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    Re: The Nautical Day
    From: George Huxtable
    Date: 2004 Feb 9, 11:44 +0000

    Kieran Kelly said-
    >I have followed the debate about the nautical day and whether it caused
    >confusion or not. I am surprised, as Cook as been used as an example, that
    >no one mentioned that he got his days confused when he crossed the position
    >of what we now call the International Date Line on his journey west from
    >Tahiti towards New Zealand.
    >As a consequence he was one day out from this part of the journey onwards
    >and it was not rectified until he arrived back in England.
    >An interesting consequence is that he placed New Zealand a little to the
    >west of its actual location i.e. closer to Australia. This would cause
    >consternation to both modern Australians and New Zealanders and thankfully
    >has been rectified.
    I don't believe it. I think Kieran has got it wrong. The international date
    line is a man-made construct which avoids confusion when voyagers travel
    round the world.
    As long as Cook, when he reached the end of one day in his nautical
    almanac, went immediately on to the next day, without jumping or missing
    any days, everything would work out all right. As long as he kept on
    reckoning his hour-angles and time-differences and longitudes as steadily
    increasing Westward from Greenwich, all would be well. If a circumnavigator
    did make such a switch in his times, then he had to adjust his date
    accordingly at that same moment, and the International Date Line is simply
    an agreed place for that to happen.
    True, when he arrived at Batavia, he would find that they would be
    celebrating Sunday on a different day than he had been, because they had
    arrived there from Europe by travelling Eastwards. Their days, when
    travelling, had been shorter than 24 hours, whereas Cook's had been longer.
    If, when Cook arrived, New Zealand or Australia had previously been settled
    by the Dutch who had arrived there by travelling Eastwards, they would have
    disagreed about his day-of-the week and his date-of-the month. Perhaps that
    may cause confusion to this day when celebrating Cook anniversaries in
    those parts.
    But that would have no effect at all on Cook's calculated positions, in
    either New Zealand or Australia, nor in their distance of separation. If
    their inhabitants now wish to put more sea-room between them, they will
    have to do find better reasons than that.
    One reason for Cook to have got longitudes wrong was the result of errors
    in the Nautical Almanac. In Beaglehole's "The Journals of Captain Cook, vol
    1", on the first voyage, in Endeavour, there's a footnote on page cclxxv.
    This refers to Sailing Directions by Capt. Horsbrugh (1811), who notes with
    respect to the longitude of the island of Savu-
    "Captain Cook in his first voyage round the world, made it 30 miles more to
    the eastward; but after his arrival in this country, the lunar tables were
    found to require a correction of 2 minutes, or 30 miles westerly, at the
    time the observations were taken at Savu."
    There's an interesting paper by Nicholas A Doe in "The Journal of
    Navigation" vol 48 no 3 (September 1995), "Captain Vancouver's longitudes
    1792". This showed, rather conclusively, by modern calculations from the
    JPL ephemeris, and also, interestingly, by analysis at Greenwich
    observatory in 1848 of their old Moon observations, that there were errors
    in the predicted Moon longitudes of 1792. These varied, with the phase of
    the Moon, between about 5 and 45 arc-seconds, which would put longitudes
    out by 2 to 22 arc-minutes to the East.
    Remember, this was 13 years after Cook's first voyage, and during that
    interval one might expect Moon predictions to have improved. So back in
    1768, Moon predictions may have been worse still. Of course similar errors
    would apply to longitudes of both New Zealand and Eastern Australia.
    So unless it's been done already, I suggest there's an interesting project
    for anyone wishing to repeat those JPL calculations, but applied to the
    Cook era, and put them with the 1848 Greenwich analysis, and use those
    figures to correct Cook's longitudes.
    The reference to the Greenwich analysis (which I haven't seen, but it
    sounds most interesting) is given by Doe as-
    Lord Commissioners of the Admiralty, (1848) "Reduction of Observations of
    the Moon made at Greenwich from 1750 to 1830" RGO archives, Cambridge
    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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