why eclipses happen ？

why eclipses happen ？迎接08年日食! A total solar eclipse—when the moon passes in front of the sun and blocks it completely—is an amazing sight. To see a total solar eclipse, you have to be in just the right spot on the earth. When you look up in the sky at the sun and the moon, you notice a strange coincidence—both look the same size in the sky. Both the sun and the moon look about one-half degree in diameter. Now, they're not really the same size. The sun's diameter is actually 400 times the moon's diameter. But, you must also take into account that the sun is also 400 times further away from the earth, reducing its apparent size to the same as the moon's. Because of this relationship, when you are standing on the earth, looking up at the two, you must be in a very limited zone to see the moon cover the entire face of the sun. If you were to move a little north, the sun would peek out over the top of the moon; a little south, and the sun shines past the southern limb of the moon. The match is so good that the "path of totality" is never more than 167 miles in diameter, and is usually less. This means that very few people have seen a total eclipse because the shadow only covers a very small area on the earth.  
[size=-1]This diagram (wildly out of scale) shows a side view of the alignment. From anywhere in the grey penumbra, you will see some part of the sun shining from behind the moon. The penumbra is the area of partial eclipse. Only from within the tiny area where the dark umbra touches the earth will you see the sun completely covered and witness a total eclipse.
[size=-1]The earth and the moon are not fixed objects. The moon is busy orbiting the earth. The earth is busy orbiting the sun and additionally rotating on its axis. This means that the spot on the earth where the umbra falls is always in motion and actually traces out a path.
  

[size=-1]This diagram shows the path of the umbra for an eclipse on December 4, 2002. Only the central blue lines mark out the path of the umbra. The much wider area shows the path of the larger penumbra, where a partial eclipse can be seen. [size=-1]Click here[size=-1] for a closer map of the umbral path over Africa.

[size=-1]The shadow first touches down in the Atlantic Ocean east of South America. It travels eastward and first sees land on the west coast of Angola. The shadow proceeds east through central Angola, then progresses through Zimbabwe and Mozambique. The shadow leaves the African continent and crosses the southern tip of Madagascar near sunset.
Why Eclipses Happen Continued

[size=-1]Seen from the moon, the moon's shadow looked like this when we were in total eclipse in Zambia, June 21, 2001. The large grey [size=-1]circle is the penumbra from which only a partial eclipse will be seen; the small circle in the center is the tiny umbra from which the total eclipse will be seen.
[size=-1]See a Quicktime movie of the moon's shadow on the earth as it traces out this eclipse:
View of the whole globe 1.9MB Close up of Africa 2.6 MB [size=-1](Note that the earth is rotating and that the stars seem to move behind the earth because of the moon's revolution in its orbit.) This is seen as if viewed from the moon. Just by coincidencen the beautiful Trifid Nebula is in our view!


[size=-1]Total eclipses seem to happen infrequently. Why doesn't the moon get between the sun and the earth every month at new moon and produce an eclipse? Because, I've over-simplified matters. The real situation is a little more complicated. We need to discuss the orbit of the earth around the sun and the orbit of the moon around the earth. The orbits of both are not circles, but rather slightly oval-shaped ellipses. Also, these orbits do not lie parallel to each other in the same plane.
[size=-1]As the earth orbits the sun, taking one year to complete one circuit, it appears to us on earth that the sun moves around our sky once against the background of stars. Here's an analogy--If you walk around a campfire (the sun) looking at your friends on the other side (the stars), to you it would look like the campfire moves past your friends. Likewise, from earth, it looks like the sun moves against the background of stars, making one circuit of the sky in one year.
[size=-1]If the sun could draw a line as it moved against the stars, we would see a great circle called the ecliptic. If we could ask the moon to also draw a line in the sky as it orbited the earth, we'd notice that the two lines would be close to each other, but the moon's path is tilted about 5 degrees to the path of the sun.
[size=-1]This is why the moon doesn't eclipse the sun every month. Most of the time, the moon passes over or under the sun. An eclipse can happen only when both the sun and the moon arrive near one of the crossing points (these are called nodes). There are two of these nodes on opposite sides of the sky, one where the moon crosses from south to north, and one where the moon passes from north to south. Since there are two crossing points in the sky, eclipses happen during two "eclipse seasons" separated by about six months.
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[size=-1]The sun does not have to be exactly on the node when the moon arrives there, only close enough for the moon to block some portion of the sun. This leaves a "window" of about 18.75 days before and after the sun gets to the nodes. During this 37.5-day period, the moon can cause an eclipse. Since the moon takes 29.5 days to go from new moon to new moon, this means that an eclipse of some kind is guaranteed about every six months.

[size=-1]The type of eclipse that does occur depends on several things. First, if the eclipse happens when the sun is further from the node, it is more likely that the eclipse will be a partial one. In this type of eclipse, the dark umbra passes above the North Pole or below the South Pole, never touching the earth. All we ever see is part of the sun covered.

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[size=-1]There's another variable, though. Remember that the orbits of the earth and moon are not perfect circles, but rather ellipses. Note that in the diagram above (again wildly out of scale), the earth is sometimes closer to the sun and sometimes further. The same is true for the moon--sometimes it's closer to the earth and sometimes it's further. See the table below:
Close Distance Far Distance Sun 147,101,455 km 152,098,155 km Moon 356,749 km 406,282 km

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