Question re the moon and its rotation

[MazDan]

While out camping recently and observing the moon, the question suddenly struck me..........why do I always see the same patterns on the moon. This suggests that it does not spin on its axis.

So I did a little research and also some quick experiments with round objects.

I learnt that the reason the moon always shows its same side is that it "spins" on its axis once every 27.5 days and is in synchronicity with the earth.

However I found that explanation rather faulty in that in all my experiments, bar one as explained below, I could not manage to get the moon to spin on its own axis and yet still only display the same face to the earth.

What I did find was that the moon in fact spun or rotated on an axis on the earth.

The only other possibility was that if spinning on its own axis then a continuation of that axis would be directly towards Earth which would also mean that in the US for example as opposed to here in Australia, the moon would actually appear upside down.

Am I on the right track here?

I dont seem to be able to find any explanations that really make much sense, well to me anyways.

 

[Foreigner]

I think your information source was inaccurate. According to this, the dark side of the moon is permanently turned away from us. The moon never rotates, so the same side is always facing us no matter what side of the earth the moon is on.

 

[Belisarius]

The Moon *does* rotate; it just rotates at the same rate that it orbits the Earth. Or rather, it's rotating east (like the Earth) at a rate that keeps pace with its westward motion in its orbit. Outside of the Moon's orbit, you would be able to see all sides of it, just like any other rotating body.

Take new moon, for instance; let's assume that (as is actually the case), the near side is all we can see at this time. If the Moon's orientation were fixed with respect to the stars, at first quarter we would "lose" half the near side and see about half of the (current) far side, because the Moon will have moved 90 degrees in its orbit. At full Moon, we'd see only the current far side, and not the near side at all. The reason we don't is because the Moon's rotation is not fixed; it rotates through 90 degrees between new moon and first quarter, and 180 degrees from new moon through full moon, etc. If you try this with say, a baseball with a distinct mark on one side, and turn it around you to keep that mark in the same position, you will *have* to turn that ball to keep that mark in the same relative position; if you don't it will rotate out of sight, and you will see all sides of the ball. This example works even better with a globe, if you have one handy.

 

[MazDan]

OK so the way I understrood it was that the moon rotates around the earth but is also spinning on its own axis as is shown in this picture.......http://mail.colonial.net/~hkaiter/Rotation_Revolution.html
But the only way that I can see that happenning and yet still allow only one side to be facing the earth at all times is if the axis on which it is spinning is an axis facing earth.
When I try it with two balls it seems the only explanation.

 

[MazDan]

I just found a utube video that seems to display it exactly as I first thought............ie the moon revolves around the earth but instead of turning on its own axis, its actually turning on Earths axis.
Similar to a bicycle wheel, the valve always points in but its spinning on the wheels axis not its own axis.

Sooo thinking this through and trying to understand the science side of things, it appears my understanding of axis is not correct.............My understanding of axis is basically an axle arround which something spins.

If we were to provide an axle for the moon and earth then the moon would be spinning on an axle through the centre of earth would it not?

 

[alasdairm]

explains it very simply (and visually):



the moon is rotating about its own axis, synchronously with its orbit around earth.

alasdair

 

[MazDan]

Yeah I guess I can see how it is actually rotating on its own axis although I wonder if they would describe it the same way if it was an odd shape.
Also I guess that if we consider an axle a "fixed" point then yes it must rotate relative to that fixed point.

I do now fully understand, Im just not sure I would describe it as rotating on its own axis so much as rotating on Earths axis. Although I can also see the problem with doing that is that earth itself is spinning on its own axis hence you could not simply join the two with spokes or a permanent staircase for example unlike a bikes wheel.

Oh this was the utube vid that explained it best. http://www.youtube.com/watch?v=OZIB_leg75Q

One further thought is that relativity appears to play the crucial role, meaning it depends on what it is relative to in order to determine axis and wether it rotates or not. Relative to Earth the moon does not rotate but relative to the universe it does. I think I got that right.

 

[rickolasnice]

It is rotating on it's own axis..

Saying the moon rotates on earths axis is like saying we are rotating on the suns.. which is obviously incorrect. If you took the earth away and just watch the moon in a fixed position you would clearly see it rotating.

And I relativity has nothing to do with it.. on that logic the sun rotates around the earth.

 

[alasdairm]

Yeah I guess I can see how it is actually rotating on its own axis although I wonder if they would describe it the same way if it was an odd shape.
Also I guess that if we consider an axle a "fixed" point then yes it must rotate relative to that fixed point.

i think you may be mixing up axle and axis (although they're essentially synonymous here).

I do now fully understand, Im just not sure I would describe it as rotating on its own axis so much as rotating on Earths axis.

the definition of an axis is: "a straight line about which a body or a geometric figure rotates or may be supposed to rotate". the moon is rotating on its own axis. it is not rotating about earth's axis - it is orbiting earth.

alasdair

 

[MazDan]

i think you may be mixing up axle and axis (although they're essentially synonymous here).
the definition of an axis is: "a straight line about which a body or a geometric figure rotates or may be supposed to rotate". the moon is rotating on its own axis. it is not rotating about earth's axis - it is orbiting earth.

alasdair

Yeah cheers mate, I get it now.