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    Re: Calibrating a sextant scale
    From: Alexandre Eremenko
    Date: 2007 Nov 25, 12:26 -0500

    
    Dear George,
    
    > Not sure that it was me, though it was a good question.
    
    I am about 70% sure this were you, but now that you
    made the question more precise we can safely refer to it
    as a "George's question" :-)
    
    > by measuring star-star distances or by any other
    > method, ever discovered reproducible errors,
    > outside the terms of a
    > calibration certificate or maker's warranty,
    
    In this I failed, even if the second part of
    this question is omitted, I mean the part beginning
    with the word "outside".
    
    > Karl's new book, "Celestial Navigation in the GPS age", devoted several
    > pages to helping users to calibrate or check their own sextants that way. He
    > selected 12 pairs of bright stars
    
    Let me recall (I already posted this once on the list
    that a Russian book proposes a simpler scheme for
    moderate latitudes of Northern hemisphere.
    The book lists several distances of selected stars
    from Polaris.
    Then for each star, a little supplemental table
    is provided giving refraction as a function of
    this star altitude and Polaris altitude (=your latitude).
    As all altitudes are only needed tith 10 degrees accuracy,
    the whole set of tables occupoies only two pages.
    
    I even wanted to post these tables on the list, but they
    are made for the epoch of 1960. Anyway, everyone can
    quickly compute such tables for any epoch.
    
    But after many attempts I decided that this method of
    arc correction does not give the results accurate enough
    for the Lunar business, unless, perhaps you have a
    very good 12x telescope.
    
    Alex.
    
    
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