Richard Green Parker.

A school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of online

. (page 30 of 38)
Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 30 of 38)
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minor planets, all situated between the planets Mars and Jupiter.
What opinion ^ ut tnese m i nor planets are so small, and their
has been formed paths or orbits vary so little, that it has been
con J ectured that the y originally formed one
large and resplendent orb, which, by the opera-
tion of some unknown cause, has exploded and formed the
minor planets that revolve in orbits very near that of the original

1234. Of these thirty-five small bodies, which are quite invisible
without the aid of a good telescope, ten were discovered by Mr.
Hind, of Mr. Bishop's private observatory, Regent's Park, London;
seven by De Gasparis, of Naples ; three by Chacornac, at Marseilles ;
three by Luther, at Bilk, Germany ; two by f)lbers, of Bremen ; two
by Hencke, of Dries^en, Germany ; two by Goldschmidt, at Paris ;
and one each by Piazzi, of Palermo; Harding, of Lilienthal, Ger-
many; Graham, at Mr. Cooper's private observatory, Markree
Castle, Ireland ; Marth, of London ; and Ferguson, of Washington.

1235. The paths or orbits of the planets
What ts the i -. iv ^ i

shape of the * re no * exactly circular, but elliptical.

orbits of the They are, therefore, sometimes nearer to
p a the sun than at others. The mean distance

is the medium between their greatest and least distance.
Those planets which are nearer to the sun than the
earth are called inferior planets, because their orbits are
within that of the earth ; and those which are further
from the sun are called superior planets, because their
orbits are outside that of the earth.

Qwe the rda- 1236 The relatiye size o f t h e srm t he

live size of the , -,

tun, moon, moon and the larger planets, as expressed by

and primary the length of their diameters, is as follows :



Sun . .
Moon . .
Mercury .
Venus . .
Earth .

. 852,000
. 2,153
. 2,962
. 7,510

Mars . .
Jupiter .
Saturn .
Uranus .
Neptune .



How large are
the minor
planets ?

1237. The size of the minor planets has been
so variously estimated, that little reliance can be
placed on the calculations. Some astronomers
estimate them as a little over 1000 miles, while others place
them much below that standard. Vesta has been described as
presenting a pure white light ; Juno, of a reddish tinge, and
with a cloudy atmosphere ; Pallas is also stated as having a
dense, cloudy atmosphere ; and Ceres, as of a ruddy color.
These four undergo various changes in appearance, and but
little is known of any of them, except their distance arid time
of revolution.

Explain 1238. Fig. 182 is a representation of the com

/<%. 182. parative size of the larger planets.

Fig. 182.

.Sir J. F. W. Herschel gives the following illustration of the com-
parative size and distance of the bodies of the solar system. " On
a well-levelled field place, a globe two feet in diameter, to represent
the Sun ; Mercury will be represented by a grain of nmstafd-seed..
on the circumference of a circle 164 feet in diameter for its orbit
Venus, a pea, on a circle 284 feet in diameter; the Earth, also u
|V:a, on a eirvle of 4ijO feel , Mars, a. rather largo pin's head, on a


drr'e of 654 feet; Juno, Ceres, Vesta, and PaLas, grains of sand,
in orbits of from 1,000 to 1,200 feet; Jnpiter, a moderate-sized
orange, in a circle nearly half a mile in diameter ; Saturn, a small
orange, on a circle of four-fifths of a mile; Uranus, a full-sized
cherry, or small plum, upon the circumference 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.

" To imitate the motions of the planets in the above-mentioned
orbits, Mercury must describe its own diameter in 41 seconds;
Venus, in 4 minutes and 14 seconds; the Earth, in 7 minutes;
Mars, in 4 minutes and 48 seconds ; Jupiter, in 2 hours, 66 minutes ;
Saturn, in 3 hours, 13 minutes; Uranus, in 12 hours, 16 minutes;
and Neptune, in 3 hours, 30 minutes."

1239. The Ecliptic is the apparent path
What is the , ,, , ,1 /. Ti ,1
Ecliptic, and * the sun > or tne rca * P atn * the earth.
why is it so It is called the ecliptic, because every

eclipse, whether of the sun or the moon,
must be in or near it.

1240. The Zodiac is a space or belt, six-
Zoctiac? ^ een degrees broad, eight degrees eacli side

of the ecliptic.

It is called the zodiac from a Greek word, which sig-
nifies an animal, because all the stars in the twelve
parts into which 'the ancients divided it were formed
into constellations, and most of the twelve constellations
were called after some animal.

1241. Sir J. F. W. Herso.hel, in his excellent treatise on As-
tronomy, says : " Uncouth figures, and outlines of men and iiion-
sters are usually scribbled over celestial globes and maps, and
serve, in a rude and barbarous way, to enable us to talk of groups
of stars, or districts in the heavens, by names which, though alv-uird
or puerile in their origin, have obtained a currency from which it
would be difficult to dislodge them. In so far as they have really
(as some have) any slight resemblance to the figures called up in
imagination by a view of the more splendid ' constellations, they
have a certain convenience; but as they are otherwise entirely ar-
bitrary, and correspond to no natural subdivisions or groupings
of the stars, astronomers treat them lightly, or altogether'disregard
them, except for briefly naming remarkable stars, as 'Alpha, Le&nis?
'Beta Scorpii,' &c., by letters of the Greek alphabet attached to them.

" This disregard is neither supercilious nor causeless. The con-
stellations seem to have been almost purpose b nained and delineated



to cause as much confusion and inconvenience as possible. Ir*
numerable snaked twine through long and contorted areas of tho
heavens, where no memory can follow them ; bears, lions, and
fishes, large and small, northern and southern, confuse all nomen-
clature, &c. A better system of constellations might have been a
material help as an artificial memory."

What arc the 1242. The zodiac is divided into twelve
sign* of i he signs, each sign containing thirty degrees of
many degrees in tuc g reat celestial circle. The names of these
wch ? signs are sometimes given in Latin, and

sometimes in English. They are as follows :

Latin. English.

I) Aries, The Ram.

2) Taurus, The Bull.

3) Gemini, The Twins.
: I) Cancer, The Crab.
|5) Leo, The Lion.
'()) Virgo, The Virgin.

Latin. English.

(7) Libra, The Balance.

(8) Scorpio, The Scorpion.

(9) Sagittarius, ^ The Archer.

(10) Capricornus, The Goat.

(11) Aquarius, The Water-bearer.

(12) Pisces, The Fishes.

1243. The signs of the zodiac and the various bodies of the
solar system arc often represented, in almanacs and astronomical
works, by signs or characters.

In the following list the characters of the planets, &c.. are
I ('presented.

The Sun. The Earth. Ceres.

< The Moon. Mara. $ Pallas.

Mercury. g Vesta. % Jupiter.

9 Venus. cj> Juno. \i Saturn.

$ Uranus.

The following characters represent the signs of the Zodiac*

\, Leo. $ Sagittarius.

TTJJ Virgo. >J Capricornus.

Libra. zz Aquarius.

TH. Scorpio. X Pisces.

Prom an inspection of Fig. 183 it appears that when the earth

K Aries.

y Taurus.

U Gemins.

IB Cancer.


a? rieeu from the SUD, is in any particular constellation, the sun
as viewed from the earth, will appear in the opposite one.

Hare the signs 1244. The constellations of the zodiac do not

of the zodiac now reta j n ^ c { r original names. Each con-

3/ioayt remain- ;-',. . , ^/T i

sd the same, stellation is about 30 degrees eastward of the

and why? sign of the same name. For example, the con-

stellation Aries is 30 degrees eastward of the sign Aries, and the
constellation Taurus 30 degrees eastward of the sign Taurus, and
so on. Thus the sign Aries lies in the constellation Pisces ; the
sign Taurus, in the constellation Aries ; the sign Gemini, in the
constellation Taurus, and so on. Hence the importance of dis-
tinguishing between the signs of the zodiac and the constellations
of the zodiac. The cause of the difference is the precession of
the equinoxes, a phenomenon which will be explained in its propei

t 1245. The orbits of the other planets

orbits of the T ' i i ,1 , c ^ ^

planets situated are inclined to that of the earth ; or, m

with respect to other words, they are not in the same
that of the

earth? P lane '

Explain Fig. 183 represents an oblique view of the plane

Fig. 183. of the ecliptic, the orbits of all the primary planets,
and of the comet of 1680. That part of each orbit which is
above the plane is shown by a white line ; that which is below
it, by a dark line. That part of the orbit of each planet where
it crosses the ecliptic, or, in other words, where the white and
dark lines in the figure meet, is called the node of the planet,
from the Latin nodus, a knot or tie.

Ejcplain 1246. Fig. 184 represents a section of the plane

Fig. 184. O f the ecliptic, showing the inclination of the orbits
of the planets. As the zodiac extends only eight degrees on
each side of the ecliptic, it appears from the figure that the
orbits of some of the planets arc wholly in the zodiac, while
those of others rise above and descend below it. Thus the
orbits of Juno, Ceres and Pallas, rise above, while those of all
the other planets are confined to the zodiac.




When is a 1247. When a planet or heavenly body

heavenly body . . ,, , ,, .^ ,., , . ,

w m tnat P art < lts orDlt which appears to

constellation f fa near ail y p ar ticular constellation, it is said
to be in that constellation.

This, in Fig. 147, the comet of 1680 appears to approach the
aun from the constellation Leo. .

mat is meant 1248 - The perihelion* and aphelion*
by the perihelion of a heavenly body express its situation with

? re ard to the sun - When a **% is nearest

gee, of a heavenly to the sun, it is said to be in its perihelion-
When furthest from . the sun, it is said to
be in its aphelion.

1249. The earth is three millions of miles nearer to the
flun in its perihelion than in its aphelion.

The apogee * and perigee * of a heavenly body express
its situation with regard to the earth. When the body is
nearest to the earth, it is said to be in perigee ; when it is
furthest from the earth, it is said to be in apogee.

Where i the 1^50. The perihelia of the planets, an seen from

77- j the sun, are in the following signs of the zodiac,

perineiionana namel Mercury in Gemini, Venus in Leo, the

Irthf Earth in Cancer, Mars in Pisces, Vesta in Sagitta-

rius, Juno in Taurus, Ceres in Leo, Pallas in Leo,

Jupiter in Aries, Saturn in Cancer, Uranus in Virgo, and Neptune

in Taurus.

What is meant 1251. When a planet is so nearly on a

V the inferior ,. .11 i , ^

and superior une wl ^" * ne eai> th and the sun as to pass

conjunction and between them, it is said to be in its inferior

opposition of a . ^ . , 11-1,1 . i

vianet ? conjunction ; when behind the sun, it is said

* The jhiral of Perihelion is Perihelia, and of Aphelion is Aphelia. The
words perihelion, aphelion, apogee, and perigee, are derived from the Greek
language, and have the following meaning :

Perihelion, near the sun

Aphelion, from the sun.

Perigee, near the earth.

Apogee, fr<,m the earth



to be in its superior conjunction ; but when behind the
earth, it is said to be in opposition.

1252. The axes of the planets, in their
revolution around the sun, are not perpen-
dicular to thtir orbits, nor to the plane of the
ecliptic, but are inclined in different degrees

1253. This is one of the most remarkable
circumstances in the science of Astronomy,
because it is the cause of the different seasons,

What is the in-
clination of the
axes of the
planets to the
plane of their
orbit t ?

What causes
the seasons ?
What causes
the differences
in the length of
the days and
nights ?

spring, summer, autumn and winter ; and

because it is also the cause of the difference in
the length of the days and nights in the different parts of
the world, and at the different seasons of the year.

. 1254. The motion of the heavenly bodies is not uniform.
They move with the greatest velocity when they are in
perihelion, or in that part of their orbit which is nearest
to the sun; and slowest when in aphelion.

1255. It was discovered by Kepler, and proved by
Newton, that if a line is drawn from the sun to either
of the planets, this line
passes over or describes
equal areas in equal times.
This line is called the ra-
dius-vector. This is one of
Kepler's great laws.

Explain In Fig. 185,

Fig. 185. let g represent the

sun, and E the earth, and the
ellipse or oval, be the earth's
orbit, or path around the sun.
By lines drawn from the sun

at S to the outer edge of the ..

figure, the orbit is divided


into twelve areas of different shapes, but each containing the
same quantity of space. Thus, the spaces E S A, A S B, D S C,
&c., are all supposed to be equal. Now, if the earth in the
space of one month will move in its orbit from E to A, it will
in another month move from A to B, and in the third month
from B to C, <fec., and thus its radius vector will describe equal
areas in equal times.

The reason why the earth (or any other heavenly body) moves
with a greater degree of velocity in its perihelion than in its
aphelion may likewise be explained by the same figure. Thus :

The earth, in its progress from F to L, being constantly urged
forward by the sun's attraction, must (as is the case with a fall-
ing body) move with an accelerated motion. At L, the sun's
attraction becomes stronger, on account of the nearness of the
earth ; and consequently in its motion from L to E the earth
will move with greater rapidity. At E, which is the perihelion
of the earth, it acquires its greatest velocity. Let us now detain it
at E, merely to consider the direction of the forces by which it
is urged. If the sun's attraction could be destroyed, the force
which has carried it from L to E would carry it off in the dotted
line from E to G, which is a tangent to its orbit. But, while the
earth has this tendency to move towards Gr, the sun's attraction
is continually operating with a tendency to carry it to S. Now,
when a body is urged by two forces, it will move between them ,
but, as the sun's attraction is constantly exerted, the direction
of the earth's motion will not be in a straight line, the diagonal
of one large parallelogram, but through the diagonal of a num-
ber of infinitely small parallelograms ; which, being united, form
the curve line E A.

It is thus seen that while the earth is moving from L to E the
attraction of the sun is stronger than in any other part of its
orbit, and will cause the earth to move rapidly. But in its
motion fiom E to A, from A to B, from B to C, and from
C to F, the attraction of the sun, operating in an opposite direc-
tion, will cause its motion from the sun to be retarded, until, at
F, the direction of its motion is reversed, imd it begin? again to


approach the sun. Thus it appears that in its passage from thd
perihelion to the aphelion the motion of the earth, as well as
that of all the heavenly bodies, must be constantly retarded,
while in moving from their aphelion to perihelion it is con
stantly accelerated, and at their perihelion the velocity will be
the greatest. The earth, therefore, is about seven days longer
in performing the aphelion part of its orbit than in traversing
lie perihelion part ; and the revolution of all the other planets
being the result of the same cause, is affected in the same man-
ner as that of the earth.

What are the 1256 ' The ther tw S reat laWS iis '

three laws of covered by Kepler, on which the discoveries

^ ' of Newton, as well as the whole modern

theory of the planets, are based, are

1257. (1.) That the planets do not move in circles,
but in ellipses, of which the sun is in one of the foci.

1258. (2.) In the motion of the planets, the squares
of the times of revolution are as the cubes of the mean
distances from the sun.

It was by this law that, in the want of other means, the
distance of the planet Uranus from the sun was estimated.
How much nearer 1259. The earth is about three millions

mnl7ummer he of miles nearer to tbe sun in wintcr thar
than in the win- in summer. The heat of summer, there-


in this question.} the distance oi the earth from the sun.

The sun is nearest to the earth in the summer of the southern
hemisphere, but the heat is not more intense there than in cor-
responding latitudes of the north. This is due to the greater
amount of land in the northern hemisphere, which by its radiat-
ing power heats the atmosphere more thoroughly.

When is the 1260. On account of the inclination of

heatofthe-sun the earth's axis.- the rays of the sun fall

more or less obliquely on different parts


rf the earth's surface at different seasons of the year.
The heat is always the greatest when the sun's rays fall
vertically ; and the more obliquely they fall, the" fewer
of them fall on any given space.

This is the reason why the days are hottest in summer, although
the earth is further from the sun at that time.
Explain 1261. Fig. 186 represents the manner in which

the rays of the sun fall upon the earth in summer and
in winter. The north pole of the earth, at all seasons, constantly
points to the north star N ; and, when the earth is nearest to the
sun, the rays from the sun fall as indicated by W in the figure ;

Fig. 186.

and as their direction is very oblique, the amount of heat for
any particular area of this portion of the surface is much less.
Hence we have cold weather when the earth is nearest to the sun.
But when the earth is in aphelion, the rays fall almost vertically
or perpendicularly, as represented by S in the figure ; and al-
though the earth is then nearly three millions of miles further
from the sun, the heat is greatest, because the rays fall more
directly, and have a less portion of the atmosphere to traverse.

This may be more familiarly explained by comparing summer
rays to a ball or stone thrown directly at an object, so as to


strike it witn ill its force ; and winter rays to the same ball ci
stone thrown obliquely, so as merely to graze the object.
Why is it cooler 1262. For a similar reason we find, even in
mornin* than 8ummer > taat earl j in tne morning and late in
in the middle the afternoon it is much cooler than at noon,
of the day ? because the sun then shines more obliquely.

The heat is generally the greatest at about three o'clock in the
afternoon ; because the earth retains its heat for some length of
time, and tht additional heat it is constantly receiving from tho
sun causes an elevation of temperature, even after the rays
begin to fall more obliquely.

What causes the 1263. It is the same cause which occasions
mUtfes^n Differ- the variet J of climate in different parts of the
ent parts of the earth. The sun always shines in a direction
world? nearly perpendicular, or vertical, on the equator,

and with different degrees of obliquity on the other parts of the
earth. For this reason, the greatest degree of heat prevails at
the equator during the whole year. The further any place is
situated from the equator, the more obliquely will the rays fall
at different seasons of the year, and, consequently, the greater
will be the difference in the temperature.

What places will 1264 ' lf the axis f the earth were P er P en -
nave the coolest dicular to its orbit, those parts of the earth
temperature? which lie under the equator would be constantly
opposite to the sun ; and as, in that case, the sun would, at all
times of the year, be vertical to those places equally distant from
both poles, so the light and heat of the sun would be dispersed
with perfect uniformity towards each pole ; we should have no
variety of seasons ; day and night would be of the same length,
and the heat of the sun would be of the same intensity every
day throughout the year.

What effects are 1265. It is, therefore, as has been stated,
inclination of *"' owing" to the inclination of the earttis
the earth's axis? a3: l s (/iaf we have, the agreeable variety


of the seasons, days and nights of different lengths ^
and that wisely -ordered variety of climate which causes
S'O great a variety of productions^ and which has afford-
ed so powerful a stimulus to human industry.

12GG. The wisdom of Providence is frequently displayed in appar-
ent inconsistencies. Thus, the very circumstance which, to the
short-sighted philosopher, appears to have thrown an insurmountable
barrier between the scattered portions of the human race, has been
wisely ordered to establish an interchange of blessings, and to bring
the ends of the earth in communion. Were the same productions
found in every region of the earth, the stimulus to exertion would
be weakened, and the wide field of human labor would be greatly
diminished. It is our mutual wants which bind us together.

1267. In order to understand the illustration of the causes of
the seasons, &c., it is necessary to have some knowledge of the
circles which are drawn on the artificial representations of the
earth. It is to be remembered that all ^f these circles are
wholly imaginary; that is, that there are on the earth itself no
such circles or lines. They are drawn on maps merely for the
purpose of illustration.

Krp/ain 1268. Fig. 187 represents the earth. N S is the

axis, or Imaginary line, around which it daily

* ' '

Fig. 187.

N is the north pole, S is the south pole.
These poles, it will be seen, aie the
extremities of the axis N S. CD
represents the equator, which is a cir-
cle around the earth, at an equal dis-
tance from each pole. The curved
lines proceeding from N to S are me-
ridians. They are all circles sur-
rounding the earth, and passing through
! he poles. These meridians may be multiplied at pleasure.

The lines E F, I K, L M, and G H, are designed to reprcsei t
Circles all of them parallel to the equator, and for this reason
'.hey are called parallels of latitude. These also may be mul
t'ipliei at pleasure.

Bu*, in the figure lines, which are parallel to the equator,


and which are at a certain distance from it, have a different
name, derived from the manner in which the sun's rays fall on
the surface of the earth.

Thus the circle I K, 23^ degrees from the equator, is called
the tropic of Cancer, and the circle L M is called the tropic oi
Capricorn. The circle E F is called the Arctic Circle. It
represents the limit of perpetual day when it is summer in the
northern hemisphere, and of perpetual night when it is winter.

On the 21st of March the rays of the sun fall vertically on
the equator, and on each succeeding day on places a little to the
north, until the 21st of June, when they fall vertically or places
23^ degrees north of the equator. Their vertical direction then
turns back again towards the equator, where the rays again fall
vertically on the 23d of September, and on the succeeding days
a little to the south, until the 21st of December, when they fall
vertically on the places 23^ south of the equator. Their verti-
cal direction then again turns towards the equator Hence the
circles I K and L M are called the tropics of Cancer and Cap-
ricorn. The word tropic is derived from a word which signifies
10 turn. The tropics, therefore, are the boundaries of the sun's
apparent path north and south of the equator, or the lines at
which the sun turns back. .- .

The circle G H is the Antarctic Circle, and represents thu
limit of perpetual day and night in the southern hemisphere.
The line L K represents the circle of the ecliptic, which, as has
already been stated, is the apparent path of the sun, or the rea.
path of the earth. This circle, although it is generally drawn
on the terrestrial globe, is, in reality, a circle in the heavens ;
and differs from the zodiac only in its width, the zodiac ex-
tending eight degrees on each side of the ecliptic.
Explain 1269. Fig. 188 represents the manner in which the

. tg. lo . gun s h} nes on the earth in different parts of its orbit ,

Online LibraryRichard Green ParkerA school compendium of natural and experimental philosophy : embracing the elementary principles of mechanics, hydrostatics, hydraulics, pneumatics, acoustics, pyronomics, optics, electricity, galvanism, magnetism, electro-magnetism, magneto-electricity, astronomy : containing also a description of → online text (page 30 of 38)