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    Re: Lunar eclipse report
    From: Herbert Prinz
    Date: 2004 Oct 31, 18:37 -0500

    Alexandre Eremenko wrote:
    > The problem of measuring "elapsed time"
    > was soleved in "antiquity" very well,
    > as I undrstand.
    > By means of "clepsydra, the water clock".
    > The sophisticated design of the water clocks
    > I've seen, suggests that they were capable to
    > measure time to the precision of less than 1 minute
    > per day, I suppose.
    Dear Alex,
    Where does this 1 minute come from? It's outrageously speculative. There
    are some Arab water clocks that allow a reading to nominally 4 minutes,
    which by no means indicate that the actual achievable accuracy was in
    that order of magnitude. Have you ever seen a clepsydra _in operation_?
    The "sophisticated design" pertains partly to gimmicks such as dropping
    balls and whistling birds on the hour and only partly to regulating the
    water flow. As far as the latter is concerned, most effort went into
    making the water clock, which inherently measures uniform time, show
    seasonal hours!!!  This indicates  that  we are mostly talking about
    household items rather than astronomical instruments. Only those water
    clocks that show equinoctial hours and where the regulation of the water
    flow attempts uniformity by addressing the problem of varying water
    level (normally by putting two containers in series) can be considered
    to have an astronomical purpose.
    > Unfortunately none of these survived,
    > and I know of no historians who seriously invetsigated
    > the question of their precision.
    > Any suggestions?
    Nobody can measure the accuracy of a device that is not extant.
    Historians are therefore confined to analysing what contemporary sources
    say about water clocks.
    Ptolemy, on the use of water clocks in antiquity (Almagest, V 14,
    Translation by Toomer, p. 252): "Of the various methods used to solve
    the latter [i.e. the timing] problem, we have rejected those claiming to
    measure the luminaries by measuring [the flow of] water [...], since
    such methods cannot provide an accurate result for the matter in hand."
    Tycho Brahe picked up the idea, replacing the water by mercury.
    (Progymnasmatum Prima Pars, Opera Omnia, Tomus II, ed. Dreyer, pp157ff).
    He makes no quantitative statements about accuracy. Nothing must have
    come of it, otherwise we would find a description of such a clock in his
    Astronomiae instauratae mechanica. If you want to blame me of an
    argumentum ex silentio, I shall take it in stride. By contrast, for
    Brahe's experiments with mechanical clocks around 1580, we do have the
    numbers. He found their precision way below his capability of measuring
    angles. Therefore, he did not use them either to time observations.
    Hevelius apparently investigated water clocks and dismissed them as
    insufficient. (see Repsold, Zur Geschichte der Astronomischen
    Messwerkzeuge, 1908).
    Herbert Prinz

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