The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (thus hours having varying length throughout the year) and the night was divided into four watches. 1500 BC, these clocks divided their respective times of use into 12 hours each. Both an Egyptian sundial for daytime use and an Egyptian water clock for night-time use were found in the tomb of Pharaoh Amenhotep I. The 12-hour clock can be traced back as far as Mesopotamia and ancient Egypt. Noon itself is rarely abbreviated today but if it is, it is denoted "m." This eventually evolved into the two 12-hour periods which are used today, one called "a.m." starting at midnight and another called "p.m." starting at noon. 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. Clock systemĮxeter 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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