G. P. (George Payn) Quackenbos.

A natural philosphy: embracing the most recent discoveries in the various branches of physics .. online

. (page 38 of 42)
Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 38 of 42)
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TTJi Virgo, the virgin.



^ Libra, the balance.

TTL Scorpio, the scorpion.

Sagittarius, the archer.

\3 Capricornus, the goat.

? Aquarius, the water-bearer.

K Pisces, the fishes.



978. The Change of Seasons. It has been stated that
the Earth is nearer the Sun at one period of its revolution
than at another. The change of seasons, however, is en-
tirely independent of this fact, and is produced by the sun's
rays falling on a given point of the Earth's surface with
different degrees of obliquity at different parts of its orbit.

nox? What is the Autumnal Equinox? 976. What is the Zodiac? How is it di-
vided? What makes it peculiarly interesting to us? From what is the zodiac so
called ? What are Constellations ? How are the signs of the zodiac now situated
relatively to the constellations from which they were named ? To what is this ow-
ing? 977. Name the signs of the zadiac. 973. By what is the change of seasons pro-



THE CHANGE OF SEASONS. 387

When the Sun is vertical, or directly overhead, its heat is
most intense ; and the less its rays deviate from a vertical
line in striking the surface, the more heat they impart to it.
The angle at which the Sun's rays strike a given part
of the Earth's surface keeps constantly varying, in conse-
quence of the Earth's revolving with its axis. always point-
ing in the same direction, or, as it is generally expressed,
everywhere parallel to itself. This will be understood from
Fig. 334.

In Fig. 334 the Fig. 334.

Earth is repre-
sented as moving
round the Sun,
which is in one of
the foci of her el-
liptical orbit. The
dotted line is the
zodiac, divided in-
to its twelve signs.
N S is the Earth's
axis, which main-
tains the same di-
rection in the four
positions shown,
and at every other
part of the orbit.

At the vernal equinox (March 21), the equator is directly opposite the
Sun ; the solar rays fall at the same angle on the northern hemisphere as on
the southern, and it is spring in the former, autumn in the latter. The
Earth's axis is inclined neither to nor from the sun ; consequently, half the
surface, from pole to pole, is enlightened at a time, and day and night are of
equal length all over the globe.

As the Earth moves eastward, the rays of the Sun no longer fall verti-
cally on the equator, but on places north of it. This continues till June 21st,
when the sun is vertical to places 23 28' north (this being the obliquity of
the ecliptic), and his rays extend over the same distance beyond the north
pole. It is now summer in the north and winter in the south, for in propor-
tion as the solar rays full less obliquely on the former, they must fall more
obliquely on the latter. It will be observed, also, that a space extending
2l> 28' around the south pole is totally dark.

duced? When is the Sun's heat most intense? Why does the angle at which the
Sun's rays strike a given part of the Earth's surface keep varying ? What does Fig.
834 represent ? Describe the position of the Earth and the circumstances attending




388 ASTRONOMY.

The Sun is never directly overhead to any place farther north of th
equator than 23 28'. As the Earth continues her course eastward, it be-
comes vertical to places more and more to the south, and by the 22d of Sep-
tember, or thereabouts, it is vertical to the equator just as it was six months
before. This is the period of the autumnal equinox. The Earth again pre-
sents a full side from pole to pole to the Sun, and the days and nights are
once more equal. We have now the southern spring and the northern
autumn.

From this point, the solar rays become more and more oblique in the
north and fall vertically on places farther and farther south, till the same
limit of 23 28' is attained, which takes place about December 21, and marks
the northern winter and the southern summer. Beyond this limit the Sun is
never directly overhead. As the Earth keeps on to the east, his vertical
rays fall on latitudes nearer and nearer to the equator, till finally on the 21st
of March places on the equator have the Sun in their zenith as they had six
and twelve months before.

979. The explanation just given shows that there are
two points of the ecliptic in which the Sun is about 23 de-
grees from the equator, and from which he seems to turn
back towards that line. These points are called Solstices
(standing-points of the Sun), because the Sun appears to
stand still for several days at the same place in the heav-
ens before taking an opposite direction. The solstice
reached in June is called the Summer Solstice ; that in De-
cember, the Winter Solstice.

980. Circles on the Earth's surface about 23|- degrees
north and south of the equator form the limits beyond
which the Sun's rays are never vertical. These circles are
called Tropics (from a Greek word meaning to turn), be-
cause on reaching them the vertical rays turn back towards
the equator. The northern tropic is called the Tropic of
Cancer, because when the Sun reaches this line he is seen
from the Earth in the sign Cancer, as will be apparent from
Fig. 334. For a similar reason the southern tropic is called
the Tropic of Capricorn.

981. It appears from Fig. 334 that from March 21 to September 22 the
north pole is constantly illuminated and the south pole in darkness, notwith-
standing the revolution of the Earth on its axis ; while from September 22

it, at March 21. At June 21. At September 22. At December 21. 979. What are
the Solstices ? Why are they so called ? How are they distinguished ? 980. What
are the Tropics? Whence is their name derived? What is the northern tropic



THE MOON. 389

to March 21, darkness reigns at the north pole and the south pole enjoys
continual light. At the summer solstice there is a space of 2y'/a degrees
about the north pole on which the Sun does not set, and at the winter sol-
stice a corresponding space about the south pole. The lines that bound
these regions are called the Polar Circles. The one near the north pole is
called the Arctic Circle ; that near the south pole, the Antarctic Circle.

982. If, instead of being inclined, the Earth's axis were perpendicular to
the plane of its orbit, the regions on the equator would have the Sun con-
stantly in their zenith, day and night would always be equal over the wholo
globe, there would be no variety of seasons, and a given place would have
about the same temperature from one year's end to another. Something of
this kind must be the case on the planet Jupiter, whose axis is nearly per-
pendicular to the plane of its orbit. On the other hand, the more the axis
of a planet is inclined, the greater are the extremes of temperature incident
to its several seasons.

983. THE MOON (3). The Earth is attended by one
satellite called the Moon, a beautiful orb which ' rules the
night ' with its gentle brilliancy, produces in part the tides,
and sensibly affects the Earth's motions by its attraction.

984. Size. The Moon's diameter is 2,165 miles, but its
apparent size is almost equal to the Sun's in consequence of
its nearness to our planet. Its density is not much more
than one-half that of the Earth, and it contains about one-
eightieth as much matter.

985. Motions. The Moon is 240,000 miles from the
Earth, and revolves about the latter so as to reach the same
point relatively to the fixed stars in 27 days, 8 hours. To
reach the same point relatively to the Sun requires 29 days,
13 hours, since the Earth has itself meanwhile advanced in
its orbit. When nearest the Earth, the Moon is said to
be in her Per'-i-gee, and when farthest from it in her Ap'-
o-gee.

The terms perigee and apogee (which mean near tlie Earth and away from
the Earili) are also applied to the apparent position of the Sun. When the
Earth is at its perihelion, the Sun is said to be in perigee ; and when the
Earth is at its aphelion, the Sun is in apogee.

called, and why ? The southern ? 931. What are the Polar Circles ? What is the
one near the north pole called ? That near the south pole ? 982. If the Earth's axis
\\-ereperpendicularto the plane of its orbit, what would follow ? What is said of
Jupiter? 9 S3. By what is the Earth attended ? 934. How great is the Moon's diam-
eter? Its density? Its mass? 935. How far is the Moon from the Earth? What
is the period of her revolution ? When is the Moon said to be in perigee ? In ap-



390 ASTRONOMY.

The Moon also turns on its axis in exactl j the same time
that it takes to* revolve round the Earth, and in the same
direction. The consequence is that she always presents the
same side to the Earth. Nearly one-half of our fair at-
tendant we never see, and to the inhabitants of half her
surface, if she has any, we are invisible.

986. Phases. The Moon is non-luminous, and shines
only by the reflected light of the Sun; hence the hemi-
sphere presented to the Sun is bright, while the opposite
one is dark. As the Sun, Moon, and Earth are constantly
taking different positions relatively to each other, the por-
tion of illuminated lunar surface presented to us is as con-
stantly changing. Hence arise what are called the Phases
of the Moon.

When new, the Moon lies between the Earth and the Sun, near a line con-
necting their centres. Her dark side is then towards us, and she is invisible.
Soon, however, she gets so far east of the Sun as to appear in the west
shortly after his setting. A bright crescent then becomes visible on the side
nearest the Sun, the rest of her circular disk being just discernible, not by
sun-light directly received, but by sun-light reflected from the Earth to the
Moon, and by her reflected back to us. The crescent gradually grows larger,
until, when the Moon is 90 degrees from the Sun, or in quadrature, half her
disk is illumined. She is then said to. be in her First Quarter.

Each succeeding night now finds the enlighteneji portion larger and
larger, and the Moon is said to be gibbous. At last she reaches a point at
which she is again almost in a line with the Sun and the Earth, but this time
the Earth is in the middle. The Moon rises in the east as the Sun sets in the
west ; the whole of her enlightened hemisphere is therefore turned towards
us, and she is said to befall.

After this the Moon again becomes gibbous, and we see less and less of
her enlightened surface, till at length half of her disk is dark, when she is
said to be in her Third Quarter. Advancing beyond her third quarter, she
wanes still further to a crescent, and at length on arriving in conjunction
with the Sun disappears entirely, to go through the same phases again as
she makes another revolution in her orbit.

987. To the inhabitants of the Moon, if any there be, the Earth presents
the same phases that the Moon does to us, but in reversed order. When the
Moon is new to us, the Earth is full to them, a splendid orb, thirteen times

ogee ? When is the Sun said to be in perigee ? In apogee ? How long is the Moon
in turning on her axis ? What is the consequence ? 9SG. What is said of the Moon's
light? What causes her to present different phases to the Earth? Describe the
phases uccessively presented. 987. What phases does the Earth present to the



THE MOON. 391

as large as the full Moon. When she is in her first quarter, the Earth is in
her third quarter, &c.

988. The Moon has either no atmosphere at all, or one
exceedingly rare, and not extending more than a mile from
its surface. Hence it must be destitute of water, for any
liquid on its surface would long since have been dissipated
by the heat of the lunar days, there being no atmospheric
pressure to check evaporation. If there were any water
on the surface of the Moon, clouds would certainly be ob-
served at times dimming its face.

989. Viewed through a telescope, the surface of the Moon appears ex-
ceedingly rough, covered with isolated rocks, deep valleys, yawning chasms,
craters of extinct volcanoes, in some cases more than 100 miles in width, and
lofty mountains, several of which are from three to four miles high and cast
their shadows a great distance over the rugged plains. Every thing is deso-
late in the extreme. Several of the earlier astronomers thought that they
discerned volcanoes in a state of eruption ; but later observers are of the con-
trary opinion, attributing the peculiar brightness of the supposed volcanic
summits to phosphorescence, or superior reflective properties.

Names have been given to the various mountains and spots visible on the
Moon, and a map has been prepared of the whole side presented to us, which
has been pronounced " vastly more accurate than any map of the Earth we
can yet produce." The great telescope of the Earl of Rosse shows with dis-
tinctness every object on the lunar surface that is 100 feet in height. It has
brought to light, however, no signs of life or habitation.

990. MARS ( $ ). Mars, the fourth planet from the Sun,
is 4,546 miles in diameter. Its day is of nearly the same
length as ours, its year about twice as long. The incli-
nation of its axis to the plane of its orbit does not differ
much from the Earth's, and its seasons are therefore simi-
lar to ours. It is surrounded by an atmosphere of mod-
erate density.

Mars is easily distinguished in the heavens by his red fiery light, which
is supposed to owe its color to the soil from which it is reflected. The tele-
scope distinctly shows continents of a dull red tinge, like that of sand-stone,

Moon ? 9SS. What is said of the Moon's atmosphere ? Why is the Moon sup-
posed to be destitute of water ? 989. How does the Moon look, when viewed through
a telescope? What is now thought respecting the supposed volcanic eruptions for-
merly observed ? How high objects does the Earl of Rosse's telescope distinctly
show ? 990. Which is the fourth planet from the Sun ? What is the length of its
diameter? Its day? Its year? How do its seasons compare with ours ? How may
Mars be distinguished ? What does the telescope show ? What are seen about tho



392 THE ASTEROIDS

washed by seas of a greenish hue. Bright white spots are seen about the
poles, which are no doubt occasioned by the reflection of the sun's light from
the snow and ice collected there. It is observed that as each pole is turned
towards the sun the spots about it diminish in size, owing to the melting of
the snow by the solar heat.

991. THE ASTEROIDS. The Asteroids are so small that,
with the exception of one or two which have been seen
without a telescope, they are invisible to the naked eye.
Their diameters have not yet been accurately determined ;
some exceed 1 00 miles, and others probably fall somewhat
under that mark. A number of them are provided with
extensive atmospheres. The Asteroids are supposed by
some to be the wreck of one large planet, which they be-
lieve to have originally revolved between Mars and Jupi-
ter, and by some tremendous catastrophe to have burst
into fragments. Many similar bodies probably remain to
be discovered in this region.

The Asteroids are comparatively so diminutive that the force of gravity
on their surfaces must be very small. A man placed on one of them would
spring with ease 60 feet high, and sustain no greater shock in his desceot
than he does on the earth from leaping a yard. On such planets giants may
exist ; and those enormous animals which here require the buoyant power
of water to counteract their weight, may there inhabit the land.

992. JUPITER (U). Next to the asteroids is Jupiter,
the largest of the planets, which exceeds the Earth in bulk
nearly 1,300 times. Its revolution round the Sun is per-
formed in about 12 years, and that around its axis in less
than 10 hours. Jupiter is attended by four satellites, which
revolve about it from west to east.

All of these satellites but one exceed our Moon in size. The largest would
sometimes be visible to the naked eye as a very faint star, were it not lost in
the superior brightness of its planet. Three of them are totally eclipsed
during every revolution by the long shadow which the planet casts, and the
fourth is very often eclipsed. The relation between their orbits and motions
is such that for many years to come Jupiter will never be deprived of the
light of all four at the same time.

poles ? By what are they supposed to be caused ? 901. Are the Asteroids visible to
the naked eye? What is the length of thi-ir diameters? What are the Asteroids
thought by many to be ? What is stated with respect to the force of gravity on their
surface? 992. How does Jupiter rank in size? How does it compare in bulk with
the Earth? What is the length of its year? Its day? By what is it attended?



SATURN. 393

So large is Jupiter, and so short a time is it in revolving on its axis, that
every point on its equator must turn at the rate of 450 miles a minute. The
result is an immense excess of centrifugal force at the equator; and this is
seen to have operated before the mass of the planet became hard, by flatten-
ing it very much at the poles. Jupiter's disk is always- crossed with a num-
ber of dark parallel belts, as shown in Fig. 331. They vary in breadth and
situation, but are always parallel to the equator of the planet ; hence they
appear to be connected with its rotation on its axis, and are no doubt pro-
duced by disturbances in its atmosphere.

992. SATURN ( b ). Saturn, which is next to Jupiter in
distance from the Sun, is also next to it in size, having a
diameter of 76,791 miles, and consequently a bulk nearly
1,000 times that of the Earth. Its day is not half so long
as ours ; but it is 29^ of our years in making one complete
revolution in its orbit.

Saturn has eight moons, seven of which were known for sixty years be-
fore the eighth was discovered. The largest of them has a diameter about
half as large again as our Moon. Saturn's disk, like Jupiter's, is frequently
diversified with belts; spots arc of rare occurrence. An atmosphere of con-
siderable density is supposed to surround the planet.

Saturn has a remarkable appendage, consisting of three bright, flat, and
exceedingly thin rings, encircling its equator, and revolving with it around
its axis in about the same time in which the planet itself revolves. The
whole breadth of these rings is 27,000 miles, while their thickness does not
exceed 100 miles. They are supposed to consist of a mixture of gases and
vapors, sufficiently substantial to cast a shadow. The three rings are de-
tached from each other, and lie in the same plane very close together, while
the inner one is l',,000 miles from the surface of the planet. They are pre-
vented from falling in upon the planet by the centrifugal force "generated by
their rapid revolution.

993. URAXUS ( r ^). Uranus, the next planet to Saturn,
revolves about the Sun in 84 of our years. There being no
spots on its surface, we are unable to fix the period of its
revolution on its axis. It is attended by six moons, which
move from east to west (unlike the satellites of the other

What is the size of the largest of these moons ? What relation subsists between their
orbits and motions? What is the shape of Jupiter? What has caused the flattening
at the poles? With what is Jupiter's disk crossed? To what are these belts to be
attributed? 992. What is the next planet to Jupiter? What is Saturn's dumeu-r?
How does its bulk compare with the Earth's? Its day? Its year? How many
moons has Saturn? How is its disk diversified? What remarkable appendage has
Saturn? Describe its rings. 993. What is the next planet to Saturn ? What is tho
length of tho year of Uranus ? Its clay ? By what is it attended ? How do its light

17*



394 ASTRONOMY.

planets) in orbits nearly perpendicular to that of the planet.
The solar heat and light of Uranus are only ^o of ours.

994. NEPTUNE (T). Neptune, the most remote planet
of the solar system, is invisible to the naked eye. Seen
through the telescope, it looks like a star of the eighth
magnitude. The diameter of Neptune is 39,800 miles,
which is 4,500 more than that of Uranus. Its rev-
olution around, the Sun is performed in about 165 of our
years. Neptune has at least one moon, distant from it about
as far as ours is from us.

The discovery of Neptune is one of the greatest triumphs of which sci-
ence can boast. Comparing observations on Uranus, while it was still
thought to be the most distant member of the solar system, astronomers
found certain perturbations or irregularities, in its motions, which could be
accounted for only on the supposition that there was some unknown planet
beyond it by whose attraction it was affected. Le Verrier thoroughly inves-
tigated the subject, and even went so far as to compute the size and distance
of the suspected planet, and to predict in what part of the heavens it would
be found at a given date. A letter from the French astronomer, embracing
the results of his calculations, reached Berlin, September 13, 184(1 ; and that
very evening, sweeping the heavens with his powerful telescope, according
to Le Verrier' s instructions, Dr. Galle discovered what was apparently a star
of the eighth magnitude not laid down on his chart, but was proved by its
change of place on the following evening to be a planet. It is just to add
that Adams, an English astronomer, had, about the same time with Le Ver-
rier, made similar calculations, and with nearly the same result.

995. REAL AND APPARENT POSITION OF THE HEAVENLY
BODIES. We seldom see the heavenly bodies in their real
position. This is owing to two causes, Refraction and
Parallax.

996. Effect of He fraction. Refraction, which has been
explained in the chapter on Optics, bends rays of light en-
tering our atmosphere from a rarer medium, and causes the
body from which they proceed to appear higher than it
really is. The Sun is thus made visible a few moments be-
fore he actually rises and after he sets. The effect of re-
ami heat compare with ours? 09 i. "What is the most remote planet of the solar sys-
ti-m ? How floes Neptune look, when seen through the telescope ? What is its diam-
eter? What is the period of its revolution ? How nmny moons has Neptune ? Give
an account of the circumstances under which Neptune was discovered. 995. Why do
wo not see the heavenly bodies in their real position ? 996. What is the effect of re-



PARALLAX. 395

fraction is greatest when a body is on the horizon, and
diminishes as it ascends towards the zenith, at which point
it entirely disappears.

997. Effect of Parallax. A planet seen from different
points of the Earth's surface appears to lie in different
positions. This is evident from Fig. 333.

The planet C to an observer . OOK

Jt 1 1". Coo.
at A seems to lie at F ; to one .

at B it appears to lie at D. To / ^-

avoid the inconsistencies which ( J2 1 "~ "~ ~- *- - - v. : :

would otherwise exist in obser- \^_^/ '" .jj

rations made at different places,

the centre of the earth is taken as a standard point ; and the true position
of a heavenly body is that point of the celestial arch which would be cut
by a line connecting the centre of the Earth with the centre of the body in
question, infinitely produced.

Parallax is the angle made by a line from a heavenly
body to the Earth's centre and another line from the same
body to the eye of an observer.

It is evident that, the nearer a heavenly body is, the greater is its parallax
The fixed stars are so remote that they have no appreciable parallax. The
Earth, if visible to them, would be nothing more than a minute point of light.
The parallax of a heavenly body is greatest when it is on the horizon. At
the zenith it would be nothing, because from that point the lines to the ob-
server's eye and the centre of the Eurth would coincide.

998. ECLIPSES. By an Eclipse of the Sun or Moon is
meant its temporary obscuration by the interposition of
some other body. An eclipse is called Total, when the
whole disk is obscured ; and Partial, when only a portion
is darkened.

999. An eclipse of the Sun is caused by the Moon's get-
ting between it and the Earth, and intercepting its rays.
This can happen only at new Moon, because, when between
us and the Sun, the Moon must present to us her unenlight-
ened side.

fraction ? 997. How does a planet seem to lie, when observed from different parts of
the Earth's surface ? Illustrate this with Fig. 3:15. What is the true position of a
heavenly body? What is Parallax? What is said of the parallax of the fixed stars?
What would be the effect of refraction and parallax on the apparent position of a
body in our zenith ? 993. What is an Eclipse ? When is an eclipse called Total, and
when Partial ? 909. What causes an eclipse of the Sun ? When alone can this bap-



Online LibraryG. P. (George Payn) QuackenbosA natural philosphy: embracing the most recent discoveries in the various branches of physics .. → online text (page 38 of 42)