From: Ronald Wong (ronwong@inreach.com)
Date: Fri Sep 28 2001 - 00:29:23 PDT
Message-Id: <l03102802b7d8914fcf6c@[209.209.18.104]> Date: Fri, 28 Sep 2001 00:29:23 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: re: moon approaching earth
jeremy eddy <jerem@rocketmail.com> asked:
> Does anyone know the rate at which the moon is
> approaching the earth? In what year are they
> predicted to collide? ...
In general, the moon is orbiting about the earth along an elliptical path
and the earth is at one of the focal points of this ellipse. As a result,
there are times when the moon is moving closer to the earth and times when
it is moving away. It isn't "approaching the earth" in the absolute sense
so we don't have to worry about a collision with our moon unless something
quite catastrophic occurs that upsets the present situation.
If you study earth-moon system in greater detail, you will discover that
the moon is slowly but surely moving away from the earth.
The reason for this is twofold: tidal friction and the conservation of
angular momentum.
The tides created on the earth due to gravity lead to frictional forces
acting on the earth. This comes about because the earth rotates under the
tidal humps. As a a consequence, the earth is gradually slowing down and,
as a result, the length of the days are getting longer (about 1 second
every 100 000 years). Since angular momentum must be conserved, the loss of
the earth's angular momentum must be accompanied by an increase in the
moon's angular momentum. The moon achieves this by receding from the earth
(the greater the distance from the point of rotation - the earth - the
greater angular momentum).
In time, the earth's rotation will have slowed down to where it, like the
moon, ends up with only one side of it's face facing the moon (synchronous
rotation). At this point the tidal effects become frozen in time and the
earth will no longer experience tidal friction. If there is no more tidal
friction, then there is no more increase in the distance between the earth
and the moon.
At this point, the length of a day for the earth will have become about 50
times longer than what it is today and the moon will be about 50% further
away.
Actually, the time scale for all of this to happen is so great that, long
before this final outcome is reached, both our planet and it's moon will
have been absorbed by the sun as it went through the red giant phase of
it's life cycle.
So much for all the number crunching.
Cheers - ron
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