# Why doesn't the moon rotate backwards? **URL:** \ **Category:** Factual Questions\ **Created:** [August 16, 2006, 6:36am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918 "2006-08-16T06:36:48Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 16, 2006, 6:36am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/1 "2006-08-16T06:36:48Z")
I (think) I understand why the moon always keeps one side pointed towards the earth. It was not always so but over time tidal drag slowed the moon’s rotation till it became tidally locked and points one side at the earth (essentially one “day” being one orbit on the moon). My understanding is this occurred because the moon is slightly ahead of the tidal bulge it raises on the earth so the bulge, having extra mass and thus extra gravitational pull, caused a darg on the moon’s rotation and slowed it down. Assuming the above is correct (and certainly correct me if I am wrong) then why hasn’t the drag continued slowing the rotation to the point of causing the moon to rotate backwards? Ok…backwards may not be the best description but if it slowed enough it would appear to rotate backwards from the earth wouldn’t it? ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [August 16, 2006, 6:42am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/2 "2006-08-16T06:42:44Z")
You are confusing the mechanics of why the Moon is receeding from the Earth, with the phenomenon of tidal locking. The dynamic forces that cause tidal locking decrease as the period of rotation comes closer to the the period of the orbit. When they are the same, the forces keep it from slowing down more as well. The receeding thing continues on and on, although at a pace that grows ever slower. The arithmatic sucks, though. Tris ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [August 16, 2006, 6:49am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/3 "2006-08-16T06:49:59Z")
In fact, wouldn’t tidal forces have sped the moon’s rotation up, had it been rotating slower than it’s orbital period? ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [August 16, 2006, 6:58am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/4 "2006-08-16T06:58:47Z")
> [@Gorsnak](#): > > In fact, wouldn’t tidal forces have sped the moon’s rotation up, had it been rotating slower than it’s orbital period? Well, yes. In fact, as you ask, I think that might well have been what happened, given the magnitude of the forces involved in the creation of the Moon. Tris ---
**Author:** ![jimbeam](https://avatars.discourse-cdn.com/v4/letter/j/858c86/32.png) [@jimbeam](https://boards.straightdope.com/u/jimbeam)\ **Post date:** [August 16, 2006, 7:00am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/5 "2006-08-16T07:00:00Z")
Tidal friction. [http://curious.astro.cornell.edu/question.php?number=142](http://curious.astro.cornell.edu/question.php?number=142) > [@](#): > > The Earth’s gravity pulls on the closest tidal bulge, trying to keep it aligned with Earth. As the Moon turns, feeling the Earth’s gravity, this creates friction within the Moon, slowing the Moon’s rotation down until its rotation matches its orbital period exactly, a state we call tidal synchronization. In this state, the Moon’s tidal bulge is always aligned with Earth, which means that the Moon always keeps one face toward Earth. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [August 16, 2006, 7:00am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/6 "2006-08-16T07:00:05Z")
No, wait, I changed my mind. It would have to slow down, as it moved farther away, and its orbital period increased. duh. Tris ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 16, 2006, 8:03am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/7 "2006-08-16T08:03:07Z")
> [@Whack-a-Mole](#): > > Assuming the above is correct (and certainly correct me if I am wrong) then why hasn’t the drag continued slowing the rotation to the point of causing the moon to rotate backwards? Ok…backwards may not be the best description but if it slowed enough it would appear to rotate backwards from the earth wouldn’t it? Because the force is _drag_ - friction; it’s working to reduce relative movement between two bodies. It doesn’t tend to spin the moon the other way for the same reason that car brakes don’t make the car go backwards after stopping it. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [August 16, 2006, 11:39am UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/8 "2006-08-16T11:39:40Z")
> [@Triskadecamus](#): > > You are confusing the mechanics of why the Moon is receeding from the Earth, with the phenomenon of tidal locking. The dynamic forces that cause tidal locking decrease as the period of rotation comes closer to the the period of the orbit. When they are the same, the forces keep it from slowing down more as well. The receeding thing continues on and on, although at a pace that grows ever slower. > > The arithmatic sucks, though. > > Tris Tris, I was curious, without getting into the math, do you have a cite to verify that these are independent phenomena? I will have to recheck my Asimov, but, IIRC, he once wrote on this very subject that the Moon, once its rotation is in-synch with its revolution, will begin to receed from earth as the tidal bulge continues to have an effect. The latter is simply an extension of the former - all a manifestion of the conservation of angular momentum. Let me dig up Asimov from my personal collection… and, I shall return! - Jinx ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 16, 2006, 12:35pm UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/9 "2006-08-16T12:35:02Z")
> [@Mangetout](#): > > Because the force is _drag_ - friction; it’s working to reduce relative movement between two bodies. It doesn’t tend to spin the moon the other way for the same reason that car brakes don’t make the car go backwards after stopping it. Well…my use of the term “backwards” isn’t entirely appropriate but rather my question was would it appear to rotate backwards as viewed from the earth. My idea being that tidal forces would continue to slow the moon’s rotation so it rotated less than one rotation per orbit. ---
**Author:** ![Triskadecamus](https://avatars.discourse-cdn.com/v4/letter/t/b19c9b/32.png) [@Triskadecamus](https://boards.straightdope.com/u/Triskadecamus)\ **Post date:** [August 16, 2006, 1:29pm UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/10 "2006-08-16T13:29:36Z")
> [@Jinx](#): > > The latter is simply an extension of the former - all a manifestion of the conservation of angular momentum. Let me dig up Asimov from my personal collection… and, I shall return! - Jinx Okay, fight my ignorance! Tris ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 16, 2006, 1:29pm UTC](https://boards.straightdope.com/t/why-doesnt-the-moon-rotate-backwards/368918/11 "2006-08-16T13:29:38Z")
> [@Whack-a-Mole](#): > > Well…my use of the term “backwards” isn’t entirely appropriate but rather my question was would it appear to rotate backwards as viewed from the earth. My idea being that tidal forces would continue to slow the moon’s rotation so it rotated less than one rotation per orbit. No, because the tidal forces are _braking the Moon’s rotation_ with respect to the Earth, not reversing it. If the Earth and Moon stick around long enough, they will eventually both be tidally locked, so that the moon is in geostationary orbit around the Earth - the rotation of the Earth will have to slow down a great deal before this can happen, but that’s what tidal forces are doing right now, slowly.