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    Re: Refraction
    From: Marcel Tschudin
    Date: 2005 Aug 17, 01:19 +0300

    Refering to my comment:
    > "Probably one should  calculate refractive index for the dominant
    > wavelength
    > depending on the  object's zenith distance?"
    Frank wrote:
    > 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.
    The idea behind my original thoughts went in the following direction. If
    refraction would be calculated by integration (the transcribed Basic
    program) the calculation would require a value for the refractive index
    which itself depends on the wavelength and the dominant visible wavelength
    from the zenith angle. The visible light of the sun at noon is apparently
    greenish yellow (probabely around 540nm) and at sunset oringe/red.(probabely
    araound 600nm). The most simple approximation of the dominant visible
    wavelength would in this case have been e.g. 595nm minus half of the
    altitude angle. But as already mentioned, I do not - at least for the
    moment - intend to use this program to calculate refraction values.
    Frank wrote further
    > 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.
    And how about the moon, the subject of your workshop? This reminds me of an
    approximation I made some years ago on the visiblity of the moon. A
    surprising result - at least for me, living normaly in the mountains - was,
    that it seems not possible to see a moonrise or moonset at sea level. It was
    only here, at my temporary domicile, that I actually could verify this. As
    soon the moon gets closer to the horizon its light turns rust coloured and
    is finally "switched-off" around one to two degrees about the horizon. Or,
    has anyone of you ever seen a real moonrise or moonset, where the moon
    really touched the sea at the horizon? I mean whether you have seen it from
    your faverd location afloat and not from those locations

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