# 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 astronomy.stackexchange.com/questions/16/… with much more detailed answers. And its dangerous to say the moon doesn't rotate at all without a reference frame for context.
– Dean
Commented Nov 30, 2016 at 15:44
• @Dean good point. A tidal lock still rotates. I've deleted the comment. Commented Nov 30, 2016 at 20:34

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. Commented Dec 2, 2016 at 22:03
• 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. Commented Dec 3, 2016 at 3:56

The Moon indeed "wobbles" about in its orbit because it goes around the Earth in an ellipse and not a circle. From our point of view, it wobbles a little back and forth such that over a lunar cycle, we end up seeing about 59% of its surface. The effect is referred to as lunar libration. You can find a number of videos that show how the Moon looks to us over the course of a cycle.

• It really sounds like a Liberace album title! :) Commented Dec 2, 2016 at 11:48

The current popular theory, that the Moon formed from accretion, would nearly eliminate any rotation of the Moon (relative to Earth). I say 'nearly', because objects striking the forming (accreting) Moon on the side further from Earth would have added slightly more rotational energy than objects striking the nearer side. This would give the Moon a slightly (negligible) retrograde rotation in its early history.

Since the Moon has been rotationally locked to the Earth since its formation (current popular theory), no rotational energy has been lost due to tidal forces, so the term 'tidally locked' is misapplied to the Moon.

• What do you mean by "formed from accretion"? Are you referring to the giant impact hypothesis? Commented Oct 15, 2017 at 17:20

If The moon's movement relative to Earth is due to it's residual rotation rather than due to the effect of the pacific, the residual rotation would be decreasing similar to a pendulum thats almost still, imagine you are swinging a pendulum around, so the moon's movement is probably a flat ellipse, a figure of 8 or a resonating spirograph.

When the moon formed from an accretion disk, it's material was less spherical and more fluid and under the influence of our gravity. If it did manage to keep a rotation at that stage.

Perhaps there isn't a measurement of the moon's rotation, because it's difficult to measure, and it may in fact be a slow rocking motion due to the pacific rather than residual motion from it's formation.

To measure the movement of the moon is technically challenging, perhaps you would have to place a camera on the moon and take a photo of a place on earth at the same time every day and then see if the variation is due to the pacific or the residual rotation. perhaps the cheapes way to measure it's movement is to measure the length of the flagpole, or to send a digital flagpole there. The shadows of the moon shift so much, it's not practical to use it's topography for reference.

• "...rather than due to the effect of the pacific,..." Huh? Tidal locking does not work like a pendulum. How can an orbiting satellite be in resonance to the body about which it orbits? No, the moon did not start swinging back and forth without rotating; again, that's not how tidal locking works. (If this "answer" gets downvoted, it wasn't me.) Commented Dec 2, 2016 at 22:12
• Hi sorry, that was poorly worded about the resonance. If you throw a turnip into orbit, it will also tend towards tidal locking similar to a pendulum. A pendulum also tends towards central alignment due to gravity. If there exists a state of movement transitioning from free rotation to synchronous rotation at a time where a moon is not molten, I used the comparison pendulum to describe the movement of the moon when it is not rotating any more and it is approching tidal locking, because the momentum of it's previously free orbit will send it past the tidal locked alignment many times. Commented Dec 3, 2016 at 3:13
• What does the pacific have to do with it? ;) Commented Jan 8, 2019 at 10:13