Wal posted:

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Yes, the moon is moving around the Earth, but its orbital path is very slowly
expanding, increasing the moon's average distance very slightly each year.
The reason for this is tidal forces. The moon raises water tides on the
Earth, since its gravitational pull is slightly higher on the side of the
Earth which is pointed towards the moon and slightly lower on the part of the
Earth which is away from the moon. The action of these water tides is to
eventually (and very gradually) slow the rotation of the Earth almost like a
friction brake. However, the mass of these tides also acts to very slowly
accelerate the moon in its orbit (conservation of angular momentum). As the
Earth slows, some of the angular momentum of the Earth is transfered to the
moon. Thus, over the years, the moon's orbit expands. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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