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Solar Eclipse
A total solar eclipse requires the umbra of the Moon's shadow to touch the surface of the Earth. Because of the relative sizes of the Moon and Sun and their relative distances from Earth, the path of totality is usually very narrow (up to 270 kilometers across). The following figure illustrates the path of totality produced by the umbra of the Moon's shadow.

    If you are in the path of totality the eclipse begins with a partial phase in which the Moon gradually covers more and more of the Sun. This typically lasts for about an hour until the Moon completely covers the Sun and the total eclipse begins. The duration of totality can be as short as a few seconds, or as long as about 8 minutes, depending on the details.
As totality approaches the sky becomes dark and a twilight that can only be described as eerie begins to descend. Just before totality waves of shadow rushing rapidly from horizon to horizon may be visible. In the final instants before totality light shining through valleys in the Moon's surface gives the impression of beads on the periphery of the Moon (a phenomenon called Bailey's Beads). The last flash of light from the surface of the Sun as it disappears from view behind the Moon gives the appearance of a diamond ring and is called, appropriately, the diamond ring effect (image at right).
    As totality begins , the solar corona (extended outer atmosphere of the Sun) blazes into view. The corona is a million times fainter than the surface of the Sun; thus only when the eclipse is total can it be seen; if even a tiny fraction of the solar surface is still visible it drowns out the light of the corona. At this point the sky is sufficiently dark that planets and brighter stars are visible, and if the Sun is active one can typically see solar prominences and flares around the limb of the Moon, even without a telescope (see image at left).
    The period of totality ends when the motion of the Moon begins to uncover the surface...