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