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It is my understanding that the tidal forces of the Moon acting on Earth cause it to slow down its rotation and, because angular momentum is conserved, the Moon's orbit subsequently expands. This continues until the Earth's rotation is synchronous with the Moon's orbit leaving both bodies tidally locked to each other.

However once that happens, how will the tidal forces exerted by the Sun on Earth (which are weaker than the Moon's) affect the Earth's rotation? Would they cause the Earth to start rotating again and thus cause the Moon's orbit to shrink?

P.S. I do realize the Earth is not destined to be tidally locked to the Moon until after the Sun evolves into a red giant, so for the sake of simplicity assume the Earth-Moon system survives unscathed without getting disturbed.

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  • $\begingroup$ Regarding the PS, you are asking us to scientifically explain something that cannot scientifically happen. That doesn't make sense. Voting to close. $\endgroup$ – David Hammen May 20 '20 at 13:04
  • $\begingroup$ I’m voting to close this question because the question asks us to use science to explain something that science says will not happen. $\endgroup$ – David Hammen May 20 '20 at 13:05
  • $\begingroup$ Although hypothetical scenarios are typically off-topic for this site, this seems like an answer that could get interesting answers. The evolution of the Sun into a red giant has no impact on the tidal locking of the Earth and the Moon. So tidal locking can be considered independently from the Sun's evolution. $\endgroup$ – usernumber May 20 '20 at 13:34
  • $\begingroup$ All problems in physics make some simplifying assumptions. This one doesn't seem exuberant, so I vote to leave open. $\endgroup$ – usernumber May 20 '20 at 13:36
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    $\begingroup$ @usernumber - One key reason that the Earth will never become tidally locked to the Moon is that the Earth's oceans are responsible for almost all of the Earth's rotational deceleration. Thank's to the ever increasing so-called solar constant, the Earth will lose its oceans in about a billion years, well before the Sun turns into a red giant, and well before the Earth could ever become tidally locked to the Moon. $\endgroup$ – David Hammen May 20 '20 at 13:48

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