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Subject: Re: Calculated Altitudes for Lunars
From: Herbert Prinz (hprinz@XXX.XXX)
Date: Tue Oct 22 2002 - 19:28:18 EDT
Bruce,
I was about to respond to your reply to my previous posting, just as your latest
one came in. It suddenly turns out that, apart from a few technicalities and
terminology, we seem to be pretty much on the same wavelength. You work your
lunar distance not much differently from the way I do.
Please understand that from your concluding remark about iterating towards GMT in
your post of Oct. 12, I (and probably many of us) naturally assumed that you
intended to propose a direct method of finding GMT from a lunar distance
observation with computed altitudes. I pointed out that this is an impossibility.
Sure enough, in your latest post you offer a variant of the classical iterative
approach, as it is described, for instance in Chauvenet's Manual, or in Cotter's
History: Starting from an assumed value for GMT you obtain an improved one.
The one thing that leaves me dumb-founded is your remark that the described
method would offer a 30 times improvement over "standard procedures" in accuracy
(obtained in the first pass of iteration, I presume). What standard are you
referring to? I thought what you proposed WAS more or less the standard
procedure. Can you give us a reference for the flawed procedure you have in mind
and elaborate on how it would give an error in GMT as big as you indicate? Is
it something that has been discussed here on the list and I missed? Why would
such an obviously inadequate procedure have become a standard, anyway, when
working procedures were readily available, well documented and commonly
practised?
Herbert Prinz
Bruce Stark wrote:
> This is a continuation of the October 12th "Re: use of Sun Sights . . ." [...]
> Using a rough guess at GMT, take the equation of time from the Almanac and
> apply it, with reverse sign, to change LAT to LMT. Convert your uncertain DR
> longitude to arc. If it's west, add it to LMT. If it's east, subtract. The
> result is the best you can do at present for GMT.
>
> With that GMT take out the sun's and moon's elements from the Almanac, [...]
> The advantage of this approach is that only a rough estimate of GMT is needed
> to get an acceptable altitude of any body other than the moon and, with the
> moon, the error in local hour angle cause by an error in GMT is only about
> one-thirtieth what it would be using standard procedures.
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