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Why does the moon drift away from earth?

It seems to me that, due to conservation of energy, the moon would drift away from the Earth if humans began extracting large amounts of energy from the ocean due to the shift in water levels due to the tides. This would occur to decrease the gravitational energy in the system. Some have said that it would cause the rotation of the moon to slow down, but that doesn't seem right to me.

Edit: To make it certain that something must happen, assume that the energy extracted from the tides is 100% radiated outside of the closed system that includes all volume surrounding the moon and the earth. The inside of that system must lose energy somehow.

Edit: If your answer involves the rotation of the moon and/or earth, what would happen if neither were rotating?

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This is very nearly a duplicate of Are tidal power plants slowing down Earth's rotation?, though approaching the problem from the other side. See the "Related" sidebar from some other Earth--Moon/tidal effect questions. –  dmckee Aug 16 '12 at 17:51
    
Possible duplicate: physics.stackexchange.com/q/9290/2451 –  Qmechanic Aug 16 '12 at 18:06
    
Thank you. My second edit doesn't make sense unless you are considering forces due to the moon's orbit around the earth which are much smaller but theoretically there. –  barry barrett Aug 16 '12 at 18:25
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marked as duplicate by Qmechanic, Manishearth Dec 11 '12 at 11:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1 Answer

The answer is, I believe, less obvious than it first appears. The first thing to realise is that the moon already is moving away from the Earth for exactly this reason. The moon moves water around the Earth. This generates kinetic energy, which is dissipated into heat and eventually radiates away, and the overall effect is to make the moon move away from the Earth. (Actually I suspect that most of this effect is associated with squishing the Earth's rocky interior than moving the oceans around, but I'm not sure of the numbers. The oceans surely play a role though.)

The question then becomes, what happens if, instead of being dissipated by friction in the oceans, the energy generated by the tides is first turned into electricity by humans and then turned into heat?

A naïve first answer to this new question would be something along the lines of "nothing - there's a set amount of energy being generated, and it doesn't matter whether it's used up by humans or by tidal friction, so the moon will continue to recede from the Earth at the same rate that it already does". However, this doesn't take account of feedbacks. If you extract that much energy from the tides you're bound to change their behaviour in some way. I would guess that the tides would become shallower, resulting in less energy being turned to heat overall, meaning that the moon's rate of recession from the Earth would slow down slightly. But it isn't obvious, at least to me, and I suspect it's a problem that would require some fancy fluid dynamics in order to solve.

The question about rotation can be answered by considering the angular momentum of the Earth-moon system. If the rotation speeds of the moon and the Earth stayed constant, the system would gain angular momentum as the moon moved further away. But the total angular momentum of the Earth-moon system cannot change, so the Earth's rotation slows down in order to counteract this. The moon's rotation has already slowed down as much as it can do - this is why it always shows the same face to the Earth. In theory the Earth will eventually become locked to the moon in the same way, but I've no idea if this will happen before or after the sun expands and destroys them both.

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Thank you. I need to make an account to accept this answer. –  barry barrett Aug 16 '12 at 18:27
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