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    Re: Refraction
    From: Frank Reed
    Date: 2008 Jul 04, 04:57 -0400

    Dave Walden, you wrote:
    "They are functions of pressure, temperature and wavelength.  Two sets of
    conditions can be calculated for easy comparisons.  Those included are for
    the Nautical Almanac conditions and a more conventional set to conditions:
    760mb, 0 deg C, lamda .54 microns.   The K =60.4 sec, should look familiar.
    The Naut Alm STP yields 58.2, also familiar."
    
    Aha. Very nice! So that does appear to explain it. Kent, did you believe
    that your refraction number was for 0 deg C or 10 deg C?
    
    And you wrote:
    "The formulas can be found in Spherical Astronomy by Green.  This is the
    "successor" to Smart's work.  Green was coauthor with Smart on later
    editions and finally wrote his own.  (To my way of thinking, it's better.)"
    
    Damn. Another book to buy. :-) What's the full title of Green's book?
    
    And:
    "Also attached is a plot of errors of Bennett and simpler formulas (minutes
    refrac vs degrees alt).  There is indeed, nothing magic about Bennett.  It
    get 34 min at 0 deg in case you've forgotten, but the added complication to
    do so only makes things worse at higher altitudes."
    
    A simple way to keep the best of Bennett (it is indeed more convenient than
    typing in the whole table) is to retain it for altitudes below 15 degrees
    (or some other cut-off) and use some simple function of tan(z.d.) for higher
    altitudes.
    
    Anyone have any thoughts about the best color of light to use for the
    refraction formulae/tables? At very low altitudes, the star images really
    are smeared out substantially. What's the standard for astronomy? Is it
    appropriate for sextant users?
    
     -FER
    
    
    
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