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Clocks and Watches with a 24 hour analog dial are marked with 24 numbers or hour marks around the edge. The hour hand makes 1 complete revolution, 360°, in a Day . The more familiar 12-hour Clock (and watch) dial shows 12 hours of the day at a time, and the hour hand makes two complete revolutions in a day. The 24 hour dial doesn't need to indicate A.m. or p.m. 24 hour analog clocks and watches are used today by pilots, scientists, and the military, and have often been preferred by scientists and specialists because of the unambiguous representation of the 24-hour Clock system. Note that this definition refers to the use of a complete circular dial to represent a whole day, not to the use of the numbers from 1 to 24 (or 0 to 23), the 24 hour clock system. 24 hour analog dials in medieval Northern Europe used the 12-hour time system rather than the 24 hour time system, even though the hour 'hand' made a single revolution in a day. HISTORY The Egyptians divided the day into 24 hours, and you can see diagrams of circles divided into 24 sections in the astronomical ceiling in the tomb of Senemut . Sundials use the 24 hour dial, because they depend on the position of the sun in the sky. However, few sundials mark the location of hours occurring during the night (for obvious reasons). Medieval clocks often used the 24-hour analog dial. In Northern Europe, the Double-XII system was preferred: two sets of the Roman Numerals I to XII were used, one on the left side for the night and morning hours, and another set on the right side of the dial to represent the afternoon and evening hours. In Italy, the numbers from 1 to 24 (I to XXIIII in Roman numerals) were used in a similar layout, leading to the widespread use of the 24 hour system in that country. A famous example is the clock at St Marks, Venice. On Italian clocks, though, the I was often shown at the right side of the dial, rather than the top. This probably reflects the influence of the Italian timekeeping system of the time, which started counting the hours of the day at sunset or twilight. In northern Europe, the Double XII system was gradually superseded during the 14th and 15th centuries by the single XII (12-hour system), leading to the widespread adoption of the 12-hour dial for popular use. The 24 hour analog dial continued to be used, but primarily by technicians, astronomers, scientists, and clockmakers. John Harrison , Thomas Tompion , and Mudge all built clocks with 24 hour analog dials, and even the famous Big Ben clock in London has a 24 hour dial as part of the mechanism, not visible from the outside. In the 20th century, the 24 hour analog dial was adopted by radio amateurs, pilots, submariners, and military personnel. TODAY 24 hour analog watches and clocks are still being manufactured today, and are sought after by collectors and enthusiasts. One of the best known examples is the Glycine Airman watch. Other manufacturers who make 24 hour analog watches include Breitling , Raketa , Vostok , Fortis , Poljot , Swatch , and many others. DESIGN The major variation in the design of 24 hour analog dials is the location of midnight and noon. Although always opposite each other, 180° apart, some prefer noon at the top, others prefer midnight at the top. A few rare variants place noon and midnight at the right and left sides. There is no ambiguity if the 24 hour clock is used. MISCELLANEOUS FACTS George Orwell makes ironic use of the 24 hour analog dial in his novel 1984 .
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