Initially I thought they meant “ante-meridian” and “post-meridian” with the meridian being the longitudinal line at any given point on Earth dividing the sky in half and running north to south (see figure 4 in the following link; the meridian is the N/S line). [1]
As the sun rises from the east it is before or “ante” meridian (A.M.).
At noon, or mid-day, the sun is at it’s zenith or on the meridian line.
It is neither A.M. or P.M.
As the sun begins to set it moves beyond the meridian (“post” or after the meridian) and you are in P.M.
At midnight the sun, relative to the initial observation point, is at the polar opposite of its zenith (the nadir).
Again it is neither A.M. or P.M.
1 second later the cycle starts again.
In actuality A.M. and P.M. stand for “ante-meridiem” & “post-meridiem;” Latin for before mid-day and after mid-day.
In the old days mid-day may have meant when the sun was at it’s zenith at a given location but today it means local clock time 12:00 noon. [2]
A.M. starts 1 second after mid-night & runs to 1 second before to mid-day.
P.M. starts 1 second after mid-day and runs to 1 second before to midnight.
12:00 noon & 12:00 mid-night are neither A.M. nor P.M.
Depending on where you live and the time of year the solar noon, or when the sun is at its zenith and on the meridian line, may or may not equal local clock-time 12:00 noon.
The two are often not the same and in some parts of the world the sun never actually sets during certain seasons and doesn’t rise during others. [3]
So while the terms A.M. and P.M. may have their origin in the position of the sun relative to the meridian in common use today they simply define the time between local clock-time 12:00 noon and local clock-time 12:00 mid-night at a given location.
Its enough to make you want to switch everyone to the 24 hour clock.
