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    Re: The Nautical Day
    From: George Huxtable
    Date: 2004 Feb 7, 10:43 +0000

    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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