Eclipse

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Eclipse->>LUNAR ECLIPSES

The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi).

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hours. If it does not pass through the center, the period of totality is less and may last for only an instant if the moon travels through the very edge of the umbra.

A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or minor eclipse, when only a small portion of the earth’s shadow is seen on the passing moon. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Before the moon enters the umbra in either total or partial eclipse, it is within the penumbra and the surface becomes visibly darker. The portion that enters the umbra seems almost black, but during a total eclipse, the lunar disk is not completely dark; it is faintly illuminated with a red light refracted by the earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when the earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the moon is invisible during totality.

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