[thedoc (OP)]

Leichhardt House  asked the Naked Scientists:
   Dear Chris,
 
I love your show!
 
Perhaps you can help me with an astronomical question that has bugged me for ages.  Why does the Moon always have the same face towards us?   Is it purely chance that it rotates on its axis at just the right rate for this to happen as it orbits the Earth, or is there some force that tends to hold it in this orientation, and if so, what is it?
 
Many thanks in advance...
 
Bob Jones
 
Toowoomba,
Queensland
Australia
What do you think?

[Janus]

Leichhardt House  asked the Naked Scientists:
   Dear Chris,
 
I love your show!
 
Perhaps you can help me with an astronomical question that has bugged me for ages.  Why does the Moon always have the same face towards us?   Is it purely chance that it rotates on its axis at just the right rate for this to happen as it orbits the Earth, or is there some force that tends to hold it in this orientation, and if so, what is it?
 
Many thanks in advance...
 
Bob Jones
 
Toowoomba,
Queensland
Australia
What do you think?

The answer is tidal forces.  Just like the Moon causes tides on the Earth, the Earth stretches and pulls on the Moon. This creates an effect called tidal braking. While the Moon was rotating faster than it is now, this slowly decreased its rotation until the it matched the orbital period. This known as being tidally locked.  Our Moon is not the only body in the Solar system many of the nearer Moons of the gas giants share this characteristic.  Pluto and Charon are tidally locked to each other.

The Moon exerts tidal braking on the Earth too.  But since the Moon is so much less massive than the Earth, it hasn't had time enough to tidally lock the Earth to it. ( give it enough time and it would eventually do so.)  Right now the Moon is increasing the period of the Earth's rotation by about 2 milliseconds per century.  This slowing is one of the reasons the length of the second in no longer defined as a fraction of the Earth's period of rotation. ( This means that our standard time system slowly drifts out of sync with the Rotation of the Earth and as a result from time to time we add a "leap second" to realign them.)

I should also point out that the Moon doesn't keep exactly the same face towards the Earth. While the period of the rotation matches the period of the orbit, the Moon's orbit is slightly elliptical, and thus varies its orbital speed over the course of the orbit.This means that the angular speed of the Moon's rotation sometimes is a bit faster than the orbit's, and sometimes slower.  It averages out over the whole orbit, and we see this as a slight rocking from side to side of the Moon.  The axis of rotation is also not aligned with the Axis of the orbit either. This means that when the Moon is on one side of its orbit we see a little more of its South pole and when it on the other side we see more of the North pole.  These apparent motions of the Moon are callled "librations".

[RD]

Why does the Moon always have the same face towards us? ... is there some force that tends to hold it in this orientation, and if so, what is it?

The moon isn't a perfect sphere ... https://wikipedia.org/wiki/Tidal_locking

[rmolnav]

I started to think about this question many years ago, after seeing somewhere just that it was "synchronized" ...
First thing I thought was that it could NOT be just a coincidence … Then my rumination about tides started.
I consider both Janus and RD are right.
I would add something relative to actual physical phenomena that have reduced Moon´s own rotation down to zero.
Janus and myself happen to be discussing just now about the double "bulge" of sea tides on
http://www.thenakedscientists.com/forum/index.php?topic=49715.new#lastPost
So, I´m not going to discuss the same here.
But even if the cause of those two bulges, both on Earth and Moon, were only gravitational (from the other object) due to differences in distances, the fact is that two bulges occur on the Moon too.
And that means a slight stretch of the Moon (as well as of Earth), in the direction Moon-Earth. On Earth we mainly sea water bulges, but solid earth is stretched TOO, as Moon  ...
When Moon was spinning faster, what affected by that deformation was periodically changing, continuosly having to go back to previous condition … That wastes friction energy, which only can come from own rotational momentum …
Slowly but surely, Moon ended up keeping that stretch line pointing us.
Surely what said by RD must have affected the exact final "pseudo-rest" position  Should Moon be a quite perfect and isotropic sphere, any final position would have been possible. Not being the case, actual position must be such as, besides the bulges, differences between distribution of masses across the Moon are highest in Moon-Earth direction. 

[rmolnav]

#3 (Continuation)
I said "... that have reduced Moon´s own rotation down to zero" ... I should have included the word "apparent", or have said "... down to 2PI/app.2d".

[rmolnav]

#4
Sorry: 2PI/app. 28 d. !!!

[RD]

... the exact final "pseudo-rest" position

Not quite at "pseudo-rest" : it's still rocking from our perspective ...

https://en.wikipedia.org/wiki/Libration

[evan_au]

is there some force that tends to hold it in this orientation, and if so, what is it?

Astronomers were quite surprised when the first photos of the far side of the Moon were seen, because it looked quite different from the near side.

There was a fairly recent hypothesis that the Moon may have formed from the (relatively) low-velocity impact of two smaller bodies that formed after the impact of a Mars-sized body with the Earth. It is thought that these bodies may have had quite different composition and different density, leading to the observed differences in terrain ("lunaine"?) and gravitational field observed between the "near" and "far" sides of the Moon.

The Earth impact would have thrown a lot of crustal material into Earth orbit, where it would have coalesced into one or more bodies. A body closer to the Earth's molten-rock surface would have driven out more volatiles, leaving denser materials, while bodies forming farther out would have had more "light" elements.

This could have affected where the Moon eventually came to rest when it was settling down to orbit with one side facing the Earth.

See: https://en.wikipedia.org/wiki/Gravitation_of_the_Moon

[rmolnav]

#7 evan _au
As I say on #3, as far as I can understand, the answer to the question is another.
What you say:
"The Earth impact would have thrown a lot of crustal material into Earth orbit, where it would have coalesced into one or more bodies. A body closer to the Earth's molten-rock surface would have driven out more volatiles, leaving denser materials, while bodies forming farther out would have had more "light" elements"
seems to me it would be a bizarre kind of coincidence.
What I remember to have read, or seen on TV from BBC Earth, is that those particles due to the impact, moving in a typical spiral way, coalesced into a spinning satellite, with more similar surface all around.
Initially it was much closer to Earth (causing much stronger tidal effects) and, due to what I say (or any other reason were I wrong), its spinning got synchronized with its rotation around Earth relatively soon.
Then Earth and Moon were still on the phase of crust formation from molten material. That made near Moon side cool much more slowly than far side, what originated there basalts (as far as I can remember) and other materials not so abundant on far side.