Earth, Moon and Sun:
Eclipses are due to a remarkable accident of nature. The sun is about 400 times larger than the moon; the sun is also about 400 times further away from the earth. This causes the moon and sun to have the same apparent size in our sky. Thus, when the moon passes in front of the sun, it completely blocks out the disk of its much larger heavenly companion. Total solar eclipses occur when the moon is in conjunction—between the earth and the sun. A Lunar eclipse occurs when the moon is in opposition—when the earth is between the moon and the sun. Eclipses, therefore, can only occur at new or full moon.
But there is another requirement. The sun, moon, and earth must be perfectly aligned. This happens when the moon is at a "node". This is the point where the moon crosses the earth's orbital plane. At other times, the moon is just above or just below the sun at the moment of the new lunar month, which is why we do not experience an eclipse every month. The node positions are not fixed and move by 1.5 degrees, returning to the same position every 18 years, 11.3 days. This cycle is called the "saros". This means that the eclipse of June 21, 2001 occurred under the same celestial conditions as the one on June 11, 1983. However, not at the same place on earth. Each saros touches ground further to the west due to the earth's rotation around its axis.
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Other conditions change the length and character of an eclipse. Whether the moon is at perigee (closest point in its orbit around the earth) or apogee (furthest from us) adds or detracts from the length of the eclipse by creating a larger or smaller diameter of umbra. In addition, the shadow is moving in the same direction as the earth so totality lasts longer, with a maximum of 7.5 minutes at the equator where rotational speed of the earth is greatest. An annular eclipse occurs when the moon is at apogee, making its apparent size smaller than that of the sun.
Like a line drawn across the surface of a ball, first contact occurs at sunrise. The umbra rushes towards the east at 1 km per second, with the greatest eclipse viewable where the sun is at its zenith. It ends with a black sunset, and then the moon's shadow and will have passed off the face of the planet--all in just a few hours.
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Eclipses in History:
Ancient eclipses had very powerful effects on the history of the human race. It was traditional seen as a bad omen. The word "eclipse" is derived from the Greek, "ekleipsis", which means "omission" or "abandonment". In Chinese it is "shih", which means, "to eat up".
The solar eclipse of June 15 763 bc figures in the biblical writings of Amos when he preached to the people of Nineveh. The eclipse of 413 bc terrified the Athenian army and they were defeated by the Syracusians.
In 585 bc, an unexpected eclipse ended the war between Lydia and Medes. Christopher Columbus used his foreknowledge of an eclipse of the moon on 29 of February 1504 to receive the aid of the Jamaicans by announcing that his G-d would remove the moon unless they gave him all he desired.
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| Lydia and Medes | Christopher Columbus |
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However, eclipses are wonderful news for astronomers. It has allowed us to prove Einstein's theory of general relativity. His theory predicts that when the light of distant star passes near the surface of the sun, it's path will be bent as if it were felling the effect of the sun's gravity. The positions of these stars can be predicted very precisely. Only during an eclipse can we see the effect of the sun's gravitational pull of those stars by noting the positions of those that should have been eclipsed. During the eclipse of 1919 Einstein's theory was proven correct.
