Whether spelled meridian or meridiem, the reference is to an imaginary line drawn throught any point on the earth, between the north and the south poles. The sun, in its movement across the sky, travels across such lines (perpendicular to them). When the sun is above a meridian, the solar time on that meridian is noon; the sun is neither before nor after that line in its “motion”. When the sun is before (ante) the line, the time on the meridian is “morning”; when the sun is after (post) the line, the time on the meridian is “after noon” (which, in everyday language, is afternoon or night). After midnight, 12:01AM, counting of the hour is always up, and 12:59AM is followed by 1AM. As noon is approached, the AM hours increase; as the “afternoon” progresses, the time starts at 12:01PM, progresses through 12:59PM, then 1PM, and again the PM hours increase. Logically, noon is not AM, but is M, and midnight (for reasons I don’t understand) is conventionally called 12AM).

12:00 during the day is called “noon” and 12:00 at night is called “midnight”. There is no such thing as 12 AM or 12 PM.

Unfortunately, computers don’t respect this, and so they are programmed to say silly things like “12:00 PM” and “12 AM”. Those of us who actually know wat AM and PM means should not be so simplistic.

PM post meridiem

Latin