Is the moon "perfectly" tidally locked and, if not, how long would it take us to observe it's rotation?

  • I have a general understanding of how and why a body in space can be tidally locked to it's planet or sun and I'm aware that our moon is in such a state.



    My question is, if our moon once had a rotation, has it slowed down to a point where across the course of our normal lifetimes we can't observe it's spin but across the span of hundreds, if not thousands, of years we could observe its current rotation? In other words, if it has a rotation and if I looked at the moon today and jumped into the future, how far would I have to go to see a noticeable difference?



    If the question is to relative then I'll ask it like this-- at the moons current rate of rotational speed and deceleration, how long would it take for the moon to rotate, say, 15 degrees on it's current axis? I assume this would be enough to make the moon look "different" to the naked eye so I'll go with that concrete figure.



    ...or is the moon in either a state of near-equilibrium or is it "wobbling" due to other forces outside the gravitational pull of the earth (e.g. pull of the sun, asteroid bombardment, comets passing by, etc.) having minor effects on its rotational state and thus making it's rotation inconsistent and unobservable?


    @userLTK actually if you look in the comments of that question there is a similar question on here http://astronomy.stackexchange.com/questions/16/why-is-only-one-side-of-the-moon-visible-from-earth with much more detailed answers. And its dangerous to say the moon doesn't rotate at all without a reference frame for context.

    @Dean good point. A tidal lock still rotates. I've deleted the comment.

  • Ken G

    Ken G Correct answer

    5 years ago

    The question is interesting, but I suspect the answer is that the Moon will never show its "far side" to the Earth, because there are differences between the side that faces us and the far side that suggest there is something quite permanent about its orientation. So while the rotation was locking, it either settled into a state of minimum potential that it has maintained ever since, or it built up the differences between the side we see and the side we don't after it tidally locked, but either way, this means it has showed us the same side for billions of years, so will likely continue to do that. I don't think anything would likely knock it out, but we can't rule out chaotic orbital effects so I don't really know. The Moon's orbit varies a lot with time, so maybe it's possible that the orbit itself could change, showing us the other side of the Moon. Or, an asteroid hit or some such thing might occur. A billion years is a long time, but I'll bet humanity will always see the same side from Earth.


    The moon has been tidally locked to the Earth since, at least, the Heavy Bombardment period, some 3.8 billion years ago. This is evidenced by the differences between the near and far sides of the moon. Any impact upon the moon sufficiently large enough to alter its rotation, would likely destroy the moon.

    I read a page which said that the moon may have been tidally locked practically since it coalesced, and that it was even less round then than it is today. simulations suggest that it was more than 10 times closer and 10 times larger in the sky than it is today. i's unlikely that an impact could have ejected material more than 5 earth radii, i.e. 15 times closer than today.

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