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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").
At the equinoxes sunrise is around 6 a.m. (Latin: ante meridiem, before noon), and sunset around 6 p.m. (Latin: post meridiem, after noon). In the modern 24-hour clock, counting the hours starts at midnight, and hours are numbered from 0 to 23. Solar noon is always close to 12:00, again differing according to the equation of time.
Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 midday), 12 p.m. (for post meridiem, literally "after midday"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 (military time). Solar noon is the time when the Sun appears to contact the local celestial meridian.
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]
Its application is also used in determining the time of the Morning Prayer, which must be recited between sunrise until the end of the fourth hour, [64] but post facto can be said until noon time, [65] and which times will vary if one were to rely solely on the dials of the standard 12-hour clock, depending on the seasons.
One kè was usually defined as 1 ⁄ 100 of a day until 1628, though there were short periods before then where days had 96, 108 or 120 kè. [ 2 ] kè literally means "mark" or "engraving", referring to the marks placed on sundials [ 4 ] or water clocks [ 5 ] to help keep time.
The Prague Astronomical Clock struck according to the Old Bohemian Clock until its destruction in 1945. The variant with counting from dawn is also rarely documented and used, e.g. on a 16th-century cabinet clock in the Vienna Art-History Museum. [14] German (Gallic) hours (half-dial): 2×12 hour system starting at midnight and restarted at noon.
Strictly speaking, it is incorrect to use "a.m." and "p.m." when referring to noon or midnight. The abbreviation a.m. stands for ante meridiem or before noon, and p.m. stands for post meridiem or after noon. Since noon is neither before nor after noon, and midnight is exactly twelve hours before and after noon, neither abbreviation is correct.