The moon

[Majestic13]

Can anybody help or explain why does the moon have a gravity but does not rotate on its own axle?

[imaplanck.]

Originally Posted by Majestic13

Can anybody help or explain why does the moon have a gravity but does not rotate on its own axle?

Any mass(or more correctly ''energy") exerts a gravity. The moon is "tidal locked", funnily enough; meaning the action of the oceans effect the moon, acting against it's angular momentum

[Jay-qu]

Infact it does rotate!

Have you never noticed that the same side of the moon always faces Earth! How could this be possible if it does not rotate?

Imaplank is right, it is tidally locked, forcing it to 'roll' around the Earth, keeping the same side visible to us at all times.

J

[Majestic13]

Another topic that do interest me very much is, "Could the moon be hollow" There was real discussion about this topic. The only reason why I'm saying it is interesting is, that NASA report during the Apollo project when they first landed on the moon is, that "the moon rang like a bell" Have you more information about this topic?

[imaplanck.]

Originally Posted by Jay-qu

Infact it does rotate!

Have you never noticed that the same side of the moon always faces Earth! How could this be possible if it does not rotate?

Imaplank is right, it is tidally locked, forcing it to 'roll' around the Earth, keeping the same side visible to us at all times.

J

Welllllll, one could argue that IRT the earth, the moon indeed doesnt rotate(that is complete a revolution, IRT to the earths own rotating frame).

[Majestic13]

Yes, I do understand in such way that the moon does rotate ariund its own axle, maybe my the english is not so good to express myself properly. but is it coincident that only the one side of the moon is always facing the earth

[Turtle]

Originally Posted by imaplanck.

Welllllll, one could argue that IRT the earth, the moon indeed doesnt rotate(that is complete a revolution, IRT to the earths own rotational frame).

One can always argue a mistaken point. The Moon rotates on its axis...period.

As to the Moon being hollow because it rings, this is also a mistaken idea. Tuning forks are completely solid, and even a solid straight bar of metal will 'ring' if struck.

[imaplanck.]

Originally Posted by Turtle

One can always argue a mistaken point. The Moon rotates on its axis...period.

:

No! IRT the earths rotating frame of reference, the moon doesn't complete a revolution. One can always argue that only a physicist will understand that point.

[Jay-qu]

Not co-incidence, its called tidal locking

[Boerseun]

Yep - the moon is tidally locked, rotating around its axis in exactly the same time it takes to make one orbit of the Earth. Quite easy to understand, if you think of a circular racetrack, the inner track is considerably shorter than the outer track. So, the side of the moon facing the Earth covers a shorter path in the orbit than the side facing away, the "Dark Side". If the moon revolved around its axis in less (or more) time an orbit around the Earth takes, then than means the side closest to the Earth will have to accellerate as its passing into the "outer track", and the side facing away from the Earth will have to decelerate as it passes into the "inner track". This constant accelleration/deceleration causes immense stresses in the mass, and is cancelled out by the moon simply altering its rotation to fit its orbital period.

The rings of Saturn is theorised to be a moon which was destroyed by this tidal friction, because that particular moon was too close to Saturn, and the tidal forces too strong. Also, because of that, it seems unlikely for Saturn's ring system to fall back together into a new moon again - the difference between the orbital speeds of the "inner tracks" and "outer tracks" are simply too big for it to coalesce into a new body.

The Earth, of course, is also subject to the same forces, but being a much larger body with much more momentum, and contains the barycenter of the Earth-Moon system, it will take billions upon billions of years before the Earth is tidally locked to the system, although the Earth's rotation is slowing down because of tidal friction.

The other issue, the moon "ringing like a bell", is purely metaphorical. It refers to seismic shocks transmitted through the moon's solid body in experiments conducted by the Apollo astronauts on their lunar sojourns. It is another case where the facts are confused by language: "Tidal friction" has nothing to do with the oceans, although the language is appliccable to both. And the moon isn't hollow, although the image created by the bell metaphor can quite understandably convey the idea.