Ohhhh! Good grief, of course! I've been thinking the elliptical orbit
center and the center of [mass of] the moon are in the same place, rather
than the center of the moon corresponding to one of the focii of the
ellipse=2E Well, I feel damn silly, but relieved=2E :)
Too much Teletubbies and Sesame Street lately=2E :)
Thanks for the explanation, Grant=2E
Clint=2E
Original Message:

From: Grant Hutchison granthutchison@...=2Eco=2Euk
Date: Mon, 29 Sep 2003 17:19:35 +0100
To: cweisbrod@...=2Eca, celestiadevelopers@...=2Esourceforge=2Enet
Subject: Re: [Celestiadevelopers] Apollo 11 Orbit
> Obviously, this is not at all correct=2E But why isn't it? Where's the f=
ault
> in my math? Have I been dillusional for half of my life?
Your formula is correct, but your difficulty is with the numbers you're
using for semimajor axis and semiminor axis=2E The semimajor axis is half =
the
long axis of the ellipse, and the semiminor axis is half the short axis  =
so
these two measurements are at right angles to each other, each taken from
the geometrical centre of the ellipse=2E But the two numbers you've used a=
re
actually the pericentre and apocentre radii  the distances from one focus=
of the ellipse to the two ends of the long axis=2E If you average these tw=
o
you'll get the semimajor axis, but then you pretty much need to work out t=
he
eccentricity in order to calculate the semiminor axis, which isn't really
any use for orbital calculations anyway=2E
Grant

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