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Marcel, you wrote:
"OK, my thoughts were more  related to our good old planet earth, for which
the paper shows results of  sample calculations. From our atmosphere we know
that blue and green  wavelengths are, with decreasing altitude,  more and
more absorbed, so  that at low altitudes only some orange/red part of the
light source can be  seen."

I mentioned the "other planet" possibilities just for fun --to  indicate the
generality of the result. Sticking to the terrestrial atmosphere,  there are
still some options. Notice that the integration does not force you to  use any
particular index of refraction. All it requires is that you specify the  index
of refraction as a function of height --as in the function "getmu" in my
sample code. So for example, you could include variability of humidity with
height if you consider that important (it's not important in navigational
applications but it might be in high precision astronomical simulations). You  can
also look at what happens all across the visible spectrum. Consider the case  of
the star Sirius at 2 degrees altitude as seen from sea level. Its light is
white, so there are more or less equal contributions of red, green, and blue.
Run the integration for three values of the "zero level" index of refraction
for  the three frequencies corresponding to those colors and see what you get.
You  should find that the image of Sirius is considerably stretched out. It
becomes a  little vertical spectral "stripe", 20 to 30 arcseconds in length.
Setting aside  the large variability in refraction at such low altitudes, this is
another  reason why there's no use fussing over a couple of tenths of a
minute in  refraction tables very close to the horizon.

By the way, not too  surprisingly, my code lost its nice neat indentation for
the two loops when it  went through the list processor. I assume it is still
comprehensible although a  little less readable. Let me know if you have any
difficulty translating from  the version of Basic I used. I also use
Delphi/Pascal, though less often now...  The next-to-last version of the "Centennia
Historical Atlas" (see my web site)  was coded in Delphi.

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

   

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