Geoscience:Ebb and Flow
The earth and the moon and the sun have mutual gravitational attraction. And the fact that the waters of the earth in some sense are loose on the earth， they can move around — the small differences in the gravitational forces， because of the relative motion of the earth， the moon and the sun， can make those waters shift， and the results are the twice and once per day ups and downs of the tides.
- Added：2006
- Country：Canada
- Duration：30
- Time of update：2011/8/16 17:21:27