or, in other words, the cause of the change in the seasons. S
represents the sun, and the dotted oval, or ellipse, ABC I), the
whit of the earth. The outer circle represents the ?odiac with
the position of the twelve signs or constellations. On the
of June, when the earth is at D, the whole northern polar reg : on
is continually in the light of the sun. As it turns on its axis,
therefore, it will be day to all the parts which are exposed to
the light of the sun. But, as the whole of the Antarctic circle
is within the line of perpetual darkness, the sun can shine on no
part of it. It will, therefore, be constant night to all places
within that circle. As the whole of the Arctic circle is
the line of perpetual light, no parr of that circle will be turned
>rom the sun while the earth turns on its axis. To all placew,
therefore, within the Arctic circle, it will be constant day.
On the 22d of September, when the earth is at C, its axis is
aeither inclined io nor from the sun, but is sidewise ; and, o!
course, while one-half of the earth, from pole to pole, is enlight-
ened, the other half is in darkness, as would be the case if its
axis were perpendicular to the plane of its orbit; and it is this
which causes the days and nights of this season of the year to?
be of equal length.
On the 23d of December the earth has progressed in its orbit
to B, which causes the whole space within the northern polar
circle to be continually in darkness, and more of that part of th* 1
earth north of the equator to be in the shade than in the light
of the sun. Hence, on the 21st of December, at all places north
of the equator the days are shorter than the nights, and at all
places south of the equator the days are longer than the nights
Hence, also, within the Arctic circle it is uninterrupted night,
the sun not shining at all ; and within the Antarctic circle it is
uninterrupted day. the sun shining all the time.
On the 20th of March, the earth has advanced still further, and
is at A, which causes its axis, and the length of the days and
nights, to be the same as on the 20th of September.
1270. From the explanation of figure 198.
What is meant
by the Equinoxes lfc appears that there are two parts of its ore-it
and the Sol- J n which the days and nights are equal all ovei
the earth. These points are in the sign of
Aries and Libra, which are therefore called the equinoxes
Aries is the vernal (or spring) equinox, and Libra the autumna?
1271. There are also two other points, called solstices, because
the sun appears to stand at the same height in the heavens in
the middle of the day for several days. These points are in the
signs Cancer and Capricorn. Cancer is called the summer sol-
stice, and Capricorn the winter solstice.
1272 ' Da and niht are Caused b the r ta '
How are day
and night cans- tion of the earth on its axis every 24 houi s.
ed and what is It is d to that gide of the earth which k
the reason of the .
difference in towards the sun, and night to the opposite side.
'heir length.? The length of the days is in proportion to the
inclination of the axis of the earth towards the sun. It may be seen,
by the above figure, that in summer the axis is most inclined
towards the sun, and then the days are the longest. A& the uortl*
pole becomes less inclined, the days shorten, till on the 21st of De<
eember it is inclined 23 J- degrees frmn the sun, when the day.
are the shortest. Thus, as the earth progresses in its orbit, after
Lhe days are the shortest, it changes its inclination towai ds the
sun, till it is again inclined as in the longest days in the summer.
Which of the 1:>73. As the difference in the length of the
^test^ffer- da ? S and the ni g hts > and the chan g e f the
ence in its sea- seasons, &e., on the earth, is caused by the in-
sons clination of the earth's axis, it follows that al]
the planets whose axes are inclined must experience the same
vicissitude, and that it must be in proportion to the degree of
the inclination of their axes. As the axis of the planet Jupiter
is nearly perpendicular to its orbit, it follows that there can be
little variation in the length of the days and little change in the
seasons of that planet.
1274. There can be little doubt that some of the other planets
and satellites are inhabited; and although it may he thought
that some of them, on account of their immense distance from the
dun, experience a great want of light and heat, while others are so
near, and the heat consequently so great, that water cannot remain
un them in a fluid state, yet, as we see, even on our own earth, that
creatures of different natures live in different elements, as, for
instance, fishes in water, animals in air, &c., creative wisdom could,
undoubtedly, adapt the being to its situation, and with as little
exertion of power form a race whose nature should be adapted to
the nearest or the most remote of the heavenly bodies, as was re-
quired to adapt the fowls to the air, or the fishes to the sea.
WhatiitheSton, 1275 ' F THB SUN. The Sun is a
e-nd what is its spherical body, situated near the centre of
gravity of the system of planets of which
our earth is one.
H^w much larger -1276. Its diameter is 853,000 English
I* the earth tJian m ji eg w hich j s equa j to ^Qg diameters of
the sun .-,.,
\Answer care- the earth ; and, as spheres are to each
f uit y-] other in the proportion of the cube of their
respective diameters, therefore his cubic magnitude must
exceed that of the earth one million of times. It
$60 NATURAL PHILOSOPHY.
around its axis in 25 days and 3 hours. This has been
ascertained by means of several dark spots which have
been seen with telescopes on its sui face.
127T. Sir Win. Herschel supposed the spots on the
eun to be the dark body of the sun, seen through open-
ings in the luminous atmosphere which surrounds him.
1278. It is probable that the sun,* like all the other
heavenly bodies (excepting, perhaps, comets), is in-
habited by beings whose nature is adapted to their
1279. Many theories have been advanced with regard
to the nature of the sun. By some it has been regarded
as an immense ball of fire ; but the theory which seems
most in accordance with facts is, that the light and heat
are communicated from a luminous atmosphere, or at-
mosphere of flame, which surrounds the sun, at a con-
siderable distance above the surface.
What is the zo- 1280. The zodiacal light is a singular phe-
diacal light, and nomeuoii, accompanying the sun. It is a faint
light which often appears .x> stream up from
the sun a little after sunset and before sunrise. It appears
nearly in the form of a cone, its bides being somewhat curved
and generally but ill defined. It extends often from 50 to 100
in the heavens, and always nearly in the direction of the place
of che ecliptic. It is most distinct about the beginning of March,
but is constantly visible in the torrid zone. The cause of this
phenomenon is not known.
1281. The sun, as viewed from the different planets, appears
>f different sizes according to their respective distances. Fig.
189 affords a comparative view of his apparent magnitude, as
seen from all except the smaller of the minor planets.
* In almanacs the sun is usually represented by a -small circle, with the
face of a raun in it : thus,
Apparent Magnitude uf the Sun as seen from thr> Platiclf
Describe the 128& ^ F MERCURY. Mercury is the
planet Mer- Dearest planet to the sun, and is seldom seen;
' ury ' because his vicinity to the sun occasions his
being lost in the brilliancy of tie sun's rays.
How many 1283. The heat of this planet is so great
^thlplawt that water cannot exist th ere except in a
Mercury ? state of vapor, and metals would be melted.
The intensity of the sun's heat, which is in the same pro-
portion as its light, is , seven times greater in Mercury
than on the earth, so that water there would be carried
off in the shape of steam ; for, by experiments made with
a thermometer, it appears that a heat seven times greater
than that of the sun's beams in summer will make water
,, iii 1284. Mercury, although in appearance
night may only a small star, emits a bright white iight,
^een? Ty ** ^ which it may be recognized when seen.
It appears a little before the sun rises, and
again a little after sunset ; but, as its angular distance
from the sun never exceeds twenty-three degrees, it is
never to be seen longer than one hour and fifty minutes
after sunset, nor longer than that time before the sun rises.
How does Mer- 1285. When viewed through a good tele-
cury appear scope, Mercury appears with all the various
when seen J .
through a phases, or increase and decrease of light, with
tslescope ? which we view the moon, except that it never
appears quite full, because its enlightened side is turned
directly towards the earth only when .the planet is so near
the sun as to be lost to our sight in its beams. Like that
of the moon, the crescent or enlightened side of Mercury
is always towards the sun. The time of its rotation on its
axis has been estimated at about twenty-four hours.
1286. OF VENUS. Venus, the second
planet Venus. p] anet j n or( J er f rom t h e smij j g fa Q nea reat to
the earth, and on that account appears to be the largest
and most beautiful of all the planets. During a part of
the year it rises before the sun, and it is then called the
morning star ; during another part of the year it rises after
the sun, and it is then called the evening star. The heat
and light at Venus are nearly double what they are at the
1287. By the ancient poets Venus was called Phosphor, or Luci-
fer, when it appeared to the west of the sun, at which time it is
morning star, and ushers in the light of day ; and Hesperus, or
Vesper, when eastward of the sun, or evening star.
Why is Venus 1288. Venus, like Mercury, presents to us
never seen late a ll the appearances of increase and decrease
at night ? ,, , . , , , n ,
of light common to the moon. Spots are also
sometimes seen on its surface, like those on the sun. By
reason of the great brilliancy of this planet, it may some-
' imes be seen even in the day-time by the naked eye. But
t is never seen late at night, because its angular distance
from the sun never exceeds forty-five degrees. In the
absence of the moon it will cast a shadow behind an opaque
What is meant 1289. Both Mercury and Ven us sometimes
by the transit pass directly between the sun and the earth.
ofapa ^ & t k e j r illuminated surface is towards the sun,
their dark side is presented to the earth, and they appear
like dark spots on the sun's disk. This is called the
transit of these planets.
1290. The reason why we cannot see the stars and planets- in
the day-time is, that their light is so faint compared with the
'ight of the sun reflected by our atmosphere.
Describe the 1291. OP THE EARTH. The Earth OB
Earth as a . . , .. . . . . ,
which we live is the next planet in the solar
#64 NATURAL PHILObOPHY.
system, in tkc order of distance, to Venus. It is a large
globe or ball, nearly eight thousand miles in diameter, and
about twenty-five thousand miles in circumference. It is
known to be round, first, because it casts a circular
shadow, which is seen on the moon during an eclipse ;
secondly, because the upper parts of distant objects on
its surface can be seen at the greatest distance ; thirdly,
it has been circumnavigated. It is situated in the midst
of the heavenly bodies which we see around us at night,
and forms one of the number of those bodies ; and it
belongs to that system which, having the sun for its centre,
and being influenced by its attraction, is called the solar
How much It is not a perfect sphere, but its figure is
longer is the th t of an Mate sp heroid, the equatorial
polar than the . ., ,
equatorial diameter being about twenty -six miles longer
diameter of the than its olar diameter.
before you It is attended by one moon, the diameter
speak.] O f w hi c h is about two thousand miles. Its
mean distance from the earth is about 240,000 miles, and
it turns on its axis in precisely the same time that it per-
forms its revolution round the earth ; namely, in twenty-
seven days and seven hours.
1292. The earth, when viewed from the
earth as a moon, exhibits precisely the same phases that
moon. t j ie moon d oes to us, but in opposite order.
When the moon is full to us, the earth will be dark to the
inhabitants * of the moon ; and when the moon is dark to
us, the earth will be full to them. The earth appears to
them about thirteen times larger than the moon does to us.
* This observation should be qualified by the condition that the moon is
mhitbitod. Although there is abundant reason for the belief that the
pl'uiets are " the green abodes of life," there are many reasons to bel'eve
thai" the moon, in Us i-rent-nt state, i? noitltor inhabits! n.r hubitaUc
As the moon, however, always presents nearly the same
side to the earth, there is one-half of the moon which we
never see, arid from which the earth cannot be seen.
1293. As this book may possibly incite the inquiry how it is th.it
the astronomer is ible to measure the size and distances of those
immense bodies tl.3 consideration of which forms the subject of
Astronomy, the process will "here be described by which the diam-
eter of the earth may be ascertained.
1294. All circles, as has already been stated, are divided into 36(1
degr es, and, by means of instruments prepared for the purpose,
the c imber of degrees in any arc or part of a circle can be correctly
ascertained. Let us now suppose that an observer^ standing upon
any fixed point, should notice the position of a particular star, the
north or polar star, for instance. Let him then advance from his
station, and travel towards the north, until he has brought the star
exactly one degree higher over his head. Let him then measure the
distance over which he -has travelled between the two points of
observation, -and that distance will bo exactly the length of one
degree of the earth's circumference. Let him multiply that dis-
tance by 3GO, and it will give him the circumference of the earth.
Having thus found the circumference, the diameter may readily be
found by the common rules of arithmetic.
This calculation is based on the supposition that the earth is a
perfect sphere, which is not the case, the equatorial diameter being
about twenty^six miles longer than the poJar. But it is sufficiently
near the truth for the present purpose. The design of this work
not admitting rigid mathematical demonstrations, this instance of
the commencement of a calculation is given merely to show that
what^he astronomer and the mathematician tell us, wonderful as
it may appear, is neither bare assertion nor unfounded conjecture.
What motions 1295. It has been stated that the earth re-
have the inhabit- volves upon its axis every day. Now, as the
ants of t/ie earth ^ . , t -r A AA -i c
tl a~tl sa ear ^ n 1S about 25,000 miles in circumierenct\
planet? Sec, it follows that the inhabitants of the equator
also, No. 1296. are carr i e( j around this whole distance in about
twenty-four hours, and every hour they are thus cariied through
space in the direction of the diurnal motion of the earth at the
rate of ^ th of 25,000 miles, which is more than 1000 miles in
1296. But this is not all. Every inhabitant travels with the
earth through its immense orbit, the diameter of which is about
<J66 NATURAL PHILOSOPHY.
lions of miles every year. This will give him, at the same time,
<!. motion of more than 68 000 miles in an hour in a different
direction. If the question be asked, why each individual is not
sensible of these tremendously rapid motions, the answer is,
that no one ever knew what it is to be without them. We can-
not be sensible that we have moved without feeling our motion,
as when in a boat a current takes us in one direction, while a
gentle wind carries us, at the same time, in another direction.
It is only when our progress is arrested by obstacles of some
kind that we can perceive the difference between a suite oi
motion and a state of rest.
What ivould 1297. The rapid motion of a thousand miles in
be the conse- an hour is not sufficient to overcome the centri-
yuenceifthe peta j f orce cause d by gravity; but, if the earth
earth should \ . , / '
revolve on its should revolve around its axis seventeen times in
axis once in a day, instead of once, all bodies at the equator
an hour 1 WQuld be lifted ^ and ^ attract j on O f gravita-
tion would be counterbalanced, if not wholly overcome.
1298. Certain irregularities in the orbit of the earth have
been noticed by astronomers, which show that it is deviating
from its elliptical form, and approaching that of a circle. In
this fact, it has been thought, might be seen the seeds of decay.
But Laplace has demonstrated that these irregularities proceed
from causes which, in the lapse of immensely long periods,
counterbalance each other, and give the assurance that there is
no other limit to the present order of the universe than the will
of its great Creator.
Describe the 1299. OF MARS. Next to the earth is
planet Mars, the planet Mars. It is conspicuous for its
fiery-red appearance, which is supposed by Sir John
Itersclier* to be caused by the color of its soil.
* Sir John Hcrschcl is the son of Sir William Herschei, the discovo.r^r
oi' *he planet Uraruw.
The degree of heat and light at Mars is less than half of
that received by the earth.
1300. OF THE MINOR PLANETS. It.has already been mention
ed that between the orbits of Mars and Jupiter one hundred and
thirteen small bodies have been discovered, which are called the
minor planets. It is a remarkable fact, that before the discovery
of Bode's law (see No. 1232) certain irregularities observed in the
motions of the old planets induced some astronomers to sup-
pose that a planet existed between the orbits of Mars ar.d Jupi-
tei. The opinion has been advanced that these small bodies
originally composed one larger one, which, by some unknown
force or convulsion, burst asunder. This opinion is maintained
with much ingenuity and plausibility by Sir David Brewster.
(See Edin. Encyc., art. ASTRONOMY.) Dr. Brewster further
supposes that the bursting of this planet may have occasioned
"the phenomena of meteoric otones ; that is, stones which have
fallen on the earth from the atmosphere.
Describe the 1301. OF JUPITER. Jupiter is the largest
planet Jupiter. pj anet O f tne solar system, arid the most bril-
liant, except Venus. The heat and light at Jupiter art
about twenty-five times less than that at the earth. This
planet is attended by four moons, or satellites, the shadows
of some of which are occasionally visible upon his surface.
1302. The distance of -those satellites from the planet are
two, four, six and twelve hundred thousand miles, nearly.
The nearest revolves around the planet in less than two days ;
the next, in less than four days ; the third, in less than eight
days ; and the fourth, in about sixteen days.
These four moons must afford considerable light to the inhab-
itants of the planet ; for the nearest appears to them four times
the size of our moon, the second about the same size, the third
somewhat less, and the fourth about one-third the diameter of
1303. As the axis of Jupiter is nearly perpendicular to its
orbit, it has no s-ensible change of season?.
1304. The satellites of Jupiter often pass be
What use has
been made of nm( i tne body of the planet, and also into its
the ecfipses of shadow, and are eclipsed. These eclipses are of
,- j' use in ascertaining the longitude of places on the
earth. By these eclipses, also, it has been ascer-
tained that light is about eight minutes in coming from the sun
to the earth ; for an eclipse of one of these satellites appears
to us to take place sixteen minutes sooner when the earth is in
that part of its orbit nearest Jupiter than when in the part
furthest from that planet. Hence, light is sixteen minutes in
crossing the earth's orbit, and of course half of that time, 01
eight minutes, in coming from the sun to the earth.
What is the ap- 1305. When viewed through a telescope,
"viler" seen *"" several belts or bands are distinctly seen, some-
through a tele- times extending across his disk, and sometimes
scope? interrupted and broken. They differ in dis-
tance, position, and number. They are generally dark; but
white ones have been seen.
On account of the immense distance of Jupiter from the sun
and also from Mercury, Venus, the Earth and Mars, observer?
on Jupiter, with eyes like ours, can never see either of the a^ove
named planets, because they would always be immersed in the
Describe the 1306. OF SATURN. Saturn is the sec-
planet Saturn* ond in size, and the last but two in dis-
tance from the sun. The degree of heat and light al
this planet is eighty times less than that at the earth.
How is Saturn 1307. Saturn is distinguished from the
particularly other planets by being encompassed bv
t^ee large luminous rings. They reflect
the sun's light in the same manner as his
moons. They are entirely detached from each other, ard
from the body of the planet. They turn on nearly the
'game axis with the planet, and in nearly the same tiino
1308. These rings move together around the planot,
but are about three minutes longer in performing their
revolution about him than Saturn is in revolving about
his axis. The edge of these rings is constantly at right
angles with the axis of die planet. Stars are said to
have been seen between the rings, and also between the
inner ring and the body of the planet. The breadth of
the two outer rings is about 57,000 miles, and the dis-
tance of the second ring from the planet is about 19,000
miles. As they cast shadows on the planet, Sir AViu
Herschel thought them solid.
1309. The surface of Saturn is sometimes diversified,
like that of Jupiter, with spots and belts. Saturn has
eight satellites, or moons, revolving around him at dif-
ferent distances, and in various times, from less than
one to eighty days.
1310. Saturn may be known by his pale and steady light.
The eight moons of Saturn revolve at different distances around
the outer edge of his rings. Sir William Herschel saw them
moving along it, like bright beads on a white string. They do
not often suffer eclipse by passing into the shadow of the planet,
because the ring is in an oblique direction.
Describe the 1311. OF URANUS. Uranus, the fourth
planetUranus. in size, is the most remote of all the old
planets. It is scarcely visible to the naked eye. Tho
light and heat at Uranus are about 360 times less than
that of the earth.
1312. This planet was long known by the name of
Ilcrschel, the discoverer, who, in announcing his dis-
covery, named it the " Georgiura Sidus," in honor ot
King George III. The name of Uranus was given to it
by the continental astronomers.
370 NATURAL PHILOSOPHY.
It was formerly considered a small star, but Sir William
Herschel, in 1781, discovered, from its motion, that it is a
By how many 1313 ' IJranuS is attended b J foUT moons >
moons is Uranus or satellites, all of which were discovered
by Sir William Herschel, and all of them
revolve in orbits nearly perpendicular to that of the planet.
Their motion is retrograde.
w , . 1314. It appears to be a general law of sat-
general law of ellites, or moons, that they turn on their axis
the rotation of ^ n t j ie same ^ me i n w ]ii c fi they revolve around
their primaries. On this account, the inhabit-
ants of secondary planets observe some singular appearances,
which the inhabitants of primary planets do not. Those who
dwell on the side of a secondary planet next to the primary will
always see that primary ; while those who live on the opposite
Fide will never see it. Those who always see the primary will
see it constantly in very nearly the same place. For example,
those who dwell near the edge of the moon's disk will always
gee the earth near the horizon, and those in or near the centre
will always see it directly or nearly overhead. Those who dwell