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Subject: Re: What were the Lunar Distances for - the request to Dan Allen
From: George Huxtable (george@XXX.XXX)
Date: Mon Dec 30 2002 - 17:18:59 EST
John McKeel asked a guestion about art. 308 in an early Bowditch-
>
>"Greenwich Date. -- Correct the chronometer time for its error form
>Greenwhich time and deduce the Greenwich date, i.e., the Greenwich day and
>hour (mean time), reckoning the hours in succession from 0 to 24, beginning
>at noon."
>
>Does that mean that in the 1800's the 24 hour clock began at noon? That 2300
>hr. would be 11 AM? I suppose that makes sense since it was much easier to
>determine noon rather than mid-night. Can anyone give me a synopis of 24
>hour time?
Response from George Huxtable.
The Astronomical Day, used by navigators and in the Nautical Almanac until
1925, did indeed start at noon and ran over 24 hours to the next noon
(without using the terms am and pm). I imagine that it was contrived that
way to suit astronomers, who did their night's observing from dusk to dawn,
then went to bed in the daytime. Thair day lagged 12 hours behind the civil
day that everyone else used, so in the mornings the date was one day less
than the civil date, but jumped on at noon so the dates became the same.
Astronomers compiled the tables that navigators used, so navigators had to
follow their convention.
So 11 am today, 30 Dec, Civil Time, would have been 23 h on 29 Dec,
Astronomical Time.
It was quite a tricky business, then, for a navigator (especially one far
removed in longitude from Greenwich) to work out what his Greenwich Date
should be, and to work out whether or not his chronometer reading was
indicating in the 0 to 12 hour time interval, or should have another 12
hours added to its reading. Many navigational slipups must have resulted
from getting these matters wrong. It would have been a simple and sensible
matter to make chronometers with a 24-hour dial instead of 12 hours, to
lessen the ambiguity (which remains to the present time, whenever one reads
a watch or clock). Norie's Navigation devoted quite a lot of space to this
matter of getting the Greenwich Date right when using the almanac.
It's hard to credit this, but in addition to the Civil Day and the
Astronomical Day, there was also an abortion called the Nautical Day, which
you may discover was in use on some old voyages (but not in any almanacs,
as far as I know). Again, this changed at noon, but instead of being 12
hours behind the civil day, it was 12 hours ahead, so Nautical Time was
always 1 day in advance of Astronomical Time.
To complicate matters even further, the day as recorded in the Ship's Log
may have commenced at midnight (as was Royal Navy practice in the 1800s) or
noon (as was the practice in the merchant service). Cook's logs started the
day at midnight when he was in harbour, but when navigating at sea they
changed to starting from noon. Readers of old voyage-accounts need to be
aware of these various possibilities.
Sorry about all that complexity, but that's the way it was.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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