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    Re: Calculated Altitudes for Lunars
    From: Herbert Prinz
    Date: 2002 Oct 22, 23:28 +0000

    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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