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History
12-hour clock
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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. and p.m.. Each period consists of 12 hours numbered: 12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.
Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme
Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme
Typical analogue 12-hour clock
Typical analogue 12-hour clock
Typical digital 12-hour alarm clock indicating p.m. with a dot to the left of the hour
Typical digital 12-hour alarm clock indicating p.m. with a dot to the left of the hour
Roman timekeeping
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In Roman timekeeping, a day was divided into periods according to the available technology. Initially, the day was divided into two parts: the ante meridiem and the post meridiem. With the advent of the sundial circa 263 BC, the period of the natural day from sunrise to sunset was divided into twelve hours.
Sundial at the Temple of Apollo (Pompeii)
Sundial at the Temple of Apollo (Pompeii)
A Roman era sundial on display at a museum in Side, Turkey
A Roman era sundial on display at a museum in Side, Turkey