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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:
 http://books.google.com/books?id=6JlzJHd15vQC&pg=PA56&output=html

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
are".

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.

George.

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.


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