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    Re: Fw: Nautical Day
    From: George Huxtable
    Date: 2004 Feb 9, 09:51 +0000

    Trevor Kenchington said-
    
    Is there any reason to think that the astronomical day was _ever_ used
    for keeping the ship's time at sea (as distinct from being used when
    consulting an almanac, which it clearly was)?
    
    Now there's a thought, which I must admit I hadn't considered, and which
    may indeed turn out to be correct, though I haven't yet come to accept it.
    
    I've recently looked up "The Practice and Theory of Navigation", by the
    Earl of Dunraven, 1908, where in vol 1 he describes the "day's work". This
    consists in summimg up the courses and distances using a traverse table to
    arrive at a new DR position. And he gives several illusrations of completed
    forms for noting this day's work, which unfortunately don't offer any
    entries for day and date. But it's quite clear, in every case illustrated,
    that the day's reckoning was always from noon to noon, times being given on
    the form as 1 to 12 hours, then 1 to 12 hours again. The form itself could
    be used either for noon-to-noon or midnight-to-midnight reckoning, as
    there's no a.m. and p.m. printed with the hours, but in each case the word
    "midnight' has been entered in the remarks column, halfway through the
    "day's work".
    
    Did this noon-to-noon "day's work" cover the same period as one whole day
    in the ship's reckoning of time, with the date changing at noon? That's not
    conclusively shown, but seems likely.
    
    Let's sum up what we can now agree on.
    
    There are three possible time-scales involved for the ship's day-
    
    1. The civil day, midnight-to-midnight, that's always used now, used by the
    Royal Navy sinse 1805. and which the Nautical Almanac has followed since
    1925.
    
    2. The astronomical day, noon-to-noon, 12 hours behind the civil day, used
    by the Almanac until 1925.
    
    3. The nautical day, noon-to-noon, 12 hours ahead of the civil day, used by
    the Royal Navy until 1805, and the East India Company until the 1820s.
    
    The ship's date would change at the end of the appropriate day.
    
    There seems to be strong evidence that merchant vessels continued to reckon
    their days from noon to noon, until well into the 20th century. If so, were
    they using the astronomical day, which would correspond with their almanac
    predictions? Or were they using the nautical day, 1 day ahead of their
    almanac, from which date they would have to subtract 1 when looking it up
    in the almanac? That's the question we are discussing.
    
    It's surprising, to me, that so far we haven't found an authoritative
    answer, about sea practice in a period that isn't far beyond living memory.
    Perhaps the answer is, that some mariners were doing it one way, and some
    another. It would be surprising, to me, if after the almanac finally
    changed to civil time in 1925, any but the most conservative diehards would
    still be using the noon-to-noon day. But then, Trevor has quoted an example
    from the "Herzogin Cecilie" in 1937, where they still appeared to be
    keeping a noon-to-noon day when at sea. But then that was in the Erikson
    fleet of Mariehamn, probably the most conservative diehards in existence.
    
    What do the navigation textbooks tell us? I will repeat some earlier quotes-
    
    ========================
    
    Thomson's "Lunar and Horary Tables", 52nd ed., 1857, says-
    
    "4. The astronomical day begins at the instant that the nautical day (of
    the same date) ends, consequently nautical time is always 24 hours ahead of
    astronomical time...."
    
    And a footnote reads- "Nautical Time has generally been abolished as
    inconvenient, the only advantage it possessed, was finishing the "day's
    work", and date, together".
    
    He goes on to say-
    
    "5. The noon of the astronomical day is at the instant that it begins, and
    the noon of the nautical day is at the instant when it ends; and as both
    these take place on the noon of a civil day, of the same date, it is plain
    that the same noon answers for any given day in the three methods of
    reckoning time."
    
    ================
    
    And Norie in 1900 stated-
    
    "In the Royal Navy the time is reckoned as on shore, and has been kept in
    civil time since 1805, by order of the Admiralty. In the Merchant Service
    it has been the custom to begin the day at noon, and presumably this
    continues to be the case in the majority of ships, and with most Masters.
    But there can be no doubt that the ship's log should be kept in Civil Time,
    24 hours to the day, - the first 12 hours being a.m., and the second 12
    hours p.m.; by this mode of reckoning, the noon of the Date by Log agrees
    with the astronomical day, and there can be no confusion in the correction
    of the elements taken from the Nautical Almanac. The civil day begins at
    midnight and ends at the midnight following; the astronomical day begins at
    noon and ends at the noon following: but the noon of any given date, as May
    12, or November 20, is the same in both methods of reckoning; hence the
    civil day is 12 hours in advance of the astronomical day.
    
    The barbarism of reckoning by a nautical day, 12 hours in advance of the
    civil, and 24 hours in advance of the astronomical day, cannot be too much
    deprecated, and must have frequently led to errors in the computation of
    the astronomical data. Besides, two modes of reckoning must surely be
    enough, without the complication of a third, and wholly useless, date."
    
    =====================
    
    Is Norie distinguishing between two different systems which may both be in
    use on the same vessel at the same time, when he says "it has been the
    custom to begin the day at noon", but then goes on to say "there can be no
    doubt that the ship's log should be kept in civil time". Is there a subtle
    distinction between these two that I am missing? Would it be possible to
    "begin the ship's day at noon" (whatever that means) AND ALSO keep the log
    in civil time? Or is he saying that it's about time for the remaining
    diehard mariners to switch to civil time?
    
    Thomson, as far back as 1857, seems to consider that Nautical Time is
    already a dead duck.
    
    ==================================
    
    If we can't find an authoritative answer in a book, it might be interesting
    to study some ship's logs, from say the early 1900s, so see if (and in what
    way) they shifted their time-scale from and to civil time, as they departed
    from port and as they returned.
    
    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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