Eclipse

I

Introduction

Eclipse, astronomical event in which one body in space cuts off the light from another body, particularly the light from the Sun or from a planetary satellite. Two kinds of eclipses involve Earth: those of the Moon, or lunar eclipses; and those of the Sun, or solar eclipses. A lunar eclipse occurs when Earth is between the Sun and the Moon, and Earth’s shadow darkens the Moon. A solar eclipse occurs when the Moon is between the Sun and Earth, and the Sun’s shadow moves across the face of Earth. An eclipse is called a total eclipse if the light is completely blocked or a partial eclipse if the light is only partly blocked.

Transits and occultations are similar astronomical phenomena. A transit happens when a planet or other body in space passes across the disk of a much larger body as seen by an observer. Planets in our solar system may transit across the disk of the Sun or moons may transit across the disk of the planet they orbit. Viewed from Earth, solar transits only happen with the inner planets Mercury and Venus and certain asteroids. Transits may also be recorded with extrasolar planets that orbit distant stars if the line of sight from Earth is close to the orbital plane of the planet.

An occultation happens when a body in space hides another body from the view of an observer. A solar eclipse is an actually an occultation of the Sun by the Moon. However, the term occultation is typically used when the Moon, a planet, or another type of solar system body passes in front of a star or sometimes a planet when viewed from Earth or a spacecraft.

On Earth, eclipses are relatively rare events. The plane in which the Moon orbits Earth is tilted about 5.1 degrees with respect to the plane of Earth’s orbit around the Sun. If the Moon orbited in exactly the same plane that Earth orbits the Sun, a lunar eclipse would occur at every full moon and a solar eclipse at every new moon. Because of the tilt of the Moon’s orbit, lunar eclipses only happen about twice a year and solar eclipses about two to five times a year. Total eclipses are rarer than partial eclipses.

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II

Lunar Eclipses

Earth casts a long, conical shadow in space as the planet is lit by the Sun. 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 Earth, it has a diameter of about 9,170 km (about 5,700 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 hours. 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 Earth’s shadow is seen on the passing moon. Historically, the view of Earth’s circular shadow advancing across the face of the Moon was the first indication of the shape of 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 Earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when Earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the Moon is invisible during totality.

III

Solar Eclipses

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 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 Earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of 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 Earth, is about 4,828 km (about 3,000 mi). At certain times when the Moon passes between Earth and the Sun, its shadow does not reach 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 Earth in an easterly direction. Because Earth is also rotating eastward, the speed of the Moon shadow across Earth is equal to the speed of the Moon traveling along its orbit, minus the speed of Earth’s rotation. The speed of the shadow at the equator is about 1,706 km/h (about 1,060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3,380 km/h (about 2,100 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 minutes, but these are rare, occurring only once in several thousand years. A total eclipse is usually visible for about 3 minutes 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 hour 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.

Narrow bands of moving shadows can be seen on the ground just before, just after, and sometimes during totality. These shadow bands are not fully understood but are thought to be caused by irregular refraction of light in the atmosphere of Earth. Before and after totality, an observer located on a hill or in an airplane can see the Moon’s shadow traveling eastward across Earth’s surface like a swiftly moving cloud shadow.

IV

Frequency of Eclipses

If 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 6,585.3 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 Earth’s surface, because of the rotation of Earth during the third of a 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 the 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 took place: 228 solar and 147 lunar. The last total eclipse of the Sun visible in the United States occurred over the state of Hawaii on July 11, 1991. The prior such eclipse occurred over the state of Washington on February 26, 1979. The next total solar eclipse will be visible from the United States on August 21, 2017. The path of totality includes the states of Oregon, Idaho, Wyoming, Nebraska, Missouri, Tennessee, North Carolina, and South Carolina.

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