CHAPTER IX.

OF THE PLANETS AND THEIR SATELLITES.

WILL ILL you, my dear mamma, oblige me today, with a general description of the planets? I have collected a number of questions, which I am anxious to propose to you.

That is right: you are sure to make progress, if you let what I impart, stimulate you to thought, when you are alone. The subject you have selected is appropriate; and I will begin by informing you, that the planets, like our earth, are all opaque spherical bodies, which have no proper light of their own, but shine by means of the borrowed light, which they receive from the sun.

How is it known that their light is borrowed? By telescopical observations, it is found, that only that side of them which is turned towards the sun, is ever illuminated; while the opposite side, which its rays cannot reach, remains continually dark.

Have the planets a motion round their axes, which corresponds with the diurnal motion of the earth?

From the regular appearance and disap

pearance of several dark spots, visible on their bodies, it is obvious they have such a motion.

Have they a progressive motion round the sun, which answers to the annual revolution of the earth in its orbit?

Their progressive motion round the sun, is ́ proved from their seeming sometimes to be stationary, and from their going backwards at other times.

Which of the planets is the nearest the sun?
Mercury.

How long is he going round the sun, or what is the length of his year?

Eighty-seven days twenty-three hours.

How long is he turning on his axis, or what is the length of his day?

This is not yet ascertained.

What is Mercury's distance from the sun?
Thirty-seven millions of miles.

When is Mercury visible?

A little after sunset, and a little before sunrise.

Which planet is second in order ?

Venus.

How long is she going round the sun?

Two hundred and twenty-four days, and seventeen hours.

How long is she turning on her axis ?
Twenty-three hours, and twenty minutes,

What is the distance of Venus from the sun?
Sixty-eight millions of miles.

With what degree of velocity does she move? At the rate of seventy-six thousand miles an hour.

When is Venus visible?

When she rises in the morning before the sun, and is west of him; and then she is called the morning star. When she appears east of him, and shines in the evening after he sets, she is named the evening star. She is alternately in each situation, about two hundred and ninety days.

Which is the planet next above Venus?
The earth.

How long is it going round the sun?

Three hundred and sixty-five days, five

hours, and forty-nine minutes.

How long is it turning on its axis?

Twenty-four hours.

What is its distance from the sun?
Ninety-five millions of miles.

With what degree of velocity does it move? At the rate of fifty-eight thousand miles an hour; which is but little more than half the velocity with which Mercury travels.

Which is the next planet above the earth?
Mars.

How long is he going round the sun?
Rather less than two of our years.

How long is he turning on his axis? Twenty-four hours, and thirty-nine minutes. What is Mars's distance from the sun? One hundred and forty-four millions of miles.

With what degree of velocity does Mars move?

At the rate of fifty-five thousand miles an hour.

When is he visible?

He rises when the sun sets, and sets when the sun rises.

What is the next planet above Mars?

Jupiter.

How long is Jupiter in making the circuit of the sun?

Rather less than twelve of our years. How long is he turning on his axis? Nine hours and fifty-six minutes. What is Jupiter s distance from the sun? Four hundred and ninety millions of miles. With what degree of velocity does he move? At the rate of twenty-nine thousand miles an hour.

At what time does Jupiter appear the largest, and most luminous ?

When in opposition to the sun; because then he is much nearer to the earth.

What is the length of his days and nights?

They are each about five hours.

What is the next planet above Jupiter?
Saturn.

What is the distance of Saturn from the sun?

Nine hundred millions of miles. And here I cannot but remark, how admirably situated are these immense planets Jupiter, and Saturn.

Why? what would have been the effect of a different situation?

If they had been situated in much lower spheres, they would certainly have disturbed the motions of the other planets.

And have they no effect now?

At their immense distances, they act almost equally on the sun, and on the inferior planets. And does that prevent their influence from being detrimental?

Their acting equally, reduces it, in some respects, to be very nearly the same thing as if they did not act at all; while, their happily selected situation may, in other respects, serve to bind and consolidate the whole of the solar system.

The admirable selection of the situations of these planets is the clearer to my apprehension from what you before told me of the inequalities in the moon's motion, occasioned, if I rightly recollect your statement, by her near approach to the earth, and sun, combined with