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    Re: Timing Lunars with a Rock
    From: Frank Reed CT
    Date: 2005 Jul 19, 18:25 EDT

    Alex wrote:
    "I repeat that using a pendulum  of fixed length
    is not a good standard of time.
    Because the period depends  on the amplitude.
    This was already well-known in XVIII century.
    Thus the  US proposal (as explained in the following message)
    was not scientifically  sound."
    There's a way around this objection. All you have to do is define  the
    "pendulum-meter" to be the length that yields a two second period in the  limit of
    zero swing amplitude. This is relatively easy since the error in the  period
    depends quadratically on the amplitude. Note that the "pendulum-meter"  would
    also have to be defined in the limit where the mass of the suspending  string is
    zero. Neither one of these is achievable in practice, but in both  cases we
    know how to subtract out exactly the effects of non-zero swing  amplitude and
    non-zero string mass [non-zero string mass?! drat. I swore I would  never get
    into string theory...]. As a standard of measurement, the biggest  objection to
    a "pendulum-meter" is the local variability of the Earth's  gravitational
    field even along one parallel of latitude. I don't think this was  any worse than
    the geographic definition which the French originally adopted,  and it may
    have been better.
    42.0N 87.7W, or 41.4N  72.1W.

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