Eileen Fishel, age ll, of Des Moines, Iowa, for her question:

What causes eclipses of the moon?

If you travel around the world to the right spot at the right time, you can seeat least two eclipses of the moon every year. During certain years you may catch up with three, four or even five lunar eclipses. some will cast shadows on only part of the moon and some will be total eclipses.

During a lunar eclipse, a rusty, brown shadow creeps across part or all of the full moon. This shadow belongs to the earth and the eclipse depends upon angles and triangles created out in space. Our big, round globe casts a shadow about 900,000 miles long. It tapers into space in a long cone from the night side of the earth which is turned away from the sun. We do not notice it unless it falls on some object out in space.

Every month the moon orbits around the earth at an average distance of 235,000 miles. every month it passes around the side opposite from the sun where the earth's shadow points out into space. We should get a lunar eclipse every month  but we do not. We get only two to five lunar eclipses a year, and in most of them the earth's shadow falls on only part of the moon.

    This is because orbits tend to wobble. Instead of traveling on a flat, level path, the orbit of the moon tips up and down. sometimes it is nearer and sometimes farther from the earth. The changeable angles and distances usually enable the moon to dodge around the earth without passing through its shadow. Some months it passes above and some months below the shadowy triangle.

But once in a while the moon's orbit cuts right through the shadow of the earth and We get a lunar eclipse. This must occur when the moon is on the side of the earth opposite the sun. The side of the moon facing us then is bathed in sunshine and a lunar eclipse can occur only at full moos.

The moon travels its orbit at about 2,000 miles an hour and gradually we can watch it move into the shadow of the earth. slowly the shadowy finger creeps over the golden face of the full moon, changing it to dusky gray or bronze. The angle of the orbit may cause the shadow to fall on only part of the moon and we see a partial eclipse. But sometimes the entire moon must pass through the shadowy triangle and the full moon may be totally eclipsed for more than an hour. Then, as the moon orbits on its way, the shadow is left behind and We see it inching back from the bright lunar landscape.

The encroaching shadow of the earth bites in a curved path across the surface of the moon. Its edge also is curved as it recedes from the opposite edge of the moon. These curves should tell us something about the ahape of the earth. They are part of a huge circle and in order to cast a circular shadow, an object must be round. During a lunar eclipse, the earth's creeping shadow gives us proof that our globe is round.