(ēklĭps', ĭ–) [Gr.,=failing], in astronomy, partial or total obscuring of one celestial body by the shadow of another. Best known are the lunar eclipses, which occur when the earth blocks the sun's light from the moon, and solar eclipses, occurring when the moon blocks the sun's light from a small portion of the earth. Occasionally a double or binary star system is aligned so that one star eclipses the other as seen from the earth; these stars are known as eclipsing binaries. Also important to science have been the eclipses of Jupiter's satellites; in 1675 the Danish astronomer Ole Roemer used these eclipses to calculate the speed of light. Observations of starlight passing near the sun during the 1919 solar eclipse were of particular value in validating Einstein's general theory of relativity.
Since the earth and moon shine only by the reflected light of the sun, each casts a shadow into space in the direction away from the sun. The shadow consists of a cone-shaped area of darkness called the umbra, where all light from the sun is cut off, and a larger area of partial darkness called the penumbra, which surrounds the umbra and receives light from a part of the sun's disk. Lunar eclipses can occur only when the moon is in its full phase, i.e., when the earth is between the sun and the moon. These eclipses may be total or partial, depending on whether the moon passes completely into the umbra of the earth's shadow or remains partly in the penumbra. Since the moon cuts the umbra close to the base, it can experience long periods of total eclipse ranging up to 1 hr, 42 min. A partial eclipse (when it passes through the penumbra) can last more than 2 hr, and the entire lunar eclipse may continue for as long as 4 hr. Some light is refracted, or bent, by the earth's atmosphere into the umbra, so that the moon at totality, instead of appearing black, ranges from a dull gray to a coppery color, depending on the amount of dust in the earth's atmosphere.
A total solar eclipse can occur only when the moon is in its new phase. At this time the moon is between the sun and the earth and cannot be seen until it moves across the sun's disk. At the onset of totality, parts of the sun may be seen shining brightly between the high points of the moon's irregular edge, a phenomenon known as Baily's beads; the disk of the moon appears black and is surrounded by the sun's corona, out of which shoot immense, flamelike spurts called prominences. The sky darkens to twilight, the brightest stars become visible, and there is a noticeable drop in temperature. Baily's beads are seen again as the sun reappears and the sky grows lighter.
At apogee (when the moon is at its farthest point from the earth) the umbra of its shadow is too short to reach the earth's surface, causing the apparent diameter of the sun's disk to be larger than that of the moon. Where the moon would otherwise block the sun entirely, now the sun is seen as a bright ring completely surrounding the moon's disk; this eclipse is known as an annular, or ring, eclipse. The longest possible duration of totality for a solar eclipse is 7 min, 40 sec at or near the equator when the sun is directly overhead; the duration decreases with increasing latitude. The eclipse of June 20, 1955, lasted 7 min, 8 sec, which was the longest duration of totality in 1,238 years; an eclipse almost as long occurred on July 11, 1991.
If the plane of the moon's orbit about the earth coincided with that of the earth about the sun, a solar eclipse would be observed each month when the moon is new and a lunar eclipse when the moon is full. However, the moon's orbital plane is tilted at an angle of about 5°10' to the earth's orbital plane, making eclipses possible only when the three bodies are aligned (at new or full moon) and when the moon is crossing the earth's orbital plane (at a point called the node). Within a given year, a maximum of seven eclipses can occur, either four solar and three lunar or five solar and two lunar. Despite the fact that there are more solar than lunar eclipses each year, over time many more lunar eclipses are seen at any single location on earth than solar eclipses. This occurs because a lunar eclipse can be seen from the entire half of the earth facing the moon at that time, while a solar eclipse is visible only along a narrow path on the earth's surface.
From their observations of eclipses the Chaldaeans (fl. 1000 B.C.–540 B.C.) discovered that similar eclipses of the sun recur in cycles of 18 years, 111/3 days; this cycle, called the saros, is an interval in which the sun, earth, and moon return to nearly identical relative positions. Since the orbits of the earth and moon are quite accurately known, eclipses can be predicted far in advance, both in time and location. Similar calculations can determine the time and place of past eclipses; this information is useful for dating historical events that are known to have occurred at the same time as an eclipse.
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