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Subject: Re: Calculated Altitudes for Lunars
From: Herbert Prinz (hprinz@XXX.XXX)
Date: Thu Oct 24 2002 - 11:26:44 EDT
Bill,
You write
> If you knew your longitude but only guessed GMT, then the error in
> computed LHA of any body would be about equal to the error in GMT
> (converting time to arc).
But if I know my longitude, I do not need a lunar distance observation!!! I
just take the altidude of any celestial body that I find in the almanac,
and I am done. It is a totally unrealistic scenario to assume that I know
my position, but not GMT, and take a lunar exclusively for the latter. This
only occurs in backyard navigation, where the goal is to practice the
distance sight itself and one needs a quick way of reducing the sight in
order to evaluate it.
In the real world (of times long gone, to be sure), how would a captain at
sea know his longitude, but not local time? He would have to see land (or
trust his DR 100%) while having no chronometer and no usable horizon for
taking altitudes. How would he then work a lunar distance anyway and why
would he bother?
At any rate, GMT from lunar distance at known terrestrial position and GMT
from lunar distance at known celestial position (i.e. latitude and time)
are two entirely different problems that require different methods for
their solution. It does not make sense to compare the efficiency of the
respective algorithms. You have no choice between them.
Herbert Prinz
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