ie Awan 1D

 

THE EARTH. AS A. PLANET.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig, 1, The Earth in Space,

CHAPTER lL
Mathematical Geography.

5. The Earth moves through empty space
around the sun. The old idea of the earth
resting on, or being supported by something, is
erroneous. The earth rests on nothing.

A book or other inanimate object placed on a
support will remain at rest until something or
somebody moves it, because it has no power of
self-motion. This property. is called iertia.

Inertia is not confined to bodies at rest. If
the book be thrown up through the air, it ought
to keep on moving upward for ever, because it
has no more power to stop moving than to begin
to move. We know, however, that in reality it
stops very soon, and falls to the earth; because—

(1.) The earth draws or attracts it; +

(2.) The falling body gives some of its motion
to the air through which it moves.

Were the book thrown in any direction through
the empty space in which the stars move, it would
continue moving in that direction for ever, unless
it came near enough to some other body which
would attract it and cause it to change its motion.

Our earth moves through empty space on ac-
count of its inertia, and must continue so moving
for eternities. There are ample reasons for believ-

10

 

ing that all heavenly bodies continue their mo-
tion solely on account of their inertia. The curved
paths in which the earth and the other planets
move are resultant paths produced in a-manner
that will be explained hereafter.

Space is not absolutely empty, but is everywhere filled
with a very tenuous substance called ether, which trans-
mits to us the light and heat of the heavenly bodies.
Wherever the telescope reveals the presence of stars we
must believe the ether also extends.

_ 6, The Stars.—The innumerable points of light
that dot the skies are immense balls of matter
which, like our earth, are moving through empty
space. Most of them are heated so intensely that
they give off heat and light in all directions.
They are so far from the earth that they would
not be visible but for their/immense size. Beyond
them are other balls, also self-luminous, but too
far off to be visible except through a telescope.
Beyond these, again, we have reason to believe
that there are still others. These balls of matter
are called stars.

All the heavenly bodies, however, do not shine
by their own light. A few—those nearest the
earth—shine by reflecting the light of the sun.
These are called planets, and move with the earth
around the sun.

7. The Solar System comprises the sun, eight
large bodies called planets, and, as far as now
known, three hundred .and eighty-four smaller
bodies called planetoids or asteroids, besides nu-
merous comets and meteors. Some of the planets
have bodies called moons or satellites moving
around them. These also belong to the solar
system. s

Fig. 2 represents the solar system. In the
centre is the sun. The circles drawn around
the sun show the paths or orbits of the planets.
These orbits are represented as circular, but in.
reality they are slightly flattened or elliptical.
The elongated orbits mark the paths of the comets.