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Name: David C.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2002

where is the center of gravity for the earth-moon system, what effect does this have on the orbit of the earth around the sun?

The center of gravity of the earth-moon system is inside the earth on the line between the centers of the two bodies. This center of mass proceeds around the sun in an elliptical orbit.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

These web sites offer some insight into what appears to be a simple question. In the crudest approximately the center of gravity of the moon/earth system is about 2900 km or about 75% of the radius from the center of the earth. However, things are much more complex than that.

The sun cannot be neglected; in fact it has a large effect on tides. So it is the sun/moon/earth system that must be taken into account. In addition, there are at least 10 more drags, wobbles, various wiggles caused by many sources including the planets, the tidal drag of the oceans, and the fact that the center of earth rotation changes.... It is all very complicated and still there are small earth/sun/moon motions for which there is no good explanation as yet.

Vince Calder

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