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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 14, 21:09 +0300

    Fred,
    
    Here the influence of humidity on the refractive index of air in figures:
    
    This page from ESO describes the procedure on how the index can be
    calculated:
    http://www.ls.eso.org/lasilla/sciops/2p2/E2p2M/FEROS/Projects/ADC/references/refraction/
    
    The tabulated values on page 17 of the following manuscript from Stanford
    correspond to the "dry part" of the index, as veryfied with the above
    "procedure" for standard condition.
    http://www.stanford.edu/group/efmh/FAMbook/Chap10.pdf
    
    Finally Andrew Young shows on his Web page the refractivity of air at
    standard condition including the "wet part" as calculated by Peck and Reeder
    http://mintaka.sdsu.edu/GF/explain/optics/disp.html
    
    Now, what effect does this have on refraction?
    Using 0? altitude at sea level (height=0m), standard temperrature (=15?C)
    and pressure (=1013.25hPa)  and a wavelength of 595nm (orange, e.g. sunrise
    or sunset).
    
    By integration over the light path:
    Index with "dry part" only =1.000277  -> Refraction = 30.27 minutes
    Index with "dry part" and "wet part" =1.000293  -> Refraction = 32.17
    minutes
    using a generally mentioned
    approximative index of 1.0003 -> Refraction = 33.03 minutes
    and, last but not least, Bennett's formula -> Refraction = 34.48 minutes
    
    At this point it really would be interesting to know to what values Bennett
    fitted the approxiation formula.
    
    Marcel
    
    
    

       
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