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    Re: chronometer question.
    From: Lars Bergman
    Date: 2006 May 15, 07:16 -0500

    George Huxtable wrote:

    > Departing from Shetland, with its Northerly point in sight, at a
    > certain time-by-chronometer, an alt. of the Sun (supposing the N. end
    > of Shetland to lie in long. 38'W) gave the Chronometer slow of
    > Greenwich 3 minutes 39 seconds ...
    > However, even in British home waters, the less-frequented parts had
    > not all been well surveyed by 1818, and the charts carried may have
    > been somewhat defective. Anyway, a modern atlas puts the N end of
    > Shetland at about 0 deg 50' W, rather than 0 deg 38' W as had been
    > assumed; quite a difference.
    > In the light of that information, what should the chronometer error
    > have really been?

    There was a reply from Fred Hebard with the answer 0:5:27. I do not
    agree. With the time sight you calculate your local apparent time which
    is then converted to local mean time, LMT. Now

    LMT = GMT + easterly longitude

    Knowing longitude you can determine GMT. Comparing chronometer time with
    the determined GMT gives the chronometer correction. The chronometer was
    3m39s slow on GMT when longitude 38'W was used. Using a longitude that
    is 12' further west, or 48s of time further west results in a GMT that
    must be 48s larger than in the first case (in order to keep the observed
    LMT). If the chronometer is 3m39s slow of GMT in the first case, it must
    thus be 48s more slow in the second case, that is 4m27s.

    In 1968 I made a voyage to the Shetlands in my farther's sailing yacht,
    a gaff-rigged 25 tons displacement double ender, but have no memory of
    the actual longitude. Anyway, on our then brand new Admirality chart of
    the Faeroes there was a small note at the bottom stating something like
    "According to the latest Danish surveys the longitude of this chart
    should be moved 25 seconds of arc to the east" or maybe it was west, I
    do not remember. So, also in rather modern times (pre GPS at least), the
    longitudes could be a little uncertain.

       /Lars 59N 18E

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