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Subject: Lunar by altitudes
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Tue Feb 26 2002 - 15:12:25 EST
I just got signed on to the List, and probably shouldn't jump in so soon.
The idea of comparing the altitude of the moon with that of one or more other
bodies to get Greenwich time has been around almost continually for at least
two centuries. It's such an appealing idea that many of the best navigation
authorities have been blind-sided by it.
First: Consider what you know about the reliable accuracy of altitudes taken
from the sea horizon, especially from a small boat. You will be combining the
errors of several such altitudes.
Second: Consider what it is you are trying to do. You are trying to measure
the moon's position in her orbit, NOT the rotation of the earth. If your
latitude is less than 30° there may be times when you'll actually see the
moon when her orbital motion is perpendicular to the horizon. At higher
latitudes, never.
The moon's is always within about 5° of the ecliptic. Her enlightened limb
points to the sun and shows the path of her orbit. How often do you see the
moon when her horns are pointing straight up or straight down?
Bruce Stark
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