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    Re: Reducing back sights
    From: Bill Noyce
    Date: 2004 Aug 11, 10:41 -0400

    George Huxtable asks:
    
    > A further question now arises: what does Jim Thompson
    > wish to use an octant backsight FOR?
    
    I had a different reading of the question.  I assumed Jim was
    using a sextant to measure the altitude of a body high in the
    sky, but without a suitable horizon under it.  He's measuring
    the altitude from the opposite horizon instead.  In that case
    he has the procedure exactly correct.
    
    David Weilacher raises a concern about differing refraction
    between UL and LL corrections.  I think Jim's procedure works
    correctly, as long as he remembers that what looks like a LL
    measurement in the backsight is really measuring the (supplement
    of the) altitude of the body's LL, and vice versa.  The key is
    to perform all the corrections for the body itself *after*
    subtracting the reading from 180.  Jim is correct that index
    error and dip corrections should be applied *before* subtracting
    from 180.  After correcting for dip and index error, and
    subtracting the result from 180, the result is just what you
    would have gotten from a normal forward sight after the same
    corrections.
    
            -- Bill
    
    
    

       
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