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    Re: Digital Camera Celestial Navigation
    From: Paul Hirose
    Date: 2008 Jul 06, 21:12 -0700

    Marcel Tschudin wrote:
    > Why not measuring the sun's horizontal diameter? This measure is
    > (almost?) not affected by refraction.
    I discovered the "almost" a few days ago, when programming a function to
    compute refracted semidiameter. The values at the 12 and 6 o'clock
    position angles (in the horizontal frame) were exactly what I expected
    for a Sun-sized object, but at 3 and 9 o'clock they were about .2" too
    After some debugging, I realized there was nothing wrong with my code.
    Refraction reduces the semidiameter in every direction. When the left
    and right limbs refract up, they move on great circles which converge at
    the zenith. With a body of 16' semidiameter, and sea level refraction,
    this decreases the semidiameter about .27". I tested this at several
    altitudes from 10 degrees to 80, and it didn't vary more than .01".
    In this case, the algorithm was smarter than its designer.
    Later I remembered that Jean Meeus ("Astronomical Algorithms", chapter
    on refraction) says the Sun's horizontal diameter is reduced slightly by
    refraction. His figure (credited to Danjon) is .6", practically
    independent of altitude.
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