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    Re: Celestial Navigation without a sextant.
    From: Frank Reed
    Date: 2008 Mar 09, 19:52 -0400

    Peter F, you wrote:
    "Here's a thought: if the time of apparent sunrise/sunset was observed
    regularly; the extent of the difference or inaccuracy shown by observation
    compared to calculated data could be evaluated on a regular basis and
    contrasted with other information about position."
    
    I see where you're going, but as George has already described, the geometric
    situations only appear to be similar. And the small differences in
    circumstances are actually large differences. Refraction above three degrees
    altitude, which is about as low as you can ever observe the stars, is almost
    completely unaffected by the low level structure of the atmosphere. But
    refraction right at the horizon can easily be shifted by several minutes of
    arc in either direction by common changes in the low level temperature
    profile. The refraction that affects the horizon itself is only dependent on
    the extreme lowest layer of the atmosphere.
    
    I can think of one case where you would have relatively similar refraction
    circumstances at nearly the same time, and your idea might work. Imagine
    watching for moonrise right around the time of Full Moon. First we watch the
    Sun setting... We record the exact time and reduce that to yield a
    longitude. Then, let's say fifteen minutes later, we observe the Moon just
    resting on the horizon on the opposite side of the sky (a difficult
    observation). We reduce that to get a longitude, too. The refraction
    circumstances should be similar so could we use one "sight" to remove the
    error in the other? It seems like that should work. The average of the two
    longitudes should cancel out the highly variable refraction right at the
    horizon. Or do I have the signs backwards??
    
     -FER
    
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