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    Re: The Nautical Day
    From: Joel Jacobs
    Date: 2004 Feb 7, 06:59 -0500

    The internet is probably the best research tool man ever had. It would be
    worth everyone's while to learn how to use it. Of course to get targeted
    results you do have to know how to couch you're query.
    
    Doug and Stacy, isn't it true that the Nautical Day is not dead? When I was
    doing my navigating, and by training, it was and likely still is the
    convention of most all mariner's to calculate their "Day's Run" from "Noon
    to Noon". Logically, there doesn't seem to be much sense behind that
    tradition because, as has been discussed here in great depth, there is
    difficulty in using the noon sight to determine longitude. So unless your
    course is mainly N or S, you get little help from the LAN sight except as to
    advance your morning LOP, the result being an EP, not a fix. Now if you have
    the time to do so, and change you course to put the sun dead astern, you
    will always get an accurate LOP from which to calculate the Day's Run.
    
    In any case, the tradition is still with us, and as I pointed out it comes
    from a British Admiralty mandate around the time of Cook's voyages.
    
    Joel Jacobs
    
    
    ----- Original Message -----
    From: "George Huxtable" 
    To: 
    Sent: Saturday, February 07, 2004 5:43 AM
    Subject: Re: The Nautical Day
    
    
    > The Nautical Day
    >
    > What a wonderful source of information is the internet in general, and
    > Nav-l in particular!
    >
    > Thanks for all the informative replies to my question about the Nautical
    > Day. They have certainly corrected some erroneous notions of mine on that
    > topic. The references to the website
    >  were
    > particularly helpful.
    >
    > I thought I was reasonably familiar with Cook's journals of his three
    great
    > voyages, in the Beaglehole edition, and was aware that he had changed his
    > time-scale on arrival at King George's Island (his name for Tahiti) in
    > April 1769. What he wrote then in his journal was-
    >
    > "Note. The way of reckoning the Day in Sea Journals is from Noon to Noon.
    > but as the Most material transactions at this Island must happen in the
    Day
    > time this method will be attended with inconveniences in inserting the
    > transactions of each Day; for this reason I shall during our stay at this
    > Island but no longer reckon the day according to the civil account, that
    is
    > to begin and end at midnight."
    >
    > And so he did. But there's an ambiguity in Cook's wording, which fooled
    me.
    > In that passage he doesn't spell out whether the days in the sea-journal
    > were kept 12 hours in advance of the civil day (the nautical day), or 12
    > hours lagging behind it (the astronomical day). He presumed that every
    > reader would realise that it was the former. Because, for his navigation,
    > he was relying on the new-fangled Nautical Almanac, I had wrongly presumed
    > that his sea journal would refer to the astronomical day, because that was
    > what the almanac used as its time-scale throughout.
    >
    > Looking at the detailed sequence of dates as he arrived at Tahiti, and
    > departed, it is indeed clear that he was using the nautical day when at
    > sea. In fact, Beaglehole makes the matter perfectly obvious in a "Note on
    > the dating", placed just before Cook's Journal starts in Vol. 1, in which
    > he says-
    >
    > "The dating of the Journal is according to 'ship time', by which the
    > twenty-four hour day begins twelve hours before the day of civil time, and
    > runs from noon to noon. Cook's p.m. therefore precedes his a. m., and his
    > a.m. alone is identical with civil a.m. Thus hid Friday, May 27 (with
    which
    > the Journal opens) corresponds with civil Thursday, May 26 p.m. and civil
    > Friday, May 27 a.m.; his Saturday begins on civil Friday afternoon, ; and
    > so on. At Tahiti, as he explains,, he abandons this convention for civil
    > time, reverting thereafter to ship time for the rest of the voyage."
    >
    > So, really, there's no excuse for any reader to misunderstand.
    >
    > ====================
    >
    > It opens up another question, however.
    >
    > Assume that mariners were indeed universally accustomed to using the
    > nautical day in their logs. Considering that the new Nautical Almanac had
    > been especially prepared for their use, and indeed the Greenwich
    > Observatory had been set up primarily for the benefit of nautical
    > astronomy: then why were the astronomers so bloody-minded as to use the
    > astronomical day, rather than the nautical day, for the Nautical Almanac
    > tabulations? Conservatism, presumably. "That's the way we astronomers have
    > always done it."
    >
    > Once mariners' navigation started to depend on accurate numbers extracted
    > from an almanac, it seems that they then had to take account of a new
    > time-scale, a day away from their normal accounting for time, with its
    > additional likelihood of confusion and error. What a mess!
    >
    > I wonder what time-scale had been used in the tables provided to mariners
    > previously? The most essential would have been the daily tables of Sun
    > declination. I have a modern reprint of Matthew Bourne's "A Regiment for
    > the Sea", which, two centuries before the Nautical Almanac, provided Sun
    > declination tables for each day at noon over a four-year cycle starting
    > 1574: taken, I suppose, from Pedro Nunez. Presumably, the noons given in
    > that table were for civil days, and corresponded to the last moment of the
    > same-numbered nautical day, and the first moment of the same-numbered
    > astronomical day.
    >
    > The problem would become more critical when intermediate times, between
    > noons, were interpolated into the predictions. The French "Connaissance du
    > Temps" which appeared much earlier than the Nautical Almanac, was intended
    > mainly for astronomers and surveyors. Was its time-scale based on the
    > astronomical day, I wonder?
    >
    > ======================
    >
    > The next question is- when did the noon-to-noon nautical day drop out of
    > use? We know that the noon-to-noon astronomical day remained the basis for
    > nautical tables until 1925.
    >
    > The Admiralty may have directed that it should be dropped by the Royal
    Navy
    > in 1805, but it was certainly still alive-and-kicking in the Arctic
    whaling
    > fleet in 1813, and a reference quoted by Chuck Taylor states that it
    > persisted into the 1820's in the East India Company..
    >
    > Trevor Kenchington says-
    >
    > >The Nautical Day, 24 hours early than the Astronomical Day, was long
    > >established and of general application -- at least in the
    > >English-speaking world. (Smyth, for example, treated it as an
    > >established fact, without reservations or caveats.) I'm not sure when it
    > >entirely ended. I have a vague memory that it was still used in the
    > >grain trade (under sail) in the 1930s, though I would be pushed to find
    > >a reference to that.
    >
    > I wish only to question that last sentence. If the nautical day was indeed
    > being used so late as the 1930's, I would expect to see some references to
    > it in my collection of textbooks and tables dating from the mid 1800s, and
    > (from indexes only, not cover-to-cover scanning) I can find no such
    > references. Not that such lack-of-evidence is in any way conclusive. It
    > would be interesting to learn more, if only to avoid errors when examining
    > old logs.
    >
    > 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.
    > ================================================================
    
    
    

       
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