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    The Online Nautical Almanac- beware!
    From: George Huxtable
    Date: 2004 Jul 4, 21:36 +0100

    Mike Burkes has been perceptive enough to note discrepancies between
    predictions in "The Online Nautical Almanac" for dates in the 1700s, and
    has drawn this to my attention.
    You can access this online almanac via-
    and then typing in the date that interests you in a box at the bottom of
    the page.
    I have looked into the matter and discovered that the predictions show a
    jump of one day (plus an hour) between 0h and 1h on 28 Feb 1800. For
    earlier dates than that, the data printed is relevant to the day before
    that implied by its date-label. Data relevant to the day of 27 Feb 1800 is
    missing altogether, and in its place is the data for 26 Feb, and so on.
    After 1h on 28 Feb 1800, the predictions correspond correctly with the
    date, but before that, it's all a day out. Users beware!. To get the right
    results before 28 Feb 1800, you should obtain the predictions, not for the
    date you really want, but those marked for a day later. And there's no way
    (except by interpolation) to get predictions for the day 27 Feb 1800.
    To show the "jump", Sun declination is predicted as S 8deg 30.0', for 0h on
    28 Feb 1800, but an hour later it has suddenly changed to S 8deg 06.4'
    (quite impossible in an hour!)
    Of course, 1800 is one of those years which divide by 4 yet which isn't a
    leap year. The Online Almanac program gets that non-leap-year right for
    1800, in not printing output for 29 Feb, and getting the day-sequence
    correct as Thursday 27th, Friday 28th, Saturday 1st March. But somehow the
    computation of Julian Day Number must have slipped here by a day, for dates
    preceding that leap year.
    This is a bit ironic. I think it was Frank Reed who pointed out that when
    Hamilton Moore produced his "Practical Navigator", a few years prior to
    1800, but with predictions spanning that period, he failed to allow for
    1800 being a non-leap year. That allowed Bowditch, in his own first
    edition, to scoff at all the errors that ensued in Moore's predictions, for
    dates after 28th Feb 1800. Of course, Moore and Bowditch were predicting
    ahead from an earlier era, whereas the Online Almanac is working backwards
    to 1800 from a later era.
    The web page referred to above states clearly that "valid dates are in the
    range 1950 to 2050", so those using the almanac outside that interval do so
    at their own risk. Yet I have found the predictions to be usefully accurate
    right back to the early 1800s.
    I know that Ken Muldrew has been investigating David Thompson and other
    travellers in Canada around this period, and one of his postings
    specifically refers to data from the Online Nautical Almanac for October
    1800. This is close in time to the moment when the Almanac goes haywire so
    Ken needs to take care that none of the data he uses (or has used) relates
    to that period before 28 Feb 1800.
    I am notifying Omar Reis (at omar@tecepe.com.br, in Brazil) about these
    difficulties that historical researchers face, and will pass on any
    reactions from him. I suspect that he never envisaged his predictions being
    used for the purposes we are putting them to, so could quite legitimately
    respond by pointing out that we are working outside his stated validity
    period of 1950 to 2050. But I hope he will be more constructive than that.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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