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    Re: Timing Lunars with a Rock
    From: George Huxtable
    Date: 2005 Jul 18, 07:58 +0100

    At 02:37 18/07/2005, Jared wrote:
    >Frank, iirc  if the length of
    >the pendulum is some precise length (vaguely16 inches?) the period *will* be
    >one second. So if you can find out the correct pendulum length and carry
    >that bit of strong with your sextant......
    >Obtaining a willing "native" may be a bit harder these days.
    Much confusion here. The author quoted by Frank got it right when he said
    that a pendulum that "beats" seconds was 39 inches long. (it varies
    slightly with position on the Earth, because gravity varies, but not enough
    to make a significant difference for that purpose). Beating seconds means
    that it takes a second to swing left-to-right, then another second to swing
    right-to-left; a total period, for a complete oscillation, of 2 seconds.
    That provides a useful rate that's relatively easy to count.
    Instead, Jared has referred to a PERIOD of one second, which is quite a
    different matter. Half a second each way: much more frantic, and
    oscillations will die out much quicker.
    And then he has quoted a "precise length" as "vaguely 16 inches" for that
    pendulum. Far from it.
    It's very simple. The period is proportional to the square root of the
    length. So halve the period, quarter the length. So the length of the
    faster pendulum that Jared refers to is NOT "vaguely 16 inches", it's a
    quarter of the 39-inch length feferred to in Frank's quotation, or just
    less than 10 inches.
    If it had been Jared who found himself without a watch by an African lake,
    he would have made rather a mess of the observation.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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