How do we know the moon is still rotating.?

Question

Thankyou to everyone who put me straight on the moons synchronus rotation around its own axis and not the earths.
My son is very happy to be an IOU millionaire at 13 ! ( see my original question)
As I understand it, the slowing of the moons `natural` speed of rotation was caused by the earth and moons gravitational pull.
If the moon were to break free of the earth`s pull would it start spinning quicker, stay the same or fly off like a rock.
Ah you can see I`m still a bit sceptical. So the crux of my question is how do we know it is still rotating. after all this time in synchronous orbit how do we know it hasn`t just stopped

Thanks for the tips and I did understand the imagery and the concept,its just I had another image in my minds eye.
If I held a tennis ball at arms length and rotated around my own axis , the same face of the ball would face me at all times without any synchronous rotation in my hand. Also if the moon is heavier on one side and this side is stongly attracted to the earth so that that face always points to the earth it wouldn`t need to rotate around its axis to produce the same effect.
So perhaps you can see where I was coming from. However If this were the case I guess the moon would be fixed at the same point in the sky at all times and people on the `far side ` of the earth wouldnt have known about it. But once again that is assuming the gravitational force is linear at one point and pressumeably it is reasonably even in all directions around the earth?

Answer 1

We can be sure that the Moon is still rotating around itself. We know that the Moon is orbiting once around earth in one lunar month. So if the moon would have stopped its rotation, the sun would shine more or less onto the same side of the month during one lunar month. The observable result from the earth would be that through the phases of the moon the line between darkness and sunshine on the moon would be along the same craters and maria. But we can observe that during one lunar month the lunar phases reveal that the terminator (border between day and night on the moon) is wandering around the moon. This is a proof that the moon is still rotating.

If somebody could move the Moon out of the orbit of earth (or to orbit twice as high) you would notice that the rotation of the moon would be once in a month while the lunar month would be much longer.

Answer 2

We can see that the Moon always has the same face towards the Earth.

At new Moon, when the Moon is roughly between the Earth and the Sun, its Earth-pointing face must be pointing away from the Sun. At full Moon, the same face must be pointing towards the Sun. So we can see that the Moon is pointing in a different direction, which means that it must have rotated.

The friction of the tides is slowing down the Earth's rotation, and we can see direct evidence of this in the structure of old sedimentary rocks. Eventually, the Earth would also be rotating only once a month, but long before that it is expected that the Sun will have turned into a red giant and swallowed all the inner planets.

Answer 3

This is a bit more of a trickier question to answer - but I'll do so in two parts.

Firstly, we do know that it is still rotating by virtue of the fact that it presents the same face to the Earth at all times (in, as you correctly say, synchronous rotation). If it wasn't rotating, we would see all sides of the Moon as it rotated around us.

Secondly, if it did ever break free of the Earth's gravity, the amount of rotation it would have would be entirely dependent on how this happened. Newton's Laws of Motion lead us to the conservation of angular momentum (i.e. energy) - whereby the energy involved in rotation cannot disappear, but can be converted to other forms of energy.

For example, if the Earth/Moon system were to be in collision with another body which broke the moon free, then the sum rotational energy of all three bodies would have to be preserved - some (quite a bit) would convert to light, heat and sound, but the balance would be retained by what was left of the bodies as angular momentum (i.e. rotation). Think of a snooker or a pool game - the energy of the cue ball being hit into a stationary target ball will dissipate as sound (the "click" as the balls contact) and motion as the two balls go off in different directions.

Hope this helps.

Answer 4

If you carried out the experiment I suggested (walking around your son and always facing him) you would see that, as a previous poster has said, the very fact that we always see the same face of the moon IS proof that it is still rotating.

If the moon were not rotating, then when it travelled to the opposite side of the earth from where it is now people there would see the far side of the moon. But they don't. The moon has turned to face them!

If you can't get your son to go along with this, just place a kitchen chair in the center of a room. Face the chair, then walk to the opposite side of the chair without turning. Your "far side" is now facing the chair. In order to keep facing the chair, you have to rotate.

Answer 5

Because if it had we would see different features on its face as it orbits around the Earth over the course of a month. It does wobble a bit so sometimes we see a little bit more or a little bit less at a given "edge" (limb) but otherwise we look up and see the same features, though the changing illumination angle from the Sun does change the contrast in what we see and brings out particular features at given phases.

Answer 6

The moon is gravity gradient sync with its rotation around the earth. This is to say that always the same side is what we see ,because the side that is near the earth is heavier ,

Answer 7

Do you know how much crap we left on the moon? Check periodically to reference the items on the list as to change of position.

Answer 8

when it change its shape