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    Re: Reducing back sights
    From: Bill Noyce
    Date: 2004 Aug 11, 17:03 -0400

    re halftant / puptent (David Weilacher):
    
    > Problem is that in the afternoon, the limb that I am bring down is
    > the upper limb, refraction wise, but appears as lower limb against
    > east horizon through the eyepiece of my puptent.
    
    Agreed.
    
    > Our marine almanac combines semidiameter and refraction together as
    > one figure.  Thats what makes our lower and upper limb corrections
    different.
    
    Agreed.  But the Almanac doesn't provide corrections for Hs greater
    than 90 degrees, so you can't use it directly in the afternoon anyway.
    
    Once you correct for dip and index error, and then subtract from
    90 degrees, the result is exactly what you would have gotten if you
    had measured the upper limb against the western horizon (assuming
    today's
    dip matches the tables, of course).  From this result, you need to
    adjust for semidiameter and refraction, and the upper limb table entry
    is correct for both.
    
    There would indeed be a problem if you tried to apply either or both
    of semidiameter / refraction corrections *before* subtracting from 180.
    If there's a problem I don't see, can you demonstrate it with numbers?
            -- Bill
    
    
    

       
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