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    Almanac errors. was: Navlist 4576, Re: Flinders' survey of Australia
    From: George Huxtable
    Date: 2008 Mar 3, 20:34 -0000

    Clive wrote, (in Navlist 4576, Re: Flinders' survey of Australia) in
    response to my question about lunar-distance errors in the Nautical Almanac,
    in the period 1801-1803 -
    | There is a passing reference to the faults in the Nautical Almanac etc
    | "The Almanacs, Yesterday, Today and Tomorrow by Seidelmann.PM et. al."
    | which is item 60 in the index of "The Journal of the Institute of
    | Navigation" disk. I believe you have a copy of this.
    Thanks to Clive for pointing out to me that interesting paper, which is
    indeed in the ION disc of selected celestial papers, which I have. It was
    one of several papers I had noted down as to-be-read, but hadn't yet got
    round to doing so.
    The paper itself appeared in "Navigation", vol 24 No 4, winter 1976-77,
    pages 303 to 312. One of the authors, Seidelmann, achieved later fame as
    editor of the revised "Explanatory supplement to the Astronomical Almanac"
    of 1992.
    As Clive says, it's no more than a passing reference to almanac accuracy.
    The paper refers to Maskelyne's death in 1811, and goes on "After Maskelyne,
    the Nautical Almanac suffered a decline in accuracy, and in prestige.
    Although the accuracy defects were easily remedied ..."
    At his death, Maskelyne was still Astronomer Royal, and still firmly in
    control, so any defects that came to Flinders' attention relating to his
    voyage of 1801-03 were firmly within Maskelyne's watch at Greenwich, and
    before the later decline that Seidelmann noted.
    In this context, it's worth drawing attention to an interesting paper, one
    that I have mentioned before, by Nicholas A Doe, in "Journal of Navigation",
    vol 48 No.3, September 1993, pages 374 - 388, "Captain Vancouver's
    Longitudes-1792". He attributes discrepancies between longitudes carefully
    observed by Vancouver, and present day values, at least in part as due to
    errors in the lunar distance predictions of the Nautical Almanac. By
    comparing those predictions with modern calculations of Moon position, he
    has found errors of up to 50 arc-seconds, varying rather cyclically over a
    month. Those errors, in themselves, would give rise to unavoidable errors in
    longitude of 25 minutes or so, even if Vancouver's observations had been
    Indeed, Vancouver did have a chronometer, in working order, which had been
    checked at Tahiti and Hawaii. The locations of both those locations had been
    well-determined earlier, by Cook, with whom Vancouver had previously sailed.
    In the light of his lunar observations, Vancouver had distrusted the rate of
    his chronometer, but it may be that it was marking better time that his
    lunar observations could check, because of those almanac errors.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
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