Eclipse
| I | INTRODUCTION |
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).
| II | LUNAR ECLIPSES |
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.
| 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 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.
| IV | FREQUENCY OF ECLIPSES |
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.
| V | OBSERVATION OF ECLIPSES |
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.
See also Astronomy.
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