What is the Moon Like?

            The moon is our nearest neighbor in the solar system. When the moon is full, it seems to be about the same size as the sun. Actually, the diameter of the moon is only about a quarter that of the earth. So the diameter of the sun is 400 times greater than the diameter of the moon. The moon appears as big as the sun because it is so much closer to us. It is only about 239,000 miles from the earth. This is a small distance compared with the 93 million miles between the earth and the sun.
            The moon reflects light from the sun just as the planets do. When the moon is full, you can see dark, irregular spots on its surface. Before the telescope was invented, no one knew what these spots were. But through a telescope, you can see that they are really mountains, craters, and vast plains. The mountains on the moon are steeper than those on the earth. Some of them rise almost straight up from the plains to a height of more than 5 miles.
            The moon revolves around the earth in the same way that a planet revolves around the sun. And the moon spins on its axis like a planet. But the moon takes just exactly as long to turn once around on its axis as it takes to revolve once around the earth. So the same side of the moon is always turned toward the earth. No one has ever looked directly at the other side of the moon. However, pictures of the far side of the moon taken from a rocket and sent back to the earth by television seem to show that the far side of the moon has about the same appearance as the side that we see. The surface of the moon is scorching hot where the sun shines and bitterly cold where does not. There is apparently no air or water on the moon. So probably there are no living things there either. The most unusual things we can see on the moon are its craters. Over 30,000 of these have been counted on the side of the moon that we can see from the earth. Most of the moon's craters are 20 to 30 miles in diameter, and one has a diameter of about 140 miles. These craters were probably formed when meteorites struck the moon.

Why Does the Moon Seem to Change Its Shape?

            The moon seems to change its shape from one night to the next. Sometimes it looks like a full circle, at other times like a half circle, and at still others like a thin crescent. And sometimes we do not see the moon at all. We know that the moon does not actually change its shape. What really changes is the amount of its lighted part that we can see. Like our earth, the moon is a sphere. And like the earth, only half of the moon is lighted by the sun at any one time. But we cannot always see all of this lighted half. Sometimes we cannot see any of it.
            As the moon revolves around the earth once in about 29½ days, we see different amounts of the half of its surface that is lighted by the sun. When the moon is on the side of the earth toward the sun, we are looking toward the half that is not lighted. So we cannot see the moon at all. But when the moon is on the side of the earth away from the sun, we can see all of its lighted half. Then we say that the moon is full. In other positions around the earth, we see different amounts of the lighted half of the moon. The shapes the moon seems to have are called its phases. The thin strip of the moon that we see in the west just after sunset is commonly called the new moon. But scientists call the phase that is not visible from the earth, the new moon.
            Occasionally when a full moon is shining, the moon will begin to darken. Part or all of its surface may become dark for a time. Sometimes, in bright daylight, a similar thing will happen to the sun. Its light will be shut of partly or completely. The cutting off of the light of the moon or the sun in this way is an eclipse. Eclipses frightened the ancient peoples because they did not understand what was happening. But eclipses are easily explained.
            When the earth is directly between the moon and the sun, the earth's shadow falls on the moon. So the moon is partially or totally darkened. This is called a lunar eclipse. A lunar eclipse takes place only when the moon is full. But there is not always a lunar eclipse at the time of full moon. This is because the orbit of the moon around the earth is tilted, compared with the orbit of the earth around the sun. So at the time of full moon, the moon is usually either above or below the earth's shadow. There may be as many as two or three lunar eclipses in a year, but sometimes there is none.
            When the moon is directly between the earth and the sun, the moon's shadow falls on the earth. So the sun is partially or totally hidden behind the moon. This is called a solar eclipse. A solar eclipse takes place only during the time of a new moon. But there is not always a solar eclipse at this time. There may be from two to five solar eclipses a year. To see a solar eclipse, you must be in a rather small area where the moon's shadow falls on the earth.