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Fred,

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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