When is midnight? Nomenclature question

Paul Eggert eggert at CS.UCLA.EDU
Mon Apr 24 17:17:17 UTC 2006

"Dave Cantor" <Dave at Cantor.mv.com> writes:

> Using 12:00 M for noon is what I learned in junior high school, 
> back in the 1950's.  We were also taught that midnight was 
> properly called 12:00 p.m. (it is the last minute of the ending 
> day, and is therefore 12 hours after noon, which is what 'p.m.' 
> literally means).

Thanks for your recollections.

Older editions of the Chicago Manual of Style (CMS) indeed recommended
that '12 m.' means noon and '12 p.m.' means midnight.  However, this
recommendation was removed sometime during the 14th edition of the
CMS.  Here's my source:

  Over the years, we issued about ten printings of the fourteenth
  edition. Each time we reprinted, we corrected errors that had been
  pointed out since the last printing. In section 8.48 we wanted to
  eliminate two examples that seemed more confusing than helpful (and
  12:00 P.M. is in fact noon). For the fifteenth edition, we have made
  several clarifications, including the recommendation that numerals
  not be used for midnight and noon except in the twenty-four-hour
  system. Note also that the fifteenth recommends writing "a.m." and
  "p.m.," though the more traditional small capitals are still
  accepted (but now without periods: AM and PM).

So, I don't know the origin of the tradition you were taught in junior
high school, but the CMS is now saying that it was an error!

I doubt whether that tradition came from the Romans themselves, since
the original Latin tradition was that 3 a.m. was three hours before
noon (which is what "a.m."  literally means), so the Roman 3 a.m. was
our 09:00.  Hence, in the original Latin tradition, I guess '12 a.m.'
and '12 p.m' would both have meant midnight.

Wikipedia says that one should use '12 m.n.'  instead, where the
'm.n.' means 'media nox', but I'm dubious of this as well.  I suspect
that the actual Romans almost invariably wrote either 'meridies' for
noon, or 'media nox' for midnight, and didn't bother with the 'xii'.
For a (modern) example of this, the motto of the RAF No. 409 Nighthawk
Squadron during World War II was "Media nox meridies noster"
("Midnight is our noon")
<http://www.rcaf.com/squadrons/400series/409squadron.php>.  The RAF
didn't bother with 'xii', and I'll bet the Romans didn't either, and
that "12 m." and "12 p.m." are neologisms, and confusing ones as well.

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