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    Re: Star-star distances for arc error
    From: George Huxtable
    Date: 2009 Jun 26, 14:30 +0100

    Gary asked-
    I don't follow the discussions of lunar distance very carefully. What
    is the expected accuracy of the derived longitude in practice?
    There's an unavoidable  factor of about-30 comes in, from the ratio between
    the length of the month and the length of the day. So the Moon travels round
    the sky, with respect to the stars, at only about half a degree per hour.
    Greenwich time is determined from measuring that motion. So if you can
    measure Moon position to 1 minute of arc, with respect to the star
    background, then you can measure Greenwich Time to about 2 minutes-worth,
    thus longitude to about 30 minutes.
    If you could measure Moon position to half a minute of arc, which was about
    the best anyone could hope for at sea, even in good conditions, then if
    there were no other errors (in prediction, in correction, in deducing local
    time from another observation), you could get longitude to about 15' of arc,
    15 miles in the tropics. Dyson's estimate of 20 miles wasn't far out, in my
    view. Dyson was careful to state (from his 1920s perspective)
    "Unfortunately, even with perfect tables, , it is found that the most
    skilful mariner cannot obtain avery accurate position of his ship in this
    manner. With great pains and elaborate calculations he can be correct to
    within 20 miles."
    And so, Frank's qualification to that statement is irrelevant "And don't
    forget that an error of 20 miles included the error from the lunar distance
    tables in the Nautical Almanac. Since those were about 0.3' for the latter
    half of the period when lunars were commonly used, the implied error in the
    observations is even a little less.". It wasn't. Dyson was assuming perfect
    tables, and said so.
    Frank denigrates the Astronomer Royal as "not knowledgeable on lunars". I
    have asked Frank before now, on several occasions, to explain to us his own
    qualifications and experience in determining lunars fron a vessel AT SEA. He
    has never responded.
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Navigation List archive: www.fer3.com/arc
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