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A clock (from the Latin cloca, " bell") is an instrument for measuring time. The clock in its most common, modern form (in use since at least the 14th century) displays the time in hours, minutes, and often seconds during a 12- or 24-hour period.

Clocks used for technical purposes, of very high accuracy, are usually called chronometers. A common portable timekeeping instrument for personal used is the pocket or wrist watch.

True clocks also have an announcing or striking mechanism that sounds after each set interval of time, usually by ringing a bell (which, as previously indicated, originally gave the clock its name), chimes, or gong. A silent clock without a striking mechanism is traditionally known as a timepiece, a term sometimes used by horologists and other specialists to describe ordinary wrist watches and other timekeeping devices lacking a striking mechanism (see Baillie et al., p. 307; Palmer, p. 19; Zea & Cheney, p. 172).


A replica of an ancient Chinese incense clock
A replica of an ancient Chinese incense clock

The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes. The sundial, which measures the time of day by the direction of shadows cast by the sun, was widely known in ancient times.

Candles and sticks of incense that burn down at, approximately, predictable speeds were also used as to estimate the passage of time. In an hourglass fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary time.

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) long.
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) long.

Water clocks

The historian Vitruvius reported that the ancient Egyptians used a clepsydra, a time mechanism using flowing water. Herodotus had mentioned an ancient Egyptian time-keeping device that was based on mercury. By the 9th century AD a mechanical timekeeper had been developed that lacked only an escapement mechanism. Later years saw the rise of automated water clocks in Arabia, China, and Korea.

Early mechanical clocks

No clocks survive from medieval Europe but various mentions in church records reveal some of the early history of the clock.

Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be strictly regulated. This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in combination. Important times and durations were broadcast by bells, rung either by hand or by some mechanical device such as a falling weight or rotating beater.

The word 'horologia' (from the Greek hora, hour, and legein, to tell) was used to describe all these devices but the use of this word (still used in several romance languages) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a ‘horologe’ but the mechanism used is unknown. In 1198, during a fire at the abbey of St Edmundsbury (now Bury St Edmunds) the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.

These early clocks may not have used hands or dials but “told” the time with audible signals.

A new mechanism

The word 'clock' (from the Latin word for "bell") which gradually supersedes 'horologe' suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century.

Between 1280 and 1320 there is an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take the driving power from falling weights. This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power - the escapement - marks the beginning of the true mechanical clock.

These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modelling the solar system. The former purpose is administrative, the latter arises naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with the religious philosopy of the time. The astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system.

Simple clocks intended mainly for notification were installed in towers, and did not always require dials or hands. They would have announced the canonical hours or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including Italian hours, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata.

In 1283 a large clock was installed at Dunstable Priory; its location above the rood screen suggests that it was not a water clock. In 1292, Canterbury Cathedral installed a 'great horloge'. Over the next 30 years there are brief mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322 a new clock was installed in Norwich, an expensive replacement for an earlier clock installed in 1273. This had a large (2 metre) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two technicians for two years.

Early astronomical clocks

The clocks constructed by Richard of Wallingford in St Albans by 1336, and by Giovanni de'Dondi in Padua from 1348 to 1364, no longer exist but detailed descriptions of their design and construction survive, and modern reproductions have been made. They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena.

Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition it had a wheel of fortune and an indicator of the state of the tide at London Bridge. Bells rang every hour, the number of strokes indicating the time.

Dondi's clock was a seven-sided construction, 1 metre high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and movable feasts, and an eclipse prediction hand rotating once every 18 years.

It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture.

Elements of the mechanical clock

These 14th century clocks show the four key elements common to all clocks in subsequent centuries, at least up to the digital age:

  • the power, supplied by a falling weight, later by a coiled spring
  • the escapement, a periodic repetitive action that allows the power to escape in small bursts rather than drain away all at once
  • the going train, a set of interlocking gear wheels that controls the speed of rotation of the wheels connected between the power supply and the indicators
  • indicators, such as dials, hands, and bells

Later developments

Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried.

Spring-driven clocks were developed during the 15th century, and this gave the clockmakers many new problems to solve, such as how to compensate for the changing power supplied as the spring unwound.

The first record of a minute hand on a clock is 1475, in the Almanus Manuscript of Brother Paul.

During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg and Augsburg, and, in France, Blois. Some of the more basic table clocks have only one time-keeping hand, with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The cross-beat escapement was developed in 1585 by Jobst Burgi, who also developed the remontoire. Burgi's accurate clocks helped Tycho Brahe and Johannes Kepler to observe astronomical events with much greater precision than before.

The first record of a second hand on a clock is about 1560, on a clock now in the Fremersdorf collection. However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working.

The next development in accuracy occurred after 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.

A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, which would be virtually useless on a rocking ship. Many European governments offered a large prize for anyone that could determine longitude accurately; for example, Great Britain offered 20,000 pounds, equivalent to millions of dollars today. The reward was eventually claimed in 1761 by John Harrison, who dedicated his life to improving the accuracy of his clocks. His H5 clock is reported to have lost less than 5 seconds over 10 days.

The excitement over the pendulum clock had attracted the attention of designers resulting in a proliferation of clock forms. Notably, the longcase clock (also known as the grandfather clock) was created to house the pendulum and works. The English clockmaker William Clement is also credited with developing this form in 1670 or 1671. It was also at this time that clock cases began to be made of wood and clock faces to utilize enamel as well as hand-painted ceramics.

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.

Alexander Bain, Scottish clockmaker, patented the electric clock in 1840. The electric clock's mainspring is wound either with an electric motor or with an electro-magnet and armature. In 1841, he first patented the electromagnetic pendulum.

The development of electronics in the twentieth century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the vibration of a tuning fork, the behaviour of quartz crystals, the decay of radioactive elements or resonance of polycarbonates. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.


Clocks can be classified by the type of time display, as well as by the method of timekeeping.

Time display methods

Analog clocks

A linear clock at London's Piccadilly Circus tube  station. The 24 hour band moves across the static map, keeping pace with the apparent movement of the sun above ground, and a pointer fixed on London points to the current time.
A linear clock at London's Piccadilly Circus tube station. The 24 hour band moves across the static map, keeping pace with the apparent movement of the sun above ground, and a pointer fixed on London points to the current time.

Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and often also as a scale of 60 seconds – though many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. Of these alternative versions, the 24 hour analog dial is the main type in use today. The 10 hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch chiming 24 times uses more power).

Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon. Sundials use some or part of the 24 hour analog dial.

There also exist clocks which use a digital display despite having an analog mechanism - these are commonly referred to as flip clocks.

Digital clocks

A digital clock outside Kanazawa Station displays the time by controlling valves on a fountain.
A digital clock outside Kanazawa Station displays the time by controlling valves on a fountain.

Main Article: Digital clock

Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:

  • the 24-hour notation with hours ranging 00–23;
  • the 12-hour notation with AM/PM indicator, with hours indicated as 12AM, followed by 1AM–11AM, followed by 12PM, followed by 1PM–11PM (a notation mostly used in the United States).

Most digital clocks use an LCD or LED display; many other display technologies are used as well ( cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, digital clocks without a backup battery or capacitor either start counting from 00:00, or stay at 00:00, often with blinking digits indicating that time needs to be set. Some newer clocks will actually reset themselves based on radio or internet time servers which in turn are tuned to national atomic clocks.

Basic digital clock radio.
Basic digital clock radio.

Auditory clocks

For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. The sound is either spoken natural language, (e.g. "The time is twelve thirty-five"), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour like the clock Big Ben).

Timekeeping methods

Most types of clocks are built around some form of oscillator, an arrangement that goes through an endless sequence of periodic state changes, designed to provide a continuous and stable reference frequency. The periods of this oscillator are then counted and converted into the desired clock display.

  • Mechanical clocks use a pendulum as their oscillator, which controls the rotation of a system of gears that drive the clock display.
  • Electrical clocks use electrical current to run, rather than requiring manual winding and weights.
  • Crystal clocks use an electronic quartz crystal oscillator and a frequency divider or counter. Most battery-powered crystal clocks use a 215 Hz = 32.768 kHz oscillator.
  • Atomic clocks use a microwave oscillator ( maser) tuned by the energy transitions of elements such as caesium, rubidium or hydrogen. These are the most precise clocks available. Atomic clocks based on caesium are used as the official definition of time today.
  • Mains power clocks count the 50 or 60 hertz periods of their AC power.
  • Radio clocks receive time signal broadcasts from a radio transmitter (which may be hundreds of kilometres away). The clock can decode the transmission and adjust its hands or display for perfect accuracy. The broadcast radio signals received are generated by an atomic clock. These clocks are used extensively by mariners, especially short-wave radio clocks which use simultaneous bursts of time-signals, often encoded or encrypted – not to be confused with number stations.
  • Sundials observe the apparent rotation of the Sun around the Earth as their reference oscillation. They are observed with a solar tempometer.


Clocks are in homes and offices; smaller ones (watches) are carried; larger ones are in public places, e.g. a train station or church. A small clock is often shown in a corner of computer displays or mobile phones.

The purpose of a clock is not always to display the time. It may also be used to control a device according to time, e.g. an alarm clock, a VCR, or a time bomb (see: counter). However, in this context, it is more appropriate to refer to it as a timer or trigger mechanism rather than strictly as a clock.

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. The internal computer clock is generally kept running by a small battery. Memory of this kind is often referred to as "non-volatile". Many computers will still function even if the internal clock battery is dead, but the computer clock will need to be reset each time the computer is restarted, since once power is lost, time is also lost.

Ideal clocks

An ideal clock is a scientific principle that 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 desk clock
A desk clock

This leads to the following definitions:

  • 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 by ships 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 chronometer in the mid-18th century. The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers.

Specific types of clocks

A windup, mechanical, spring-driven alarm clock.
A windup, mechanical, spring-driven alarm clock.
  • Alarm clock
  • Analog clock with digital display
  • Astronomical clock
  • Atomic clock
  • Balloon clock
  • Binary clock
  • Bracket clock
  • Carriage clock
  • Cartel clock
  • Chiming clock
  • Clock network
  • Clock of the Long Now
  • Countdown clock
  • Cuckoo clock
  • Data clock for timescapes created with time-technology
  • Digital clock
  • Doll's head clock
  • Electric clock
  • Flip clock
  • Floral clock
  • Game clock
  • Hourglass
  • Japanese clock
  • Lantern clock
  • Lighthouse Clock
  • Longcase (or "grandfather") clock
  • Mantel clock
  • Master clock
  • Paper clock
  • Pedestal clock
  • Pendulum clock
  • Projection clock
Digital clock display in an oven.
Digital clock display in an oven.
  • Quartz clock
  • Railroad chronometers
  • Reference clock
  • Rolling ball clock
  • Shelf clock
  • Sidereal clock
  • Skeleton clock
  • Slave clock
  • Stopwatch
  • Striking clock
  • Sundial
  • Tall-case clock
  • Tide clock
  • Time ball
  • Time clock
  • Tower clock
  • Torsion pendulum clock
  • Watch
  • Water clock
  • Wall clock
  • World clock

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