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Marcel you wrote:
"Probably one should  calculate refractive index for the dominant wavelength
depending on the  object's zenith distance?"

Depends on your goal. If you're trying to  portray the appearance of stars at
very low altitudes in the atmosphere, then  you should calculate refraction
for each color of the spectrum (split up as you  choose). When you see stars
very low in the sky, they do, in fact, look like  little "French flags" because
the red light is refracted more than the blue  light. For navigational
purposes, this is irrelevant, but for the project you  briefly described earlier it
might be interesting to include. But be sure to  include extinction. NO stars
are visible to the unaided eye right at the horizon  since the extinction is on
the order of twelve magnitudes. Fortunately, if you  do a proper numerical
integration for refraction, you get the extinction "for  free".

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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