Re: Will the Moon Crash Into Earth?

In article <1172604583.214659.119870@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Stuart" <bigdakine@xxxxxxx> wrote:

On Feb 8, 5:31 pm, Jo Schaper <jospamnotschaper34@5socket78dot9net>
wrote:
effac...@xxxxxxxxxxx wrote:
I asked a science prof if the moon will eventually move away or
into the earth.

He said neither, it will remain revolving the earth for eternity.

Obviously after this insulting response I stopped taking him too
serious.

Well, barring a collision with an asteroid, the moon probably will
continue moving away (as it is currently by a few cm a year.) It
likely will remain in orbit at some distance, until both it and
earth are crisped to cinders by the expanding sun, a couple of
billion years in the future. A couple of billion years is close
enough to an eternity for me.


Not quite. There comes a time when the moon and the Earth will have
perfect spin-orbital resonance; i.e., the same part of the will face
the same part of the earth. At this point the orbital period of the
moon and the rotation period of the Earth will be 48 days.

You mean the same side of the Earth will face the moon, as one side of
the moon already faces the Earth.

This will happen because the moon's pull on the Earth, through the
tides, slows the Earth's rotation.

When this situation is achieved, the tide becomes static and
dissipation ceases, hence left to its own devices the system will not
evolve futher. However, there is another actor here, and that is the
Sun. And the influence of the Sun will be to reverse this and send
the Earth-Moon system spiraling back towards each other.

How will this happen?

A nice discussion of this can be found in Frank Stacey's book
"Physics of the Earth".

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