What's up with our lunar activity; why doesn't the Moon spin?

Question

I don't know why I've never thought about this before; I used to be a physics major-being interested in astronomy and astrophysics.

But what is wrong with our Moon?

Answer 1

The moon does spin!  The rotation is once a "lunar" month.  We always see the same side of the moon but if you look at the sun's point of view, it rotates.  That is why there are lunar phases.

Answer 2

Nothing is wrong with it.  And it does spin.  It turns around once per orbit, so that the same side faces Earth all the time.  It's called being tidally locked.

Think of it this way - If you had a plastic bottle, and stuck a skewer through it so that it was perfectly balanced on the skewer, you could spin it around and it would stop pointing in any random direction.  But if you moved the skewer just a bit towards the top of bottle, it would tend to stop with the neck pointing up when you spun it, right?  That's because the heavier end would tend to point down.

The moon is like that bottle.  It's weight is not perfectly distributed.  The part facing towards us is just a bit heavier than the far side.  Over the millions of years, the moon's rotation gradually slowed until one side pointed directly towards us all the time.

Answer 3

the moon does spin.  It just happens to spin at the perfect rate so that the same side always faces the earth.  I don't know exactly why it is the same.  Probably having to do with magnetic fields.

Answer 4

The moon does rotate on its axis. It rotates at the same rate it orbits the Earth, thus we only see one side of the moon.

Answer 5

ahh, actually the moon does spin. however it just happens to spin at the just teh right rate so that one side is always facing the earth.

Answer 6

Tidal locking is a fun concept.

Kepler's laws of orbital motion relate orbital altitude very strictly to orbital velocity.  Consider the Moon as having two portions, the Near portion and the Far portion.  The Near portion, being in a lower orbit than the center of the Moon, wants to go faster to stay up.  But it can't because it's connected to the rest of the Moon.  So it has a tendency to "fall" toward Earth because it's going too slow for its altitude.  Conversely, the Far portion, being in a higher orbit than the center of the Moon, wants to go slower.  But since it too is connected to the rest of the Moon, it's constrained to move along at the same average speed.  So because it's moving too fast for its altitude, it wants to fly away from Earth.

This makes the Moon want to stretch along its orbital radius.  That's the "gravity tide" force.  If the Moon is itself turning at a different rate than its orbital period, the elastic deformation caused by the tidal force isn't a consistent direction through the Moon's bulk, and instead moves around.  The Near and Far portions are constantly changing.  Such a system will naturally tend toward a lowest-energy state, which in this case is when the axis of deformation changes at the same rate as the Moon's orientation in space -- its rotational period.

Answer 7

There's nothing really *wrong* with the moon... .but, consider: The center of mass of the moon isn't directly in the center.  It's about 100 miles from the center, pointing at Earth.   This is probably due to the way the moon formed;  it was  a collection of debris resulting from an Earth-impact that ejected the material that formed the moon.

Now... if you were to slowly spin a dumbell weight in space in orbit, there's the two weights at either end of the dumbell would continually be more attracted to the center of mass, and the other end would be less attracted.  Eventually, over the years, this tidal effect slows the spinning, until finally, one end is pointed at the body it's orbiting.... this is essentially what happened with the moon - it probably rotated faster than it's orbit, but now, it's what we call "Tidally locked" - always facing one side toward us.  

Answer 8

The moon does spin, its just that it spins at a slower revolution than earth, that to the human eye it looks like the moon doesn't spin.