From: joe@montebello.soest.hawaii.edu (Joe Dellinger)
Subject: Re: the moon
Reply-To: joe@montebello.soest.hawaii.edu
Organization: School of Ocean and Earth Science Technology
Date: Sat, 22 Aug 1992 00:56:13 GMT

|> >[Richard.Mathews@West.Sun.COM's] argument seems to suggest that the moon 
|> >would deccelerate, move closer to the earth and violate conservation of
|> > angular momentum.
|> >-Scott

	Sometimes its useful to remember that you can DERIVE conservation
of momentum from the relevant physical laws (like F=MA, Newton's law of
Gravity, etc). Once you've done that, working out what will happen in
certain problems using conservation of angular momentum is an extremely
convenient short cut. BUT... you could, in theory, just work out the whole
thing by following all the myriads of interactions between the particles making
up the Moon and the Earth!

	In this case, if the Earth rotated retrograde the tidal bulge would
always precede the Moon in it's orbit, decelerating it, and thus bringing
the moon closer and closer in. [I suspect you were thinking of running
the scenario in reverse. There are two problems with that: 1) then the
moon would be retrograde too; 2) the tidal friction dissipated as heat
in the forward scenario becomes heat miraculously being used up maintaining
a bulge in the Earth in the wrong place in the reverse scenario.]


|> As the moon moves into a closer orbit, the tidal forces increase (inverse
|> cube of distance?), rapidly increasing the deceleration until CRASH!
|> Donald Arseneau      asnd@reg.triumf.ca

	In fact, the Earth wouldn't have to go retrograde. Just as long
as it turned forward on its axis slower than the moon orbits it would be
enough. Mars' moon Phobos is in just this situation now. It will continue to
spiral IN to Mars because of tidal effects, and must eventually crash.
Mars seems to have an excess of low-angle impact craters around its equator;
it has been suggested that perhaps Phobos is merely the last survivor of a
number of former Martian moons left in low orbits as the result of some
improbable near-Mars asteroid collision / breakup. All the others have
already crashed, each leaving behind an equatorial low-angle impact
crater as a sort of tombstone.

	In fact, if the Sun let the Earth survive that long, the Earth's
moon would eventually crash too. Here are the stages on the way:
1) Today: Moon relatively close, tidal forces moving Moon away from Earth.
2) Moon moved away. Length of day very close to length of month.
3) Tidal forces of SUN slow the Moon down some more. Length of day becomes
   LONGER than length of month. EARTH-MOON tidal forces start pulling
   the moon in again.
4) Moon moves in and in, month gets shorter and shorter; day gets shorter
   and shorter too, but always longer than month. Eventually moon gets
   too close and breaks up due to tidal disruption. Earth has a ring.

	Supposedly stages 2, 3, and 4 will never happen though because
we all get fried by the sun long before.

     /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
    /  \  /  \  /Hawaii Institute of Geophysics, Honolulu\/\/\.-.-....__
___/    \/    \/Joe Dellinger, Internet: joe@montebello.soest.hawaii.edu\/\.-.__
____________________________________________________
Objectivity is not an unobtainable emptying of mind |
but a willingness to abandon a set of preferences   |----> Stephen J. Gould
when the world seems to work in a contrary way.     |