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    Re: chronometer dials: 12 or 24 hours?
    From: George Huxtable
    Date: 2009 Jan 19, 13:34 -0000

    Paul Hirose wrote-
     Chauvenet (1863) said chronometers should have 24 hour dials, because the
    AM/PM ambiguity of a 12 hour dial made it more difficult to read the correct
    Greenwich time. See the footnote on this page:
    Why not? Chauvenet's point seems logical to me.
    I agree. There seems little sense in the traditional pattern. And there's no
    technical difficulty, in changing the gearing between the minute hand and
    the hour hand, from 12:1 to 24:1.
    And Lecky, in "Wrinkles", from about 1890, agrees too. He writes, in page
    404 of my 1920 print-
    "...he may make the not uncommon blunder of taking out the Nautical Almanac
    elements for the wrong day, and so get adrift of his true position.
    Here the marking of the chronometer face from 0 up to 23 hours would be of
    great service. As the dial is figured at present, there is no way of
    distinguishing the XII noon from XII midnight; wherein, if marked as
    suggested, 0 hour would always refer to noon, and 12 hours to midnight."
    However, in those last few words, we might glimpse a problem behind making
    such a change as Chauvenet and Lecky advocated .
    Until the early 19th century, navigation was bedevilled by three differing
    measures of time. (Here, I am bypassing the differences, up to a
    quarter-hour, between mean and apparent time; everything that follows will
    relate to mean time).
    First, there's the Civil Day that a vessel adopted when in port, to
    correspond with that of its surroundings, in which the date changed at
    midnight, and midday was at 12; just as it is today.
    However, at sea, the navigator's day ended with his noon sight, when he
    concluded his "day's work" by working-up his dead reckoning from log and
    compass readings, and traverse tables, and when the date changed. He used
    the Nautical Day, which happened to be 12 hours AHEAD of civil time.
    But for many years, astronomers, perhaps because they went back to bed in
    the mornings, had also used a measure of time that continued unbroken
    through each night, and for them, also, the date changed at noon.
    Unfortunately, the Astronomical Day, which had  a much longer documented
    history than that of the mariner, was 12 hours BEHIND civil time. And as
    astronomical tables (which were based on the Astronomical Day) became more
    and more important to navigation, mariners had to cope with the resulting
    1-day difference between their own reckoning of dates, and that of the
    tables that they used. What a recipe for chaos!
    In the early 19th century, the Nautical Day was gradually dropped, and
    instead mariners, when at sea, adopted the Astronomical Date and Time that
    corresponded to their tables. But still, in port, they reverted to Civil
    Time and Date, as used on land. This was the time-world that was familiar to
    both Chauvenet and Lecky.
    And now you can see a problem inherent in Chauvenet's and Lecky's proposal.
    Lecky, as he says, would have set his clock to read 24h at Greenwich noon,
    but that would be quite different from how we would set a Greenwich Civil
    Time 24-hour clock on land, here in Britain, when a "railway time" of 24
    hours would correspond to midnight. [You might think of that as a mere
    parochial concern, here in Britain. After all, there would be close
    agreement with local Civil Time in New Zealand. But London, rather than
    Auckland, was where such things mattered, in those days.] Local Times and
    dates, throughout the World, have long been based on, and offset from,
    Greenwich Civil Time, changing at midnight.
    A 12-hour chronometer, by its very ambiguity, avoids having to make such a
    distinction, between noon and midnight. Not a conclusive argument against a
    24-hour chronometer, but an addition to the inertia of "keep things as they
    Not until 1924 was the situation resolved, when the Almanac for that year
    stated that from then on, Greenwich Mean Time would be measured on a 24-hour
    basis starting at midnight, corresponding to Greenwich Civil Time. From then
    on, a 24-hour chronometer would have been unambiguously useful.
    Frank Reed wrote, in [7112]
    "Chauvenet was a mathematician with some limited sea experience, but he was
    not a practical navigator."
    A mathematician is what he was, but that denigration might be compared with
    what Wikipedia has to say about Chauvenet, as follows-
    "In 1841 he was appointed a professor of mathematics in the United States
    Navy, and for a while served on Mississippi. A year later, he was appointed
    to the chair of mathematics at the naval asylum in Philadelphia,
    Pennsylvania. He was instrumental in the founding of the United States Naval
    Academy at Annapolis, Maryland."
    I've little doubt that his navigational experience on "Mississippi" far
    exceeded mine, or Frank's.
    Frank adds-
    " I believe that the concern he has would only rarely arise in practice."
    and adds later "....Assuming that the navigator who does the noon Sun sight
    is the same as the one who works other sights (morning or afternoon time
    sights specifically), it's hard for me to imagine a case where they would
    look at the chronometer and get the hours wrong by twelve."
    Here, Frank is making invalid assumptions, in thinking about a small craft
    with one single "navigator", and in temperate climates, who observes a noon
    Sun each day.
    He might otherwise imagine himself as a second officer of a vessel in Winter
    in high latitudes, working watch-and-watch. He has been dragged out of his
    pit to go on watch for four hours, in pitch darkness and overcast weather.
    With short daily sunlight, his previous watch might well have been the same.
    In rough conditions, he hardly knows which way is up, to start with, never
    mind what Greenwich Time it happens to be. In those conditions, it would be
    easy to look at the chronometer, and get it wrong by 12 hours. Navigation
    has to take place under such conditions, as well as in sunny Summer cruises.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Navigation List archive: www.fer3.com/arc
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