IN WHICH THE SCIENCE IS CONSIDERED PRINCIPALLY IN ITS RELATION TO THE
EARTH.
[Illustration: THE SEASONS.]
ASTRONOMY is the science which treats of the System of the Universe.
The system of which the Earth is a part, is called the SOLAR SYSTEM;
and is composed of the _Sun_, the _Planets_, _Satellites_, and _Comets_.
The SUN is the source of light and heat to the whole system, and is the
centre of motion, round which all the other bodies revolve from west to
east.
The PLANETS are opaque or dark bodies, which revolve round the Sun at
different distances. There are eleven planets, viz. _Mercury_, _Venus_,
_Earth_, _Mars_, _Jupiter_, _Saturn_, _Herschell_, _Ceres_, _Pallas_,
_Juno_, and _Vesta_.
SATELLITES are opaque bodies moving round the planets, in company with
the planets round the Sun. They are generally called moons. There are
eighteen Satellites, viz. the Earth has _one_, Jupiter _four_, Saturn
_seven_, and Herschell _six_.
COMETS are bright bodies moving round the Sun. Coming from a far
distant part of the universe, they approach very near the Sun, and then
return with equal swiftness. Their nature is but little known.
The paths of the planets and comets round the Sun, and of the
satellites round their planets, are called their ORBITS.
Those stars which always appear in the same situation in relation to
each other are called FIXED STARS. About 1000 can be seen at once by
the naked eye, and by the help of the telescope many millions may be
seen. Their sizes are little known, but probably some of them are _many
million times larger_ than the earth. _They are not considered a part
of our Solar System_, but are supposed to be Suns of other systems,
round which other planets revolve.
A planet _turns on its axis_ (like a wheel upon an axletree,) and each
_turn_ is the cause of day and night. It is _day_ to that half of the
planet which is toward the Sun, and _night_ to the other half. Each
_revolution of a planet round the Sun_ makes its year; but _because the
orbits are not equal_, the years of all the planets are not the same.
Thus Herschell’s year is more than _eighty-three of our years_.
The axis of the earth is not perpendicular while the earth is revolving
round the sun, but slanting, as is seen in the cut above. Whilst the
earth is passing between the points marked March 20, and Sept. 23, the
north pole inclines toward the sun, and then the northern hemisphere
enjoys more of his rays than the southern. Consequently it is the warm
season in the northern, and the cold season in the southern hemisphere;
and from 23d Sept. to 20th March, as the south pole is toward the sun,
it is the warm season in the southern, and the cold season in the
northern hemisphere.
The EARTH is nearly a round body, and revolves on its axis, _once every
twenty-four hours_, _from west to east_, which causes the Sun and stars
to appear to _rise in the east, and set in the west_.
The earth’s surface contains about _two hundred millions of square
miles_, and its orbit is _ninety-six millions of miles from the Sun_.
The time occupied by the earth in its revolution round the Sun, is
about _365 days_, which space of time constitutes its year.
Planets and satellites do not shine with their own light, but merely
_reflect the light of the Sun_; of course only one half can be bright
at a time, which must be that half which is _toward the Sun_.
When the moon is on the side of the earth opposite to the Sun, the
whole bright side of the moon is toward the earth, and it is then _full
moon_; but when the Sun and moon are on the same side of the earth, the
dark side of the moon is toward the earth, and it is _new moon_.
The earth’s satellite or _moon_, revolves round the earth in about
_twenty-nine and a half days_, and this causes the _phases_ or changes
of the moon. It turns on its axis in the same space of time.
[Illustration: PHASES OF THE MOON.]
The inner circle represents the moon in her different positions as
enlightened by the Sun.
The outer circle represents the moon’s appearance in her different
positions as seen from the earth.
An ECLIPSE means a temporary obscuration of the Sun or moon, or a part
of either, from our sight. When the Sun or moon is wholly hid from our
sight, the eclipse is called _total_, and when only a part is obscured,
it is called _partial_.
_An eclipse of the Sun_ is caused by the passage of the moon between
the Sun and the earth, and can never take place except at the time of
_new moon_.
To understand this more fully, observe the cut below, and suppose a
person standing on the earth at a place where the shadow of the moon
falls upon the earth; he cannot see the Sun, because the moon is
between him and the Sun.
[Illustration: ECLIPSE OF THE SUN.]
The moon being an opaque body, does not shine, when by any cause the
rays of the Sun are prevented from reaching her surface. Thus when the
moon arrives in her orbit at a point where the rays of the Sun are
intercepted by the earth, she is in the shadow of the earth, and is not
enlightened by the Sun. Therefore;
_An eclipse of the moon_ is caused by the moon’s passing through the
shadow of the earth. This can never happen except at the time of _full
moon_.
[Illustration: ECLIPSE OF THE MOON.]