Everybody knows that an A.M. hour is a morning hour and a P.M. hour is an afternoon or evening hour. But did you ever wonder what A.M. and P.M stand for?
Meridies was the Latin word for “midday” or “noon.” Since noon is the moment when the sun is directly overhead, it was natural to divide the day into two parts, one before noon, when the sun was rising, and one after noon, when the sun was traveling toward sunset.
In Latin, ante means “before” and post means “after.” So the hours before noon became known as ante meridies hours, and that term was abbreviated A.M. Hours after midday were called post meridies hours, and that term was shortened to P.M