1. ## Spinning Moon

How much of the Earth's moon's mass would have to be lost for the satellite to overcome it being tidally locked and begin rotating?

2. The objects that are tidally locked are medium-sized. If the Moon had a much larger mass, it could have resisted tidal locking the way the Earth has, or if it had a very small mass, its size would be too small to feel much in the way of tidal forces, and again it would not be locked. But to get to that situation, you would need to take away most of the Moon's mass.

3. Have you been listening to Stephen Hawking? I think the media put a very bad twist on his last interview and focused on his daughter.

I could be wrong on phaishazamkan's original question, but I had the same thought.

4. This tidal locking would be dependent on rate of rotation, the mass of and the composition of wouldn't it.?

A molten core might react differently than a solid. ?
The distance from and gravity affect.?

5. Yes, but one can imagine controlling all those other variables.

6. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by phaishazamkhan
How much of the Earth's moon's mass would have to be lost for the satellite to overcome it being tidally locked and begin rotating?
If I read this question correctly, it's worth pointing out that simply removing (or adding) mass will not make the moon "begin rotating". It will retain its currently synchronized rate of rotation, unless acted on by some outside force. Relatively small forces would be all that were required to maintain sychronization as the moon migrated tidally. So a moon that would not have dropped into tidal locking if it were originally rotating briskly, might drop easily into tidal locking if it happened to be rotating near the synchronous rate.

Grant Hutchison

7. ...what is tidal locked?

8. Tidal locked means the Moon rotates once per orbit. If you think about it, this means we always see the same side of the Moon (the side that is sometimes described as "the man in the moon", the other side mislabeled as "the dark side"). The reason for this is that, once the tidal bulges form on the Moon, they can stay aligned with Earth without any rocks having to be lifted or squeezed like they would if the Moon had a different rotation.

9. A broader definition of "tidally locked" would also include resonance, such as Mercury, where the rotation and revolution are in a ratio of 3:2.

10. Yeah, that applies to highly elliptical orbits. Eventually Mercury's orbit will be circularized, I would imagine, and then it will be in a normal tidally locked situation.

11. Order of Kilopi
Join Date
Sep 2005
Location
Metrowest, Boston
Posts
4,054

## tidally aligning

I think we might use the term tidal aligning..or some such...since the effect, though long lived ...is not permanent. Eventually, the resonance detunes, and the rotation drifts to another one. Students reading tidal-locking will think it can never change, which is not quite the case for a teaching forum. Pete.

12. Originally Posted by trinitree88
I think we might use the term tidal aligning..or some such...since the effect, though long lived ...is not permanent. Eventually, the resonance detunes, and the rotation drifts to another one. Students reading tidal-locking will think it can never change, which is not quite the case for a teaching forum.
Nah, nothing's ever permanent. Everybody knows that.

13. I don't see "locking" as being permanent. I lock my door when I leave home.

14. So if the Moon is receding from us, and its orbit becoming longer, I wonder how that will affect its rotational period.

15. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by umop ap!sdn
So if the Moon is receding from us, and its orbit becoming longer, I wonder how that will affect its rotational period.
It'll stay locked to its orbital period, so it will continue to turn one face towards Earth all the time.

Grant Hutchison

16. Established Member
Join Date
Mar 2005
Posts
1,233
I wonder how much force would have to be applied to the Moon to unlock it. If we could apply enough force to increase its equatorial rotaion speed by 1 cm / s, would it be able to resist? Or would we see it slowly, over the course of several years, turn its far side towards us? Just how delicate is its tidal lock?

17. Here is a neat list of tidally locked objects we know of...

Click

18. Yes it would begin to rotate. It would not require a huge force to begin such a rotation. Once rotation began it would require an equal force to stop it. A fleet of unused shuttles could do it maybe. All you need to do is get them and, their fuel there. . . This is not a good idea.
Gravity is not a strong force but it is relentless and very persistent.

19. In essence, one could literally knock anything with an orbit loose with enough force?

So the Sun (no "orbit") would be out of the question, right?

20. No. Not at all. The sun is in a orbit around the galactic core. It would only require a object of conciderable mass to nudge it loos. A near star or passing black hole could do it easy.

21. Originally Posted by astromark
No. Not at all. The sun is in a orbit around the galactic core. It would only require a object of conciderable mass to nudge it loos. A near star or passing black hole could do it easy.

No wonder those science challenged people freak out about stuff like this! Me, being one of them.

22. Originally Posted by Māori

No wonder those science challenged people freak out about stuff like this! Me, being one of them.
While it is theoretically possible, space is big, and unless it was a really close interaction with something very massive, you'd barely be able to tell except by instruments.

23. I never said it would. . . just could,

Do not give it any chance. This system has endured for Four and a half billion years. I will not loose any sleep because of this daft idea. Nor should you.

24. Much like the asteroid belt, in which pretty much all the orbits that remain are stable with respect to each other and the planets.

*thinks for a second about near Earth asteroids*

Well the galaxy has been around longer and operates on larger timescales, so it's certainly not a major concern over the next few billion years.

25. The side of the moon that is facing the Earth has probably been warped slightly in this direction through gravity. The resulting bulge would make it permanently "top-heavy" so that its center of gravity no longer lies near its axis. This would cause the heavier side (the bulge) to fall more directly toward the Earth at all times in the same way that objects on the Earth will turn to place their heavier side down. In this way, the same side will always face the Earth and its rotation time will match its revolution. This is probably eventually true of all satellites. The time it takes for this to occur would depend on its elasticity.

26. Originally Posted by grav
The resulting bulge would make it permanently "top-heavy" so that its center of gravity no longer lies near its axis.
Axis of what? As the moon revolves, it also rotates. The axis of rotation is essentially through the center of mass. There are mascons (mass concentrations) on the earth-side, but their distribution is not "pointed" directly at earth--I seem to remember it's 25-30 degrees off.

27. Originally Posted by tony873004
... If we could apply enough force to increase its equatorial rotaion speed ...
Perhaps if the Chinese could transport enough of its people to the moon and then have them all jump off of chairs at the same time ...

28. Originally Posted by hhEb09'1
Axis of what? As the moon revolves, it also rotates. The axis of rotation is essentially through the center of mass.
The moon still rotates (once per revolution), just not relative to the Earth. Its axis would run through the center of rotation, which is different from its center of mass if it is warped. You can visualize this with a ball on a string analogy. If we rotate the entire system (swing it around) from the end of the string, then the end of the string is the axis but the center of the ball is approximately the center of mass. But since this looks too close to the ball revolving and could cause confusion, I suppose you could also simply consider a wheel whose axis of rotation is slightly offset from its center. Or better yet, imagine a uniform wheel set vertically with the axis at its center. Now add a weight to it. The wheel will turn until the weight eventually rests on the bottom.

Originally Posted by hhEb09'1
There are mascons (mass concentrations) on the earth-side, but their distribution is not "pointed" directly at earth--I seem to remember it's 25-30 degrees off.
This is interesting. It may show a delay in reaction for the rotation of the moon to "catch up" with the gravitational pull. This should then be caused by the resistance of the mass to change so that the heavier side is constantly "falling" toward Earth but never quite makes it because of the speed of its orbit. This may mean something. Do you know of any good links about this?

29. I have been thinking about this some more. Now don't get me wrong, I'm no expert on this by any means, so my thoughts can't be taken too literally, just more on philosiphical grounds.

The rotation of the moon matches that of its revolution, but not relative to the Earth. If we look at it relative to the Earth, it does not appear to be rotating at all. This can be seen more directly if we take, say, half a dozen points on the moon and then forget about the moon altogether except for those points. The points are not connected at all and will each orbit freely about the Earth. If we neglect the slightly greater velocity that the points closer to the Earth would have, they will all orbit together in the same way that they are originally lined up. That means they are always facing the same way relative to the Earth, the left points will stay on the left and the right points on the right all the way around their orbits. If we line these back up with the moon, then to an observer that is not on Earth, it will appear to rotate once per revolution. It appears there are different ways of looking at things.

I am also thinking that if this is the case, then the apparent rotation of the planets is different than what we think because of their revolutions around the sun. That is to say, for instance, if the Earth rotates about 365 times per revolution around the sun (per year), and if one of these is caused by the orbit itself, then it is really rotating 364 or 366 times depending on whether it rotates with or against the revolution. This would also be true of all the planets, although the offset of some of their axii (Earth, too) would make this relation more complicated.

This also makes me wonder, however, about the way I described the tidal locking. I said that the top-heavy side would turn to face the Earth ans then remain that way because of gravity. But the moon also feels the centrifugal force caused by its orbit, which means that all points on the moon should then be in equilibrium and experience "free-fall" (except for the gravity of the moon itself). In this case, it would not feel top-heavy in the same respect as a stationary object on Earth that experiences the full force of gravity. But the moon still feels gravity, of course, in order to stay in orbit. So I'm in a quandary. Perhaps someone else can answer this.

If the moon does feel gravity in the same way as an object that feels the full force on Earth, then the top-heavy side would "fall" until it faces toward us but then continue to turn until it reaches some highest point, and then began to fall again in the opposite direction. It would continue doing this in a sort of pendulum action unless some sort of friction is created between the Earth and moon to bring it eventually to a halt. If this is the case, then the moon might have originally had some rotation which was stopped by the friction involved. I was wondering what sort of friction this might entail. Could it be tidal friction, some sort of friction in space, or perhaps something that we relate to the idea of gravitational waves? Or maybe some combination or does this friction exist at all? Any ideas on this?

30. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by grav
That is to say, for instance, if the Earth rotates about 365 times per revolution around the sun (per year), and if one of these is caused by the orbit itself, then it is really rotating 364 or 366 times depending on whether it rotates with or against the revolution.
That's right: the Earth rotates 366 times relative to the stars (what's called a "sidereal day" in the time it takes to rotate 365 times relative to the sun.

Originally Posted by grav
But the moon also feels the centrifugal force caused by its orbit, which means that all points on the moon should then be in equilibrium and experience "free-fall" (except for the gravity of the moon itself).
On average, the moon feels just enough gravity to keep it in orbit around the Earth. But the parts of the moon farthest from the Earth feel less gravity, and the parts closest to the Earth feel more gravity than that average. So there's a gradient of gravity across the moon, which pulls the far side away from the Earth, and the near side towards the Earth: two tidal bulges.
If the moon were rotating relative to the Earth, those bulges would move constantly over its surface, generating movement, friction and heating. They'd also lag a little behind the direct line connecting Earth and moon, and the Earth's gravity would haul on them, adjusting the moon's rotation speed. If it rotated too slowly, tides would speed it up; if it rotated too quickly, tides would slow it down. The final result is that the moon's rotation is constantly adjusted by the tides to match its period of revolution around the Earth.

Grant Hutchison

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•