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Subject: Re: Explanation by Mr. Stark needed
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Thu Jun 12 2003 - 13:11:59 EDT
Doug,
Please don't hold me to my statement: ". . . the moon being east or west of
you doesn't mean you are west or east of her." I was thinking DUE east and
west. In other words, a body's position angle is hardly ever the reciprocal of
the azimuth. This is easily demonstrated with a piece of string on a globe.
The ideal situation for getting local time or longitude is when a body is due
east or west. That's when it is perfectly in line with the motion you want to
measure, the earth's rotation. But to get GMT from the moon you have to
measure the moon's motion along her orbit. The earth's rotation has nothing to do
with it.
Years ago, I picked a time from the Almanac and set up a hypothetical
situation where a boat was expecting to be entering the English Channel in a few
days. Night was falling and weather making up, but the single-handed navigator got
a good cut with two stars, and the altitude of the moon. His LOP lunar put
the boat well out in the Atlantic. Reassured, he went below.
Trouble was, he was just west of the Scilly Islands. Although the moon was
within 5° of due west, and all three altitudes more accurate than could be
expected, the observation was a disaster. The moon's orbital motion, as I recall,
was out of line with her altitude by 79°.
Bruce
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