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    From: Marcel Tschudin
    Date: 2005 Aug 4, 19:20 +0300

    Yes, how do I get in this? Just trying to cover in a self-made program the
    situation from an object at the horizon (over sea level) as seen from a
    mountain or air craft.
    The "real" calculation is done via integration. But since this is not very
    practical one uses approximative formulae like e.g. the one from Bennett
    which Meeus mentions in his book Astronomical Algorithms. All tables on
    refraction I found so far do end at 0? elevation and for none of the
    approximative formulae I could find an indication that they also would be
    valid for negative elevations.
    I also was wandering whether the approximate formulae could be used by
    calculating the Refraction R for e.g. -2? the follwing way:
    R(-2?)  =  R(0?)  +  (  R(0?) - R(+2?)  )
    If this would be correct then one would not need separate formula for
    negative elevations.
    Greetings from Marcel

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