Solar eclipse

A solar eclipse is an eclipse occurring when the Sun, Moon and Earth are on a single line with the Moon in the middle. Seen from the Earth, the Moon is in front of the Sun and thus part or all of the light of the Sun is hidden by the Moon. Thus it may seem that a piece has been taken out of the Sun, or that it has suddenly disappeared. There are three types of solar eclipses:

• A partial solar eclipse: Only part of the Sun and the Moon overlap
• A total solar eclipse: All of the Sun is hidden by the Moon
• A ring-formed or annular eclipse: Sometimes the Moon's umbra does not reach the Earth's surface. This can happen if the Moon is near apogee. This is similar to a penumbral eclipse. During an annular eclipse, a ring of the Sun can still be seen around the Moon. The Moon's shadow umbra often fails to reach the Earth.

Warning: Never look at the Sun directly, even during an eclipse. To do so can cause retinal damage and affect one's eyesight. Always use protection, such as welder's glasses, that are recommended for the extreme brightness of the sun, or view the image of the Sun indirectly on, say, a piece of paper, using a simple pinhole camera. Sunglasses are not sufficient!

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 an Earthly vantage point.

A solar eclipse can only be seen in a band across the 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 across. 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 diameter.

Total solar eclipses are rare events. Although they occur somewhere on the Earth every 18 months or so, it has been estimated that they recur at any given spot only every 300-400 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 per hour. Totality can never last more than 7min 40 sec, but 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. Totality will last 7 min 29 sec. (eclipse predictions by Fred Espenak, NASA/GSFC.)

For astronomers, a total solar eclipse forms a rare opportunity to watch the corona, the outer layer of the Sun. Normally this is not visible because the light of the Sun overshines the corona.

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