
tances at which the remoter regions must bo situated will snfli-
ciently account for the vast predominance of small mngnitudos
which are observed in it."— *SVr John JJerschel.

717. The fixed stars are arranged according to four
different principles:-—!. According to their brightness.
2. In constellations. — 3. According to tlieir situation in
the heavens.— 4. According to their kind, so fur as that
can be discovered.

1. Divisions of the Stars accordingr to their

Brightness.

718. The fixed stars are divided, according to their
brightness, into classes termed magnitudes. The bright-
est are said to be of the first magnitude, the next in
point of brightness of the second magnitude^ and so on
down to the sixth magnitude, which are the faintest
discernible by the naked eye. With powerful telescopes
* *.e range is continued down to the sixteenth magnitude,
./here are about 20 stars reckoned of the first mao-ni-

tUClO. Of th'^KP 11 nrn xricil'l" '"" Cl^r^..*- l>..:i..;_ o--^- -

ELEMENTS OF ABTriONOMV.

217

tlio (log.8tnr, 18 the brightest of tbo itars of tl.o first
magnitiide. There are about G5 of the secoiul uiUKi.i-
nh ^^ i *^'" *'".''*^ magnitude, and, in ull, nearly
uOOO have been registered, including the sixth magni-
tude. It 18 only at the eciuator, however, that so luriro
ft number can bo seen, for there only the spectator has
th.^ opportunity of surveying the whole heaven without
altering his jKhsitiou. At Alexandria the number of
stars visib e to the naked eye is only 4638 ; at Paris
4140; and at Berlin, 3206. The whole number of
stars already registered, including those of the seventh
magnitude, IS about 18,000, but astronomers assert that
the total number of stars visible through the best tele-
scopes, uown to those of the fourteenth maifnitude ex-
ceeds 20,000,000; while, by including those of' the
hfteenth and sixteenth magnitude, the number may
possibly amount to 500,000,000,000, or half a million
of millions 1

719. Seldom above 1000 are visible at a time to the
naked eye. Those of the fifth and sixth magnitudes
may, on a clear night, be discerned without the aid of a
telescope. The Milky Way is composed of innumerable
stars, whose average brightness is about the eleventh
magnitude, and whoso joint light is therefore separated
only by very powerful telescopes.

720. Every increase in the powers of t' .cojmj brinM new

stars .„to view; and as their distances are so groat, it is "SlI
tha there may be mynads of stars so remote froA, our system
tliat their light has never reached the earth, though they may
have been created at the same period as our system ; while others,
whose hglit still reaches us, may have been long since extin-
guished. rhere is no reason to suppose that the boundaries of
the sidereal system are within reach of even our most powerful
telescopes. 1 he most remote of the stars which the best tele-
scopes brmg into view may owe ...eir apparent minuteness, not
to interior magnitude, but to immense (fistance; and, perhaps
an observer at the furthest of these would find the same appear^
ance as wo do. star beyond star in countless myriads and at

adequate conception. •'

2\H

r.l.P.MENT8 OV ASTRONOMY.

2. Arrangement of Fixed Stars in Oonstel-
lations. 3. Aooordiuff to thoir Situation in
the Heavens.

These two iiK'tluxIs have been alroiwly described in
onr ft' count of tlio sphere of the heavens, und need not
be again discussed lierc.

4. Arrangremont of the Stars acoordingr to
other Difibrences than their apparent Bright-
ness or Situation.

721. f'onsidered with respect to other differences
thar iheir situation or brightness, the fixed stars may bo
divided into six kitids:— 1. Ordinary fixed stars; 2.
Temporary stars; 3. Variablo stars; 4. Binary stars;

5. Nebuhe ; (}. Clusters of stars.

1. Ordinary Fixed Stars.

722. Tlieso are the stars which, eitlier lo the eye uv
in tlie telescope, do not present any peculiar phenome-
non, such as variation in lustre, motion, nebulous ap-
pearance, or are not ag;,'re^ated wit)* any other stars in
a distinctlv separate cluster. T'ls is the case with
many >f Uio fixed stars.

2. Temporary Stars.

723. These are stars which have appeared for a
limited time, and then dit-'ippeared. Many stars, given
in old catalogues, are not to be seen now ; and on seve-
ral occasions, in various parts of the heavens, new stars
have suddenly come into view, anc^ disappeared at longer
or shorter intervals, shining with various degrees of
brilliancy during t!ieir short career. It is said that 't
was the sudden appearance of a new an" bright star in
the heavens, about 125 n.c, which led the illustrious
ancic.-it (istronomcr Hipparchus to the idea of makinir

ELEMRNTi OF ABTIloNoMV.

219

a CAtaloppifl of the stars, which ho did. Homo of thew;,
which have appeared atditTcrcutperiotlM, arc conjectured
to ho periodical in their viHitations, especially the stars
of dA,\ 12GI, and l/iT'i, which a[)pearod in tho same
rc^non of tiie lieavens, and have bi-ctn tiionglit to bo thfi
ame star with a period of about 300 years. The star
of 1572 appeared so ssidchMily that Tycho IJraho saw
several people looking at it, attracted by its brilliancy,
where he was curtain it was not prominent half-oii-hour
previously.

inlciurr
o

8. Variable Stars.

724. The variable stais present the singular phenomo-
non of a change in their brightness ; they undergo a
regula:- alternate increase and diminution in their lustre ;
and several altogether dis»'ppear for a time. Tiiey are
sometimes termed /;mo^/.'iu/.

725. Tho Bccond star, /9, in tho constellation Perseas, is a
vaiiftblo star, tho phenomena of which arc visible to th'i nak"(t
oyo. It is just on tho margin of tho Milky Way, on the side of
it farthest from tho north polo-Htnr, and about tiie same d'stunco
from tiiat star as Vega. It is in li. A. 45°, D. N. 40". It rnay
be found by drawing a line from tJio polo-star in the direction
of t!.o lottjrs Per, in Fig. 1, nago 13; and is siiown in Fig. 2,
pago 15, ibovo tho word " PerseuH." It is called Algol, and
usually apjXT.rs as a star of tho second magnitude!. It remains
so for 2*'- 1 1"-, when it suddenly begins to diminish in bright-
ness, and in about 3^ houra dwindles to a star of tho fourth
magnitude. It then begins again to increasf and in 3A hours
returns to its usual brilliancy, going through all its changes
in 2'> •20»»; '^8 ■»• The star tj of the southern constellation Argus
hasexhibite' very remarkable changes in lustre, from tho fourth
to tho second m... :<'"aidc, then to the fourth, then to tho second
again, and during the prcbcut century to tho first and second
magnitudes alternately.

726. This singular and regular change in the bright-
ness of Algol, is attributed to the revolution of jome
body round it, sulliciently large to cut otT a portion of
its bVht when inter nosed between the foi'tli and the

il:'%te

220

ELEMENTS OF ASTRONOMY.

/«,i>''"

star, tliongh not of sufficient maf^nitnde to eclipse the
star altofjether: in other cases it may be caused by
rotation, i he bo'^y having' a dark and bright side — or
by revohition in an elliptic orbit with the longer axis
nearly in a line with our position.

727. The foUowinjj are some of the leading variable stars
visible in Great Britain. /3 (beta) of Perseus ; 5 (delta) of
Ccpheus; /3 (beta) of Lyra, a little south from Vega; a (alpha)
of Hercules ; o (omicron) of Cctus. These have periods varying
from about 3 to 331 days, that of omicron of Cetus. The latter is
one of those variable stars which disappear altogether for a
time. It is called Mira, is a star of the second magnitude
when at its brightest, in R. A. about 32° or 2»'- lO'"-, and D. S,
between 3° and 4°. It is nearly due south of the leading stars
in Aries: and is idiowi: in Fig. 11, page 32. It appears about
12 times every 11 years, and remains tolerably bright for up-
wards of a fortnight.

4. Binary Stars.

728. Binary stars are those stars which, on examin-
ation wills the aid of a powerful telescope, and obser-
vation for a considerable time, are found to consist of
two stars nearly equal in apparent magnitude, and hav-
ing a revolution round each other, — " Sidereal systems,
composed of two stars revolving out each other in
regular orbits."

729. This great discovery was made by Sir William
Herschel, towards the close of the last century. It
was first publicly announced in papers read to the
Royal Society of London in 1803 and 1804.

730. About 6000 binary stars have been discovered,
and the periods of revolution of 700 of them have been
determined. In Castor, which is a binary star, the
revolution is completed in 252 years. In a binary star
in Corona Borealis the orbit is completed in 43 years ;
and therefore a complete period has in this instance
been gone through since the discovery by Sir W.
Herscheb The following are some of the most remark-
able of the binary stai's : y (gamma) of the constellation

ELEMENTS OF ASTRONOMY.

221

[)sc the
mvA. by
ide — or
er axis

bio stars
lelta) of
X (alpha)
varying
! latter is
er for a
agnitude
ind D. S,
ing stars
irs about
k for up-

jxamiii-
i obser-
)nsist of
nd hav-
;y stems,
)ther in

Virgo (182 years); »j (eta) of Cassiopeia (181 yearsj;
a Centaiiri (75 years) ; | Ursro Majoris (63 years) ;
7 (gamma) of Leo (1200 years); d (delta) of Cygnus
(178 years).

731. The binary stars are often coloured, each being
of a different hue ; and they usually exhibit those
tints which are called complementary, as blue and yel-
low — red and green. Triple, ?r>d even quadruple stars
have been discovered, in which three or four stars are
grouped together, forming one connected system.

7 32. The phenomena of periodical and binary stars
seem to indicate that among the fixed stars there are
the same general laws which prevail in our solar system ;
for wherever we observe motion in an orbit, there we
must infer the existence of some force analogous to that
of universal gravitation. " No doubt," says Sir John
Herschel, referring to the double star y Virginia, *' can
remain as to the prevalence in this remote system of the
Newtonian law of gravitation."

733. Stars of nearly equal magnitude, and placed
close to each other, but in which no revolution has yet
been detected, are termed double stars. Of these, there
is a very great number.

y^illiam

iry. It

to the

covered,
ve been
tar, the
ary star
\ years;
instance
Sir W.
remark-
;ellation

5. Nebulae.

734. Nebulae are faintly luminous stars, different
from either of the preceding varieties. The leading
kinds are two: — (1.) Those which are resolved by
powerful telescopes into a collection, or globular cluster^
of separate stars, densely crowded into one luminous
mass in the centre, but becoming scattered and separate
towards the border. These are regarded as systems of
suns, — as a whole world of stars, separate from other
systems ; the individuals of which are probably suns,
at great distances from each other.

735. (2.) Another kind of nebulae, to which the term

222

ELEMENTS OF ASTRONOMY.

nebulg, is most usually applied, is that in which the star
appears a thin cloudy mass, of that fleecy appearance
observed in the tail of a comet. The latter nebulae, it
wan at one time imagined, may be p^aseous matter in the
process of formation into suns with their attendant plan-
ets : but recent examination of nebula) presenting this
character by more powerful telescopes, such as that of
Lord Eosse, having entirely, or partly, resolved many
of such nebuUe into numbers of separate si* rs, it has
been presumed that they might all bo so resolved if we
had telescopes of sufficient power. The nebulous theory
of the formation of the sun, planets, and other stars and
systems, has thus lost the support it derived from the
apparently irresolvable nebula). But it still remains
as an explanation of a possible mode of formation of
the heavenly bodies, sanctioned formerly by die great
names of Sir W. Herschel and La Place.

736. Tho nebuljB exhibit every variety of form, from the
circular to that of an elongated ellipse ; such as the nebulae
in the girdle of Andromeda, which is visible to the naked eye.
Some are termed annular, from their ring-like form ; and others
planetary, from their resemblance to planets in presenting discs;
others are termed nebulous stars, appearing as bright stars sur-
roundfii jy .i faint luminosity; and Lord Kosse's telescope has
revealed anoiher class of a very remarkable character, termed
spiral nebulce, from the spiral coils of luminous matter which
they exhibit. The remarkable objects called the Magellanic
Clouds, near tho southern pole of the heavens, are two rounded
luminous spots, like a portion of the Milky Way; and are found
to be resolved by the telescope into separate stars, globular
clusters, and almost every variety of nebula). Their light is
not great, and the lesser disappears in bright moonlight.

6. Clusters of Stars.

737. Where there is a number of stars gathered to-
gether, apart from the others, and forming in a man-
ner an isolated group, they are termed a cluster^ and
are considered to belong to some system separate from
the general body of the stars. The Milky Way, tho

ELEMRNT8 OF ASTRONOMY.

223

Pleiades, Coma Berenices, the bright spot in Cancer
called Prsesepe or ^ho Beehive, are examples of those
clusters.

738. It thus appears that there are many descriptions
of fixed stars, as they are called, and that no stars are
truly fixed, but that movements and changes are going
on amongst them, which in time must greatly alter the
appearance of the heavens ; that the universe has no
bounds that we can even fancy, and that wherever
we know it, it is full of matter and of motion of every
form and variety. There is no point in space that has
not some body in it, or some influence })assing through
it. There are no voids — no objects fixed and un-
changing. Life, force, activity, and endless change
pervade the boundless realms of creation.

ELEMENTS OF ASTRONOMY.

PART VIL

SKETCH OF THE HISTORY OF ASTRONOMY.

739. As the heavenly bodies are everywhere con-
Bpicaoiis, and naturally attract the attention ; — as
many of their relative changes of position are obvious,
and must have been observed by even the rudest tribes ;
and as the coincidence of these changes with important
terrestrial phenomena could not escape observation ;
Astronomy has been cultivated from the earliest ages,
and is by far the oldest of the sciences.

740. The origin of this science cannot be distinctly
traced to any one country or people. The earliest
authentic records show that it was cultivated simultane-
ously, and wiixi considerable success, by the four great
nations of remote antiquity, the Chaldeans, Egyptians,
Indians, and Chinese. The Egyptians and Chaldeans
had divided the year into 365 days, observed the direct
and retrograde motions of the planets, and that they
were sometimes stationary, that the ecliptic was inclined
to the equinoctial, and had the zodiac divided into
twelve constellations ; * besides many other important
astronomical phenomena. The Chaldeans are said to
have been the first who divided the day into twelve
hours ; and the Egyptians first used tiie period of seven
days — the week.

741. From Egypt, which has been justly termed
the "cradle of the arts and sciences," and fromChaldea,

* Some of the ancient Oriental nations (Indians and Chinese) divided tho
zodiac jiiti) 27 portions corresponding to the moon's daily progress, called
tlie Houses or Mansions of the Moon,

ELEMENTS OP ASTRONOMY.

225

a knowlcdg-o of astronomy passed into Greece. Among
tho philosophers of that country, and of the famous
school of Alexandria, founded in Egypt by Ptolemy,
after the death of Alexander the Great, Astronomy
was cultivated with much zeal, and enriched by num-
bers of new observations, and important corrections of
former observations.

742. Thales Is the first Grecian on record who seems to havo
given a stimulus to astrenomy. lie is said to have predicted an
eclipse of tho sun, to have understood the nature of eclipses, to
have discovered the obliquity of tho ecliptic; and to have pointed
out the Little Bear by which to steer as a ^uide to tho north,
instead of the Great Pear. Ho lived about GOO b.c. Ho is said
to have been of Phoenician extraction. Tho Phoenicians steered
by the Little Bear.

743. The present view of the solav system was first
promulgated by Pythagoras, a iiimous Grecian philo-
sopher, who flourished about 500 years before the
ommencemcnt of the Christian era; and taught by his
disciple Philolaus. He supposed the sun to be in the
centre, and the earth and planets to revolve round it ;
and was persecuted for holding this opinion.

744. But this opinion was not generally entertained.
It was confined to some of the learned, and was not
confidently taught or firmly bplieved by them. The
majority of the philosophers of those days, as well as
the people, entertained the popular notion that the sun,
moon, planets, and stars, revolve daily round the earth,
supposed to be fixed. Though it is conjectured that
the true system of the universe was known to some
before Pythagoras, as Anaximander, it Las been
always termed the Pythagorean System.

745. The progress of discovery and improvement in
Astronomy, in ancient times, was greatly retarded by
assumed fancies ret,arding the perfection and immuta-
bility of the heavenly bodies ; the gross and corrupt
nature of the earth, and the entirely opposite natures of

K 2

220

ELEMENTS OP ASTRONOMY.

the earth nnd the heavenly hodies ; the pLrfect nature
of the circle ; the notion that the celestial bodies must
move in circles, and that their motions must be uniform;
and many other dop^mas which had the sanction of the
great name of Aristotle.

746. Hipparchus, who has usually been regarded as
the "Father of Astronomy," flourished about 140 n.c.
He determined with greater accuracy the length of the
year ; discovered the inequality in the rate of the sun's
motion, which he explained by supposing the earth not
in the centre of the sun's orbit ; observed the inequality
in the length of the solar day ; discovered the preces-
sion of the equinoxes;* drew up a catah)gue of the
fixed stars, with their precise positions, and determined
the positions of places on the earth's surface by their
latitudes and longitudes. He is said to have been the
inventor of spherical trigonometry.

747. Ptolemy of Alexandria, who flourished about
130 years after the commencement of the Christian era,
being born in the year 69, is the next astronomer of
note. He was the author of a work on astronomy,
called " The Great System," which is still preserved,
being known by the name of Almagest, which it received
from the Arabians, in which the whole astronomical
knowledge of the times is recorded.

748. Ptolemy upheld the popular system that the
earth is a fixed centre, round which all the heavenly
bodies revolve daily. He placed the stars, sun, moon,
and planets in the following order of distance from the
earth : — viz., Moon, Mercury, Venus, Sun, Mars, Jrpi-
ter, Saturn, and, lastly, the si)here of the fixed stars.
This was the Ptolemaic System, which so long held
possession of public opinion.

* But the precession of the equinoxes must have been known to the archi-
tect of the Great Pyraiiiid, erected n.c. 2170. (See " Facts and Dates/'
p. 131.)

ELEMENTS OF ASTRONOMY.

227

Ptolemy nrgncd tlmt if tho cnrth moved round tlio Bun, tho
IKjles of the heavens would not nlwaya remain tho Hnme, tluit
tlio fixed stars would not preserve the same figures and relative
distances,* tl .ct the earth, hy the greatness of its muss, would
move faster than tho loose bodies on its surface, so that they

Ptolemaic System.

Fig. 62.

would be left behind, and that the earth would soon move out of
the heavens. He objected to the earth's rotatory motion, that if
such were the case, clouds, birds, and bodies floating in the at-
mosphere would be left behind.

749. The apparer.tly irregular and retrograde move-
ments of the planets were explained by the theory of
Epicycles: — namely, that the heavenly bodies revolve
in small circles, the centres of which move in regular

* Tlicse objections lind been removed by the suggestions of previous
astronomers, that tlic earth's orbit miglit perhaps be a mure point iu coiu-

"aribou with the distance of the stiirSi

228

ELEMENTS OF ASTRONOMY.

orbits ronntl the earth. Thus, let E, in Fif^. C3, ho tho
earth, and P any planet revolving round tho earth. It
is supposed to revolve in a Biuall circle, P o 7, round a
centre c, which af^ain revolves in the circle a m no
round the centre E, tho earth. The small circle P ^,

in which the planet revolves, is called an epicyle, and
the large circle round which it turns the deferent. This
theory had been devised by Apollonius to explain the
retrograde movements of the planets.

750. Ptolemy also entertained the eccentric hypothesis, namely,
that the earth was not exactly the centre of the orbits of the sun
and planets, by which the apparent inequality in the rate of the
sun's motion was explained. He considered the earth to be
spherical, and a mere point in comparison with the distance of
the fixed stars.

751. It has been said that the ancient Egyptians at one period
held the opinion that Venus and Mercury had the sun, and not
the earth, Air the centre round which they revolve, i.o explain
the constant vicinity of these planets to the sun. T> is was the

ELEMENTS OF ASTRONOMY.

Ber3^tian System.

Fig. 64,

229

752. After the timo of Ptolemy, Astronomy made
little progress for more than a thousand years. It was
cultivated by the Arabs, who made some additions and
corrections, and some improvements in trigonometry ;
and through whose invasion of Spain it was introduced
into Europe in the ..inth century.

753. In Europe, Astronomy made little progress till
the restoration of the true system of the planets by
Copernicus. This distinguished man was born at
Thorn in Prussia in 1473. lie was professor of mathe-
matics at Rome, but spent his latter days in his own
country. In meditating upon the phenomena of As-
tronomy, he found that the Ptolemaic system did not
afford an adequate explanation of them, and that the
Pythagorean system accoimted satisfactorily for all the
changes and motions. He accordingly adopt-d it : and
his views were published in a work called Astronomia
luataurata, which was published only a few days

230

KLEMENT8 OP ASTRONOMY.

:i,

before Ills (l(!utlj, which took place in 1543. His
HyHtoui, culled the Copernican System, has now been
fdlly established.

754. The next astronomer of note was Tychj
Brahe, a D.ine, bom in 1540. He made many impor-
tant corrections of previous observations, drew up a
catalof,nie of the stars, discovered the refraction by tho
air, and many imjjortant points in the motions of tho
moon, and ascertained the comets not to be atmospherio
phenomena, by showing' their .,'reat distance, etc. Ho
devised a planetary system, in which the earth was
placed in the centre, and tl.e sun and moon were con-
jectured to have the earth for their centre of motion,
while all the planets were supposed to have the sun
for their centre. This, which has been called tho

Tychonic System,

Fig. 65

^ ..

was devised to accommodate better to astronomical pbe-

ELEMF.NTS OP ABTUONOMY.

231

iiomona tho i<b'.i of the ciutlj bcin^ tho centre of the
whole, to wliich Tycho Brahe adiiered.

755. Kepler was born in Wilrternlnirf? in 1571.
!io was a pupil of Tycho Braho, on whoi;c observations
his important discoveries were founded. He hinted at
some such power as gravitation, and applied it to tho
phenomena of the earth and moon and th(! tides. Ho
developed tho three lavvs stated in par. 451, — the basis
of tho scienco of Astronomy.

756. Tho celebrated Galileo mad(5 great contribu-
tions to Astronomy. Ho was born at Fh)rence in 1564.
He invented the pendidum, discovered the s[iots on tho
sun and its rotation on its axis, ascertained that Venus
had phases like the moon, as Copernicus had predicted ;
that tho moon's f-'iirface was not smooth and rounded,
but rough and irregular, like tho earth's; that tho
plarets ext)and into a broad opaque disc when viewed
through th*. telescope, while the fixed stars still appear
as mere shining points ; and discovered the satellites of
Jupiter. He was a leading advocate and promoter of
tho Copornican system, for which he suilered much per-
secution ; and died in 1642, having enriched Astro-
nomy and Mechanical Philosophy with many valuable
discoveries. His great work, " Dialogues on tho
Ptolemaic and Copernican Systems, " was published
in 1G32. .

757. Hevelius, Huyerbcns, and Cassini in the early part and
middle, and Flamsteed towards the close of tho 17th century,
hnxt considerable asristaiice to the advancement of astronomical
science. Flamsteeci drew up a catalogue ol" the stara. Hooke,
about the sniddle of tUs century, f^ave hin'.s of the theory of uni-
versal gravitation, '^''owards the close of the century Roemer
discovered the progressive motion of light: and Picard, tho
Cassinis, and others, maJc approximations towards asctsrtaining
tho true figure of the earth by measuring the Icnr'th of the
degree at different latitudes.

758. Sir Isaac Newton, the founder of the Science

232

ItEMENTg or AiTRONOMY.

lisluMl ill IfiftO liiH iiiiniortul work, in whidi he dcvfil-
omd the great hiWH ol' univerKjil jj^ruvitution, iiml iipplieil
thf'Tn t(» the ex|)huuiti()n ot'llie motioiiH of the lu'iivoiily
hodit'K. It WjiH tith'tl J'/til ).ii>f)/ii(r. IWituralis I'rincipia
Alnthrwatica. HeMitleH eHtuhlishin;,' AHtronomy on
BciontiJic principles, he vXm mft<le vulualih' a(Mition8 to
OpticH anil Mathematics. He died in tliu year 1727.

1^)\). Another di«tin<;uish('d Knj!:lisli antronoiiu'r, Dr
Edmund Halley, was horn in HIAO and die(l in 17I'J.
He HUg«i;e.sted thi* transit of Venns over tlie Kiln's disc jih
a ineanH of linding the Bim's paralhix; and calciihited
the elementfi and predicted the return of the comet
wliich now bears ins nrme. He made other vahiahlo
contributions to Astronomy.

700. Dr Bradley, born in 1092, made the two cap-
ital discoveries of tlie aberration of liglit and nutation
of the earth's axis ; besides many other improvements
in astronomical science. Clairaut, D'Alembert, Euler,
Simpson, La Place (to whom is due the credit of com-
pletiiijL,' tiie work that Newton had bej^un), La Grange,
and La Lande, made many improvements and exten-
sions in astroiHtmical science. liy these, j)articularly
La Place and La Gran«.^e, various irregularities in the
motions of the planetary system were reconciled v/ith
the theory of universal gravitation, and others pre-
viously unknown were deduced from it; and the
mutual correction of the various disturbances, so as to
preserve the ultimate stability of the system, was
proved — one of the greatest steps Astronomy made
after the development of Newton's grand disc(jveries.

761. Sir William Herschel, towards the close of
the last century, discovered Uranus and his satellites,
the two inner satellites of Saturn, and ascertained its
ring to be double. He also discovered the phenomena
of the binary stars, and extended greatly our knowledge
of the fixed stars.

762. During the present century, Astronomy has

H-

ELEMENTS Of ASTRONOMY.

S88

lias

been oiltivntetl witli great micc(>«H, and many important
a«MitionH liavo been made to our aHtrononiical know-
Icd^n'. Tlio Asteroids liavo been discovorcd, 4 in th«
onrly part of the century, and upwards of IW rticently ;
Neptune lias boon added to the lint of known pbinetn,
by L«verrier and Adams, in a manner that has aston-
ishet. the worhl by the wonderful accuracy it has proved
in aslronomical observaticns and calcuhitions. Num-
berH of cometH have been observed, and tlufir periodic
times calcuhited, ai. 1 tim phenomena of one of these
has tended to coufj. • the idea of the existence of a uni-
versal Ether dilVused tlirou^j^liout space. A second re-
turn of the comet of Halley in ISaf)— the first comet
whoso return was ^ jtlicted — luis confirmed his compu-
tations, and those of his successors wlio liad calculated
tl 3 operation of disturbing influences on its motions.
The meteors that flash across the sky are beginning to
take their place in the planetary system. 'J'he fixed
starei, which so 'ong resisted all attempts to find their
parailax, lave at last yielded to the perseverance of
Henderson, Bessel, Struve, and others, and parallaxes
have been found for many of these remote suns; ^o that
we can now assign a definite distance for theni,^ which
will be made the basis of further discoveries. Govern-
ments, associations, and individuals have alike conspired
to promote astronomical research. Sir Jolin Herschel
visited and resided long in the sonth of Africa, for the
purpose of exploring the southern skies ; and extended
immensely our knowledge of these interesting regions
of the heavens. The famous telescope of Lord Rosse
has made its wonderful revelations. New catalogues of
the stars have been prepared, and observatories founded.
By these means a mass of observations has been accu-
mulated, which must extend our knowledge of the most
remote part;: of creation, and pave the way for new in-
ductions and discoveries. -A niong very recent discover-
ies there are two which rival the must bfilllc.nt of any

23J

ELEMENTS OP ASTHONOMV.

former time, viz. :—Jirst, the discovery of the earth's
true distance from the sun ; and, second, the splendid
invention of spectrum analysis, by which we have
come to know tijat the materials of which the sun,
planets, and stars ai'e composed do nr' differ in any
respect from those which prevail in i e world which
we inhabit.

INDEX.

Aberration,
Algol, .
Alphcrat, .
Andromeda,
Angle,
Aphelion,
Apogee, .
Apsides,
Arcturus, .
Aries, .
Ascension,
Asteroids,

rage

. 192
34, 219

. 34

19,22

. 108

. 109
. 108

. 34
. 27, 29

. 27
. 161

Astronomy, History of, . 224
Atmosphere, Influence of, 98

Conjunction,
Ccnstellations, Northern,

Southern,

Zodiacal, .
Corona Borcalis, .
Cygnus, .

Page

110

. 33

37
. 36

35
. 34

Auriga,
Axis,

. 16, 34
9,24

Benetnasch, .

Bootes,

. 34

Cancer,
Capella,
Capricorn,
Cassiopeia, .
Ccpheus, .

. 29

. 15, 34, 44

. 29

. 15, 33

. 33

Cetus, .
Circles,

35
. 8,21, 22

Arctic and Antarctic, 48

Hour, . . .25

Climate, ... 64

Comets. . . . 105, 180

Day, Sidereal and Solar, 79

Lunar, . . .86

Day and Night, . . 49

Equal over the Earth, 56

Change in the Length of, 57

Causes of Differences

and Changes in, . 60

Declination, . . 27

Definitions, . 8-18, 46, 106

Diameter, ... 8

Disc, . . . .111

Draco, . . . .36

Dubhe, ... 33

Dragon, . . • .12

Earth, .... 150
Ax's, . .. . 152

Density, . • 153

Diameter, . . .151
Distance from Sun, 150
Form, . . 151, 201
Gravity, Force of, . 154
Kevolution, . 152, 210
notation on Axis, . 205

236

INDEX.

Pngo
Eccentricity, . . 107
Eclipses, . . . .92
Ecliptic, . . 28, 109
Ellipse, . . . .100
Equator, ... 40
Equinoctial, . . . 2f)
Equinoxes. . . .50, 59
Precession of, . . 195
Vernal or Spring, . 27

Fomalhaut, ... 35
Force, . . . 112, 114
Attractive, . . 110
Gravity, . . .118
Heat, . . .121
Projectile, . . .114
FoiTii of Heavenly Bodies, 130

Gravity, Centre of, . .113

Heat, . . . 65, 121
Heavens, Sphere of the, . 8

Extent visible at any

Place, ',)

Hemisphero, . . 23

Northern, Long Day, 52
„ Short Day, 54

Southern, State of, .50
Hercules, ... 35
Horizon, .... 8

Sensible and Rational, 42
Horizontal, ... 20
Hour-Circlcs, , . .25

Inclination, .

Jupiter, .
Satellites,

. 163
166

Pnpfo

Latitude, . . . 31, 47

Light, Velocity of, . 167

Zodiacal, . . .183

Linear Eccentricity, . 107

Longitude, . . 31, 47

Lyra, . . . . 16, 34

Mars, . . . .157

Mercury, .... 147

Meridian, ... 46

Celestial, . . . 39

Meteoric Systems, . 184

Milky Way, . . 39, 217

Month, ... 80

Moon, . . . .155

Diameter, . . 155

Distance from the

Earth, . . .155
Eclipses and Occulta-

tions of, . . .92
Octants of, . . 92
Orbit, . . .156
Penumbra, . , 94
Phases, . . .90
Quadratures, , . 92
Revolution, , . 155
Rotation, . , 156
Syzigies of, . 92

Umbra, . . 94

Motion, .... 9
Apparent, of the Heav-
ens, ... 10
Composition of, . .113
Mean, . . . Ill
Orbitual, of Planets,
Satellites, and Com-
ets, ... 122
Rotatory, . . .130

INDEX.

237

157

.147

. 39

184

39, 217

.165

155

. 155

ta-

. 92

. 156

. 90

. 155

156

. 9

av-

. 113

111

im-
122
.130

■

ruRe

I'ngo

Ncbulro, t .

. 22 1

Rotation, . • ,

. 9

Ncptuno, , , •

. 173

Nodes, .

. 109

Satellites, , , .

105

Nortli Polar Star, .

12-13

Saturn,

. 168

North Polo, .

Satellites of, .

171

Nutation of Earth's A

xis, 199

Seasons, . . . '

49,67

Semicircle, .

Occultatioii,

. Ill

Shooting Stars,

. 184

Opposition, .

. 110

Signs, ....

Orbits, , . .

. 107

Solstices, .

. 29

Solar System,

105

Parallels,

General facts rclatin

of Declination, .

. 26

to, .

135

of Latitude, ,

General illustrations

Parallax, .

. 188

of, .

176

Pegasus, . .

Sphere, . . .

8,24

Pendulum,

. Ill

Spheroid, . ,

111

Perigee,

. 109

Stars,

. 212

Perihelion,

. 108

Binary, .

220

Perpendicular,

Brightness of, .

.216

Perseus, .

31, 219

Fixed, .

218

Phases,

. Ill

Temporary,

.218

Plane Figure, .

. 19

Variable,

219

Planets,

17,105

Sun,

.139

Inferior,

. 109

Annular Eclipse of,

Superior,

. 110

Atmosphere of, .

. 145

Polar Regions, .

. 48

Axis,

141

Poles,

21, 25, 40

Dial,

. 83

Elevation of.

. 39

Distance of, .

139

Zenith, Distance

! of, 40

Eclipse of, ,

. 94

Positions of Objects

in the

Facula),

141

Heavens, .

Macula}, , .

. 141

Precession of Equinoxes, . 195

Magnitude, .

140

Physical constitution, 148

Radius, .

Rotation, . .

140

Vector, .

. 12G

Shape,

. 139

Reflection, . •

. 102

Spots, .

140

Refraction,

. 99

i Tangent, . ..

. 107

238

INDKX.

Terminator, ,

Tides,

Time, Divisions of,

Standurds of,
Transit,
Tropics, .
Twilight, .

Uranus, .

Satellites of, .
Ursa Major, ,

Minor, . .

Vega,

Page
51,68

Venus, .

. 71

Vertical, .

Vulcan,

. 89

111
29,48

Winds,

103

. 172

. 173

. 12

15, 34, 44

Year, .

rape
148

. 20
146

. 69

Zenith, .... 8

Zodiac, ... 29

Signs of, . . . 30

Constellations of, . 36

Zones, ... 48

THE END.

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Ath^nn-um — " It is snpovior to most works of the kiiul."
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SPALDma'S ENGLISH LITERATURE.

lieviscd and Continued. New Edition,

HISTORY OF ENGLISH LITERATURE; With an

OutUr of the Orlj,'in and Growth of the Enplish LanRuage.
Illustrated b 'extract

By Professor "Al. )i
Tlio wh(il<i work I'lis uii
chapters on the Lan ^aix/
brought into harm ny
toricdllnvetitigatioiu'
to the present tiuiM.
pendix, with the v' •
dons anil illusf

" It is not f
work in tli*-

chapters cf
ciatlvc c. i" ^,

field (»■ /:„ -"'
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gets a: (
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resulti I
texturi ,'
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'•» Students.

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work the

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(1853, but
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Inowledge
wlmistreaa.

, post-ftee.
*ver anu. Boyd.

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