The first known use of ‘ante meridiem’ was in 1563. It’s from Latin: ‘meridiem’ means midday or noon. ‘Ante’ means before and ‘post’ means after. So ante meridiem or a.m. is before noon—the hours between midnight and noon. Post meridiem or p.m. is afternoon—the hours between noon and midnight.
A.M. and p.m. are used to describe hours on a 12-hour clock. 10:15 a.m. means 10:15 in the morning; 10:15 p.m. means 10:15 at night. In the military and in Europe, they use a 24-hour clock, so 13:00 means 1:00 p.m.
‘Meridian’ is a different word with 2 meanings. The first meaning is to make an adjective out of meridiem. To say, “She’s wearing an anti-meridian dress” means she’s wearing a dress suitable for the morning. Nobody talks like that nowadays.
The second meaning is to describe a line of longitude. The Prime Meridian is Point Zero of east or west. It’s only ad 1563 in this history so we have to wait 200 years before somebody figures out longitude—and where in the world the Prime Meridian is located…
Back to the beginning of The Western Civ User’s Guide to Time & Space