Why doesn't the moon rotate backwards?

[Whack-a-Mole]

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?

[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

[Gorsnak]

In fact, wouldn't tidal forces have sped the moon's rotation up, had it been rotating slower than it's orbital period?

[Triskadecamus]

Originally Posted by 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

[jimbeam]

Tidal friction.

http://curious.astro.cornell.edu/que...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.

[Triskadecamus]

No, wait, I changed my mind. It would have to slow down, as it moved farther away, and its orbital period increased.

duh.

Tris

[Mangetout]

Originally Posted by 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.

[Jinx]

Originally Posted by 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

[Whack-a-Mole]

Originally Posted by 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.

[Triskadecamus]

Originally Posted by 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

[Mangetout]

Originally Posted by 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.