Re: Moon's synchronism

[William Beaty <billb@eskimo.com>]

On Tue, 17 Mar 1998, LUDWIK KOWALSKI wrote:

> Our Moon spins in its axis at exactly the same rate (T=27.3 days) as 
> its period of revolution about the Earth. That is why we never see its 
> other side from Earth. Is this a pure coincidence? I suspect that there 
> must be some profound reason for this. What is it?
>                                                     Ludwik Kowalski

"tidal locking".  The "tidal" stretching of the moon by the earth's field
gradient causes internal heating as the moon rotates, therefor its
rotation must slow down as energy goes from KE to the internal heating
(and the moon must also orbit faster/higher, if I understand correctly.) 
In the distant past the moon's "day" was presumably much shorter.  As the
rotation slowed, the "day" lengthened until it was near the orbital
period, and then it became stuck at that period.  I think I remember
hearing that because of its slight asymmetry, the moon's rotation didn't
just "lock on" to the orbital period, instead it started overshooting and
undershooting, swinging with a rotary pendulum motion with respect to the
earth, until tidal "friction"  damped that motion too.  The moon is still
supposed to be swinging with a slight rotary oscillation, but I don't know
if that is left over from the original swinging, or if it's from
interaction with the sun, etc.  I think there are moons of Jupiter or
Saturn in a similar tidally-locked situation. 

If we wait long enough, the same should happen to the earth, and the 24
hour "day" will stretch to become months long, lock to the moon's period,
and then the earth will always present the same face to the moon. 

Since I picked all this up from "hard SF", somebody else here might wish
to speak up and correct anything I got wrong.

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