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George,

Actually the earth is an oblate spheroid, sort of squashed at the top
and stretched at the equateor, and the moon librates (quite a lot) in
its orbit.

Lisa


>
> ====================
>
> This is due to the ellipsoidal shape of the Earth. If the Earth were a
> uniform sphere, then the gravitational attraction of the Moon (and, to a
> lesser extent, the Sun) would act exactly through the Earth's centre. But
> the Earth's ellipsoidal shape upsets that simple geometry. You can think of
> the Earth as a sphere with a sort of "spare tyre" added round its equator.
> The total attraction on that extra ring does not pass exactly through its
> centre, so now there's a slight torque which is trying to align the Earth's
> equator with the averaged plane of the Moon's orbit. The Moon's orbit
> wobbles about a lot,

   

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