Eclipse
Eclipse
Eclipse, in astronomy, the obscuring
of one celestial body by another, particularly that of the sun or a planetary
satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar
eclipses; and those of the sun, or solar eclipses . A lunar eclipse occurs when
the earth is between the sun and the moon and its shadow darkens the moon. A
solar eclipse occurs when the moon is between the sun and the earth and its
shadow moves across the face of the earth. Transits and occultations are
similar astronomical phenomena but are not as spectacular as eclipses because
of the small size of these bodies as seen from earth (see Transit).
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LUNAR ECLIPSES
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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 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 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.
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SOLAR ECLIPSES
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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/h
(about 1060 mph); near the poles, where the speed of rotation is virtually
zero, it is about 3380 km/h (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
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. 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.
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FREQUENCY OF ECLIPSES
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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 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
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 eclipse will be visible from the U.S. in 2017.
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OBSERVATION OF ECLIPSES
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Total Solar Eclipse
During a solar eclipse, the moon moves
between the sun and the earth. The light from the outer part of the sun’s
atmosphere, called the corona, became visible during a total solar eclipse on
July 11, 1991, in La Paz, Baja California, Mexico. The moon’s shadow on earth
appeared only as a thin band not more than 269 km (167 mi) wide.
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 (see Relativity).
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.
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