CHAP. XIX.

ABOUT THE CHANGES OF THE MOON.

Now that we have in some degree considered the sun, we may talk again of the changes of the moon, or changes of its phases or appearances. They depend upon the sun.

I have told you that the moon is a large round body, with an uneven surface. Its bulk is nearly a fiftieth part as large as the earth; and is two hundred and forty thousand miles distant. If a swift bird could fly constantly from the earth toward the moon, it would perform the journey, from the one to the other, in eighty days and nights.

Different measurements are stated of the moon, according to different considerations. Its diameter, or the measure of a line which should be carried entirely through it, from side to side, at its equator, is called from two thousand one

hundred and sixty miles, to two thousand one hundred and eighty; and, since the earth, similarly measured, is less than eight thousand miles, therefore the diameter of the moon is much more than equal to a third part of that of the earth. But the moon, measured as to the side or surface of the disc which it presents to the earth, is only equal to a thirteenth part of the earth, respecting the same surface; and further, the moon, measured as to its bulk or volume, or, by other terms, its total magnitude or size, is equal to no more than a forty-eighth, or less than a fiftieth, of the bulk or volume of the earth; and, finally, measured as to its mass, which means the quantity of matter it contains, it is only equal to about a seventy-fifth part of the mass, or amount of matter, that is contained in the earth.

I have told you that the moon turns on its axis once in about twenty-nine days and a half,

How long does it take the moon to turn round on its axis?

and revolves round the earth in the same period of time; and, further on, I shall explain both those motions. You have observed that the moon sometimes appears round, and sometimes only partly round; I have before spoken of these changes, and I am now going to tell you their

reason.

The moon is a dark body, and therefore cannot shine of itself; but the sun shines upon it, and makes it bright, and then the moon shines upon the earth.

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But you will ask me how the sun can shine. upon the moon when it is night, and the sun is gone? You must remember, that although the sun is gone from us at night, yet the sun really exists, at the same time, at a great distance, round the other side of the earth. There it is

How long does it take the moon to revolve round the earth? Does the moon always appear round?

Describe the moon's various appearances.

Does the moon shine of itself like a candle, or not
Why does it shine?

always shining; and thus from the situation of the moon, (though our part of the earth is then gone or turned away from the sun,) it still lights the moon, and renders it visible to us, and is the means of shedding moonlight upon the earth. Well! sometimes the sun shines upon one side of the moon, and sometimes upon the other. When one side of the moon is bright, the other is dark. We see but one same side of the moon, and we can see only so much of that as the sun shines upon. Sometimes, we can see only the edge of the light which the sun casts upon the moon. Then, it looks like a bow in the sky, and we call it the new moon.

The next night, we can see a little more of the light on the moon; and the next night more, and the next more still; till at length the

Where does the sun shine in the night?

Does the sun shine at one time upon all parts of the moon?
Can we see that part of the moon that is not lighted by the sun?
What part of the moon do we see?

What causes the new moon, as it is called?

sun shines upon the whole of that side of the moon which is always turned toward us, and the moon appears round or full.

Here is a picture of the sun shining upon the moon, and the moon shining upon the earth. I

am obliged to draw it as if the side of the earth that is next the sun were dark; but you will understand that the sun is more behind the earth than I can make it appear upon paper, and that therefore the light side of the earth is really toward the sun. You remember that the

What causes the full moon, as it is called?