ECLIPSE

The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi).

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hr. If it does not pass through the center, the period of totality is less and may last for only an instant if the moon travels through the very edge of the umbra.

A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial 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. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Before the moon enters the umbra in either total or partial eclipse, it is within the penumbra and the surface becomes visibly darker. The portion that enters the umbra seems almost black, but during a total eclipse, the lunar disk is not completely dark; it is faintly illuminated with a red light refracted by the earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when the earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the moon is invisible during totality.

The length of the moon’s umbra varies from 367,000 to 379,800 km (228,000 to 236,000 mi), and the distance between the earth and the moon varies from 357,300 to 407,100 km (222,000 to 253,000 mi). Total solar eclipses occur when the moon’s umbra reaches the earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the earth, so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower. The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4828 km (about 3000 mi). At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon.

The shadow of the moon moves across the surface of the earth in an easterly direction. Because the earth is also rotating eastward, the speed of the moon shadow across the earth is equal to the speed of the moon traveling along its orbit, minus the speed of the earth’s rotation. The speed of the shadow at the equator is about 1706 km/hr (about 1060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3380 km/hr (about 2100 mph). The path of a total solar eclipse and the time of totality can be calculated from the size of the moon’s shadow and from its speed. The maximum duration of a total solar eclipse is about 7.5 min, but these are rare, occurring only once in several thousand years. A total eclipse is usually visible for about 3 min from a point in the center of the path of totality.

In areas outside the band swept by the moon’s umbra but within the penumbra, the sun is only partly obscured, and a partial eclipse occurs.

At the beginning of a total eclipse, the moon begins to move across the solar disk about 1 hr before totality. The illumination from the sun gradually decreases and during totality (and near totality) declines to the intensity of bright moonlight. This residual light is caused largely by the sun’s corona, the outermost part of the sun’s atmosphere. As the surface of the sun narrows to a thin crescent, the corona becomes visible. At the moment before the eclipse becomes total, brilliant points of light, called baily’s beads, flash out in a crescent shape. These points are caused by the sun shining through valleys and irregularities on the lunar surface. Baily’s beads are also visible at the instant when totality is ending, called emersion. Just before, just after, and sometimes during totality, narrow bands of moving shadows can be seen. These shadow bands are not fully understood but are thought to be caused by irregular refraction of light in the atmosphere of the earth. Before and after totality, an observer located on a hill or in an airplane can see the moon’s shadow traveling eastward across the earth’s surface like a swiftly moving cloud shadow.

If the earth’s orbit, or the ecliptic, were in the same plane as the moon’s orbit, two total eclipses would occur during each lunar month, a lunar eclipse at the time of each full moon, and a solar eclipse at the time of each new moon. The two orbits, however, are inclined and, as a result, eclipses occur only when the moon or the sun is within a few degrees of the two points, called the nodes, where the orbits intersect.

Periodically both the sun and the moon return to the same position relative to one of the nodes, with the result that eclipses recur at regular intervals. The time of the interval, called the saros, is a little more than 6585.3 days or about 18 years, 9 to 11 days, depending on the number of intervening leap years, and 8 hours. The saros, known since the time of ancient Babylonia, corresponds almost exactly to 19 returns of the sun to the same node, 242 returns of the moon to the same node, and 223 lunar months. The disparity between the number of returns of the moon and the number of lunar months is the result of the nodes moving westward at the rate of 19.5° per year. An eclipse that recurs after the saros will be a duplicate of the earlier eclipse but will be visible 120° farther west on the earth’s surface, because of the rotation of the earth during the third day included in the interval. Lunar eclipses recur 48 or 49 times and solar eclipses 68 to 75 times before slight differences in the motions of the sun and moon eliminate the eclipse.

During one saros about 70 eclipses take place, usually 29 lunar and 41 solar; of the latter, usually 10 are total and 31 partial. The minimum number of eclipses that can occur in a given saros year is 2, the maximum 7, and the average is 4.

During the 20th century 375 eclipses have taken or will take place: 228 solar and 147 lunar. The last total eclipse of the sun visible in the U.S. in this century occurred over the state of Hawaii on July 11, 1991. The prior such eclipse occurred over the state of Washington on Feb. 26, 1979. The next total eclipse will be visible from the U.S. in 2017.

Many problems of astronomy can be studied only during a total eclipse of the sun. Among these problems are the size and composition of the solar corona and the bending of light rays passing close to the sun because of the sun’s gravitational field. The great brilliance of the solar disk and the sun-induced brightening of the earth’s atmosphere make observations of the corona and nearby stars impossible except during a solar eclipse. The coronagraph, a photographic telescope, permits direct observation of the edge of the solar disk at all times. Today, scientific solar eclipse observations are extremely valuable, particularly when the path of the eclipse traverses large land areas. An elaborate network of special observatories may provide enough data for months of analysis by scientists. Such data may provide information on how minute variations in the sun affect weather on earth, and how scientists can improve predictions of solar flares.