Editing Clock (section)

==Purposes==

[[File:Robbins NC Clock.jpg|thumb|upright|Many cities and towns traditionally have public clocks in a prominent location, such as a town square or city center. This one is on display at the center of the town of [[Robbins, North Carolina]] ]]

[[File:Collections of Wang Yung-Chang Clock&.Watch-02.2023-09-05.jpg|thumb|upright|A clock on sale in the store from [[Taipei]], [[Taiwan]].]]

[[File:Rellotge al museu de Belles Arts de València.JPG|thumb|upright|A [[Napoleon III style|Napoleon III]] mantel clock, from the third quarter of the 19th century, in the [[Museu de Belles Arts de València]] from Spain]]

Clocks are in homes, offices and many other places; smaller ones (watches) are carried on the wrist or in a pocket; larger ones are in public places, e.g. a railway station or church. A small clock is often shown in a corner of computer displays, mobile phones and many [[MP3 player]]s.

The primary purpose of a clock is to ''display'' the time. Clocks may also have the facility to make a loud alert signal at a specified time, typically to waken a sleeper at a preset time; they are referred to as ''[[alarm clock]]s''. The alarm may start at a low volume and become louder, or have the facility to be switched off for a few minutes then resume. Alarm clocks with visible indicators are sometimes used to indicate to children too young to read the time that the time for sleep has finished; they are sometimes called ''training clocks''.

A clock mechanism may be used to ''control'' a device according to time, e.g. a central heating system, a [[VCR]], or a [[time bomb]] (see: [[digital counter]]). Such mechanisms are usually called [[timer]]s. Clock mechanisms are also used to drive devices such as [[Solar tracker#Chronological tracker|solar trackers]] and [[Telescope mount#Equatorial mounts|astronomical telescopes]], which have to turn at accurately controlled speeds to counteract the rotation of the Earth.

Most [[digital computer]]s depend on an internal signal at constant frequency to synchronize processing; this is referred to as a [[clock signal]]. (A few research projects are developing CPUs based on [[asynchronous circuit]]s.) Some equipment, including computers, also maintains time and date for use as required; this is referred to as time-of-day clock, and is distinct from the system clock signal, although possibly based on counting its cycles.

===Time standards===

{{Main|Time standard|Atomic clock}}

For some scientific work timing of the utmost accuracy is essential. It is also necessary to have a standard of the maximum accuracy against which working clocks can be calibrated. An ideal clock would give the time to unlimited accuracy, but this is not realisable. Many physical processes, in particular including some [[Electronic transition|transitions]] between atomic [[energy level]]s, occur at exceedingly stable frequency; counting cycles of such a process can give a very accurate and consistent time—clocks which work this way are usually called atomic clocks. Such clocks are typically large, very expensive, require a controlled environment, and are far more accurate than required for most purposes; they are typically used in a [[Metrology#Laboratories|standards laboratory]].

===Navigation===

Until advances in the late twentieth century, navigation depended on the ability to measure [[latitude]] and [[longitude]]. Latitude can be determined through [[celestial navigation]]; the measurement of longitude requires accurate knowledge of time. This need was a major motivation for the development of accurate mechanical clocks. [[John Harrison]] created the first highly accurate [[marine chronometer]] in the mid-18th century. The [[Noon gun]] in [[Cape Town]] still fires an accurate signal to allow ships to check their chronometers. Many buildings near major ports used to have (some still do) a large [[Time ball|ball]] mounted on a tower or mast arranged to drop at a pre-determined time, for the same purpose. While [[satellite navigation]] systems such as GPS require unprecedentedly accurate knowledge of time, this is supplied by equipment on the satellites; vehicles no longer need timekeeping equipment.

===Sports and games===

Clocks can be used to measure varying periods of time in games and sports. [[Stopwatch]]es can be used to time the performance of [[track athlete]]s. [[Chess clock]]s are used to limit the board game players' time to make a move. In various sports, ''{{Vanchor|Game clock|text=game clocks}}'' measure the duration the game or subdivisions of the game,<ref>{{Cite web |last=Rizzo |first=John |date=2023-09-10 |title=NFL Clock Stop Rules: A Comprehensive Guide |url=https://www.metroleague.org/nfl-clock-stop-rules_af/ |access-date=2023-09-11 |website=Metro League |language=en-US}}</ref><ref>{{Cite web |title=sports and games > sports facilities > scoreboard image - Visual Dictionary |url=http://www.ikonet.com/en/visualdictionary/sports-and-games/sports-facilities/scoreboard.php |access-date=2023-09-11 |website=www.ikonet.com}}</ref> while other clocks may be used for tracking different durations; these include [[play clock]]s, [[shot clock]]s, and [[pitch clock]]s.