R. David Kuhn.. age 13, of San Dingo, Calif., for his question:

What are the umbra and penumbra of an eclipse?

The umbra and penumbra are shadows and you can sea how they work with the help of a baseball, a sheet of paper and an electric light bulb. Hold the ball close to the paper so that the light casts a round shadow. Raise the ball a few inches and study the round shadow. Notice it is darker in the center. This darkest area is called the umbra of the shadow cast by the light bulb. The lighter area around the edge is the penumbra. Raise the ball still higher and you will see the umbra shrink. At a certain height it disappears altogether.

A solar eclipse is a trick with shadows on a grand scale. The source of light is, of course, the sun. The shadow is cast by the moon upon the face of the earth. The lunar shadow too has an umbra and a penumbra. The umbra is a dark circle usually between 50 and 160 miles wide. It is in the center of the paler penumbra which may be a circle 6,000 miles wide. Because the earth is rotating and the moon is orbiting the earth, the umbra and penumbra together trace a curved path over the face of the earth.

The umbra of the moons shadow tapers out more than 232,000 miles, away from the sun. When conditions are just right, the pointed end of this shadow falls upon the earth. This occurs when the moon is close enough to the earth and when the new moon orbits directly between the sun and the earth. When this happens we have a total solar eclipse. But this magnificent spectacle is seen only by people in the path of the umbra.

A total eclipse of this kind was  seen in  the New World on July 20, 1963. Its path  swept across New England and Canada.

The huge penumbra, of course, covers a far greater area than the umbra. The people in this area, however, do not see the dark moon completely blot out the radiant face of the sun. But they do see part of the eclipse. For them, the dark blot of the moon is not quite so big as the bright face of the sun. They see the dark circle bite into one side of the sun and pass over its face. But when it is in the center, the dark moon is not big enough to hide all the golden glory of the sun. During the peak of the eclipse, the sun looks like a doughnut, a fiery golden ring with a hole in the middle.

During the total eclipse, the people inside the umbra are in darkness. The sky becomes so dark that the stars may be seen. In the larger area of the penumbra, the eclipse does not bring total darkness