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    Revisting refraction
    From: Bill B
    Date: 2005 May 13, 15:26 -0500

    While searching the list archives I came across an interesting posting by
    George Huxtable regarding refraction (re: refraction formula, Sept 28,
    2000).
    
    Paul Hirose had posted the formula:
    
    R = 1 / tan (H + 7.31 / (H + 4.4))
    
    R is refraction in minutes
    H is observed height in degrees (but was used as Hc by Paul)
    
    and George manipulated it as well as providing Saemundssen's Hc-to-observed
    formula--which was just what I needed.
    
    I noticed the 2005 Nautical Almanac provides a refraction formula similar to
    the one Paul posted in its sight reduction/direct computation section (page
    280):
    
    Ro = 0.0167 / tan (H + 7.32 / (H + 4.32))
    
    I believe I understand the 0.0167 vs. 1.  The formula Paul posted gives
    refraction in minutes, while the almanac formula gives refraction in degrees
    (1/60 = 0.0167).
    
    I do not understand 7.31 vs. 7.32 or 4.4 vs. 4.32.  Has the formula changed
    slightly over the years, or are there different camps on which (if any) is
    correct?  My only thoughts are that the almanac formula goes on to correct
    Ro to R with a temperature/pressure-refraction correction, while no mention
    of temp/pressure correction is noted in the formula Paul posted.  That
    reasoning does not pass my common-sense test.
    
    I would appreciate the list's views on the reason for the differences or
    which is more accurate.
    
    Thanks
    
    Bill
    
    
    

       
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