Solar eclipse

:Solar Eclipse is also an alien friend of the rubber doll Betty Spaghetty.

Eclipse frequency and cause

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Total and annular eclipses both occur when the Moon lines up with the Sun exactly, but since the Moon's orbit is not perfectly circular it is sometimes farther away from Earth and doesn't always cover the entire solar disc from Earth's point of view.

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It is one of the most remarkable coincidences of nature that the Sun lies approximately 400 times as far away from Earth as does the Moon, and the Sun is also approximately 400 times as large in diameter as the Moon. As a result, as seen from Earth, the Sun and the Moon appear to be nearly the same apparent size. The Moon orbits Earth in an elliptical, or elongated orbit, however, and not in a circular orbit. Thus during about 55-60% of its orbit the Moon is far enough from Earth ("apogee") that it is too small to cover the Sun's surface completely. During the remaining portion of its orbit, it is closer to Earth ("perigee") and large enough in apparent size to cover the Sun completely.

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When a solar eclipse occurs near apogee, there is therefore a small ring or annulus of Sun that remains uncovered even at the moment of maxiumum eclipse. This produces an "annular" eclipse, during which the brilliant and blinding uncovered ring of the Sun makes the solar corona invisible. When a solar eclipse occurs near perigee, however, the Moon is close enough to Earth and large enough in the sky that it can cover the entire bright surface (the photosphere) of the Sun completely, and the observer sees a total eclipse, at which time the ghostly white solar corona appears.

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A solar eclipse can only be seen in a band across Earth as the Moon's shadow moves across its surface, while a total or annular eclipse is actually total or ring-formed in only a small band within this band (the eclipse path), and partial elsewhere (total eclipse takes place where the umbra of the Moon's shadow falls, whereas a partial eclipse is visible where the penumbra falls). The full band is generally around 100 km in width. The eclipse path will be widest if the Moon happens to be at perigee, in which case the eclipse path alone can reach 270 km in width.

Related Topics:
Umbra - Penumbra - Perigee

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Total solar eclipses are rare events. Although they occur somewhere on Earth approximately every 18 months, it has been estimated that they recur at any given spot only every 300 to 400 years. And after waiting so long, the total solar eclipse only lasts for a few minutes, as the Moon's umbra moves eastward at over 1700 km/h. Totality can never last more than 7 min 40 s, and is usually a good deal shorter. During each millennium there are typically fewer than 10 total solar eclipses exceeding 7 minutes. The last time this happened was June 30, 1973. Those alive today probably won't live to see it happen again — on June 25, 2150. The longest total solar eclipse during the 8,000-year period from 3000 BC to 5000 AD will occur on July 16, 2186, when totality will last 7 min 29 s. (eclipse predictions by Fred Espenak, NASA/GSFC.)

Related Topics:
Millennium - June 30 - 1973 - June 25 - 2150 - July 16 - 2186

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For astronomers, a total solar eclipse forms a rare opportunity to observe the corona (the outer layer of the Sun's atmosphere). Normally this is not visible because the photosphere is much brighter than the corona.

Related Topics:
Astronomers - Corona - Photosphere

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