What does AM and PM stand for?

Definition and Meaning

The notations A.M. (abbreviating the Latin ante meridiem, also called as “before midday”) and P.M. (abbreviation of post meridiem, also called as “after midday”) denote the two 12-hour divisions of the 24 hours in a single Earth day. Each is comprised of 12 numbered hours in which the 12th acts as “Zero”. Every 24-hour cycle begins at 12 midnight (“12 a.m.”), is marked halfway at 12 noon (“12 p.m.”) and concludes with the next 12 midnight marking the start of the next day’s cycle.

The 12-hour clock originated in the time of Mesopotamia and ancient Egypt. Prior to that, one cycle tracked the sun’s position during the day and another tracked that of the moon and stars at night. The development of the 12-hour clock began circa 1500 B.C. with referring to an Egyptian sundial for telling time during the day and a water clock at night, both of which were found in the tomb of Pharaoh Amenhotep. Similarly, Romans used a 12-hour clock dividing 12 equal daylight hours that varied in length throughout the year and four divided watches of nighttime.

The 14th century’s earliest mechanical clocks used a 24-hour analog dial influenced astrolabe and sundial displays to display all 24 hours and model the sun’s apparent motion. Northern European clocks generally used the 12-hour numbering scheme displaying A.M. and P.M. in Roman numerals.

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