CHAP. III.

Of the Solar System.

OUR solar system consists of the sun, and the planets, and comets moving about it.

The planets are bodies which appear to us like stars; not that they are luminous bodies, that is, have light in themselves; but they shine by reflecting the

sun.

They are called planets from a Greek word, which signifies wandering; because they change their places, and do not always keep the same distance with one another, nor with the fixed stars, as the fixed stars do.

There are two kinds of planets, primary and secondary. The first move round the sun, and respect him only as the center of their motions. The secondary planets, called also satellites, or moons, are smaller planets, revolving round the

primary, while they, with the primary planets about which they move, are carried round the sun. The planets move round the sun at various distances, some being much nearer to him than our earth, and others being much further off.

There are nine primary planets, which are situated with respect to their distances from the sun as follows:

Ө

Mercury, Venus, The Earth, Mars, Ceres, Pallas, 24 h

H

Jupiter, Saturn, and Herschel, or the Georgium Sidus.

Of these, our earth is accompanied by one moon, Jupiter has four moons, Saturn has seven moons, and the Georgium planet has six moons. None of these moons, except our own, can be seen without a good telescope. The other five planets do not appear to have any satellites or moons.

All the planets move round the sun from west to east, and in the same direction do the moons revolve round their primaries, excepting those of the Georgium planet, which seem to move in a contrary direction. The paths in which they move round are called their orbits.

They perform their revolutions also in very different periods of time. The time of performing their revolutions round the sun is called their year, and the time of performing their revolutions on their axes their day.

The axis of a planet is an imaginary line conceived to be drawn through its center, about which it revolves as if on a real axis. The extremities of this line, terminating in opposite points of the planet's surface are called its poles. A bowl whirled from one's hand into the open air turns round such a line within itself, whilst it moves forward; and such are the lines we mean when we speak of the axes of the heavenly bodies.

Venus and Mercury being nearer to the sun than our earth, are called inferior planets, and all the rest, which are without the earth's orbit, are called superior planets.

The sun is placed near the common center, or rather in the lower* focus of

* If a thread be tied loosely round two pins stuck in a table, and moderately stretched by the point of a black-lead pencil carried round by an even motion and light pressure of the hand, an

the orbits of all the planets and comets, and turns round his axis in 25 days 6 hours, as is evident by the motion of spots seen on his surface. By the various attractions of the circumvolving planets, he is agitated by a small motion round the center of gravity of the system.

Let us suppose the earth's orbit to be a thin, even, solid plane; cutting the sun through the center, and extended out as far as the starry heavens, where it will mark the great circle called the Ecliptic. This circle we suppose to be divided into 12 equal parts, called Signs; each sign into 30 equal parts, called Degrees; each degree into 60 equal parts called Minutes; and every minute into 60 equal parts, called Seconds; so that a second is the 60th part of a minute; a minute the 60th part of a degree; and a degree the 360th part of a circle, or 30th part of a Sign. The planes of the orbits of all the other

oval ellipsis will be described; the two points where the pins are fixed being called the foci or focuses thereof. The orbits of all the planets are elliptical, and the sun is placed in or near one of the foci of each of them; and that in which he is placed is called the lower focus.

planets likewise cut the sun in halves; but extended to the heavens, form circles different from one another, and from the ecliptic; one half of each being on the north side, and the other on the south side of it. Consequently the orbit of each planet crosses the ecliptic in two opposite points, which are called the planet's Nodes.

Mercury is the first planet in the order of the system. It is computed to be about 37,000,000 miles distant from the sun, and to move at the rate of 105,000 miles an hour, completing its orbit in about 88 of our days, or little less than three months, which is the length of its year. It is not much larger than the moon, being about 3200 miles in diame

ter.

Venus is the second planet from the sun, remarkable for its brightness, it is computed to be 68,000,000 miles from it, and to move round it at the rate of 76,000 miles an hour, completing its annual revolution in 224 days 17 hours, or above 7 months. Its diameter is 7700 miles, and its diurnal revolution is performed in 23 hours 22 minutes.