NavList:

   

Reply

Henry Halboth said, in his first posting-

>Refer to Dutton, 1934 edition, where it is stated as follows ........
>
>"Formerly navigators considered the day as beginning at the instant the
>sun crossed the upper branch of the meridian, i.e., at noon. Mean time,
>with the beginning of the day at that instant, is called astronomical
>time. Navigators now use the instant of transit of the sun across the
>lower branch or the meridian .... etc."
>
>I'm not too sure as to how this came to be called the Nautical Day.

Response from George-

No, that didn't come to be called the nautical day. The nautical day was
yet another measure of the passage of time, also noon-to-noon, which
preceded the astronomical day by 24 hours, crazy though that may seem.

And in his second posting-

>For older references try Norie's Epitome, 1839 edition, commencing on
>page 152, where a relatively complete discussion will be found -
>including ...
>
>" The nautical or sea day begins at noon, or 12 hours before the civil
>day ...., etc.". But you can read it for yourself.
                                  Henry


No, can't read it for myself, as my copy of Norie's dates from 1900. By
then Norie had changed tack, in a big way, about the Nautical Day.

Following your clues, I have tracked down what Norie had to say on the
topic, on page 159 of the 1900 edition.

"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."

I can't help but agree with Norie's second paragraph about the unnecessary
"barbarism" of the Nautical Day, viewed from the perspective of 1900.

=====================

Thomson's "Lunar and Horary Tables", 52nd ed., 1857, has no index, but I
have now tracked down its reference to the nautical day.

After introducing civil time and astronomical time, Thomson 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."

For me, that final paragraph 5 has clarified a point that I wasn't sure
about before.

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.
================================================================

   

Reply