Does Moon revolves itself ?

[Sayonara]

Your problem swansont is that you never dare to question the accepted dogma of the dictionary.

[swansont]

I did not realize we had gone through the looking glass.

 

"When I use a word," Humpty Dumpty said, in a rather scornful tone, "it means just what I choose it to mean - neither more nor less."

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The moon is locked gravitationally so the situation is like a hammer thrower. The weight (moon) pulled round by the athlete (earth) applying the force via a wire. The weight is restrained from rotating about its own axis - but it clearly rotates about the athlete. Same situation!

 

OK, let's look at that situation. You have your (track & field) hammer and because we want to make sure it doesn't rotate about the axis in question, we put a gyroscope inside, perpendicular to that axis. Now, can we swing the hammer only via a force along the arm, i.e. perpendicular to the sphere?

 

I think you'll have a hard time doing that.

[Akhenaten2]

I did not realize we had gone through the looking glass.

 

"When I use a word," Humpty Dumpty said, in a rather scornful tone, "it means just what I choose it to mean - neither more nor less."

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OK, let's look at that situation. You have your (track & field) hammer and because we want to make sure it doesn't rotate about the axis in question, we put a gyroscope inside, perpendicular to that axis. Now, can we swing the hammer only via a force along the arm, i.e. perpendicular to the sphere?

 

I think you'll have a hard time doing that.

Perhaps, perhaps not. The weight doesn't rotate about its axis so the insertion of the gyroscope is merely an attempt to prevent the weight being rotated about the athlete. I doubt the gyroscope could be made large enough to prevent this though. But do we not have proof of this in the planet Uranus, which has a "gyroscopic-like" rotation but still follows a low ellipticity orbit despite its axis of rotation being almost perpendicular to its orbital plane. Is this proof that the Newtonian view of gravity, as a force between centres, is not correct?

Edited February 21, 2010 by Akhenaten2

[swansont]

Perhaps not. The weight doesn't rotate about its axis so the insertion of the gyroscope is merely an attempt to prevent the weight being rotated about the athlete.

 

Try it. Take a gyroscope and move it around a circular path, whose axis is perpendicular to the gyroscope's axis of rotation, without exerting a torque on the gyroscope. We know the gyro won't rotate, so that's what non-rotation looks like. The gyro axis maintains a fixed attitude toward a distant, fixed object. The moon doesn't do this.

[Akhenaten2]

Try it. Take a gyroscope and move it around a circular path, whose axis is perpendicular to the gyroscope's axis of rotation, without exerting a torque on the gyroscope. We know the gyro won't rotate, so that's what non-rotation looks like. The gyro axis maintains a fixed attitude toward a distant, fixed object. The moon doesn't do this.

This is all very interesting, but mostly diversionary from the original discussion about the moons orbital/rotational motion. I understand what you are saying and if the moon actually rotated additionally as a gyroscope, with axis perpendicular to axis of orbit, everyone who ever lived would swear that (from our perspective) it also rotated bodily once backwards every orbit. The question of whether it rotates would never have been asked. But the moon is gravitationally bound (which I feel is the essence of the problem) is not gyroscopic as it does not rotate about its supposed axis of rotation (at least not more than slowly) and its supposed axis of rotation is parallel to the axis of its orbit. So I cannot accept that your analogy is in the least valid.

In my reference to Uranus, which does exhibit some of the properties you describe, I ask (genuinely) will its gyroscopic-like spin orient it in the same attitude all the way round its orbit?. If so, will we describe this as one backward bodily rotation?

Edited February 21, 2010 by Akhenaten2

[swansont]

Either it rotates or it doesn't. A thought experiment that shows this is hardly a diversion nor an invalid analogy. Rotation about an axis is a fairly well-defined concept.

[John Cuthber]

If I get a good analytical balance and calibrate it at the equator then move it to the North pole it needs recalibrating. Part of that change is because the Earth isn't a perfect sphere, but part of it is because the Earth rotates.

I can do that experiment without needing to know about anything else in the cosmos or relativity and I can deduce that the Earth spins.

Similarly I can set up a Foucault's pendulum and, from its motion I can tell that the Earth is rotating.

 

What would be the outcome of doing these experiments on the Moon?

My guess is that would show the Moon to be rotating once a month.

I also guess that that is a lot nearer to the sort of answer that the OP wanted.

[Akhenaten2]

John - My guess is that it won't, but we will probably not now find out in our lifetimes.

Swansont - Thanks for your patience under duress - I thought I had mastered the posting and have just spent 2 hours composing an extensive response, which had my 3 strongest arguments that might have "stirred" you also, but this is all now somewhere in cyberspace - I can't afford any more time on this topic - we must agree to disagree. I've enjoyed our discussion.

Regards - Akhenaten

Edited February 22, 2010 by Akhenaten2

[John Cuthber]

We has people on the moon for a while; did they notice any stars rise and set?

(come to think of it, did they leave a telescope and transmitter of some sort behind?)

If not then it seems to me to be really weird that the moon is "stuck" in one orientation whereas everything else we see seems to have a spin.

[insane_alien]

you can actually tell the moon is rotating from earth, as the orbit is not perfectly circular, and the rate of rotation is constant(the torque required to change the rotational velocity of the moon this much this quicklywould be MASSIVE) you get libration

 

http://en.wikipedia.org/wiki/Libration

 

you can test this too with just an el cheapo brand telescope and a camera and a month of clear skies. heck, even just a camera with a decent optical zoom on it should work fine.

[Iggy]

We know the gyro won't rotate, so that's what non-rotation looks like.

 

Either it rotates or it doesn't.

 

I think the original question in this thread has been answered very well and I post in fear of looking like a persistent pest on this issue, but I really cannot understand this it-rotates-or-it-doesn't thing.

 

What makes this effect so interesting and important is that while other effects that I have described in this book, including the geodesic precession, have to do with such concepts as gravitational fields, curved space-time, and nonlinear gravity, this effect tells us something about the inertial properties of space-time.

 

If you ask yourself, “Am I rotating?” and you wish an answer with more accuracy than you get simply by seeing if you are getting dizzy, you usually turn to a gyroscope, for the axis of a gyroscope is assumed to be nonrotating relative to inertial space. If you were to build a laboratory whose walls were constructed to be lined up with the axes of three gyroscopes arranged to be perpendicular to each other, you would conclude that your laboratory was truly inertial (and if it were in free fall, that would be even better). However, if your laboratory happened to be situated outside a rotating body, the gyroscopes would rotate relative to the distant stars because of the dragging effect I have just described. Therefore, your laboratory can be nonrotating relative to the gyroscopes, yet rotate relative to the stars. In this way, general relativity rejects the idea of absolute rotation or absolute nonrotation, just as special relativity rejected the idea of an absolute state of rest.

 

--"Was Einstein right?", Clifford Martin Will, ch. 12

 

If you set up a coordinate system by some gyroscope axes (call them A) in the manner of the quote above and you are not rotating in that coordinate system then you are not rotating relative to A. At the same time someone else may set up a different coordinate system by gyroscope axes (call them B) while they are, let's say, a little closer to the sun. Now the person is rotating relative to A and not rotating relative to B.

 

If you are rotating relative to one gyroscope and not rotating relative to another and you define rotation by the action of a gyroscope then I fail to understand "either it rotates or it doesn't".

 

I think the gut reaction to my argument would be to point out that one set of gyroscopes is local and the other is not and say that absolute rotation is determined by the local set of gyroscopes. But, consider time. Nobody would say that time is absolute because local clocks always run the same rate. Time dilates the closer a clock gets to a massive body the same way rotation precesses the closer a gyroscope gets to a spinning body. If clocks measure time and gyroscopes measure rotation and we say that time is relative because different clocks run different rates then rotation should be relative because different gyroscopes measure different rates of rotation.

 

We has people on the moon for a while; did they notice any stars rise and set?

 

In the same respect, nobody would claim that seeing the stars move means you have velocity. If you see a star move then you have velocity relative to that star. you may not have velocity relative to a different star.

[swansont]

I think the original question in this thread has been answered very well and I post in fear of looking like a persistent pest on this issue, but I really cannot understand this it-rotates-or-it-doesn't thing.

 

I'll reiterate that when someone asks a question that is of the sort that indicates that they have never taken a physics class, then it's not really prudent (and one might even consider it impolite) to throw graduate-level physics at them. Within the realm of Newtonian physics, rotation is absolute.

 

So the question of frame-dragging isn't pertinent to the level of discussion, IMO, nor is the rotation of the moon an example of frame dragging.

[Iggy]

I'll reiterate that when someone asks a question that is of the sort that indicates that they have never taken a physics class, then it's not really prudent (and one might even consider it impolite) to throw graduate-level physics at them. Within the realm of Newtonian physics, rotation is absolute.

 

I agree completely and that is exactly what I was referring to in saying that the original question in this thread had been answered very well.

[Akhenaten2]

Swansont/Iggy - Gentlemen before you congratulate yourself too early on a job not even nearly done I have a few things to say. In no way has the original question in this thread been answered.

When I first saw Pravin.sonars simplistic question I didn't think he had just got up that morning and thought "I wonder if the moon rotates". I expected that he had been pondering it for some time and hadn't found many answers that satisfied.

I was surprised that it (initially) received only a brief, scornful response, because I felt ( as is being proved) that it was a rather interesting and profound question which was by no means unequivocal. This kind of handling is what "drives people away from science" I believe.

I have long held the view that there is a deep misconception bound up in this seemingly innocuous query (and many others). I strongly disagree that this should be viewed as a rotation about its own axis and neither of you have found any argument that convinces me otherwise.

Swansont - I feel you are peddling the "Newtonian" viewpoint because this best suits your arguments and your reluctance to "invoke relativity" is more because It doesn't.

Then we have Iggy coming in with a sledgehammer to break an egg and invoking Totally Irrelevant relativistic effects I suspect he does'nt understand a great deal more than I do; talking as if the sun were a black-hole and not really contributing to the discussion at all.

This approach too is the kind of handling which "drives people away from science"

Iggy you say you don't get this "It either rotates or it doesn't" bit, well from what you have said, I defy anyone to decide categorically whether you agree with rotation or not. I don't get why such clever people can be so ambiguous. It's almost as if this is the intention in science today and this too drives people away from science.

I haven't time tonight, but I think everyone who has commented, deserves an answer of some sort and I shall try to do this in due course before I respond more appropriately to your combined comments.

Perhaps pravin.sonar could, at this stage, say whether he is forming an opinion as to what conclusion he can draw.

Regards for now -Akhenaten

Edited February 24, 2010 by Akhenaten2

[Iggy]

I’m sorry you find my post/posts distasteful, Akhenaten. I was asking Swansont to clarify something very specific that he had said, which he did. I was not answering, or trying to answer, the opening post--and not directing my comments toward your position or your argument. If you think there is any way for me to help satisfy your contentions on this issue of the moon's rotation then please let me know.

[swansont]

Swansont - I feel you are peddling the "Newtonian" viewpoint because this best suits your arguments and your reluctance to "invoke relativity" is more because It doesn't.

 

You would be wrong, then.

[Akhenaten2]

If I get a good analytical balance and calibrate it at the equator then move it to the North pole it needs recalibrating. Part of that change is because the Earth isn't a perfect sphere, but part of it is because the Earth rotates.

I can do that experiment without needing to know about anything else in the cosmos or relativity and I can deduce that the Earth spins.

Similarly I can set up a Foucault's pendulum and, from its motion I can tell that the Earth is rotating.

 

What would be the outcome of doing these experiments on the Moon?

My guess is that would show the Moon to be rotating once a month.

I also guess that that is a lot nearer to the sort of answer that the OP wanted.

John,

I apologise, I answered this in haste and it was ill conceived. Having now partially re-examined your "Moon experiment hypothesis" I conclude that a rotation would be recorded. However, in view of the peculiar geometry of the sun/earth/moon system and its several sets of precessional effects, the moons motion becomes very complex.

Using either a gyro or pendulum alignment at the moons poles would record a lunar 'sidereal' month of 27.3 days and this is generally taken to represent a true rotational period. I contest this in a gravitationally bound situation.

The counterpart experiment on earth (viewed from its clearly rotating and therefore 'non-inertial' reference frame) introduces the "Coriolis" and 'Centrifugal' "pseudo forces" which cause the observed periodic deflections in the apparatus.

The same set up on the moon would similarly constitute a non-inertial reference frame and this would similarly introduce pseudo forces to create the deflections which would be seen in that apparatus also, but (I emphasise) this would only be recording the "revolution" of the moon about its orbit - as compelled by earth/sun gravity - and not an actual rotation of the moon about its own axis.

Regards

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We has people on the moon for a while; did they notice any stars rise and set?

(come to think of it, did they leave a telescope and transmitter of some sort behind?)

If not then it seems to me to be really weird that the moon is "stuck" in one orientation whereas everything else we see seems to have a spin.

John

Astronauts on the moon could still observe stars rising and setting due to orbit change, but they might possibly not have "noticed" them because you get a very different view of stars on the moon. There is no "Twinkle" effect.

Because stars are so distant, each is just a pinprick of light and requires at least a small time lapse to register. Normal photographic shutter speeds are probably too quick and probably our eyes are too.

I honestly don't know what they left on the moon.

The "stuck" orientation/motion you say you find weird and cannot understand, is actually quite common. A number of the larger moons in the solar system exhibit similar gravitational binding to our moon. It is also common among binary star and star/BHole systems and even evident to a degree in the planet Mercury, which (according to conventional teaching) is bound into doing 3 rotations to every 2 orbits. They all spin relative to some axis or other but not their own. In all cases they must "share" an axis with a larger body.

Edited February 26, 2010 by Akhenaten2
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[mulreay]

Just to add, people are asking about what was left on the moon. I can tell you that they left a mirror on there... not like the one at home of course. They fire a lazer at it everyonce in a while to gauge the distance between the moon and our planet. This is how we know the moon is slowly moving away from us, and will eventually break orbit and we will lose the moon. This will be hugely detrimental to our planet of course but were safe for quite some time yet.

[Sisyphus]

and will eventually break orbit and we will lose the moon.

 

This part is incorrect. The moon is indeed slowly receding, but this would never cause it to break away. It would continue to recede until tidal friction had tidally locked the Earth to the moon, and they both rotated at the same speed (and revolved around each other in the same period). At this point, the day, the month, and the lunar day would all be about 47 of our days, and it would not recede any more. However, it would take about 50 billion years for that to happen, and conditions will change long, long before then.

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Oh, and BTW, most of the answers to the questions in this thread can be found here:

 

http://en.wikipedia.org/wiki/Orbit_of_the_Moon

[Akhenaten2]

Thanks Sisyphus

I've found one or two of your comments to be a bit wide of the mark, but I agree with you on this last comment. I will leave people to consult wikipedia for the answers that I consider less contentious or (dare I say) - "beyond dispute".

I in fact, try to concentrate my comments (in language we can all understand) to material that isn't contained in such scientific edicts. This is because I have an enquiring mind and I do not find it satisfactory to be told to accept something just because a majority of people believe it to be true.

All such "wiki - type" articles are written based on the "best interpretation currently to hand" of the physics involved. But if there is a fundamental misconception running through the "establishment view" then how are we ever to understand and expose it if we are only ever allowed to "consult" the "establishment edicts"?

Regards - Akhenaten

Edited February 27, 2010 by Akhenaten2

[John Cuthber]

"The "stuck" orientation/motion you say you find weird and cannot understand, is actually quite common. A number of the larger moons in the solar system exhibit similar gravitational binding to our moon."

Gravitational locking is perfectly commonplace and I'm familiar enough with it.

 

What would be odd would be if the moon were "locked" to the rest of the universe, rather than rotating in it.

 

Incidentally, because there's no atmosphere the view of the stars from the moon should be excellent. It's silly to say "Because stars are so distant, each is just a pinprick of light " they are practically as far away from Earth and they are practically pinpoints from here too. In most cases they are so small that all the light from a given star falls onto just one retinal cell; it wouldn't matter if the Earth's atmosphere were removed- they would still be points as far as the human eye is concerned.

If you are on the moon you should be able to see them rise and set - albeit very slowly.

Of course it would be easier on the dark side of the moon (and I mean dark, not Earth-facing)

Edited February 27, 2010 by John Cuthber

[shakes]

For the OP, you might find this of interest, http://www.digipro.com/Trials/moon.html

[KennyC]

Well, that was fun....

[Akhenaten2]

"The "stuck" orientation/motion you say you find weird and cannot understand, is actually quite common. A number of the larger moons in the solar system exhibit similar gravitational binding to our moon."

Gravitational locking is perfectly commonplace and I'm familiar enough with it.

 

What would be odd would be if the moon were "locked" to the rest of the universe, rather than rotating in it.

 

Incidentally, because there's no atmosphere the view of the stars from the moon should be excellent. It's silly to say "Because stars are so distant, each is just a pinprick of light " they are practically as far away from Earth and they are practically pinpoints from here too. In most cases they are so small that all the light from a given star falls onto just one retinal cell; it wouldn't matter if the Earth's atmosphere were removed- they would still be points as far as the human eye is concerned.

If you are on the moon you should be able to see them rise and set - albeit very slowly.

Of course it would be easier on the dark side of the moon (and I mean dark, not Earth-facing)

What would be more interesting is if the universe itself turned out to be rotating - we wouldn't be able to trust any observation we ever made!

 

I'm afraid what I said about not detecting stars so readily on the moon is perfectly true; Though you would see some of the nearer/ larger ones rise and set slowly and make out larger structures reasonably (eg The Milky way galactic features).

Earths atmosphere smears out the stream of photons of light from stars and thus we are able to observe them better but the downside is that twinkling and colour scatter occurs. Being smeared out means that some refraction effects are occurring which slows some photons more than others and this holds the image together long enough for our eyes to register a larger number of photons over a larger area of receptors.

This is not the case on the moon, where the photons arrive effectively one at a time in a very narrow stream and hence only excite a small number of receptors. Each receptor only registers the signal for a short time so you only get a faint and largely incomplete image.

If earth had no atmosphere this is all we would see from here also.

 

Thanks shakes - digipro was interesting but nothing new and nothing to resolve this thread question I'm afraid.

Edited March 1, 2010 by Akhenaten2

[Mr Skeptic]

What would be more interesting is if the universe itself turned out to be rotating - we wouldn't be able to trust any observation we ever made!

 

Oh, that would be interesting indeed. If the entire universe were rotating, then it would have a center and we would be able to find it, right?