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    Re: sextant index error measurement
    From: Frank Reed CT
    Date: 2006 Nov 3, 23:03 EST
    Jim Martin, you wrote:
    "According to "Nicholls concise guide" vol.1, a close by object
    or straight line should not be used to determine index error,because it
    does not allow for sextant parallax and atmospheric refraction. I can
    give the textbook explanation if it is of interest."
    Sextant parallax, yes. That's an important issue for index correction. But refraction, no. The only time terrestrial refraction would make a difference is if you're comparing sight lines to two objects at distances differing by a good fraction of a mile. I can't think of any instance where this would matter for standard index correction observations.
    42.0N 87.7W, or 41.4N 72.1W.

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