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Under the erroneous assumption that members of the group had access to
reference material available to me, I did not fully quote from Norie's
1839 edition in my last - I apologize. Perhaps the full reference will
serve to clarify the subject somewhat; it is presnted solely as written,
with no effort at analysis.........

"The nautical or sea day begins at noon, or 12 hours before the civil
day; it is divided into two parts of 12 hours each, the former being
marked PM and the latter AM. This mode of reckoning arises from the
custom of seamen dating their log for the preceding 24 hours, the same as
the civil day; so that occurrences which happen, for instance, on Monday
21st, afternoon, are entered in the log marked Tuesday, 22nd. Hence noon
of the civil day, the beginning of the astronomical day, and the end of
the nautical day, take place at the same moment".

Regardless, It it easy to see how such recording might confuse the
researcher unless otherwise in some way clarified in the actual text,
however, the Nautical Day seems not to have been of any consequence in
the actual practice of celestial navigation which has always been based
on civil and/or astronomical time, the original Nautical Ephemeris (for
the year 1767) having utilized Greenwich time and date, whether civil or
astronomical, as it has always continued to do in American and British
practice.

I have otherwise searched American navigational references available to
me back the year 1795, find no reference whatsoever to the Nautical Day -
makes me wonder if perhaps this may have been a European custom. It does
appear that some references and writers are confusing or, for that
matter, combining the nautical and astronomical days, which is of course
technically incorrect.
HO Pub No. 220 "Navigation Dictionary" contains perhaps the most
authoritative American definition of these terms.

                                        Henry

   

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