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A question was posed at work today, "if the earth stopped spinning what would happen to the moon"? Ignoring any effects on the Earth, what actually happens to the Moon? Does it continue to orbit the now non-rotating Earth, or does it fly off into space like a massive asteroid, or more likely some other thing that I can't conceive of?

Note, the Earth stops spinning, not orbiting the sun.

For additional curiosity, what would be the differences if this were an extremely gradual slowing vs an almost instantaneous stoppage?

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  • $\begingroup$ What do you plan to do with Earth's rotational momentum (energy)? $\endgroup$
    – Mick
    Aug 6, 2019 at 18:41
  • $\begingroup$ @Mick, does it have any factor in what happens to the Moon? If not, ignore it for this purpose, if it does...then I have no idea, I'd imagine that would be an interesting facet of an answer. $\endgroup$
    – gilliduck
    Aug 6, 2019 at 18:51
  • $\begingroup$ Well, the rotational energy has to go somewhere. You can't just magic it away. $\endgroup$
    – Mick
    Aug 6, 2019 at 19:00
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    $\begingroup$ @Mick well, you certainly can for a Gedanken experiment, and that's what is being asked here. $\endgroup$
    – Carl Witthoft
    Aug 7, 2019 at 18:50
  • $\begingroup$ @CarlWitthoft thanks for the new word of the day! $\endgroup$
    – gilliduck
    Aug 7, 2019 at 19:30

2 Answers 2

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Earth's current rotation is faster (~1 day) than the Moon's orbital period around Earth (~28 days). This leads to tidal acceleration -- the tidal bulges raised by the Moon rotate ahead, pulling on (and being pulled by) the Moon. Over a long period, this converts Earth's rotational momentum into Lunar orbital momentum. The Earth's rotation slows, and the Moon's orbit expands. Eventually, if nothing else interfered, Earth and Moon would become tidally locked, with the length of a day and the length of a month equal.

If Earth weren't rotating, this effect would happen in reverse. Tidal acceleration (deceleration?) would transform Lunar orbital momentum into Earth rotational momentum. The Earth would slowly start to rotate again, and the Moon's orbit would contract, again until the two were tidally locked.

I'm not in a position to run the numbers, but it should be possible to estimate the final configuration -- how close the Moon would be, and how long the day/month would be.

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  • $\begingroup$ But in the meantime, I shudder to think of the 6-month day or night we all would experience (yeah I know it's not exactly that due to orbit's ellipticity) $\endgroup$
    – Carl Witthoft
    Aug 7, 2019 at 18:52
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There is not the slightest chance of the Earth stopping spinning, but if it did there would be hardly any effect on the moon. It would continue to orbit just as it has always done. You ask about a very gradual slowing ,and what the result of that would be. We are on very firm ground here, because there has already been a gradual slowing and we know exactly what the results have been.

The tidal interaction of the moon in its orbit around the Earth has considerably slowed the Earth's rotation over the last 4 billion years, and this rotational energy was captured by the moon and has boosted it into a higher orbit. The moon is still moving away from us at the rate of several centimetres per year. Presumably, if the Earth stopped rotating there would be no rotational energy for the moon to capture, but that is not going to happen.

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  • $\begingroup$ "No rotational energy for the moon to capture" -- but from the Moon's perpective, there would be retrograde rotation, which would lead to a reverse transfer of energy, right? See my answer below... $\endgroup$
    – jeffB
    Aug 7, 2019 at 14:59

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