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        Science | Astronomy Terms

Eclipse. An eclipse of the sun, or solar eclipse, occurs when the moon's shadow sweeps across the earth. An eclipse of the moon, or lunar eclipse, occurs when the moon passes through the shadow of the earth. Eclipses also occur on other planets and their satellites. Eclipses of Jupiter's four large moons are readily observed from the earth, even with small telescopes.

Solar Eclipses

Solar eclipses occur only at new moonthat is, when the moon passes between the earth and sun. Usually, the moon's shadow misses the earth (and there is no eclipse), because the moon swings a little above or below the plane of the earth's orbit around the sun. However, two to five times a year the moon passes through the plane of the earth's orbit at or near the time it is in the new moon phase. At these times the moon casts its shadow on the earth.

Total solar eclipses, 1985-2015
DatePath of total eclipse
Nov. 12, 1985 South Pacific Ocean
March 18, 1988 Indonesia, Philippines, Pacific Ocean
July 22, 1990 Northern Siberia, Aleutian Islandslombia
July 11, 1991 Hawaii, Mexico, Central America, Colombia, Brazil
June 30, 1992 South Atlantic Ocean
November 3, 1994 South America, South Atlantic Ocean
October 24, 1995 Southern and Southeast Asia, Indonesia
March 9, 1997 Mongolia, eastern Siberia
Feb. 26, 1998 Pacific Ocean, Central America, Colombia, Venezuela, Caribbean Sea
August 11, 1999 North Atlantic Ocean, Europe, Middle East, southern Asia
June 21, 2001 Atlantic Ocean, central Africa
Dec. 4, 2002 Antarctica, south Indian Ocean
Nov. 23, 2003 Antarctica, south Indian Ocean, Australia
March 29, 2006 West Africa, southwestern Europe
Aug. 1, 2008 Siberia
July 22, 2009 Western Pacific Ocean
July 11, 2010 Central Pacific Ocean
Nov. 13, 2012 South Pacific Ocean
March 20, 2015 Scandinavia, northern Siberia, Arctic Ocean

The shadow of the moon has two parts: the umbra, or full shadow, and the penumbra, or partial shadow, which surrounds the umbra. The umbra has a conical shape and its length is such that it either just reaches or barely misses the earth's surface. When the umbra reaches the earth, those areas that fall under it experience a total eclipsethe sun is completely hidden behind the moon. Elsewhere the eclipse is a partial eclipse. Total eclipses are preceded and followed by a partial eclipse. When the umbra fails to reach the earth's surface, an annular eclipse is observed in those areas directly behind (below) the umbra. In such an eclipse, the sun is visible as a thin ring around the moon.

A total solar eclipse is relatively rare, and when one occurs it can be observed only along a narrow path, called the path of totality. This path is traced by the tip of the umbra, which is at most 167 miles (269 km) wide at the earth's surface. Over large areas on both sides of the path of totality a partial eclipse can be observed. A total eclipse never lasts more than eight minutes at any one place, but a partial eclipse may last up to several hours.

A total eclipse of the sun is an impressive sight. During a total eclipse the sun's corona (outer atmosphere) becomes clearly visible as a pearly white glow around the moon. The temperature of the air drops noticeably and the sky becomes dark enough for planets and bright stars to appear.

Eclipses occur in cycles, enabling astronomers to determine the exact location, time, and duration of both future and past eclipses. Records of total eclipses have been made since ancient times and these records allow precise dates for certain historical events to be established.

Astronomers will often travel great distances to observe a total eclipse because it permits them to make solar observations that are otherwise extremely difficult or impossible due to the sun's brightness. During a total eclipse in 1868 the gas helium was discovered in the sun's atmospherealmost 30 years before it was found to exist on earth. During a total eclipse in 1919 observations of stars appearing near the sun (stars that would otherwise not be visible when the sun is between them and the earth) showed that the stars had apparently shifted from their known positions. This observation showed that starlight is bent by the gravity of the sun, confirming a key point in Einstein's General Theory of Relativity.

Observing An Eclipse

The sun should not be looked at directly, even during an eclipse.

Lunar Eclipses

Lunar eclipses occur only at full moon, when the moon is on the far side of the earth from the sun. There are three kinds of lunar eclipses: penumbral, total, and partial. A penumbral eclipse occurs when the moon passes through the penumbra of the earth's shadow; the moon is dimmed only slightly. A total lunar eclipse occurs when the moon passes through the umbra, in the center of the earth's shadow. During a total eclipse, the moon darkens noticeably, turning a dull orange. It usually does not darken completely, because some sunlight is refracted by the earth's atmosphere into the shadow. The moon may be totally eclipsed for up to approximately two hours. In a partial lunar eclipse, the moon passes through only an edge of the umbra.

Eclipses of the moon are visible from anywhere on the earth where the moon is above the horizon. Except for penumbral eclipses, they occur less frequently than solar eclipses.