<p><strong>Not much would happen to the Earth, but the lunar interior would heat up a bit.</strong></p>
<p><strong>The backstory:</strong> Both the Moon and the Earth exert tidal forces on each other, deforming their shapes. But the deformation has a time lag, so it is not perfectly lined up along the Earth-Moon line.</p>
<p>Long ago, the Moon rotated faster than its orbital period, and it was closer to the Earth. Gravity acting on the time-lagged tidal deformations enables the two bodies to share angular momentum. The Earth and the Moon both rotated slower and slower, and the Moon moved outward into longer and longer orbital periods. Eventually the Moon became tidally locked, and as it continues to move outward, it will continue to rotate slower and slower, maintaining the spin-orbit resonance.</p>
<p><a href="https://ase.tufts.edu/cosmos/view_picture.asp?id=1337" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Ukdx2.jpg" alt="enter image description here" /></a></p>
<p><strong>Your scenario:</strong> The Moon's new spin period would create a time-lagged tidal bulge, and the Earth's gravity would pull on that bulge until the spin-orbit resonance was restored. But the tidal forces in the Moon's interior would not be uniformly distributed, creating some friction and producing some heat. I don't have a good intuition about the amount of heat, and it would of course depend on the Moon's new rotation period.</p>
<p>The diagram above from <a href="https://ase.tufts.edu/cosmos/view_picture.asp?id=1337" rel="nofollow noreferrer">Tufts</a> is meant to explain why the Earth's rotation gradually slows, but the exact same forces would act on a Moon whose spin period is shorter than its orbital period.</p>

CC BY-SA 4.0
2022-01-24T05:37:24.283Z
2022-01-24T05:37:24.283Z
giardia
<p>While the side effects of whatever caused the moon to start rotating could be dramatic, there would be little direct consequence of the moon's rotation.</p>
<p>Newtonian gravity acts on particles and the rotation of a spherically symmetric body has no effect on its gravitational field. So the moon would stay in the same place, tides would happen as normal and the phases of the moon would be unaffected. Relativistic effects such as frame-dragging would be too small to detect.</p>
<p>Over time, the moon would be recaptured into a tidally locked rotation. This would take relatively little time, on a geological scale.</p>
<p>Of course, to change the moon's rotation by a significant amount would require a massive amount of energy. There's not much that could quickly get the moon to change rotation that would not have pretty serious side effects. on the moon and on the Earth too, but these would not be a direct effect of the moon rotating.</p>
<p>And as commenters have pointed out, the moon already does rotate, once per month, but I think we know what you mean: rotate faster (or slower) so that tidal locking is broken</p>

CC BY-SA 4.0
2022-01-23T22:16:27.043Z
Angular momentum in the Earth-Moon will be conserved, so should be addressed.
uhoh
CC BY-SA 4.0
2022-01-24T00:40:55.717Z
@uhoh: Surely whatever caused the moon to start rotating would supply its own angular momentum? Like, I don't know, big friggin rockets positioned around the moon's equator?
TonyK
CC BY-SA 4.0
2022-01-25T02:09:44.540Z
@TonyK I’m talking about “what might happen to Earth” after that. Over time “little direct consequence”’will become wronger and wronger. It’s what happens when an opinion is cast in the form of an unsupported Stack Exchange answer.
uhoh
CC BY-SA 4.0
2022-01-23T20:33:10.627Z
2022-01-23T20:33:10.627Z
James K
<p>Hypothetically speaking, what might happen to Earth if one day the moon became un-tidal locked and started to rotate?</p>

CC BY-SA 4.0
2022-01-23T13:43:44.760Z
Nothing at all.
TonyK
CC BY-SA 4.0
2022-01-23T14:41:56.923Z
Minor comment: the tidally-locked Moon rotates once per orbit.
rob
CC BY-SA 4.0
2022-01-23T14:56:39.090Z
Link-only answer: it would just lock again. See [this Physics post](https://physics.stackexchange.com/a/659775/44126), which is mostly about the tidal locking of the primary.
rob
CC BY-SA 4.0
2022-01-23T20:02:22.500Z
There is a fundamental misconception in your question: The moon does rotate already, it just rotates on its axis once in the same time it takes to orbit around Earth once.
Stuart Robbins
CC BY-SA 4.0
2022-01-24T00:38:48.990Z
@StuartRobbins, rob: don't be silly. We all know what she means.
TonyK
CC BY-SA 4.0
2022-01-24T16:02:43.117Z
What would happen if the moon stopped rotating?
sno
CC BY-SA 4.0
2022-01-25T01:52:28.560Z
@TonyK I know what she means. I work with a lot of undergraduates and general public who think the Moon does not rotate. It is not clear from the question that she knows the Moon rotates, in fact the phrasing of the question would imply she does not.
Stuart Robbins
CC BY-SA 4.0
2022-01-25T15:36:50.293Z
How would you like to me to professionally rephrase this thought in my mind if you will…
StarGirl6789
CC BY-SA 4.0
2022-01-25T15:47:47.700Z
Let’s Suppose if The Moon could rotate in position while it spins and dances alongside Earth around and around completing the yearly orbit,   and thus you see the “Dark or Farside of the moon” that it could possibly change the way people think?
StarGirl6789
<the-moon>
What if Earth’s moon started rotating?
StarGirl6789
# What if Earth’s moon started rotating?

(Asked by: username_0 on 2022-01-23)

Hypothetically speaking, what might happen to Earth if one day the moon became un-tidal locked and started to rotate?

**Tags:** the-moon

> **Comments:**
> **username_1**: Nothing at all.  
> **username_2**: Minor comment: the tidally-locked Moon rotates once per orbit.  
> **username_2**: Link-only answer: it would just lock again. See [this Physics post](https://physics.stackexchange.com/a/659775/44126), which is mostly about the tidal locking of the primary.  
> **username_3**: There is a fundamental misconception in your question: The moon does rotate already, it just rotates on its axis once in the same time it takes to orbit around Earth once.  
> **username_1**: @StuartRobbins, username_2: don't be silly. We all know what she means.  
> **username_4**: What would happen if the moon stopped rotating?  
> **username_3**: @username_1 I know what she means. I work with a lot of undergraduates and general public who think the Moon does not rotate. It is not clear from the question that she knows the Moon rotates, in fact the phrasing of the question would imply she does not.  
> **username_0**: How would you like to me to professionally rephrase this thought in my mind if you will…  
> **username_0**: Let’s Suppose if The Moon could rotate in position while it spins and dances alongside Earth around and around completing the yearly orbit,   and thus you see the “Dark or Farside of the moon” that it could possibly change the way people think?  

---

## 2 answers

### Answer by username_5 on 2022-01-24. Score: 4

**Not much would happen to the Earth, but the lunar interior would heat up a bit.**

**The backstory:** Both the Moon and the Earth exert tidal forces on each other, deforming their shapes. But the deformation has a time lag, so it is not perfectly lined up along the Earth-Moon line.

Long ago, the Moon rotated faster than its orbital period, and it was closer to the Earth. Gravity acting on the time-lagged tidal deformations enables the two bodies to share angular momentum. The Earth and the Moon both rotated slower and slower, and the Moon moved outward into longer and longer orbital periods. Eventually the Moon became tidally locked, and as it continues to move outward, it will continue to rotate slower and slower, maintaining the spin-orbit resonance.

[![enter image description here](https://i.sstatic.net/Ukdx2.jpg)](https://ase.tufts.edu/cosmos/view_picture.asp?id=1337)

**Your scenario:** The Moon's new spin period would create a time-lagged tidal bulge, and the Earth's gravity would pull on that bulge until the spin-orbit resonance was restored. But the tidal forces in the Moon's interior would not be uniformly distributed, creating some friction and producing some heat. I don't have a good intuition about the amount of heat, and it would of course depend on the Moon's new rotation period.

The diagram above from [Tufts](https://ase.tufts.edu/cosmos/view_picture.asp?id=1337) is meant to explain why the Earth's rotation gradually slows, but the exact same forces would act on a Moon whose spin period is shorter than its orbital period.


---

### Answer by username_6 on 2022-01-23. Score: 1

While the side effects of whatever caused the moon to start rotating could be dramatic, there would be little direct consequence of the moon's rotation.

Newtonian gravity acts on particles and the rotation of a spherically symmetric body has no effect on its gravitational field. So the moon would stay in the same place, tides would happen as normal and the phases of the moon would be unaffected. Relativistic effects such as frame-dragging would be too small to detect.

Over time, the moon would be recaptured into a tidally locked rotation. This would take relatively little time, on a geological scale.

Of course, to change the moon's rotation by a significant amount would require a massive amount of energy. There's not much that could quickly get the moon to change rotation that would not have pretty serious side effects. on the moon and on the Earth too, but these would not be a direct effect of the moon rotating.

And as commenters have pointed out, the moon already does rotate, once per month, but I think we know what you mean: rotate faster (or slower) so that tidal locking is broken

> **Comments:**
> **username_7**: Angular momentum in the Earth-Moon will be conserved, so should be addressed.  
> **username_1**: @username_7: Surely whatever caused the moon to start rotating would supply its own angular momentum? Like, I don't know, big friggin rockets positioned around the moon's equator?  
> **username_7**: @username_1 I’m talking about “what might happen to Earth” after that. Over time “little direct consequence”’will become wronger and wronger. It’s what happens when an opinion is cast in the form of an unsupported Stack Exchange answer.  

---

