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The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday").
Initially, the day was divided into two parts: the ante meridiem (before noon) and the post meridiem (after noon). With the introduction of the Greek sundial to Rome from the Samnites circa 293 BC, the period of the natural day from sunrise to sunset was divided into twelve hours. [1] [2] [3]
The Latin phrase “ante meridiem” means “before noon” or “before midday.” That’s why, in the 12-hour system, all times from midnight onward use this designation.
The International Date Line zigzags around the 180th Meridian. The 180th meridian or antimeridian [1] is the meridian 180° both east and west of the prime meridian in a geographical coordinate system.
This is also the origin of the terms ante meridiem (a.m.) and post meridiem (p.m.), as noted below. The Sun is directly overhead at solar noon at the Equator on the equinoxes , at the Tropic of Cancer ( latitude 23°26′09.8″ N) on the June solstice and at the Tropic of Capricorn (23°26′09.8″ S) on the December solstice .
AM/PM. Abbreviations for “ante meridiem” and “post meridiem” ASAP. As soon as possible. BOGO. Buy one, get one. BOPUS. Buy online, pick up in store
post meridiem "after midday" Used on the twelve-hour clock to indicate times after 12 midday. Example: "We will meet the mayor at 2:00 p.m." (14:00 in 24-hour clock) p.m.a. post mortem auctoris "after the author's death" p.p. per pro. per procurationem "through the agency of" PRN pro re nata "for the thing born"
The Romans originally counted the morning hours backwards: "3 a. m." or "3 hours ante meridiem" meant "three hours before noon", in contrast to the modern meaning "three hours after midnight". This ancient division has survived in the Liturgy of the Hours : Prime , Terce , Sext , and Nones are named after the first, third, sixth and ninth hours ...