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04-02-2006, 12:03 PM
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Photo taken during the 1999 eclipse.A solar eclipse occurs when the Moon passes between the Earth and Sun and the Moon obscures Earth's view of the Sun totally or partially. This configuration can only occur at New Moon, when the Sun and Moon are in conjunction, as seen from Earth. A total solar eclipse is considered by many to be the most spectacular natural phenomenon that one can observe.
Types of solar eclipses
There are four types of solar eclipses:
A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark outline of the Moon, and the much fainter corona is visible (see image above). During any one eclipse, totality is visible only from, at most, a narrow track on the surface of the Earth.
An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring surrounding the outline of the Moon.
A hybrid eclipse is intermediate between a total and annular eclipse. At some points on the Earth it is visible as a total eclipse; whereas at others it is annular. The generic term for a total, annular or hybrid eclipse is a central eclipse.
A partial eclipse occurs when the Sun and Moon are not exactly in line, and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of a central eclipse. However, some eclipses can only be seen as a partial eclipse, because the central line never intersects the Earth's surface.
The reason some solar eclipses are total and others are annular has to do with the elliptical nature of the Moon's orbit around Earth. One of the most remarkable coincidences in nature is that (i) the Sun lies about 400 times as far from Earth as does the Moon, and (ii) the Sun is also about 400 times the diameter of the Moon. As seen from Earth, therefore, the Sun and the Moon appear to be about the same size in the sky - about 1/2 of a degree in angular measure. Because the Moon's orbit around Earth is an ellipse rather than a circle, however, at some times during the month the Moon is further away, and at other times it is closer to Earth, than average.
When a solar eclipse occurs while the Moon is at its closest (near its perigee), it appears large enough to cover the bright disk, or photosphere, of the Sun completely, and a total eclipse occurs. When it is at its farthest, however (near apogee), it appears smaller, and it cannot cover the Sun completely. In that case, at the time of greatest eclipse there remains a thin annulus (or ring) of brilliant Sun left uncovered. Hence the term annular eclipse. Slightly more annular eclipses than total eclipses occur, because on average the Moon lies too far away from Earth to cover the Sun completely.
Terminology
The term eclipse is actually a misnomer: The phenomenon of the Moon passing in front of the Sun is actually an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. When the Moon disappears at Full Moon by passing into Earth's shadow, the event is properly called an eclipse, but when the Moon passes in front of the Sun, we see an occultation of the Sun by the Moon.
Eye damage mechanism
Glancing at the sun with all or most of its disc visible is unlikely to result in permanent harm, as the pupil will close down and reduce the brightness of the whole scene. If the eclipse is near total, the low average amount of light causes the pupil to open. Unfortunately the remaining parts of the sun are still just as bright, so are now brighter on the retina than when looking at a full sun. As the eye has small "sweet spot", or fovea, for detailed viewing, the tendency will be to track the image on to just this best part of the retina, causing damage.
Frequency of solar eclipses
The Moon's orbit intersects with the ecliptic at the two nodes that are 180 degrees apart. Therefore, the New Moon occurs close to the nodes at two periods of the year approximately six months apart, and there will always be at least one solar eclipse during these periods. Sometimes the New Moon occurs close enough to a node during two consecutive months. This means that in any given year, there will always be at least two solar eclipses, and there can be as many as five. However, some are visible only as partial eclipses, because the umbra passes either above or below the earth, and others are central only in remote regions of the arctic or antarctic.
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Photo taken during the 1999 eclipse.A solar eclipse occurs when the Moon passes between the Earth and Sun and the Moon obscures Earth's view of the Sun totally or partially. This configuration can only occur at New Moon, when the Sun and Moon are in conjunction, as seen from Earth. A total solar eclipse is considered by many to be the most spectacular natural phenomenon that one can observe.
Types of solar eclipses
There are four types of solar eclipses:
A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark outline of the Moon, and the much fainter corona is visible (see image above). During any one eclipse, totality is visible only from, at most, a narrow track on the surface of the Earth.
An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring surrounding the outline of the Moon.
A hybrid eclipse is intermediate between a total and annular eclipse. At some points on the Earth it is visible as a total eclipse; whereas at others it is annular. The generic term for a total, annular or hybrid eclipse is a central eclipse.
A partial eclipse occurs when the Sun and Moon are not exactly in line, and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of a central eclipse. However, some eclipses can only be seen as a partial eclipse, because the central line never intersects the Earth's surface.
The reason some solar eclipses are total and others are annular has to do with the elliptical nature of the Moon's orbit around Earth. One of the most remarkable coincidences in nature is that (i) the Sun lies about 400 times as far from Earth as does the Moon, and (ii) the Sun is also about 400 times the diameter of the Moon. As seen from Earth, therefore, the Sun and the Moon appear to be about the same size in the sky - about 1/2 of a degree in angular measure. Because the Moon's orbit around Earth is an ellipse rather than a circle, however, at some times during the month the Moon is further away, and at other times it is closer to Earth, than average.
When a solar eclipse occurs while the Moon is at its closest (near its perigee), it appears large enough to cover the bright disk, or photosphere, of the Sun completely, and a total eclipse occurs. When it is at its farthest, however (near apogee), it appears smaller, and it cannot cover the Sun completely. In that case, at the time of greatest eclipse there remains a thin annulus (or ring) of brilliant Sun left uncovered. Hence the term annular eclipse. Slightly more annular eclipses than total eclipses occur, because on average the Moon lies too far away from Earth to cover the Sun completely.
Terminology
The term eclipse is actually a misnomer: The phenomenon of the Moon passing in front of the Sun is actually an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. When the Moon disappears at Full Moon by passing into Earth's shadow, the event is properly called an eclipse, but when the Moon passes in front of the Sun, we see an occultation of the Sun by the Moon.
Eye damage mechanism
Glancing at the sun with all or most of its disc visible is unlikely to result in permanent harm, as the pupil will close down and reduce the brightness of the whole scene. If the eclipse is near total, the low average amount of light causes the pupil to open. Unfortunately the remaining parts of the sun are still just as bright, so are now brighter on the retina than when looking at a full sun. As the eye has small "sweet spot", or fovea, for detailed viewing, the tendency will be to track the image on to just this best part of the retina, causing damage.
Frequency of solar eclipses
The Moon's orbit intersects with the ecliptic at the two nodes that are 180 degrees apart. Therefore, the New Moon occurs close to the nodes at two periods of the year approximately six months apart, and there will always be at least one solar eclipse during these periods. Sometimes the New Moon occurs close enough to a node during two consecutive months. This means that in any given year, there will always be at least two solar eclipses, and there can be as many as five. However, some are visible only as partial eclipses, because the umbra passes either above or below the earth, and others are central only in remote regions of the arctic or antarctic.