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    Re: Beginner / Davis Plastic Sextants
    From: Frank Reed CT
    Date: 2005 Oct 6, 18:50 EDT

    Bill you wrote:
    " The *really* nice  three-arm-protractors like Alex's Soviet unit can be
    dialed in to 0.1' if  I
    recall, so would demand input of equal precision and accuracy for optimal
    results. "
    An angle of 1 minute of arc is (and I do mean *IS*) a ratio of  1 to 3438
    (10,800/pi to be exact). So an angle of 0.1 minutes of arc is a ratio  of 1 to
    about 34,000. The distance "across" the object(s) and the distance "out"  to the
    object(s) are in a ratio of 1 to 34,000 when the angle is 0.1 minutes of
    arc.  Given that, is there any chance that this tiny angle could be  meaningful
    in plotting with a three-arm protractor?? Assuming that the chart  itself is,
    let's say, one meter across, if I extend a line all the way across  the chart
    and shift the end-point by 1 millimeter, I'm making an angular shift  of only
    1/1000 or in other words about 3 minutes of arc. No horizontal angle (as
    between buoys) would ever need to be measured to an accuracy better than this  for
    live navigation.
    42.0N 87.7W, or 41.4N  72.1W.

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