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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.'' (from the Latin ''ante meridiem'', meaning "before midday") and ''p.m.'' (''post meridiem'', "after midday"). Each period consists of 12 hours numbered: ''12'' (acting as zero), ''1, 2, 3, 4, 5, 6, 7, 8, 9, 10,'' and ''11''. The 24 hour/day cycle starts at 12 midnight (often indicated as 12 a.m.), runs through 12 noon (often indicated as 12 p.m.), and continues to the midnight at the end of the day. The 12-hour clock was developed over time from the mid-second millennium BC to the 16th century AD. ==History and use== The natural day/night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally these were 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.〔 The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt:〔(The History of Clocks )〕 Both an Egyptian sundial for daytime use〔(Berlin instruments of the old Egyptian time of day destination )〕 and an Egyptian water clock for nighttime use were found in the tomb of Pharaoh Amenhotep I.〔(A Walk through Time - Water Clocks )〕 Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each. The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into four watches. The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, using the 24-hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the apparent motion of the Sun. In Northern Europe these dials generally used the 12-hour numbering scheme in Roman numerals, but showed both ''a.m.'' and ''p.m.'' periods in sequence. This is known as the double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter. Elsewhere 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. The 12-hour clock was used throughout the British empire. During 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. The 24-hour analog dial was reserved for more specialized applications, such as astronomical clocks and chronometers. 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. Some analog clock dials have an inner ring of numbers along with the standard 1-to-12 numbered ring. The number 12 is paired either with a 00 or a 24, while the numbers 1 through 11 are paired with the numbers 13 through 23, respectively. This modification allows the clock to be read also in the 24-hour notation. The 12-hour clock can be found in countries where the 24-hour clock is preferred. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「12-hour clock」の詳細全文を読む スポンサード リンク
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