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    Re: Timing Lunars with a Rock
    From: Frank Reed CT
    Date: 2005 Jul 19, 17:55 EDT

    Bruce Stark wrote:
    "The old navigation manuals suggested checking the  log line and half minute
    glass occasionally. One way to check the glass was by  pendulum. As I recall,
    the length of the pendulum, to the center of the musket  ball that formed the
    weight, was sometimes given as 39 and 1/4 inches, and  sometimes as 39 and 1/8
    [small typo corrected]. Count a second each time the  pendulum passed the
    bottom. I suppose you had to give the pendulum a few moments  to settle the
    length of its swing. "
    For what it's worth, the difference  in length suggested (an eighth of an
    inch) is comparable to the difference that  arises from the variation of g with
    latitude. Personally, I don't think the  length of a crude pendulum can be
    measured to better than an eighth of an  inch in any meaningful way. One problem
    is that the length refers to the  center of mass of the whole pendulum. If the
    plummet is a sphere (like the  musket ball you mentioned) that helps a lot.
    Another problem is that the length  of the string will change.
    For a two second pendulum period, I get a length of 99.6cm at the poles and
    99.1 at th equator (cos(2*lat) variation in between).
    42.0N  87.7W, or 41.4N 72.1W.

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