1. TOTAL ECLIPSE OF THE SUN. (Photo courtesy of Murray Alexander.)

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Photo taken in Zimbabwe on December 4, 2002.

A total eclipse occurs when the moon moves between the Earth and the Sun, and so blocks the Sun. Click here to see a simulation of an eclipse.

Total eclipses are only possible because of a remarkable coincidence: The Sun is 400 times the diameter of the Moon, and the Sun is 400 times father from the Earth. Because of this fortuitous geometry, the moon and the Sun appear to be exactly the same size in the sky; the moon seems to fit snugly over the Sun -- like a lense cap. If the moon were smaller than it is, or if it were farther away so that it appeared smaller, it would never completely block the Sun.