IN WHICH THE SCIENCE IS CONSIDERED
PRINCIPALLY IN ITS RELATION TO THE EARTH.

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.

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.

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.