David Copps age 13, of Metamora, Ill

How often do we get a total lunar eclipse?

A eclipse of the moon performs for a very large audience, for it may be seen from a little more than half our globe. From start to finish, the strange spectacle may last almost four hours. The total phase, when the moon is completely veiled by the earth’s shadow, may last almost two hours. Some will be total eclipses and scone partial eclipses in which the moon is only partially veiled by the earths shadow, Some will be penumbral eclipses 3n which the paler outer shadow of the earth falls upon part of the moon.

The dark shadow of the earth, called the umbra, is a tapering finger pointing some 900,000 miles out into space away from the sun. Around it, a lighter shadow called the penumbra fans outward. When the orbiting moon comes completely inside the umbra, it is totally eclipsed. One would expect this dark shadow to blot out the moon, but this does not happen. Sunlight filtering through the earth's surrounding atmosphere casts a soft glow and we see the face of the moon as a dull, coppery disk.

The moons average distance from the earth is 235,000 miles and at this distance the earth's tapering umbra casts a dark circle 5,700 miles wide.

This is more than twice the diameter of the moon, When the moon is full, we are between it and the sun. This is the time when the umbra points towards the orbiting moon, the only time we can have a lunar eclipse. It is natural to wonder why the moon does not pass into the huge umbra on every trip around the earth, giving us a lunar eclipse with every full moon.

This does not happen because of variations in the position of the sun, the earth and the moon. The distance between the earth and the sun varies by some three million miles and this makes differences ,in the size and shape of the earth’s shadow. The distance between the earth and the moon varies by some 31, 000 miles. The moon may cross through the tapering shadow where it is narrower and be only partly eclipsed. Most important, the moon's orbit is far from regular. ' It tips up or down, usually taking the moon above or below the shadow of the earth.

With all these irregularities, there is nevertheless a fairly orderly cycle of eclipses which repeats after 18 years and 11 days. This period is the Saros, discovered by stargazers of ancient Mesopotamia. In each Saros, there are 29 lunar eclipses in which the moon passes partly or completely into the tapering shadow of the earths umbra.