How exact is the rotation of the moon relative to its orbit that keeps the same side of it facing Earth?

Question

How exactly "the same side" is it?

Just to make sure the question is understood:

If you were to be on a frictionless moving platform that could move over the entire surface of the Earth and was able to keep the moon directly above you at all times, from that frame of reference, how many arc seconds or degrees or whatever would it rotate (from your point of view) in x amount of time?

Or could it really be perfect and never rotate (from your point of view if you had a magical platform)? Hard to believe it could be perfect.

Answer 1

Not really perfect. As the lunar orbit is no circle, the angular velocity around Earth is not constant, so it does wobble a bit seen from Earth. This movement is called liberation.It allows us to see 5% more of it's surface from Earth as possible when it would be perfectly showing only one side.

But one rotation of the moon still requires exactly one lunar orbit. That rotation rate is stabilized by the gravity of earth, which pulls stronger on the side pointing towards Earth.

The liberation movement is still less than theoretically allowed for a gravity gradient stabilized object alone.  

Answer 2

Well to answer the simple part, the time it takes for the moon to orbit the Earth is equal to the amount of time it takes for the moon to rotate on its axis.  This means that when it orbits, it spins at a rate that it appears that the same side always faces the planet.

Answer 3

The moons rotation on it's axis  is a constant and steady speed.  Because it's orbit is slightly elliptical (not quite a perfect circle) the moons slows down (farthest distance) and speeds up (closest distance) in it's orbit around the earth.  But it averages out. The term to look up here is "libration."  But it does even out, so that the far side of the moon, never faces the earth.  We see slightly more than 50% of the lunar surface throughout the course of it's orbit because of this.  The link below is an image that illustrates this.

http://a967.ac-images.myspacecdn.com/ima...

Answer 4

There are two effects, known as "librations" which enable an observer on Earth to see more than half of the Moon over a period of a month. The libration in longitude arises from the fact that the Moon's orbit is elliptical which means that its angular motion in its orbit is not constant. At the time of the Moon's perigee, it's orbital revolution rate is faster than its axial rotation rate and the accumulated difference amounts to a total offset of up to about 8 degrees by the time the Moon is about 90 degreees past perigee in its orbital position. Similarly, about 90 degrees after apogee, the offset can be up to 8 degrees in the opposite direction.

The libration in latitude is caused by the fact that the Moon's rotational axis is inclined at slightly under 7 degrees to the perpendicular to its orbit, so the Moon's north and south poles successively tilt towards the Earth by this amount during a month.

Answer 5

It's all a big conspiracy in my opinion. Just too farfetched to believe it could be perfect or even that close. How is it that the sun look the same size as the moon anyway?

Answer 6

The same side faces the earth due to density differences in the moon.  The denser side is drawn more strongly to the earth and so always faces it