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    Re: How Many Chronometers?
    From: George Huxtable
    Date: 2009 May 7, 10:51 +0100

    Henry Halboth gave his view, with good sense as usual-
    "Astronomical observations with a reliable, error free or error corrected,
    sextant, quintant, or even octant can be remarkably accurate as respects the
    determination of Chronometer Error, even on a good sea horizon. In my
    personal experience, I believe the Lunar Distance to be the more difficult
    observation as it probably requires the greater obsevational ability, but
    both the Equal Altitudes and Time Sight solutions are well within the
    capability of the average observer and will produce satisfactory results on
    a good sea horizon ..."
    However, he didn't point out, this time, as he had done in a previous
    message, that though the equal-altitude or time-sight operations can be
    inherently precise, they can only work if the longitude is well known, such
    as off a well-identified headland.
    Lunar distance, however, can be used to determine chronometer error at the
    time you might need it most badly, before sighting land. The problem is that
    the observation needs to be made so damn precisely. An error of 1' in the
    lunar distance itself can give rise to an error of 2 minutes in the
    chronometer time, or about 30' in the resulting longitude. So it provides a
    rough-and-ready answer, which may help to avoid disaster but not a precise
    correction to the chronometer. It's nowhere-near good enough to allow a new
    rate to be deduced.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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