<P> The 12 - hour time convention is common in several English - speaking nations and former British colonies, as well as a few other countries . </P> <P> The natural day - and night division of a calendar day forms the fundamental basis as to why each day is split into two cycles . 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). This would eventually evolve into the two 12 - hour periods that started at midnight (a.m.) and noon (p.m.) which are used today . Noon itself is rarely abbreviated today, but if it is, it is denoted M . </P> <P> The 12 - hour clock can be traced back as far as Mesopotamia and Ancient Egypt . 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. Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each . </P> <P> 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 . </P>

Who came up with the 12 hour clock
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