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    Re: Manual calculation of compass variation
    From: Frank Reed CT
    Date: 2004 Oct 16, 17:35 EDT
    George H wrote:
    "That raises another interesting question. How did Cook manage to take a
    compass bearing of the Sun (or even the average of several) to sufficient
    accuracy that he could quote the result to a minute of arc; to be 20deg
    59', rather than 21deg? Compass bearings can't be taken to that accuracy
    now, nor could they then. Cook was simply quoting the answer that his
    calculations gave him. We have to take the 59' part of his measured
    variation with a large pinch of salt."

    Yep! I've seen variations in logbooks quoted to the nearest 10 arcseconds. The concept of "significant digits" was apparently unknown in that era. Almost 40 years later, there is a letter to Thomas Jefferson, which is available on the Library of Congress web site, giving longitudes measured by the rate of change of the Moon's declination (not a great idea) quoted to the nearest hundredth of a second of arc. That's about one foot on the ground!

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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