Expert: Jayendra Upadhye - 3/26/2008

Question
Hello,
What physical rules govern the moon's orbit so that we can
see only one side of it.
In other words:What would prevent the moon from spinning like
many other universe objects do?
Many thanks

Answer
Dear shalom,
That is due to tidal "phase locking".
That is a general rule applying to even planets that orbit close to their stars. If mercury had not been in 3:2 resonance, it would have been tidally phase locked like our moon. (showing same face to the sun).

The moon and some other moons of other planets in the solar system are tidally phase-locked by gravitational bulges / distortions in their bodies.

There is a clear gravitationsl gradient (variation of gravitational intensity (g) from the satellite's / orbiting planet's opposing point and closest point to the main gravitating body. This distorts the satellite to make it non-spherical and pear shaped!

As the satellite rotates, newer and newer areas have to undergo this deformation, at the expense of the angular momentum of the satellite's / planet's spin.

[Energy of deformation comes at the expense of angular momentum].

In the end the satellite thus slows down and stops.
[Some energy is lost in non-regenerative lossy heat processes, that keep their cores molten, as in the case of some icy satellites of our gas giants].

The closest example on earth is running a car on tyres with less air pressure. You will notice, since the tyres are flatter, lot of energy is wasted in deforming the rim of the tyre that "comes under the car on the road". As each spin means that deformation has to travel along the whole periphery of the tyre, a lot of energy is wasted, and engine is loaded.
[we can directly experience deformational loading in bicycles with less tyre pressure].

In some cases, the satellite may "speedup" to get phase locked too.

The other benefit of tidal locking is that is that to conserve overall system angular momentum, the satellite picks up orbital speed and hence actually "recedes" from its parent, as our moon is doing now at approx 4 cm / year!!

reference:-
1 - http://en.wikipedia.org/wiki/Mercury_(planet)
2 - http://en.wikipedia.org/wiki/Tidal_locking

hope that suffices.

Please do rate the answer.

Jayen