Thursday, June 11, 2009
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Paper 1999
4. How the solar and lunar eclipses are caused?
Two kinds of eclipses involve the Earth: eclipses of the Moon, or lunar eclipses, and those of the Sun, or solar eclipses. A lunar eclipse occurs when the Earth is between the Sun and the Moon and its shadow darkens the Moon. A solar eclipse occurs when the Moon is between the Sun and the Earth and its shadow moves across the face of the Earth. Eclipses are rare, however; most of the time, the shadows do not overlap.
A partial solar eclipse can be seen when part of the Earth is within the penumbra, or the outer portion of the Moon’s shadow. The inner portion, the umbra, is formed by tangents to the Sun and Moon, and the outer portion, the penumbra, is formed by tangents that intersect between the Sun and the Moon. These geometric regions occur because light travels in straight lines and casts shadows even at great distances.
A partial lunar eclipse occurs when only a part of the Moon enters the umbra of the Earth’s shadow, leaving a portion of the Moon in total darkness. The extent of a partial lunar 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.
During a total solar eclipse, the Moon completely blocks out the Sun for as long as seven to eight minutes. This type of eclipse is visible from the area on the Earth within the umbra, the inner part of the Moon's shadow. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the Earth. As a result, the area in which a total solar eclipse is visible from the Earth is never wider than that, and it is usually considerably narrower.
A total lunar eclipse occurs when the Moon passes completely into the umbra of the Earth’s shadow. If the Moon moves directly through the center, it is obscured for about two hours. If it does not pass through the center, the period of totality is shorter; it may last for only an instant if the Moon travels through the very edge of the umbra.
total lunar eclipse
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