The Earth.

The next planet in our system, nearest the sun, is the Earth. Her diameter is 7912 miles. This planet revolves around him in 365 days, 5 hours, and 48 minutes; and at the distance of 95 millions of miles. It turns round its own axis once in 24 hours, making a day and a night. (The Earth's revolution round the sun is called its annual, or yearly motion, because it is performed in a year; while the revolution around its own axis, is called the diurnal, or daily motion, because it takes place every day. The figure of the earth, with the phenomena connected with her motion, will be explained in another place.

The Moon.

The Moon, next to the sun, is, to us, the most brilliant and Interesting of all the celestial bodies. (Being the nearest to us of any of the heavenly orbs, and apparently designed for our use, she has been observed with great attention, and many of the phenomena which she presents, are therefore better understood and explained, than those of the other planets.

While the earth revolves round the sun in a year, it is attended by the Moon, which makes revolution round the earth once in 27 days 7 hours and 43 minutes. The distance of the Moon from the earth is 240,000 miles, and her diameter about 2000 miles.

Her surface, when seen through a telescope, appears diversified with hills, mountains, valleys, rocks, and plains, presenting a most interesting and curious aspect: but the explana tion of these phenomena are reserved for another section.

Mars.

The next planet in the solar system, is Mars, his orbit sur rounding that of the earth. The diameter of this planet is upwards of 4000 miles, being about half that of the earth. The revolution of Mars around the sun is performed in nearly 687 days, or in somewhat less than two of our years, and he turns on his axis once in 24 hours and 40 minutes. His mean

How long does it take the earth to revolve round the sun? What is meant by the earth's annual revolution, and what by her diurnal revolution? Why are the phenomena of the moon better explained than those of the other planets? In what time is a revolution of the moon about the earth performed? What is the distance of the moon from the earth? What is the diameber of Mars? How much longer is a year at Mars than our year?

distance from the sun is 144 millions of miles, so that he moves in his orbit at the rate of about 55,000 miles in an hour. The days and nights, at this planet, and the different seasons of the year, bear a considerable resemblance to those of the earth. The density of Mars is less than that of the earth, being only three times that of water.

Mars reflects a dull red light, by which he may be distinguished from the other planets. His appearance through the telescope, (is remarkable for the great number and variety of spots which his surface presents.

Mars has an atmosphere of great density and extent, as is proved by the dim appearance of the fixed stars, when seen through it. When any of the stars are seen nearly in a line with this planet, they give a faint, obscure light, and the nearer they approach the line of his disc, the fainter is their light, until the star is entirely obscured from the sight.

This planet sometimes appears much larger to us than at others, and this is readily accounted for by his greater or less distance At his nearest approach to the earth, his distance is only 50 millions of miles, while his greatest distance is 240 millions of miles; making a difference in his distance of 190 millions of miles, or the diameter of the earth's orbit.

The sun's heat at this planet is less than half that which we enjoy.

To the inhabitants of Mars, our planet appears alternately as the morning and evening star, as Venus does to us.

Vesta, Juno, Pallas, and Ceres.

These planets were unknown until recently, and are therefore sometimes called the new planets. It has been mentioned, that they are also called Asteroids.

The orbit of Vesta is next in the solar system to that of Mars. This planet was discovered by Dr. Olbers, of Bremen, in 1807. The light of Vesta is of a pure white, and in a clear night she may be seen with the naked eye, appearing about the size of a star of the 5th or 6th magnitude. Her revolu tion round the sun is performed in 3 years and 66 days, at the distance of 223 millions of miles from him.

What is his rate of motion in his orbit? What is his appearance through the telescope? How is it proved that Mars has an atmosphere of great density? Why does Mars sometimes appear to us larger than at others? How great is the sun's heat at Mars? Which are the new planets, or astemids? When was Vesta discovered? What is the period of Vesta's annual revolution?

Juno was discovered by Mr. Harding, of Bremen, in 1804 Her mean distance from the sun is 253 millions of miles Her orbit is more elliptical than that of any other planet, and in consequence, she is sometimes 127 millions of miles neare the sun than at others. This planet completes its annual revolution in 4 years and about 4 months, and revolves round its axis in 27 hours. Its diameter is 1400 miles.

Pallas was also discovered by Dr. Olbers, in 1802. Its distance from the sun is 226 millions of miles, and its periodic revolution round him, is performed in 4 years and 7 months.

Ceres was discovered in 1801, by Piazzi, of Palermo. This planet performs her revolution in the same time as Pallas, being 4 years and 7 months. Her distance from the sun 260 millions of miles. According to Dr. Herschel, this planet is only about 160 miles in diameter.

Jupiter.

Jupiter is (89,000 miles in diameter, and performs his annual revolution once in about 11 years, at the distance of 490 millions of miles) from the sun. This is the largest planet in the solar system/being about 1400 times larger than the earth. His diurnal revolution is performed in nine hours and fiftyfive minutes, giving his surface at the equator, a motion of 28,000 miles per hour. This motion is about twenty times more rapid than that of our earth at the equator.

Jupiter, next to Venus, is the most brilliant of the planets, though the light and heat of the sun on him is nearly 25 times less than on the earth.

This planet is distinguished from all the others, by an ap pearance resembling bands, which extend across his dise Fig. 184.

When was Juno discovered? What is her distance from the sun? What te the period of her revolution, and what her diameter ?

These are termed belts, and are variable, both in respect to number and appearance. Sometimes seven or eight are seen, several of which extend quite across his face, while others appear broken, or interrupted.

These bands, or belts, when the planet is observed through a telescope, appear as represented in fig. 184. This appearance is much the most common, the belts running quite across the face of the planet in parallel lines. Sometimes, however, his aspect is quite different from this, for in 1780, Dr. Her schel saw the whole disc of Jupiter covered with small curved lines, each of which appeared broken, or interrupted, the whole having a parallel direction across his disc, as in fig. 185 Fig. 185.

Different opinions have been advanced by astronomers respecting the cause of these appearances. By some, they have been regarded as clouds, or as openings in the atmosphere of the planet, while others imagine that they are the marks of great natural changes, or revolutions, which are perpetually agitating the surface of that planet. It is, however, most probable, that these appearances are produced by the agency of some cause, of which we, on this little earth, must always be entirely ignorant.

Jupiter has four satellites, or moons, two of which are sometimes seen with the naked eye. They move round, and

What is said of Pallas and Ceres? What is the diameter of Jupiter? What is his distance from the sun? What is the period of Jupiter's diurnal revolution? What is the sun's heat and light at Jupiter, when compared with that of the earth? For what is Jupiter particularly distinguished? Is the appearance of Jupiter's belts always the same, or do they change? What is said of the cause of Jupiter's belted appearance? How many moons has Jupiter, and what are the periods of their revolutions ?

attend him in his yearly revolution, as the moon does our earth. They complete their revolutions at different periods. the shortest of which is less than two days, and the longest seventeen days.

hese satellites often fall into the shadow of their primary, in consequence of which they are eclipsed, as seen from the earth. The eclipses of Jupiter's moons have been observed with great care by astronomers, because they have been the means of determining the exact longitude of places, and the velocity with which light moves through space. How longitude is determined by these eclipses, cannot be explained or understood at this place, but the method by which they become the means of ascertaining the velocity of light, may be readily comprehended. An eclipse of one of these satellites, appears, by calculation, to take place sixteen minutes sooner, when the earth is in that part of her orbit nearest to Jupiter, than it does when the earth is in that part of her orbit at the greatest distance from him. Hence light is found to be sixteen minutes in crossing the earth's orbit, and as the sun is in the centre of this orbit, or nearly so, it must take about 8 minutes for the light to come from him to us. Light, therefore, passes at the velocity of 95 millions of miles, our distance from the sun, in about 8 minutes, which is rearly 200 thousand miles in a second.)

Saturn.

The planet Saturn revolves round the sun in a period of about 30 of our years, and at the distance from him, of (900 millions of miles. His diameter is 79,000 miles, making his oulk nearly nine hundred times greater than that of the earth, but notwithstanding this vast size, he revolves on his axis once in about ten hours. Saturn therefore performs upwards of 25,000 diurnal revolutions in one of his years, and hence his year consists of more than 25,000 days; a period o. time equal to more than 10,000 of our days. On account of the remote distance of Saturn from the sun, he receives only about a 90th part of the heat and light which we enjoy on the earth. But to compensate, in some degree, for this vast distance from the sun, Saturn has seven moons, which revolve

What occasions the eclipses of Jupiter's moons? Of what use are these eclipses to astronomers? How is the velocity of light ascertained by the eclipses of Jupiter's satellites? What is the time of Saturn's periodic revolution round the sun? What is his distance from the sun? What his diameter? What is the period of his diurnal revolution? How many days make a year at Saturn?