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    Re: Astronomical Refraction: Computational Method for All Zenith Angles
    From: Frank Reed CT
    Date: 2005 Aug 18, 16:47 EDT

    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.
    42.0N 87.7W, or 41.4N  72.1W.

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