A clock from the Latin cloca, "bell" is an instrument for measuring time. A clock is usually a physical instrument, an especially accurate one is called a chronometer. The clock in its modern form 24 hour clock has been in use since at least the 14th century. Clock can refer to an abstract system of time measurement ed. see calendar for other measurements.

Practically all computers depend on an accurate internal clock signal to allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuits). Some computers also maintain time and date for all manner of operations whether these be for alarms, event initiation or just to display the time of day.

Ideal uses
An ideal clock measures the ratio of the duration of natural processes, and thus will give the time measure for use in physical theories. Therefore, to define an ideal clock in terms of any physical theory would be circular. An ideal clock is more appropriately defined in relationship to the set of all physical processes.

A clock is a recurrent, periodic process and a counter.
A good clock is one which, when used to measure other recurrent processes, finds many of them to be periodic.
An ideal clock is a clock (i.e., recurrent process) that makes the most other recurrent processes periodic.
The recurrent, periodic process (a metronome) is an oscillator and typically generates a clock signal. Sometimes that signal alone is (confusingly) called "the clock", but sometimes "the clock" includes the counter, its indicator, and everything else supporting it.

This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances. While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes all individual physical processes which are proposed for consideration. Since atoms are so numerous and since, within current measurement tolerances, they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical processes. However, they are not so designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition.

Navigation
Accurate navigation by ships beyond the sight of land depends on the ability to measure latitude and longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrison created the first, highly accurate marine chronometers in the mid-18th century. The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers.

History

An ancient Chinese incense clockThe history of the time telling device can be traced to antiquity. Vitruvius reports that the ancient Egyptians used clepsydras, a time mechanism run by flowing water. Historians disagree over the Antikythera mechanism but this is largely thought to be an early mechanical clock. By the 9th century AD a mechanical timekeeper had been developed that lacked only an escapement mechanism. There is a record that in 1176 Sens Cathedral installed a ‘horologe’—the word still used in French for large clocks. It is derived from the Greek hora meaning ‘hour’ and legein meaning 'to tell'. This word has led scholars to believe that these tower clocks did not employ hands or dials, but “told” the time with audible signals.

The earliest reasonably accurate clocks are the 13th century tower clocks probably developed for (and perhaps by) monks in Northern Italy. These were used to nnounce the canonical hours or intervals between set times of prayer. Canonical hours differ in length, and varied as the times of sunrise and sunset shifted.

The earliest table clocks that survive in any quantity are mid-16th century ones from the metalworking towns of Nuremberg and Augsburg. These clocks have only one hand. The dial between the hour markers is divided into four equal parts making the clocks readable to the nearest 15 minutes.

The massive clock on Big Ben, London, England. The 5 foot 4 inch (1.63 m) person "holding on" to the six o'clock marking has been inserted into the picture at correct scale. The hour hand is 9 feet (2.7 m) long and the minute hand is 14 feet (4.3 m) longThe next major development in accuracy occurred in 1657 with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to propel the motion of a time telling device earlier in the 17th century. Christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (99.38 cm or 39.13 inches for the one second movement) and had the first pendulum driven clock made. In 1670, the English clockmaker William Clement created the anchor escapement, an improvement over Huygens' crown escapement. Within just one generation, minute hands and then second hands were added.

The excitement over the pendulum clock attracted the attention of designers resulting in a proliferation of clock forms. Notably, the longcase clock was created to house the pendulum and works. The English clockmaker William Clement, inventor of the anchor escapement, is credited with developing this form in 1670. It was also at this time that clock cases began to be made of wood and clock faces to employ enamel. On November 17, 1797, Eli Terry received his first patent for a clock. Terry is known as the founder of the American clock-making industry.

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