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Chris Fellbaum, age 12, of Kingston, Ont., Canada, for his question:     

HOW DO UMBRAS AND PENUMBRAS DIFFER?

Umbras and penumbras are super size shadows, and the average eclipse creates one of each kind. They form a pair of shadowy cones, one inside the other. The umbra is the very dark central cone, tapering outward from either the earth or the moon. Around it is the huge, dusky penumbra that fans outward from the earth or the moon.

The earth and the moon are solid globes, and as they rotate one side is always lit by the beaming sun. The opposite side casts a long tapering shadow out into space. This is the dark umbra. It is shaped like a long thin cone, with its base centered on the night side of the earth or the moon.

Out there in space, sunlight filters around the umbra,creating    a much larger,

less shadowy shadow. This is the dusky penumbra that fans out from the earth and the moon, spreading wider and wider through space. The dark tapering umbra is centered in the huge dusky penumbra.    

Several times during an average year the earth and moon manage to cast their shadows upon each other. Then we get some sort of eclipse. When only the penumbra touches down, the eclipse is partial. A total eclipse is seen from only within the dark shadow of the umbra.

Eclipses occur only when the sun is a straight line with both the earth and the moon. When the new moon passes directly between us and the sun, it casts its shadow on the earth and 'we get a solar eclipse. Its midnight umbra is no wider than 167 miles. Along its fast moving path, the dark moon blots out the dazzling sun and viewers behold a total solar eclipse. Meantime the surrounding penumbra casts a dusky shadow, maybe 3,000 miles wide, where viewers behold only a partial solar eclipse.

A lunar eclipse occurs when the earth is directly between the sun and the full moon. Then our shadow falls on the lunar landscape. At the moon's distance, our larger umbra is about 5,700 miles wide. A total lunar eclipse may last an hour and 40 minutes, while the partial lunar eclipse may last almost four hours.

In theory the orbiting moon should give us one solar and one lunar eclipse every month. But it travels a wobbly path and its average distance is too short to cast its shadow upon the earth. So eclipses occur only when its wobbly path comes a little closer just when the sun, earth and moon can form a perfect lineup.

 

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