Re: Reason for Procession of the Equinox?

From: Dr John Stockton (spam_at_merlyn.demon.co.uk)
Date: 12/02/04 

Date: Thu, 2 Dec 2004 16:04:27 +0000

JRS: In article <col453$hdv$1@zot.isi.edu>, dated Wed, 1 Dec 2004
18:56:35, seen in news:sci.astro.amateur, Brian Tung <brian@isi.edu>
posted :
>
>The only way that the Moon's center of orbit could be at the center of the
>Earth is if it had essentially zero mass. Otherwise, the center of the
>orbit is some 10,000 km (the eccentricity) away from the center of gravity,
>which is located about 5,000 km away from the center of the Earth.

The eccentricity of an (elliptical) orbit is a pure number.

Both Moon & Earth orbit their common centre of gravity, with minor
corrections due to other parts of the Universe. If the separation is
400,000 km, and the ratio of masses 1:79 (almost, but not quite,
correct), then the centre of the Earth is 5000 km away from the centre
of gravity (the Earth's radius is nearly 6400 km, for those still using
the Imperial System of units).

I doubt the significance of 10,000 km, except as the diameter of the
Earth's monthly orbit.

The Earth-Moon distance is not constant; both orbits are in fact
basically ellipses (of the same eccentricity) with a focus at the
C of G.
  

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