Originally there were two cycles: one cycle which could be tracked by the position of the Sun (day), followed by one cycle which could be tracked by the Moon and stars (night). The natural day-and-night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. History and use Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme There is no widely accepted convention for how midday and midnight should be represented: in English-speaking countries, "12 p.m." indicates 12 o'clock noon, while "12 a.m." means 12 o'clock midnight.
The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries. The 12-hour clock was developed from the second millennium BC and reached its modern form in the 16th century. (from Latin post meridiem, translating to "after midday"). (from Latin ante meridiem, translating to "before midday") and p.m. The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m.
For the meridian 180° both east and west of the prime meridian in a geographical coordinate system, see antimeridian.
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