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    Re: Refraction
    From: Peter Fogg
    Date: 2005 Aug 7, 20:16 +1000

    I thought these mentions of his work on refraction might be of interest to
    George Bennett, and so passed them along. His comment:
    
    "The evaluation of Astronomical Refraction is an interesting subject. I see
    that Marcel Tschudin has chosen one of the empirical formulas that I derived
    when I was working at Greenwich. In my article I confined the apparent
    altitudes to range of zero to ninety degrees. I would be interested to know
    the result of any investigation into the validity of my formulas at small
    negative altitudes. As a lead you might write to HM Nautical Almanac Office
    who in turn could direct any enquiries to Dr A T Sinclair. He is an expert
    on the subject and although retired now he may offer some helpful
    information. Cheers, George."
    
    ________________________________________
    From: Navigation Mailing List [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]
    On Behalf Of jcs
    Sent: Friday, 5 August 2005 7:26 AM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Refraction
    
    Hi Marcel  Welcome to the list;
    
    Be careful there are nearly as many different formula for refraction as
    there are members of the list. There is not to my knowledge a definitive
    formula. Almost all of them will work well at sextant altitudes above about
    15 deg but  begin to fail at low angles, particularly negative angles, due
    to the unpredictable behaviour of the atmosphere close to the sea or ground.
    
    G.G.Bennet  wrote a short article in the (UK) Journal of the Institute of
    Navigation.Vol 35 No 2. in which he compared several calculator programs
    with a Fortran program written by G Garfinkel, described in The Astronomical
    Journal,Vol 72,No 2  (1967), However it is not an easy method to use. I
    believe his program was written for the benefit of Nasa space program but I
    may be wrong about this. .
    
    The following  is one I use,
    
    Ref = Tan  (90-Altitude - 0.999139 * Altitude - (7.31 / (Altitude + 4.4)))
    
    between -20 < Air Temp < +40, and  970 
       
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