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Moon's orbit and tidal locking

by DHF
Tags: moon, orbit, tidal lock
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May20-13, 03:02 PM
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P: 66
The Moon is close enough to Earth that it is tidally locked, however it is also slowly but steadily moving into a wider orbit around Earth. Will there be a point in tile when the Moon is no longer locked and able to freely rotate and if so how far apart would that need to be?

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May20-13, 03:30 PM
P: 742
Hi Don,

No that won't happen. Once locked, a body stays like that forever, unless some external force acts on it and supplies some extra angular momentum. In a two-body system like that of the Moon-Earth any such interaction is pretty unlikely(think huge asteroid impact, or rogue planet/star passing close by).

What will happen, given enough time, is that the Earth's rotation will get tidally locked to the Moon's revolution, with both bodies eventually presenting always the same sides to each other.
At that point in time, the Moon will also stop moving away in its orbit, as this process is simply the other side of the same coin. The same forces that cause Moon to move away cause Earth to slow down in its rotation. It has to be like that, else the angular momentum is not conserved.

In other words, Moon-Earth will end up like Charon-Pluto.
May20-13, 03:34 PM
DHF's Avatar
P: 66
Thank you very much for that very well explained answer.


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