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Most analog clocks and watches today use the 12-hour dial, on which the shorter hour hand rotates once every 12 hours and twice in one day. The 24-hour analog dial was reserved for more specialized applications, such as astronomical clocks and chronometers. The 12-hour clock was used throughout the British empire.ĭuring the 15th and 16th centuries, the 12-hour analog dial and time system gradually became established as standard throughout Northern Europe for general public use. This is known as the double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter.Įlsewhere in Europe, particularly in Italy, numbering was more likely to be based on the 24-hour system (I to XXIV), reflecting the Italian style of counting the hours. In Northern Europe these dials generally used the 12-hour numbering scheme in Roman numerals, but showed both a.m. The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, used the 24-hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the Earth's apparent motion around the Sun. 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. īoth an Egyptian sundial for daytime use Īnd 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 would eventually evolve into the two 12-hour periods that started at midnight (a.m.) and noon (p.m.) which are used today. 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. Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme