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## Our Spinning Moon

I have read various articles on the net regarding whether or not our moon spins on it's own axis. Funny thing is, most professional astronomy sites seem to be trying to tell us that it does spin around it's own axis about once for every orbital period.

My questions are these....If one should drill a hole in the center of a baseball from top to bottom and run a string through it, tie it off with enough left over to swing it about one's head from a distance of a few feet, will it rotate around the string or 'pole/axis' running top to bottom through the center? Or will one side continually face you as it orbits your head? If it does not spin about it's own axis but instead move it's whole body through space around your hand, is not the moon actually fixed and stationary in reference to it's own axis therefore moving only around your hand as the external axis point?

If one can accept the obvious as true, then many astronomy sites that claim the moon spins on it's own axis are teaching error. A car driving around you in a circle is hardly "spinning on it's own axis". Even accounting for that "once for every rotation" gibberish. An arrow with a string tied to it's middle will present one side to you as you swing it around your head, it too is hardly spinning around it's own axis. These objects, like our Moon, are simply moving in a circular path around an axis point, which I'm sure we all can agree, quite external to themselves.

So who's right? Those laymen who see the moon as not spinning at all, instead merely wobbling around it's own axis a bit and merely travelling around a distant axis point....or those professionals who 'seemingly' try to bamboozle us that it does?

Or what secret could they be trying to hide?

On the far side.....

TS
Last edited by Techstuf; 2007-Jul-05 at 04:12 AM.

2. Speaking of the back side, do you think that an astronomer on the back side of the Moon would say it is not spinning? He/She would see the same stars pass overhead every 655 hours. There is no question that he/she would say it is spinning on its axis.

3. My inclination is to analyze the Moon as rotating about an axis through its center and poles, at a rate that is synchronized with the orbital motion by tidal locking. This axis is orbiting around the barycenter with the Earth. To me this is the optimum vector combination. If the Moon were spinning faster it would be nonsynchronous but otherwise not fundamentally different, and there would be no question about its spin axis. The tidally induced synchronous rotation is in my opinion merely a special case, not geometrically unique.

The ball on a string is not a good analogy because the string imposes torques and constraints on rotational components in ways that gravity would not.

If you think I am condescending, fire away. I am pretty flameproof.

4. The moon definitely does rotate once around its own axis - it's just rotating at the same rate as its orbital period. It orbits around the earth, but it rotates around its own axis.

5. Originally Posted by Techstuf
A car driving around you in a circle is hardly "spinning on it's own axis".
Take a compass and put it on your dash and drive around in a circle. Your compass needle will swing around one revolution per circuit around the circle. So, yes indeed, the car is indeed rotating once per revolution so to speak.

Say the car is going around counterclockwise around you in a circle. When the car is at the north part of that circle, the hood ornament is facing west. When the car has gone around 1/2 of the circuit and is at your south, the hood ornament is facing east. When it gets back to your north position, the hood ornament is facing west again. The car rotates one complete circle around the compass points each trip around the circle.

To see what the motion would be like without rotating, imagine keeping that compass needle pointed in the same direction while the car goes around the circle. One could not do this with the steering system of the car, of course, but imagine a car going around a point in a circle but keep the hood ornament pointed in the same direction the whole time.

If the moon did not rotate, we would see all sides of it over the course of one orbit, or revolution around the earth.

-Richard

6. Bad astronomy's explanation should suffice.

If not, take two objects and using your hand move one in circles around the other on a table. The object in your hand does not rotate since the side facing you always faces you, but it presents every side to the central object, right? You actually have to rotate the object in your hand as it goes around in circles to keep one side facing the central object. This is like the earth/moon system as seen from above.

7. One can explain this via frames of reference, but I think there's also something linguistic going on.

This is similar to an old philosophical chestnut about the hunter and the squirrel. A hunter circles a tree. On the tree is a squirrel which also circles, crawling around the trunk, so as to keep the tree's trunk between itself and the hunter. The question is whether the hunter is going 'around' the squirrel or not, and it has led to many long and frustrating arguments.

The point of division seems to be how one defines 'around'. One side (A) says that if you draw a circle so that it contains some object, you have drawn it around that object. The other side (B) says that if you really go around something, you'd see its front, side, back, side again, front... And if you haven't seen that, you haven't gone 'around' it.

I think that some people are raised with one definition of 'around' taken for granted, and so they find it hard to understand when someone else uses it in a different sense. Since the average participants in bar room debates rarely define their terms, the confusion spreads and perpetuates.

I favor (A) myself, and I think that the fallacy of (B) can be demonstrated by having someone (X) walk in a circle about someone else (Y). Y stands still for a minute, then turns constantly to face X, then stands still again. Does it really make sense to say that X was only walking around Y when Y was not moving?
Last edited by jja; 2007-Jun-29 at 10:40 PM. Reason: Minor syntax correction.

8. Originally Posted by Techstuf

So who's right?
The BA, of course.

Those laymen who see the moon as not spinning at all, instead merely wobbling around it's own axis a bit and merely travelling around a distant axis point....or those professionals who seemingly try to bamboozle us that it does? Could that many well known astronomers simply that bad at simple physics?
Welcome to the dark side, Techstuf!

9. And that is the point of this missunderstanding... Its our point of reference. From the Earth the moon does not look like its rotating. After all its the same face we see all the way round. So no its not rotating.. Wrong. Yes it is. Take your point of view further away and watch the moon rotate once per revolution about Earth. If the moon did not rotate at all it would look like it was from here. Over the period of its orbit we would see it all. giving the allusion of spin. This argument would not go away.

10. Originally Posted by Techstuf
So who's right? Those laymen who see the moon as not spinning at all, instead merely wobbling around it's own axis a bit and merely travelling around a distant axis point....or those professionals who seemingly try to bamboozle us that it does?
That bamboozling Einstein!

The problem is, you're couching your question in terms of Euclidean geometry, where space is fixed-and-absolute. But in GR, each observer establishes his/her/its own reference frame. Thus, the moon is its own reference frame, and it spins about its own axis.

You have choosen the center-of-mass of the earth-moon system as your point-of-reference, but this is completely "arbitrary." You could have choosen the sun: how would you describe the moon's rotation with respect to the sun? How about the galaxy?

Try this thought experiment: A baseball is in orbit around the earth, one side always facing the earth, like luna. An astronaught fires a BB gun at the baseball, delivering a glancing blow. What happens? The BB will deliver some angular momentum to the baseball, and the ball will begin spinning...about its own axis, right?

Now, we take that same baseball in orbit around the earth, only you have it by a string--a long string--as described in your OP. It is rotating...about the center of the earth--as you would have it. Our astronaut fires a BB at it, as before. Does it begin to spin?

No, because you have it fixed by a string!

The string-thing confuses the issue; whirling it around on a string is not the same as gravity holding it.

11. Originally Posted by jja
One can explain this via frames of reference, but I think there's also something linguistic going on.
I think this is almost entirely a linguistic problem. We all know what is going on, I think. I don't think anybody seriously thinks that a person on the far side of the moon would always see the same stars. It's really a question of how, in words, we describe it, isn't it?

12. Makes me wonder if those pesky experts are right about the Sun not going around the Earth...

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As previously mentioned, it's all about the reference frame. If I put my keys on my dresser before I go to bed, when I wake up 12 hours later, my keys are are on the opposite side of me because the Earth rotated 180 degrees (pretend I live at the north pole to simplfy this). But the entire room has spun, so the keys are still on my dresser seemingly right where I put them. Living on Earth, we are used to treating our rotating frame of reference as a standard reference. If we didn't, giving directions would be a nightmare. Imagine someone asking you directions to the store. You point down the block and say "if you leave now, its 200 yards that way, but if you leave in 12 hours, it will be 200 yards in the opposite direction." Even though the latter is technically correct, no one will consider you wrong for failing to phrase it that way.

In the case of the Moon, it does not rotate in a rotating frame of reference pivoting on Earth's position, and having the same period as the Moon's orbital period. But it does wobble a bit. But in a non-rotating frame the Moon certainly does rotate. If it had an atmosphere with clouds, they would be subject to a coriolis force. The Moon's diameter at its equator is slightly larger than at its poles because of its rotation.

14. I'll take a bit of issue with it's all linguistics. In the strictest sense it's a matter of defintion. Rotation, both in Newton and Einstein, have a rigorous mathematical defintion. Even in Newton, that ain't exactly as simple as you think -- if you want to play with it, consider what the 'w' vector is, and how it is defined. The defintion of rotation can be found therein. Basically, it boils down to your basis vectors having time derivatives (relative to an inertial frame) of a certain (complicated to see) form. IOW, a particular type of acceleration. In General Relativity, that is extended to *covariant* derivatives, expressing what something "is doing" relative to its own geodesic, using its own ruler and clock. Those are called Fermi-Walker derivatives.

The moon has non-zero Fermi-Walker derivatives. In flat space-time in the low velocity limit, that agrees with the Newtonian definition of rotation.

The moon is rotating according that rigorous, and physically inspired definition.

-Richard

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Trying to summarize the Newtonian view, pithily:
1) In any reference frame in which the moon [/ baseball / car / arrow] does not appear to rotate, pseudoforces (centrifugal, Coriolis) will be present, indicating that the reference frame itself is rotating.
2) In any reference frame in which rotational pseudoforces are not present, the moon [/ baseball / car / arrow] will visibly rotate.
The rotation is always there, no matter how you slice it.

Grant Hutchison

16. Just to be a stinker, I'll point out the Fermi-Walker derivatives of the earth or moon are ever so slightly, vanishingly small, different from the Newtonian rotation ("stellar inertial" as NASA calls it, relative to the fixed stars as some like to say). That's the geodetic effect, one of things Gravity Probe B was looking for and has currently verified to within 1&#37; of GR's prediction -- they've hit some snags. Nordtvedt and others are having sort of a "We don't need no Gravity Probe B" party, and saying Lunar Laser Ranging data has confirmed both the geodetic effect *and* B_g to within 0.1%.

Anyway, it's interesting that the geodetic effect is present in the earth-moon system's orbital motion, and they say they see it to 0.1% accuracy. There are very small effects indeed.

There is the slightest bit of coordinate rotation of something in orbit around the sun due to this geodetic effect, which means our actual proper rotation is very slightly different than our sidereal rotation. And it's apparently been confirmed to high precision according to Nordtvedt.

-Richard

17. Sheesh, there's always someone who comes a long and makes a really simple thing that's already confused someone a million times more complicated than it needs to be...

18. Originally Posted by Peter Wilson
An astronaught . . . .
Ahem!

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A hem??? I just used the word 'hem' in an e-mail as a third-person
pronoun to refer to a netizen of unknown gender. (Unknown sex, too.)

-- Jeff, in lessee... Lhasa

20. One way of looking at it is, what would we see if the moon did not spin, but continued to travel around the Earth every month?

We would of course see one side, then a fortnight later we would see the other side.

This does not happen.

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## Spin

Pete, you're the man with the plan! Thanks for that expose'! And Publius....whoa!

I was hung up on the 'classical' physics definition:

In physics, spin is the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point.

[source: Wikipedia]

It still seems a bit odd, as spin is defined as an object rotating about it's center mass. I mean, one could swing a broom handle around his head and pick any of a number of infinite spots on it, and, according to what I'm hearing, they are all spinning. Center masses unto themselves? Is 'spin' at least as defined by whomever wrote the Wikipedia definition, dependent on interpretation of a relative observer?

For some reason, it seems more reasonable to me that the moon, although carrying the potential to exhibit kinetic energy in the form of 'spin' according to the classical physics definition, seems constrained from doing so unless it were released from orbit. I don't currently fully understand why the definition makes the distinction between the two kinds of angular momentum if they are one and the same, qualitatively speaking.

Say I take a transparent hollow sphere half filled with water and spin it so that the water is stuck to the sides, it is empty in the middle. We can see clearly that the water is orbiting the center as per the definition of 'spin'. If I were to simply attach it to the outer rim of a wheel and spin the wheel, all the water would stay at the outside.

The mass is no longer orbiting the center, but captive to an external axis point. It is exhibiting Orbital angular momentum, but how can it now still be exhibiting intrinsic orbital momentum?

I mean, perhaps the whole 'idea' of an imaginary axis point is at the crux of the issue, for it seems we must assume the axis point to maintain absolutely fixed orientation coordinates for the moon to spin.

The two forces, Orbital angular momentum and intrinsic angular momentum seem to be in super position with one another to my current understanding.

Is it possible at all, that the moon could orbit while maintaing fixed orientation coordinates, and orbit as it does now and 'not' truly be spinning?

Perhaps only a static central observer to our Universe knows anything, for sure.

My head is spinning in more ways than one, I must stop now.

TS
Last edited by Techstuf; 2007-Jul-03 at 07:47 AM.

22. Liberation proves that MOON IS SPINNING.

23. To all those who still think it does not. I sagest you reload the logic program of your hard drive

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Originally Posted by Techstuf
I was hung up on the 'classical' physics definition:
Yay. Another explanatory triumph for Wikipedia.

Originally Posted by Techstuf
Say I take a transparent hollow sphere half filled with water and spin it so that the water is stuck to the sides, it is empty in the middle. We can see clearly that the water is orbiting the center as per the definition of 'spin'. If I were to simply attach it to the outer rim of a wheel and spin the wheel, all the water would stay at the outside.
The forces that stuck the water to the equator of the spinning sphere are still present when it's going around on the wheel; they're just overwhelmed by the field of forces arising from the "orbital" motion.
If you break down the components of force in this scenario mathematically, you find you can render them into:
1) A uniform acceleration of the whole sphere towards the centre of the wheel (which is what keeps it moving in a circle around the wheel's centre)
2) A radial acceleration of the sphere's substance (and the water) towards the centre of the sphere, which is what keeps the sphere's substance (and the water) moving in a circle around the centre of the sphere (="spin") while the centre of the sphere goes around the centre of the wheel.

Now, that's not glaringly evident in your example, because the wheel is transmitting force to the sphere, which is transmitting it to the water, so the water ends up piled up at the outer side of the sphere. But if you stuck accelerometers to the sphere, you'd see that the outside (the part farthest from the wheel hub) is accelerating harder towards the centre of the wheel than the inside (the part nearest the wheel hub), and that the difference between those two accelerations corresponds exactly to my component 2) superimposed on component 1).
If you put a moon in synchronous orbit, with gravity supplying component 1) throughout the moon's volume, then component 2) becomes very evident because the moon develops an equatorial bulge as it generates component 2) within its own substance. [It also develops a tidal distortion because gravity isn't a uniform force capable of precisely generating component 1), but that's another story. ]

So I wonder if you've got yourself into a false dichotomy as a result of the Wikipedia quote: there's no reason both "kinds" of angular momentum can't be present at the same time; it all depends on how you look at the rotations and forces involved.

Originally Posted by Techstuf
Is it possible at all, that the moon could orbit while maintaing fixed orientation coordinates, and orbit as it does now and 'not' truly be spinning?
It is: it just maintains a constant orientation relative to the stars. An observer on Earth would then see all sides of the moon during the course of a single orbit. It would not develop an equatorial bulge, because my component 2) of the accelerations wouldn't be required. [But it would develop a tidal bulge, because of the gravity thing alluded to earlier.]

Grant Hutchison

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## Wow

Wow. I now realize for certain that the best possible vantage point from which to view the infinite, is the exact center.

Amazing stuff, truly amazing.

Thanks for all the truly caring replies you guys!

Regards,

TS

26. The centre of what?

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The center of whom

TS

28. one small question though : what IS known about the dark side of the moon ??? no one seems to talk about this :S

29. Originally Posted by Twinsun
one small question though : what IS known about the dark side of the moon ??? no one seems to talk about this :S
The rotation of the moon is proved and we known everythink about dark side; http://en.wikipedia.org/wiki/Far_side_of_the_Moon
http://en.wikipedia.org/wiki/Libration

"

The animation shows a set of simulated views of the Moon over one month.

In astronomy a libration (from the Latin verb libro -are "to balance, to sway", cf. libra "scales") is a very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves. Used alone, the term usually refers to the apparent movements of the Moon relative to Earth, which can be compared to the rocking of a pair of scales about the point of balance.

Although the Moon's rotation on its axis is synchronously locked with its revolution around Earth, these librations permit a terrestrial observer to see slightly differing halves of the Moon's surface at different times. This means that a total of 59&#37; of the Moon's surface can be observed from Earth.

There are three types of libration. Libration in latitude is a consequence of the Moon's axis of rotation being slightly inclined to the normal to the plane of its orbit around Earth. Its origin is analogous to the way in which the seasons arise from Earth's revolution about the Sun. Libration in longitude is a consequence of the Moon's orbit around Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position. Finally, there is a small effect called diurnal libration. This is a consequence of Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's center to the Moon's center, allowing the observer to look first around one side of the Moon and then around the other."

That guy who posted this topic is trying to rediscover America.
The Moon's rotation was proved very loong ago.
Anyone can observe liberation.

30. w00t so it's called the FAR side ... I thought it was called the DARK side so that's why I couldn't find anything thanks

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