Although the Moon is some 400 times smaller than the Sun, it is also some 400 times closer to us, meaning that both the Sun and Moon have the same angular size, about 0.5o, as seen from the Earth. This produces one of the most impressive and yet important scientific coincidences in nature. When the Moon comes directly between the Earth and the Sun, the Moon's shadow or part of it falls on the Earth's surface. As seen from that point the Moon covers the Sun, blocking out the Sun's light. This is known as a solar eclipse.
A solar eclipse is possible only when the Moon is near a new phase. Since the plane of the Moon's orbit is inclined to the Earth's orbital plane by about 5o, the Moon must additionally be at or near one of the two points in its orbit where that orbit intersects the Earth's orbital plane. This lineup occurs at least twice each year and at most, but rarely, five times a year.
The totally dark portion of the shadow the Moon casts during a solar eclipse is called the umbra; the penumbra is the partial shadow or semidark portion. If one stands in the umbral shadow, one sees the Sun completely covered by the Moon, and this is called a total solar eclipse. If one stands in the penumbral shadow, one sees a partially covered Sun, and this is called a partial solar eclipse. The penumbral shadow covers a much larger area on the Earth's surface than does the umbral shadow, making a partial solar eclipse visible over a wider region than a total solar eclipse.
The average length of the Moon's conical shadow is not quite equal to the Moon's mean distance from the Earth. An annular solar eclipse takes place when the Moon, in addition to the conditions under which a partial or total solar eclipse occurs, is also farthest from the Earth. At this point its umbral shadow is too short to reach the Earth, so that one sees the slightly smaller, darkened disk of the Moon surrounded by a brilliant ring of still-exposed Sun.
Under the most favorable conditions in the Earth's equatorial regions, the Moon's umbral shadow is some 270 km wide, while the penumbral shadow is close to 6000 km wide. At this time, the totality of the eclipse lasts longest along the path of the eastward-moving shadow, the maximum length being about 7.5m. Total eclipses for the 20-year period from 1980 to the year 2000 are listed in the table below.
Total Solar Eclipses from 1980 to 2000 Date Duration of
Totality (min)Where Visible
on EarthFebruary 16, 1980 4.3 Central Africa, India July 31, 1981 2.2 Siberia June 11, 1983 5.4 Indonesia November 22, 1984 2.1 Indonesia, South America November 12, 1985 1.9 South Pacific, Antarctica March 29, 1987 0.3 Central Africa March 18, 1988 4.0 Philippines, Indonesia July 22, 1990 2.6 Finland, Arctic Regions July 11, 1991 7.1 Hawaii, Central America, Brazil June 30, 1992 5.4 South Atlantic November 3, 1994 4.6 South America October 24, 1995 2.4 South Asia March 9, 1997 2.8 Siberia, Arctic February 26. 1998 4.4 Central America August 11, 1999 2.6 Central Europe, Central Asia Usually, if an eclipse of the Sun occurs, an eclipse of the Moon precedes or follows it by 2 weeks. The Earth, Moon, and Sun are then sufficiently in line for the full Moon to move totally or partially into the Earth's shadow producing a lunar eclipse. Since the Earth's diameter is nearly four times that of the Moon, the conical-shaped shadow cast by the Earth is about four times wider at the base and four times longer than the Moon's shadow. Lunar eclipses may be partial or total, everyone on the dark side of the Earth seeing the lunar eclipse at the same time.
A year may bring as many as three lunar eclipses--or none at all. More often we have two eclipses of the Sun and two of the Moon in each calendar year. Centuries of observing eclipses taught the Babylonians that eclipses recur at regular intervals. After 18 years and 10d or 11d, the circumstances of an eclipse are repeated approximately. By 200 B.C., Babylonian astronomers could predict with surprising accuracy future lunar eclipses. Their prediction method came about by noting numerical relations, what we today would call an numerical algorithm, in tabulated observations, rather than devising a geometrical relationship for the Sun, Moon, and the Earth as the Greeks later did.