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Eclipse, blocking off of light from the Sun from one celestial body by another. An eclipse of the Moon (lunar eclipse) occurs when the Moon enters the shadow of the Earth. An eclipse of the Sun (solar eclipse) occurs when the Earth enters the shadow of the Moon. Other planets can also eclipse their own moons, and in the case of a double star, one star can eclipse the other. There are usually 2 or 3 lunar eclipses each year. Shadows have a central dark part called the umbra, and a less dark outer region called the penumbra. In the penumbra of the Earth's shadow, part of the light from the Sun is cut off from the Moon. In the umbra, all the Sun's light is cut off. Since the Earth's umbra is much wider than the diameter of the Moon, a total lunar eclipse can last up to 1 3/4 hours from the time the Moon first enters the umbra at one side to the time it moves out again at the opposite edge of the shadow. More often, the Moon moves through only part of the Earth's umbra, and the eclipse is somewhat shorter. The umbra of the Moon's shadow on the Earth is never wider than 170 mi (274 km), and the maximum diameter of the penumbra is 4,000 mi (6,437 km). Inside the umbra there is a total eclipse of the Sun, which can never last longer than 7 min and is usually less than half this duration. In the penumbra a partial eclipse is visible. Because of the Moon's motion, the track of a solar eclipse sweeps across the surface of the Earth at over 1,000 mph (1,600 kmph). An annular or ring eclipse occurs when the Moon is at its farthest from Earth. At this distance it is not big enough in appearance to completely obscure the Sun, and a ring of light remains surrounding the shadow. Eclipses of the Sun, particularly total eclipses, have provided a lot of information about the outer layers of the Sun (the corona and the chromosphere), and about the Earth's upper atmosphere.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Dream to Eijkman, Christiaan