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Name: M Negus
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Question:
What determines the present rotation speed of the Earth? What started the planets spinning? Why is the Earth subject to a wobble? Since the mass of the Earth is changing...why not it is rotation? Does mass/density/physical size control rotation on axis? Why do planets spin on their axes?



Replies:
Lots of good questions. Planets continue to rotate mostly because there are very few forces that act to slow them down. No one actually knows why the planets got started spinning. Most think the material that condensed was originally spinning and the spin was not much changed during formation of the planets. Wobble will occur for any object that is not exactly a sphere. It has to do with the natural way that any solid body moves in space if it is not spherical. As an example try to spin a tennis racket about each of its three axes and you will find some wobble about some axes. The rotation of a planet is only changed by forces that act perpendicular to the rotation axis (actually called torques) for a planet in space there are not many of these. These answers are brief because of space (here). Your questions are good.

Samuel P Bowen


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