comet continued to be visible till near the end of December in that year. Another was seen in 1807, and another appeared a few years ago; but neither of these were at all equal in splendour or brilliancy to that of 1811. The tail of the comet which was observed in 1807, was calculated to be above nine millions of miles in length; but the tail of the comet of 1811 was not less than thirtythree millions of miles long.

79

CHAPTER THE FIFTH.

SECONDARY PLANETS.

MOTION

OF THE

MOON ROUND THE EARTH.-REVOLUTION

OF THE MOON ON HER AXIS.-PHASES OF THE MOON, FORM OF THE MOON.-ECLIP

Does the sun give as much light to the planets that are very distant, as to those that are near him?

MOTHER.

No; the planets must have less of it, in proportion as they are more distant from him; but most of the superior planets are furnished with one or more moons or satellites, which supply, in some measure, the deficiency of the light of the

sun.

ELIZABETH.

But what is there to make up for the want of the sun's warmth-must not the inhabitants of those distant planets suffer extremely from cold?

MOTHER.

You judge from what your own feelings would be; but we may reasonably suppose, either that the planets themselves are more or less capable of receiving and retaining heat according to their situation, or that their inhabitants are endowed with the faculty of resisting heat and cold in a greater or less degree than we are; for we must not suppose that the rest of the planets are peopled with creatures incapable of enjoying their existence. Sir Isaac Newton calculated, that the sun's heat is seven times as great in Mercury as with us, and that water there would be all carried off in the form of steam; for he found, by experiments with the thermometer, that heat, seven times greater than that of the sun in summer, will serve to make water boil.

The eleven bodies we were talking about yesterday, all move in their own separate orbits round the sun, and are called Primary planets. Some of them are attended by others smaller than themselves, which move in orbits round them, and ac

company them in their progress through the heavens. These are eighteen in number, and are called Moons, Satellites, or Secondary planets. The Earth, you know, has one such moon, Jupiter has four, Saturn seven, and Herschel six. But we are not able to see any of these, except our own moon, without the assistance of a telescope, on account of the smallness of their size, and their great distance from us.

1

The moon is two hundred and forty thousand miles from the centre of the earth, and moves round it in the course of about twenty-nine days and a half, at the rate of nearly twenty-two thousand one hundred miles an hour. She revolves only once upon her axis in each of her revolutions round the earth. I leave it to you to find out the length of her day and night.

WILLIAM.

Shall we be able to do that, Elizabeth?

ELIZABETH.

It is not so difficult as you suppose. How long is the earth in turning on its axis?

WILLIAM.

Twenty-four hours.

ELIZABETH.

And what is the length of our day and night?

WILLIAM.

Twenty-four hours, of course.

ELIZABETH.

Then if the moon takes twenty-nine and a half of our days and nights to revolve on her axis, each of her days and nights must be equal in length to fourteen and three quarters of ours.

MOTHER.

You may amuse yourselves, at some other time, by finding out the number of our hours contained in the moon's day.

WILLIAM.

How much larger the moon looks than any of the stars. What is her size?

MOTHER.

The diameter of the earth is seven thousand nine hundred and seventy miles, and that of the moon about two thousand one hundred and eighty miles only. The moon appears to us so much larger than any of the other heavenly bodies, only