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    Re: determination of longitude and the prime vertical
    From: George Huxtable
    Date: 2007 Apr 12, 15:45 +0100

    I hope Geoffrey Kolbe's respose, with Bill's reply, show that the problems
    that Bill raised in Navlist 2550 are now resolved..
    But a minor matter cropped up in that mailing that worries me a bit.
    Bill quoted some predicted Sun positions, that had come from Andres Ruiz'
    I will refer just to that first set of numbers from his mailing, as follows-
    | Date:     1 May, 2007
    | UT:       20:41:45
    | Position: N40 W60
    | Hc:       24d 00!2
    | Az:       270.00
    | LHA:      71d 09!5
    | dec:      15d 09!5
    I don't have a 2007 Nautical Almanac. But I have compared those numbers with
    my own predictor, held on a pocket calculator, which differs slightly.
    That gives a Sun position, at 20:41:45 GMT on 1 May 2007 of 15 deg 09.5' N,
    exactly as Bill gave it, but a GHA of 131deg 09.8', and so with a long of
    60deg W, a LHA of 71deg 09.8', just 0.3' more than the figure Bill stated.
    That difference isn't a large one, but to me it's a bit of a worry,
    depending on which of us is in error. I expect the Sun predictions of that
    program to be generally well within a millidegree. One might expect
    divergences of 0.1', as a result of rounding errors, but not more. If my
    program is out by that amount I need to investigate why.
    So I wonder if Bill will kindly recheck that figure, and ask anyone with a
    current Nautical Amanac or computed equivalent, to provide a value for Sun
    GHA at that moment, please.
    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.
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