NavList:

In my little talk on the Nautical Almanac last month in Mystic, I mentioned
that there was some confusion, and also grumbling and complaining, among
navigators when the astronomical day was dropped starting on January 1,
1925. For a very long time, astronomers had reckoned the day from noon to
noon. There was some logic to this since it meant that one night's
observations would all fall on a single calendar date. This was called the
"astronomical day". Navigators used it, too, since it was the standard
practice in all the almanacs. Meanwhile, at sea, up until the middle or late
19th century (anyone have a date?) the day "began" at noon, and it was
common to reckon a "nautical day" from noon to noon. Great, just like the
astronomers, right? Nope. One day different. And of course there's the civil
day which we still use which begins and ends at midnight. And by the way, in
19th century logbooks, it's not uncommon to see a comment like "Thus ends
this day of 36 hours". This would happen when a vessel switched from
nautical to civil time upon arriving in port.

I took a few minutes just now to clean up the OCR text from the Google Books
edition of "Thomson's Lunar and Horary Table" from 1831. Right on page one,
he spells it all out:

"To prevent ambiguity in working the Examples, given to illustrate the use
of the Tables, the reader is requested to attend to the following Remarks:
 1. By the apparent time at Greenwich is always meant the apparent
astronomical time at that meridian, and by mean time at Greenwich the mean
astronomical time is to be understood.
 2. When the estimated civil or nautical time is given at any meridian, it
is first reduced to the estimated astronomical time at the given place, to
which the longitude of that place in time being applied by addition or
subtraction, according as the longitude is west or east, the estimated
astronomical time at Greenwich is obtained; and to this time all the
articles required from the Nautical Almanac are always reduced.
 3. As the civil time is 12 hours in advance of the astronomical time, that
is, the astronomical day commences at the noon of the civil day, of the same
date, it is plain that when the given civil time is in the afternoon, or P.
M. it answers to the astronomical time of the same date ; but when the given
civil time is before noon, (or A. M.) we must add 12 hours to it, the sum
will be the astronomical time for the day of the month preceding the given
civil day. For example,
5h. 3Om. P. M. civil time, on the 10th of May, is 5h. 30m. astronomical time
of Ihe same date. But 5h. 30m. A. M. civil time, on the 10th of May, is 17h.
30m. astronomical time, on the 9th of May ; for the 9th day of the month,
according to astronomical time, commences at the noon of the 9th civil day,
and ends at the noon of the 10th civil day, (the hours being reckoned up to
24;) and 5h. 30m. A. M. of the 10th, is 17h. 30m. from noon on the 9th.
 4. The astronomical day begins at the instant that the nautical day (of the
same date) ends, consequently nautical time is always 24 hours in advance of
astronomical time, therefore to turn nautical time into astronomical time,
we have only to reckon the hours from the preceding noon, and then change
the date to the preceding day. Thus, 5h. 30m. P. M. nautical time, on May
the l0th, is 5h. 30m. astronomical time, on May the 9th; and 5h. 30m. A. M.
nautical time, on May the 10th, is 17h. 30m. astronomical time on May the
9th, and so on.
 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 at the noon of a civil day, of the same date, it is plain
that the same noon answers for any given day in either of the three methods
of reckoning time."

I particularly like that last paragraph for confusing text. In context, it's
just fine and makes good sense. But if you read it aloud all by itself, you
can scare people. :-)

 -FER