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Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day (24 hours per revolution). The more familiar 12-hour analog dial has an hour hand that makes two complete revolutions in a day (12 hours per revolution).

Twenty-four-hour analog clocks and watches are used today by pilots,[1] scientists,[2] and the military,[3] and are sometimes preferred because of the unambiguous representation of a whole day at a time. Note that this definition refers to the use of a complete circular dial to represent a 24-hour day. Using the numbers from 0 to 23 (or 1 to 24) to mark the day is the 24-hour clock system.

Sundials use 24-hour analog dials—the shadow traces a path that repeats approximately once per day. Many sundials are marked with the double-XII or double-12 system, in which the numbers I to XII (or 1 to 12) are used twice, once for the morning hours, and once for the afternoon and evening hours. So VI (or 6) appears twice on many dials, once near sunrise and once near sunset.

Modern 24-hour analog dials—other than sundials—are almost always marked with 24 numbers or hour marks around the edge, using the 24-hour clock system. These dials do not need to indicate AM or PM.

HistoryEdit

The ancient Egyptians divided the day into 24 hours. There are diagrams of circles divided into 24 sections in the astronomical ceiling in the tomb of Senemut.[4]

File:TaschensonnenuhrVR.jpg

Sundials use some or all of the 24 hour dial, because they show the position of the sun in the sky. Sometimes, for aesthetic rather than practical reasons, all the 24 hour marks are shown.

Medieval clocks often used the 24-hour analog dial, influenced by the widespread example of the astrolabe.[5]

In the early 11th century, Ibn al-Haytham's Maqala fi al-Binkam described a mechanical water clock that, for the first time in history, accurately measures time in hours and minutes. To represent the hours and minutes, Ibn al-Haytham invented a clock face. It featured a 24-hour analog dial, including a large marker for each hour and a small marker for each minute, along with medium-sized markers to indicate half-hours and quarter-hours.[6]

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.[7] In Italy, the numbers from 1 to 24 (I to XXIV in Roman numerals) were used, leading to the widespread use of the 24 hour system in that country.[5] 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, 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.[5] The 24-hour analog dial continued to be used, but primarily by technicians, astronomers, scientists, and clockmakers. John Harrison, Thomas Tompion, and Mudge[8] built a number of clocks with 24 hour analog dials, particularly when building astronomical and nautical instruments. 24 hour dials were also used on sidereal clocks, such as on this example by Charles Frodsham:

Sidereal time by Charles Frodsham

The famous Big Ben clock in London has a 24 hour dial as part of the mechanism, although it is not visible from the outside.[9]

In the 20th century, the 24 hour analog dial was adopted by radio amateurs, pilots, submariners, and for military use.

In fictionEdit

George Orwell uses the 12-hour and 24-hour dials to symbolize the old and new worlds in his novel Nineteen Eighty-Four. The 12-hour dial is a relic of pre-revolutionary society, used to represent the desirable past; the 24-hour dial and time system is the compulsory standard imposed by the Party, and represents both conformity and the undesirable nature of the new world. This theme is famously set in the opening line:

It was a bright cold day in April, and the clocks were striking thirteen.

In the 1927 film Metropolis, the opening scene shows both a 24-hour analog clock and a 10-hour (metric) analog clock, one above the other. Both are used to convey the impression of an alien and highly efficient society.

In Jules Verne Science Fiction masterpiece,"20000 Leagues Under the Sea", Captain Nemo remarks that the clocks in the Nautilus use a 24-hour dial "Now, look at that clock: it's electric, it runs with an accuracy rivaling the finest chronometers. I've had it divided into twenty–four hours like Italian clocks, since neither day nor night, sun nor moon, exist for me, but only this artificial light that I import into the depths of the seas! See, right now it's ten o'clock in the morning." (See Project Gutenberg).

Further readingEdit

  • Bruton, Eric (2002). The History of Clocks and Watches. Grange. ISBN 1-84013-505-0. 
  • King, Henry (1978). Geared to the Stars: The Evolution of Planetariums, Orreries and Astronomical Clocks. University of Toronto Press. ISBN 0-8020-2312-6. 

See alsoEdit

ReferencesEdit

  1. "Glycine Watch"
  2. "Mumford sidereal clock for astronomers"
  3. "Marathon Marine clock designed for military use"
  4. http://www.anistor.gr/english/enback/v021.htm
  5. 5.0 5.1 5.2 Geared to the Stars: The Evolution of Planetariums, Orreries, and Astronomical Clocks, Henry C King, University of Toronto Press; (1978)
  6. Salim Al-Hassani, The Mechanical Water Clock Of Ibn Al-Haytham, Muslim Heritage
  7. Gerhard Dohrn-van Rossum (1996). History of the hour: clocks and modern temporal orders. University of Chicago Press. ISBN 0-226-15510-2. 
  8. The British Museum Clocks, David Thompson, British Museum Press, 2004
  9. "UK Parliament photograph on Flickr". http://www.flickr.com/photos/uk_parliament/4074262585/. 

External linksEdit

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