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    Re: Calibrating a sextant scale
    From: Alexandre Eremenko
    Date: 2007 Nov 21, 13:00 -0500

    
    Dear Fred,
    
    > My only
    > quibble is that you might rephrase your conclusion to,
    > "I could not
    > get it to work."  Maybe a more skilled observer
    
    Yes, of course. 4 years ago, when I joined the list, one of
    the list members (I think it was George) wrote that he
    challenges anyone to determine his (modern, metal)
    sextant arc correction from
    stars. I tried to do this. I made several hundred observations
    of stars, and failed.
    My MOST PRECISE observations are in fact lunars
    (Sun-Moon, Venus-Moon and Jupiter-Moon).
    To determine arc correction from such lunars, one has
    to make observations for few weeks of good weather
    as the distance from the Moon to the body changes over the whole
    range of the scale.
    I did it. My results are in reasonable agreement with the
    arc correction determined by factory instruments (Freiberger
    and Cassens-Plath).
    So I can say that from such Lunars I can determine the
    arc correction with 0.1-0.2 accuracy, by several months
    of observations of Sun-Moon and Moon-Jupiter distances,
    and a lot of averaging.
    
    In principle ANY observation of known angles would give
    you the arc correction.
    I tried many other methods. But nothing else works in my
    conditions. For the artificial horizon I cannot find a stable
    enough platform. A dog running 100 feet away shakes
    the artificial horizon! Not mentioning cars and trucks.
    (Have not tried
    mercury yet:-)
    For the church-steeple
    method discussed on this list (which sounds
    very reasonable to me), I do not have enough church-steeples,
    and anyway do not have a 360 degree panorama from my observation
    place.
    
    Alex.
    
    
    
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