Clock

Clock

Overview


A clock is an instrument used to indicate, keep, and co-ordinate time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The word clock is derived ultimately (via Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

, Northern French, and Medieval Latin
Medieval Latin
Medieval Latin was the form of Latin used in the Middle Ages, primarily as a medium of scholarly exchange and as the liturgical language of the medieval Roman Catholic Church, but also as a language of science, literature, law, and administration. Despite the clerical origin of many of its authors,...

) from the Celtic
Celtic languages
The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 words clagan and clocca meaning "bell
Bell (instrument)
A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

". A silent instrument missing such a mechanism has traditionally been known as a timepiece. In general usage today a "clock" refers to any device for measuring and displaying the time.
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Encyclopedia


A clock is an instrument used to indicate, keep, and co-ordinate time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The word clock is derived ultimately (via Dutch
Dutch language
Dutch is a West Germanic language and the native language of the majority of the population of the Netherlands, Belgium, and Suriname, the three member states of the Dutch Language Union. Most speakers live in the European Union, where it is a first language for about 23 million and a second...

, Northern French, and Medieval Latin
Medieval Latin
Medieval Latin was the form of Latin used in the Middle Ages, primarily as a medium of scholarly exchange and as the liturgical language of the medieval Roman Catholic Church, but also as a language of science, literature, law, and administration. Despite the clerical origin of many of its authors,...

) from the Celtic
Celtic languages
The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 words clagan and clocca meaning "bell
Bell (instrument)
A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

". A silent instrument missing such a mechanism has traditionally been known as a timepiece. In general usage today a "clock" refers to any device for measuring and displaying the time. Watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...

es and other timepieces that can be carried on one's person are often distinguished from clocks.

The clock is one of the oldest human invention
Invention
An invention is a novel composition, device, or process. An invention may be derived from a pre-existing model or idea, or it could be independently conceived, in which case it may be a radical breakthrough. In addition, there is cultural invention, which is an innovative set of useful social...

s, meeting the need to consistently measure intervals of time shorter than the natural units: the day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

; the lunar month
Lunar month
In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

; and the year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

. Devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today.

Sundials and other devices


The sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

, which measures the time of day by using the sun casting a shadow onto a cylindrical stone, was widely used in ancient times
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

. A well-constructed sundial can measure local solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

 with reasonable accuracy, and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations—it requires the sun to shine and does not work at all during the night—encouraged the use of other techniques for measuring time.

Candle clocks and sticks of incense that burn down at approximately predictable speeds have also been used to estimate the passing of time. In an hourglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...

, fine sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

 pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time.

Water clocks




Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

 and the day-counting tally stick
Tally stick
A tally was an ancient memory aid device to record and document numbers, quantities, or even messages. Tally sticks first appear as notches carved on animal bones, in the Upper Paleolithic. A notable example is the Ishango Bone...

. Given their great antiquity, where and when they first existed is not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon
Babylon
Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

 and in Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

 around the 16th century BC. Other regions of the world, including India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.

Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

 astronomer, Andronicus of Cyrrhus
Andronicus of Cyrrhus
Andronicus of Cyrrhus or Andronicus Cyrrhestes,son of Hermias, was a Greek astronomer who flourished about 100 BC....

, supervised the construction of the Tower of the Winds
Tower of the Winds
The Tower of the Winds, also called horologion , is an octagonal Pentelic marble clocktower on the Roman agora in Athens. The structure features a combination of sundials, a water clock and a wind vane...

 in Athens in the 1st century B.C.

The Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

 and Roman
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 civilizations are credited for initially advancing water clock design to include complex gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

ing, which was connected to fanciful automata
Automaton
An automaton is a self-operating machine. The word is sometimes used to describe a robot, more specifically an autonomous robot. An alternative spelling, now obsolete, is automation.-Etymology:...

 and also resulted in improved accuracy. These advances were passed on through Byzantium
Byzantium
Byzantium was an ancient Greek city, founded by Greek colonists from Megara in 667 BC and named after their king Byzas . The name Byzantium is a Latinization of the original name Byzantion...

 and Islamic times, eventually making their way back to Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

. Independently, the Chinese developed their own advanced water clocks(水鐘)in 725 A.D., passing their ideas on to Korea
Korea
Korea ) is an East Asian geographic region that is currently divided into two separate sovereign states — North Korea and South Korea. Located on the Korean Peninsula, Korea is bordered by the People's Republic of China to the northwest, Russia to the northeast, and is separated from Japan to the...

 and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, where every hour of work or rest is monitored, and work may start or finish at any time regardless of external conditions. Instead, water clocks in ancient societies were used mainly for astrological
Astrology
Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

 reasons. These early water clocks were calibrated with a sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

. While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

 in 17th century Europe.

Islamic civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the Abbasid
Abbasid
The Abbasid Caliphate or, more simply, the Abbasids , was the third of the Islamic caliphates. It was ruled by the Abbasid dynasty of caliphs, who built their capital in Baghdad after overthrowing the Umayyad caliphate from all but the al-Andalus region....

 caliph
Caliph
The Caliph is the head of state in a Caliphate, and the title for the ruler of the Islamic Ummah, an Islamic community ruled by the Shari'ah. It is a transcribed version of the Arabic word   which means "successor" or "representative"...

 of Baghdad
Baghdad
Baghdad is the capital of Iraq, as well as the coterminous Baghdad Governorate. The population of Baghdad in 2011 is approximately 7,216,040...

, Harun al-Rashid
Harun al-Rashid
Hārūn al-Rashīd was the fifth Arab Abbasid Caliph in Iraq. He was born in Rey, Iran, close to modern Tehran. His birth date remains a point of discussion, though, as various sources give the dates from 763 to 766)....

, presented Charlemagne
Charlemagne
Charlemagne was King of the Franks from 768 and Emperor of the Romans from 800 to his death in 814. He expanded the Frankish kingdom into an empire that incorporated much of Western and Central Europe. During his reign, he conquered Italy and was crowned by Pope Leo III on 25 December 800...

 with an Asian Elephant
Asian Elephant
The Asian or Asiatic elephant is the only living species of the genus Elephas and distributed in Southeast Asia from India in the west to Borneo in the east. Three subspecies are recognized — Elephas maximus maximus from Sri Lanka, the Indian elephant or E. m. indicus from mainland Asia, and E. m....

 named Abul-Abbas
Abul-Abbas
Abul-Abbas, also Abul Abaz or Abulabaz, was an Asian elephant given to Emperor Charlemagne by the caliph of Baghdad, Harun al-Rashid, in 797. The elephant's name and events from his life in the Carolingian Empire are recorded in the annales regni francorum , and Einhard's vita Karoli Magni also...

 together with a "particularly elaborate example" of a water clock.

In the 13th century, Al-Jazari
Al-Jazari
Abū al-'Iz Ibn Ismā'īl ibn al-Razāz al-Jazarī was a Muslim polymath: a scholar, inventor, mechanical engineer, craftsman, artist, mathematician and astronomer from Al-Jazira, Mesopotamia, who lived during the Islamic Golden Age...

, an engineer who worked for Artuqid king of Diyar-Bakr, Nasir al-Din
Nasir al-Din
Nasir al-Din, Nasir ad-Din or Nasiruddin may refer to:*Nasrudin or Nasreddin, legendary wise man in Islamic tradition*Abu Yusuf Bin Saamaan, also known as Nasiruddin , Sufi Saint*Nasir ad-Din Mahmud I of Great Seljuq Nasir al-Din, Nasir ad-Din or Nasiruddin may refer to:*Nasrudin or Nasreddin,...

, made numerous clocks of all shapes and sizes. The book described 50 mechanical devices in 6 categories, including water clocks. The most reputed clocks included the Elephant
Elephant clock
The elephant clock was a medieval islamic invention by al-Jazari , consisting of a weight powered water clock in the form of an elephant. The various elements of the clock are in the housing on top of the elephant...

, Scribe and Castle clock
Castle clock
A castle clock is a clock whose face is mounted outside a castle or perhaps a church tower. They may be elaborate or simple.Al-Jazari constructed an elaborate clock and described it in his Book of Knowledge of Ingenious Mechanical Devices in 1206. It was about 3,3 metres high , and had multiple...

s, all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.

Early mechanical clocks


None of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock.

The word horologia (from the Greek ὡρα, hour, and λέγειν, to tell) was used to describe all these devices, but the use of this word (still used in several Romance languages
Romance languages
The Romance languages are a branch of the Indo-European language family, more precisely of the Italic languages subfamily, comprising all the languages that descend from Vulgar Latin, the language of ancient Rome...

) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral
Cathédrale Saint-Étienne de Sens
Sens Cathedral is a Catholic cathedral in Sens, Bourgogne, eastern France. It was one of the earliest Gothic buildings in the country, and the largest of the early Gothic churches. The choir was begun in 1140. As was typical in cathedral construction work progressed westwards, building the nave,...

 installed a ‘horologe’ but the mechanism used is unknown. According to Jocelin of Brakelond, 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.

A new mechanism


The word clock (from the Latin word clocca, "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 in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

.

Outside of Europe, the escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 mechanism had been known and used in medieval China, as the Song Dynasty
Song Dynasty
The Song Dynasty was a ruling dynasty in China between 960 and 1279; it succeeded the Five Dynasties and Ten Kingdoms Period, and was followed by the Yuan Dynasty. It was the first government in world history to issue banknotes or paper money, and the first Chinese government to establish a...

 horologist and engineer Su Song
Su Song
Su Song was a renowned Chinese polymath who specialized himself as a statesman, astronomer, cartographer, horologist, pharmacologist, mineralogist, zoologist, botanist, mechanical and architectural engineer, poet, antiquarian, and ambassador of the Song Dynasty .Su Song was the engineer of a...

 (1020–1101) incorporated it into his astronomical clock-tower of Kaifeng
Kaifeng
Kaifeng , known previously by several names , is a prefecture-level city in east-central Henan province, Central China. Nearly 5 million people live in the metropolitan area...

 in 1088. However, his astronomical clock and rotating armillary sphere
Armillary sphere
An armillary sphere is a model of objects in the sky , consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic...

 still relied on the use of flowing water (i.e. hydraulics
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...

), while European clockworks of the following centuries shed this old method for a more efficient driving power of weights, in addition to the escapement mechanism.

A mercury clock, described in the Libros del saber, a Spanish work from AD 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. The first mercury powered automata clock was invented by Ibn Khalaf al-Muradi
Al-Muradi
Alī Ibn Khalaf al-Murādī, Al-Andalus, was an engineer and author of the unique technological manuscript entitled Kitāb al-asrār fī natā'ij al-afkār . It was copied and used at the court of Alfonso VI of León and Castile in Christian Spain in the 11th 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
Hydropower
Hydropower, hydraulic power, hydrokinetic power or water power is power that is derived from the force or energy of falling water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as...

 were being adapted to take their 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
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 - 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 modeling the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. 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 philosophy of the time. The astrolabe
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

 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 faces or hands. They would have announced the canonical hours
Canonical hours
Canonical hours are divisions of time which serve as increments between the prescribed prayers of the daily round. A Book of Hours contains such a set of prayers....

 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
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata
Automata
Automata is the plural form of automaton, a self-operating machine. It may also refer to:* "Automata", a short story by E. T. A. Hoffmann* "Automata", a hardboiled science fiction crime series by Penny Arcade...

.

In 1283, a large clock was installed at Dunstable Priory
Dunstable Priory
The Priory Church of St Peter with its monastery was founded in 1132 by Henry I for Augustinian Canons in Dunstable, Bedfordshire, England. St Peter’s today is a large and impressive building, but this is only the nave of what remains of an originally much larger Augustinian priory church...

; its location above the rood screen
Rood screen
The rood screen is a common feature in late medieval church architecture. It is typically an ornate partition between the chancel and nave, of more or less open tracery constructed of wood, stone, or wrought iron...

 suggests that it was not a water clock . In 1292, Canterbury Cathedral
Canterbury Cathedral
Canterbury Cathedral in Canterbury, Kent, is one of the oldest and most famous Christian structures in England and forms part of a World Heritage Site....

 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
Norwich
Norwich is a city in England. It is the regional administrative centre and county town of Norfolk. During the 11th century, Norwich was the largest city in England after London, and one of the most important places in the kingdom...

, 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 clockkeeper
Clockkeeper
A clockkeeper, sometimes seen as clock keeper, refers to a form of employment seen prevalently during Middle Age Europe involving the tracking of time and the maintaining of clocks and other timekeeping devices...

s for two years .

Early astronomical clocks



Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed by Richard of Wallingford
Richard of Wallingford
Richard of Wallingford was an English mathematician who made major contributions to astronomy/astrology and horology while serving as abbot of St Albans Abbey in Hertfordshire.-Biography:...

 in St Albans
St Albans
St Albans is a city in southern Hertfordshire, England, around north of central London, which forms the main urban area of the City and District of St Albans. It is a historic market town, and is now a sought-after dormitory town within the London commuter belt...

 by 1336, and by Giovanni de Dondi in Padua from 1348 to 1364. They 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
London Bridge
London Bridge is a bridge over the River Thames, connecting the City of London and Southwark, in central London. Situated between Cannon Street Railway Bridge and Tower Bridge, it forms the western end of the Pool of London...

. 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
Moveable feast
In Christianity, a moveable feast or movable feast is a holy day – a feast day or a fast day – whose date is not fixed to a particular day of the calendar year but moves in response to the date of Easter, the date of which varies according to a complex formula...

, 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.

Water clocks are sometimes still used today, and can be examined in places such as ancient castles and museums.

The Salisbury Cathedral clock
Salisbury cathedral clock
The Salisbury Cathedral clock, a large iron-framed clock without a dial located in the aisle of Salisbury Cathedral. Supposedly dating from about 1386, it is claimed to be the oldest working clock in the world....

, built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours.

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 appeared during the 15th century, although they are often erroneously credited to Nuremberg
Nuremberg
Nuremberg[p] is a city in the German state of Bavaria, in the administrative region of Middle Franconia. Situated on the Pegnitz river and the Rhine–Main–Danube Canal, it is located about north of Munich and is Franconia's largest city. The population is 505,664...

 watchmaker Peter Henlein
Peter Henlein
Peter Henlein , a locksmith and watchmaker from Nuremberg, is often considered the inventor of the portable timekeeper, making him the inventor of the watch, but this claim is disputed...

 (or Henle, or Hele) around 1511. The earliest existing spring driven clock is the chamber clock given to Peter the Good, Duke of Burgundy, around 1430, now in the Germanisches Nationalmuseum
Germanisches Nationalmuseum
The Germanisches Nationalmuseum is a museum in Nuremberg, Germany. Founded in 1852, houses a large collection of items relating to German culture and art extending from prehistoric times through to the present day...

. Spring power presented clockmakers with a new problem: how to keep the clock movement
Movement (clockwork)
In horology, a movement is the internal mechanism of a clock or watch, as opposed to the case, which encloses and protects the movement, and the face which displays the time. The term originated with mechanical timepieces, whose movements are made of many moving parts...

 running at a constant rate as the spring ran down. This resulted in the invention of the stackfreed and the fusee
Fusee
Used in antique spring-powered mechanical watches and clocks, a fusee is a cone-shaped pulley with a spiral groove around it, wound with a cord or chain which is attached to the mainspring barrel...

 in the 15th century, and many other innovations, down to the invention of the modern going barrel
Barrel (horology)
Used in mechanical watches and clocks, a barrel is a cylindrical metal box closed by a cover, with a ring of gear teeth around it, containing a spiral spring called the mainspring, which provides power to run the timepiece. The barrel turns on an arbor . The spring is hooked to the barrel at its...

in 1760.

Early clock dials did not use minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus, and some 15th-century clocks in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 indicated minutes and seconds.
An early record of a second hand on a clock dates back to 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.

During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg
Nuremberg
Nuremberg[p] is a city in the German state of Bavaria, in the administrative region of Middle Franconia. Situated on the Pegnitz river and the Rhine–Main–Danube Canal, it is located about north of Munich and is Franconia's largest city. The population is 505,664...

 and Augsburg
Augsburg
Augsburg is a city in the south-west of Bavaria, Germany. It is a university town and home of the Regierungsbezirk Schwaben and the Bezirk Schwaben. Augsburg is an urban district and home to the institutions of the Landkreis Augsburg. It is, as of 2008, the third-largest city in Bavaria with a...

, and in Blois
Blois
Blois is the capital of Loir-et-Cher department in central France, situated on the banks of the lower river Loire between Orléans and Tours.-History:...

, France. 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 nearer 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The cross-beat escapement was invented in 1584 by Jost Bürgi, who also developed the remontoire
Remontoire
In mechanical horology, a remontoire, is a small secondary source of power, a weight or spring, which runs the timekeeping mechanism and is itself periodically rewound by the timepiece's main power source, such as a mainspring...

. Bürgi's clocks were a great improvement in accuracy as they were correct to within a minute a day.
These clocks helped the 16th-century astronomer Tycho Brahe
Tycho Brahe
Tycho Brahe , born Tyge Ottesen Brahe, was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...

 to observe astronomical events with much greater precision than before.

A mechanical weight-driven astronomical clock
Astronomical clock
An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.-Definition:...

 with a verge-and-foliot escapement, a striking train of gears, an alarm, and a representation of the moon's phases was described by the Ottoman
Ottoman Empire
The Ottoman EmpireIt was usually referred to as the "Ottoman Empire", the "Turkish Empire", the "Ottoman Caliphate" or more commonly "Turkey" by its contemporaries...

 engineer Taqi al-Din in his book, The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), published in 1556-1559. Similarly to earlier 15th-century European alarm clocks, it was capable of sounding at a specified time, achieved by placing a peg on the dial wheel. At the requested time, the peg activated a ringing device. The clock had three dials
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

 which indicated hours, degrees and minutes. He later made an observational clock for the Istanbul observatory of Taqi al-Din (1577–1580), describing it as "a mechanical clock with three dials which show the hours, the minutes, and the seconds." This was an important innovation in 16th-century practical astronomy, as at the start of the century clocks were not accurate enough to be used for astronomical purposes.

The next development in accuracy occurred after 1656 with the invention of the pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

. Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 had the idea to use a swinging bob to regulate 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
Anchor escapement
In horology, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward...

, an improvement over Huygens' crown escapement . Within just one generation, minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

 hands and then second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

 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
Longitude prize
The Longitude Prize was a reward offered by the British government for a simple and practical method for the precise determination of a ship's longitude...

 for anyone who 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
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

, who dedicated his life to improving the accuracy of his clocks. His H5 clock was in error by less than 5 seconds over 10 weeks.

The excitement over the pendulum clock had attracted the attention of designers, resulting in a proliferation of clock forms. Notably, the longcase clock
Longcase clock
A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

 (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 face
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

s to utilize enamel
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

 as well as hand-painted ceramics.
On November 17, 1797, Eli Terry
Eli Terry
Eli Terry Sr. was an inventor and clockmaker in Connecticut. He received a United States patent for a shelf clock mechanism. He introduced mass production to the art of clockmaking, which made clocks affordable for the average American citizen...

 received his first patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

 for a clock. Terry is known as the founder of the American clock-making industry.

Starting in the U.S. in early decades of the 19th century, clocks were one of the first items to be mass produced and also to use interchangeable parts
Interchangeable parts
Interchangeable parts are parts that are, for practical purposes, identical. They are made to specifications that ensure that they are so nearly identical that they will fit into any device of the same type. One such part can freely replace another, without any custom fitting...

.

Alexander Bain
Alexander Bain (inventor)
Alexander Bain was a Scottish inventor and engineer who was first to invent and patent the electric clock. Bain installed the railway telegraph lines between Edinburgh and Glasgow.-Early life:...

, Scottish clockmaker, patented the electric clock
Electric clock
An electric clock is a clock that is powered by electricity instead of powered manually or by other sources of energy, specifically in order to wind the mainspring or to drive the pendulum or oscillator.-Classification:...

 in 1840. The electric clock's mainspring is wound either with an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

 or with an electro-magnet and armature. In 1841, he first patented the electromagnetic
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 pendulum.

The development of electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 in the 20th 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
Tuning fork
A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal . It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a...

, the behaviour of quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 crystals, or the quantum vibrations of atoms. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.

How clocks work


The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from continuous
Continuous function
In mathematics, a continuous function is a function for which, intuitively, "small" changes in the input result in "small" changes in the output. Otherwise, a function is said to be "discontinuous". A continuous function with a continuous inverse function is called "bicontinuous".Continuity of...

 processes, such as the motion of the gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

's shadow on a sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

 or the flow of liquid in a water clock
Water clock
A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...

, to repetitive oscillatory processes, like the swing of a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 or the vibration of a quartz crystal, which were more accurate. All modern clocks use oscillation.

Although the methods they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be divided into analogous parts. They consist of an object that repeats the same motion over and over again, an oscillator, with a precisely constant time interval between each repetition, or 'beat'. Attached to the oscillator is a controller device, which sustains the oscillator's motion by replacing the energy it loses to friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

, and converts its oscillations into a series of pulses. The pulses are then added up in a chain of some type of counters to express the time in convenient units, usually seconds, minutes, hours, etc. Then finally some kind of indicator displays the result in a human-readable form.

Power source


This provides power to keep the clock going.
  • In mechanical clocks, this is either a weight suspended from a cord wrapped around a pulley
    Pulley
    A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

    , or a spiral spring
    Spring (device)
    A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

     called a mainspring
    Mainspring
    A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

    .
  • In electric clock
    Electric clock
    An electric clock is a clock that is powered by electricity instead of powered manually or by other sources of energy, specifically in order to wind the mainspring or to drive the pendulum or oscillator.-Classification:...

    s, it is either a battery
    Battery (electricity)
    An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

     or the AC power line.

Since clocks must run continuously, there is often a small secondary power source to keep the clock going temporarily during interruptions in the main power. In old mechanical clocks, a maintaining power
Maintaining power
In horology, a maintaining power is a mechanism for keeping a clock or watch going while it is being wound.-Huygens:The weight drive used by Christiaan Huygens in his early clocks acts as a maintaining power...

 spring
kept the clock turning while the mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 was being wound. In quartz clock
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

s that use AC power, a small backup battery
Backup battery
A backup battery provides power to a system when the primary source of power is unavailable. Backup batteries range from small single cells to retain clock time and date in computers, up to large battery room facilities that power uninterruptible power supply systems for large data centers...

 is often included to keep the clock running if it is unplugged temporarily from the wall.

Oscillator


The timekeeping element in every modern clock is a harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....

, a physical object (resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...

) that vibrates or oscillates repetitively at a precisely constant frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

.
  • In mechanical clocks, this is either a pendulum
    Pendulum
    A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

     or a balance wheel
    Balance wheel
    The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...

    .
  • In some early electronic clocks and watches such as the Accutron, it is a tuning fork
    Tuning fork
    A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal . It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a...

    .
  • In quartz clock
    Quartz clock
    A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

    s and watches, it is a quartz crystal.
  • In atomic clock
    Atomic clock
    An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

    s, it is the vibration of electron
    Electron
    The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

    s in atom
    Atom
    The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

    s as they emit microwave
    Microwave
    Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

    s.
  • In early mechanical clocks before 1657, it was a crude balance wheel or foliot
    Foliot
    A foliot may refer to:* Part of the verge escapement for early clocks* A creature of Italian folklore , as recorded by Gerolamo Cardano and Robert Burton...

     which was not a harmonic oscillator because it lacked a balance spring
    Balance spring
    A balance spring, or hairspring, is a part used in mechanical timepieces. The balance spring, attached to the balance wheel, controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands...

    . As a result they were very inaccurate, with errors of perhaps an hour a day.

The advantage of a harmonic oscillator over other forms of oscillator is that it employs resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 to vibrate at a precise natural resonant frequency or 'beat' dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

, or quality factor, which increases (other things being equal) with its resonant frequency. This is why there has been a long term trend toward higher frequency oscillators in clocks. Balance wheels and pendulums always include a means of adjusting the rate of the timepiece. Quartz timepieces sometimes include a rate screw that adjusts a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 for that purpose. Atomic clocks are primary standards, and their rate cannot be adjusted.

Synchronized or slave clocks


Some clocks rely for their accuracy on an external oscillator; that is, they are automatically synchronized
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

 to a more accurate clock:
  • Slave clock
    Slave clock
    In telecommunication, a slave clock is a clock that is coordinated with a master clock. Slave clock coordination is usually achieved by phase-locking the slave clock signal to a signal received from the master clock...

    s, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a master clock
    Master clock
    A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...

     in the building, and periodically received a signal to synchronize them with the master, often on the hour. Later versions without pendulums were triggered by a pulse from the master clock and certain sequences used to force rapid synchronization following a power failure.
  • Synchronous electric clocks
    Electric clock
    An electric clock is a clock that is powered by electricity instead of powered manually or by other sources of energy, specifically in order to wind the mainspring or to drive the pendulum or oscillator.-Classification:...

     don't have an internal oscillator, but rely on the 50 or 60 Hz
    HZ
    Hz is the International Standard symbol for Hertz, the unit of frequencyHZ may also stand for:* Habitable zone, the distance from a star where a planet can maintain Earth-like life* Hamilton Zoo, in New Zealand...

     oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. This drives a synchronous motor
    Synchronous motor
    A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....

     in the clock which rotates once for every cycle of the line voltage, and drives the gear train.
  • Computer real time clocks keep time with a quartz crystal, but are periodically (usually weekly) synchronized over the internet to atomic clocks (UTC), using a system called Network Time Protocol
    Network Time Protocol
    The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...

    .
  • Radio clock
    Radio clock
    A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

    s keep time with a quartz crystal, but are periodically (often daily) synchronized to atomic clocks (UTC) with time signals from government radio stations like WWV
    WWV
    WWV is the call sign of the United States National Institute of Standards and Technology's HF radio station in Fort Collins, Colorado. WWV continuously transmits official U.S. Government frequency and time signals on 2.5, 5, 10, 15 and 20 MHz...

    , WWVB
    WWVB
    WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic...

    , CHU
    Chu
    Chu or CHU may refer to:Surname:* Chu , a common Chinese surname for 朱 , but it can also refer to any Chinese surname whose pinyin is "chu", such as 楚, 储, 褚, 初, 除 and other possible surnames....

    , DCF77
    DCF77
    DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH , on behalf of the Physikalisch-Technische Bundesanstalt, Germany's...

     and the GPS system.

Controller


This has the dual function of keeping the oscillator running by giving it 'pushes' to replace the energy lost to friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

, and converting its vibrations into a series of pulses that serve to measure the time.
  • In mechanical clocks, this is the escapement
    Escapement
    In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

    , which gives precise pushes to the swinging pendulum or balance wheel, and releases one gear tooth of the escape wheel at each swing, allowing all the clock's wheels to move forward a fixed amount with each swing.
  • In electronic clocks this is an electronic oscillator circuit
    Electronic oscillator
    An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

     that gives the vibrating quartz crystal or tuning fork tiny 'pushes', and generates a series of electrical pulses, one for each vibration of the crystal, which is called the clock signal
    Clock signal
    In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

    .
  • In atomic clock
    Atomic clock
    An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

    s the controller is an evacuated microwave
    Microwave
    Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

     cavity attached to a microwave oscillator
    Electronic oscillator
    An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

     controlled by a microprocessor
    Microprocessor
    A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

    . A thin gas of cesium atoms is released into the cavity where they are exposed to microwaves. A laser measures how many atoms have absorbed the microwaves, and an electronic feedback
    Feedback
    Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

     control system called a phase locked loop tunes the microwave oscillator until it is at the exact frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by digital counters to become the clock signal
    Clock signal
    In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

    .

In mechanical clocks, the low Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

 of the balance wheel or pendulum oscillator made them very sensitive to the disturbing effect of the impulses of the escapement, so the escapement had a great effect on the accuracy of the clock, and many escapement designs were tried. The higher Q of resonators in electronic clocks makes them relatively insensitive to the disturbing effects of the drive power, so the driving oscillator circuit is a much less critical component.

Counter chain


This counts the pulses and adds them up to get traditional time units of second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

s, minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

s, hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

s, etc. It usually has a provision for setting the clock by manually entering the correct time into the counter.
  • In mechanical clocks this is done mechanically by a gear train
    Gear train
    A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next.The transmission of...

    , known as the wheel train
    Wheel train (horology)
    In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.Watch...

    . The gear train also has a second function; to transmit mechanical power from the power source to run the oscillator. There is a friction coupling called the 'cannon pinion' between the gears driving the hands and the rest of the clock, allowing the hands to be turned by a knob on the back to set the time.
  • In digital clocks a series of integrated circuit
    Integrated circuit
    An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

     counter
    Counter
    In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...

    s or dividers add the pulses up digital
    Digital
    A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

    ly, using binary
    Binary numeral system
    The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...

     logic. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time.

Indicator


This displays the count of seconds, minutes, hours, etc. in a human readable form.
  • The earliest mechanical clocks in the 13th century didn't have a visual indicator and signalled the time audibly by striking bell
    Bell (instrument)
    A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

    s. Many clocks to this day are striking clock
    Striking clock
    A striking clock is a clock that sounds the hours audibly on a bell or gong. In 12 hour striking, used most commonly in striking clocks today, the clock strikes once at 1 AM, twice at 2 AM, continuing in this way up to twelve times at 12 noon, then starts over, striking once at 1 PM, twice at 2...

    s which strike the hour.
  • Analog clocks, including almost all mechanical and some electronic clocks, have a traditional dial or clock face
    Clock face
    A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

    , that displays the time in analog
    Analog signal
    An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

     form with a moving hour and minute hand. In quartz clock
    Quartz clock
    A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

    s with analog faces, a 1 Hz
    Hertz
    The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

     signal from the counters actuates a stepper motor
    Stepper motor
    A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...

     which moves the second hand forward at each pulse, and the minute and hour hands are moved by gears from the shaft of the second hand.
  • Digital clock
    Digital clock
    A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Digital clocks are often associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism...

    s display the time in periodically changing digit
    Numerical digit
    A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...

    s on a digital display
    Display device
    A display device is an output device for presentation of information in visual or tactile form...

    .
  • Talking clock
    Talking clock
    A talking clock is a timekeeping device that presents the time as sounds. It may present the time solely as sounds, such as a phone-based time service or a clock for the hearing impaired, or may have a sound feature in addition to an analog or digital face.- History :Although they would not be...

    s and the speaking clock
    Speaking clock
    A speaking clock service is a recorded or simulated human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observatory on 14 February 1933.The format of the service is...

     services provided by telephone companies speak the time audibly, using either recorded or digitally synthesized voices.

Types


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

Analog clocks



Analog clocks usually indicate time using angles. The most common clock face
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

 uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

s, which can also serve as a scale of 60 minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

s, and 60 seconds if the clock has a second hand. Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. The only other widely used clock face today is the 24 hour analog dial
24 hour analog dial
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day...

, because of the use of 24 hour time in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 organizations and timetables. The 10-hour clock was briefly popular during the French Revolution
French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...

, when the metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

 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 striking 24 times uses more power).

Another type of analog clock is the sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

, which tracks the sun continuously, registering the time by the shadow position of its gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

. 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.

Alternative systems have been proposed. For example, the Twelv clock indicates the current hour using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a moon phase
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...

.

Digital clocks




Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

 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 and Canada).


Most digital clocks use an LCD, LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

, or VFD
Vacuum fluorescent display
A vacuum fluorescent display is a display device used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Invented in Japan in 1967, the displays became common on calculators and other consumer electronics devices...

 display; many other display technologies are used as well (cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

s, nixie tube
Nixie tube
A nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode and multiple cathodes. In most tubes, the cathodes are shaped like numerals. Applying power to one cathode surrounds it with an orange glow discharge...

s, etc.). After a reset, battery change or power failure, digital clocks without a backup battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

 or capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 either start counting from 12:00, or
stay at 12:00, often with blinking digits indicating that the time needs to be set. Some newer clocks will actually reset themselves based on radio or Internet time servers that are tuned to national atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s. Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly.

Auditory clocks



For convenience, distance, telephony or blindness
Blindness
Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...

, auditory clocks present the time as sounds. The sound is either spoken natural language
Natural language
In the philosophy of language, a natural language is any language which arises in an unpremeditated fashion as the result of the innate facility for language possessed by the human intellect. A natural language is typically used for communication, and may be spoken, signed, or written...

, (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 bell Big Ben
Clock Tower, Palace of Westminster
Big Ben is the nickname for the great bell of the clock at the north end of the Palace of Westminster in London, and is generally extended to refer to the clock or the clock tower as well. It is the largest four-faced chiming clock and the third-tallest free-standing clock tower in the world...

). Most telecommunication companies also provide a speaking clock
Speaking clock
A speaking clock service is a recorded or simulated human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observatory on 14 February 1933.The format of the service is...

 service as well.

Purposes


Clocks are in homes, offices and many other places; smaller ones (watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...

es) are carried on the wrist; larger ones are in public places, e.g. a train station
Train station
A train station, also called a railroad station or railway station and often shortened to just station,"Station" is commonly understood to mean "train station" unless otherwise qualified. This is evident from dictionary entries e.g...

 or church. A small clock is often shown in a corner of computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

s, mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s and many MP3 players.

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
Counter
In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...

). However, in this context, it is more appropriate to refer to it as a timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

 or trigger mechanism rather than strictly as a clock.

Computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s depend on an accurate internal clock signal
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

 to allow synchronized processing. (A few research projects are developing CPUs based on asynchronous circuit
Asynchronous circuit
An asynchronous circuit is a circuit in which the parts are largely autonomous. They are not governed by a clock circuit or global clock signal, but instead need only wait for the signals that indicate completion of instructions and operations. These signals are specified by simple data transfer...

s.) 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. 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.

This leads to the following definitions:
  • A clock is a recurrent process and a counter
    Counter
    In digital logic and computing, a counter is a device which stores the number of times a particular event or process has occurred, often in relationship to a clock signal.- Electronic counters :...

    .
  • 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 (e.g. a metronome
Metronome
A metronome is any device that produces regular, metrical ticks — settable in beats per minute. These ticks represent a fixed, regular aural pulse; some metronomes also include synchronized visual motion...

) 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 clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s 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


Navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

 by ships and planes depends on the ability to measure latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

. Latitude is fairly easy to determine through celestial navigation
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

, but the measurement of longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 requires accurate measurement of time. This need was a major motivation for the development of accurate mechanical clocks. John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

 created the first highly accurate marine chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

 in the mid-18th century. The Noon gun
Noon Gun
The Noon Gun has been an historic time signal in Cape Town, South Africa since 1806. The gun is situated on Signal Hill, close to the centre of the city.- History :...

 in Cape Town
Cape Town
Cape Town is the second-most populous city in South Africa, and the provincial capital and primate city of the Western Cape. As the seat of the National Parliament, it is also the legislative capital of the country. It forms part of the City of Cape Town metropolitan municipality...

 still fires an accurate signal to allow ships to check their chronometers
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

.

Use of an atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

 in radio signal producing satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s is fundamental to the operation of GPS
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (Global Positioning System) navigation devices.

Seismology


In determining the location of an earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

, the arrival time of several types of seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

 at a minimum of four dispersed observers is dependent upon each observer recording wave arrival times according to a common clock.

Specific types of clocks

By mechanism: By function: By style:
  • Astronomical clock
    Astronomical clock
    An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.-Definition:...

  • Atomic clock
    Atomic clock
    An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

  • Candle clock
    Candle clock
    A candle clock is a thin candle with consistently spaced markings , that when burned, indicate the passage of periods of time. While no longer used today, candle clocks provided an effective way to tell time indoors, at night, or on a cloudy day...

  • Congreve clock
    Congreve clock
    A Congreve clock is a type of clock that uses a ball rolling along a zig-zag track rather than a pendulum to regulate the time. It was invented by Sir William Congreve in 1808...

  • Digital clock
    Digital clock
    A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Digital clocks are often associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism...

  • Electric clock
    Electric clock
    An electric clock is a clock that is powered by electricity instead of powered manually or by other sources of energy, specifically in order to wind the mainspring or to drive the pendulum or oscillator.-Classification:...

  • Flip clock
    Analog clock with digital display
    An analog clock with digital display is a clock which displays information in a digital format on a split flap display, but actually keeps track of the time in an analog fashion.-Method of operation:...

  • Hourglass
    Hourglass
    An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...

  • Incense clock
    Incense clock
    The incense clock is a Chinese timekeeping device that appeared during the Song Dynasty and spread to neighboring countries such as Japan. The clocks' bodies are effectively specialized censers that hold incense sticks or powdered incense that have been manufactured and calibrated to a known rate...

  • Mechanical watch
    Mechanical watch
    A mechanical watch is a watch that uses a mechanical mechanism to measure the passage of time, as opposed to modern quartz watches which function electronically. It is driven by a spring which must be wound periodically...

  • Oil-lamp clock
    Oil-lamp clock
    Oil-lamp clocks are clocks consisting of a graduated glass reservoir to hold oil - usually whale oil, which burned cleanly and evenly - supplying the fuel for a built-in lamp. As the level in the reservoir dropped, it provided a rough measure of the passage of time....

  • Pendulum clock
    Pendulum clock
    A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

  • Pipe organ clock
    Pipe organ clock
    A pipe organ clock was a clock that chimed with a small organ pipe built into the unit. An example is a Markwick Markham made for the Turkish market, circa 1770....

  • Projection clock
    Projection clock
    A projection clock is an analog or digital clock equipped with a projector that creates an enlarged image of the clock face on any suitable projection screen, most often the ceiling...

  • Quantum clock
    Quantum Clock
    A quantum clock is a type of clock that confines aluminum and beryllium ions together in an electromagnetic trap and cools them by lasers to near absolute zero temperatures. Developed by National Institute of Standards and Technology physicists, the clock is 37 times more precise than the existing...

  • Quartz clock
    Quartz clock
    A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

  • Radio clock
    Radio clock
    A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

  • Rolling ball clock
    Rolling ball clock
    A rolling ball clock is a clock which displays time by means of balls and rails.-History:The rolling ball clock was invented by Harley Mayenschein in the 1970s. He patented the design and founded Idle Tyme Corporation in 1978, which manufactured these clocks from solid hardwoods. Later, a license...

  • Spring drive watch
    Spring Drive
    The Spring Drive is a novel watch movement that was developed by Seiko Epson through collaboration with Seiko Instruments and Seiko Holdings.It uses a mainspring, barrel, automatic winder and stem winding like in a mechanical watch to store the watch energy...

  • Steam clock
    Steam clock
    A steam clock is a clock which is fully or partially powered by a steam engine. Only a few functioning steam clocks exist, most designed and built by Canadian horologist Raymond Saunders for display in urban public spaces...

  • Sundial
    Sundial
    A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

  • Torsion pendulum clock
    Torsion pendulum clock
    A torsion pendulum clock, or torsion clock, is a mechanical clock which keeps time with a mechanism called a torsion pendulum. This is a weighted disk or wheel, often a decorative wheel with 3 or 4 chrome balls on ornate spokes, suspended by a thin wire or ribbon called a torsion spring...

  • Water clock
    Water clock
    A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...

  • 10-hour clock
  • Alarm clock
    Alarm clock
    An alarm clock is a clock that is designed to make a loud sound at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but...

  • Binary clock
    Binary clock
    A binary clock is usually a clock which displays traditional sexagesimal time in a binary format. Originally, it showed each decimal digit of sexagesimal time as a binary value, but presently binary clocks also exist which display hours, minutes, and seconds as binary numbers. Most binary clocks...

  • Chiming clock
  • Chronometer watch
    Chronometer watch
    A chronometer watch is a specific type of watch tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them....

  • Cuckoo clock
    Cuckoo clock
    A cuckoo clock is a clock, typically pendulum-regulated, that strikes the hours with a sound like a common cuckoo's call and typically has a mechanical cuckoo that emerges with each note...

  • Game clock
    Game clock
    A game clock consists of two adjacent clocks and buttons to stop one clock while starting the other, such that the two component clocks never run simultaneously. Game clocks are used in two-player games where the players move in turn...

  • Japanese clock
    Japanese clock
    A is a mechanical clock that has been made to tell traditional Japanese time. Mechanical clocks were introduced into Japan by Jesuit missionaries or Dutch merchants in the sixteenth century. These clocks were of the lantern clock design, typically made of brass or iron, and used the relatively...

  • Master clock
    Master clock
    A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...

  • Musical clock
    Musical clock
    A Musical clock is a clock that marks the hours of the day with a musical tune played from a spiked cylinder either on bells, organ pipes, bellows, combs and even dulcimer strings....

  • Railroad chronometer
  • Slave clock
    Slave clock
    In telecommunication, a slave clock is a clock that is coordinated with a master clock. Slave clock coordination is usually achieved by phase-locking the slave clock signal to a signal received from the master clock...

  • Speaking clock
    Speaking clock
    A speaking clock service is a recorded or simulated human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observatory on 14 February 1933.The format of the service is...

  • Stopwatch
    Stopwatch
    A stopwatch is a handheld timepiece designed to measure the amount of time elapsed from a particular time when activated to when the piece is deactivated. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stopclock.The timing functions...

  • Striking clock
    Striking clock
    A striking clock is a clock that sounds the hours audibly on a bell or gong. In 12 hour striking, used most commonly in striking clocks today, the clock strikes once at 1 AM, twice at 2 AM, continuing in this way up to twelve times at 12 noon, then starts over, striking once at 1 PM, twice at 2...

  • Talking clock
    Talking clock
    A talking clock is a timekeeping device that presents the time as sounds. It may present the time solely as sounds, such as a phone-based time service or a clock for the hearing impaired, or may have a sound feature in addition to an analog or digital face.- History :Although they would not be...

  • Tide clock
    Tide clock
    A tide clock is a specially designed clock that keeps track of the Moon's apparent motion around the Earth. Along many coastlines the Moon contributes the major part of the combined lunar and solar tides....

  • Time ball
    Time ball
    A time ball is a large painted wooden or metal ball that drops at a predetermined time, principally to enable sailors to check their marine chronometers from their boats offshore...

  • Time clock
    Time clock
    A time clock, sometimes known as a clock card machine or punch clock or time recorder, is a mechanical timepiece used to assist in tracking the hours an employee of a company worked. In regards to mechanical time clocks this was accomplished by inserting a heavy paper card, called a timesheet,...

  • World clock
    World clock
    A world clock, often called a "Worldtime Clock", is a clock which displays the time for many places around the world.The display can take various forms:...

  • American clock
    American clock
    The term American clock was used in the Victorian era and afterward to refer to a particular style of clock design followed by the American clockmakers of the day...

  • Automaton clock
    Automaton clock
    An automaton clock or automata clock is a type of striking clock featuring automatons. Clocks like these were built from the Middle Ages through to Victorian times in Europe. A Cuckoo clock is a simple form of this type of clock. The automatons usually perform on the hour, half-hour or...

  • Balloon clock
    Balloon clock
    A balloon clock is a bracket clock with a waisted or balloon-shaped case. It was popular in England from the late 18th to the early 19th century....

  • Banjo clock
    Banjo clock
    The banjo clock, or banjo timepiece, is an American wall clock with a banjo-shaped case. It was invented by Simon Willard, originally of Grafton, Massachusetts, later of Roxbury, Massachusetts, and patented in 1802...

  • Bracket clock
    Bracket clock
    A bracket clock is a style of antique portable table clock made in the 17th and 18th centuries. The term originated with small weight driven clocks that had to be mounted on a bracket on the wall to allow room for their hanging weights. When spring driven clocks were invented they continued to...

  • Carriage clock
    Carriage clock
    A carriage clock is a small, spring-driven clock, designed for travelling, developed in the early 19th century in Austria. The case, usually plain or gilt-brass, is rectangular with a carrying handle and often set with glass or more rarely enamel or porcelain panels...

  • Cartel clock
    Cartel clock
    A Cartel Clock is a clock designed to hang directly on the wall. Most often used in reference to highly ornate mid-18th-century Rococo examples. These flowing, curvilinear designs are executed in gilt bronze ....

  • Cat clock
    Kit-Cat Klock
    The Kit-Cat Klock is an art deco novelty style clock. The clock is manufactured in the shape of a black cat and is hung on a wall. The tail swings as a second hand and eyes move back and forth to give illusion of it watching you. It first appeared during the 1930s...

  • Clock tower
    Clock tower
    A clock tower is a tower specifically built with one or more clock faces. Clock towers can be either freestanding or part of a church or municipal building such as a town hall. Some clock towers are not true clock towers having had their clock faces added to an already existing building...

  • Cuckoo clock
    Cuckoo clock
    A cuckoo clock is a clock, typically pendulum-regulated, that strikes the hours with a sound like a common cuckoo's call and typically has a mechanical cuckoo that emerges with each note...

  • Doll's head clock
    Doll's head clock
    Doll's head clocks, often known by their French name tête de poupée, were popular in the later half of Louis XIV's reign. They are named for their profile which resembles a head and shoulders. A doll's head clock is almost always ornamented with Boulle marquetry....

  • Floral clock
    Floral clock
    A floral clock or flower clock is a large decorative clock set into a flower bed in a park or other public recreation area.The floral clock was a form of carpet bedding set onto an operational clock-face, invented in 1903 by John McHattie of Edinburgh Parks in Princes Street Gardens, Edinburgh and...

  • French Empire mantel clock
    French Empire mantel clock
    A French Empire-style mantel clock, is a type of elaborately decorated mantel clock made in France over the Napoleonic Empire between 1804–1814/15, although the clocks manufactured throughout the Bourbon Restoration are also included within this art movement because they share subject, decorative...

  • Granddaughter clock
  • Grandfather clock
  • Grandmother clock
  • Lantern clock
    Lantern clock
    A lantern clock is a type of antique weight-driven wall clock, shaped like a lantern. They were the first type of clock widely used in private homes. They probably originated before 1500 but only became common after 1600; in Britain around 1620. They became obsolete in the 19th century.- Origin...

  • Lighthouse clock
    Lighthouse Clock
    A lighthouse clock is a type of mantel clock manufactured in the U. S. from 1818 through 1830s by the American clockmaker Simon Willard, having the dial and works exposed beneath a glass dome on a tapered, cylindrical body....

  • Longcase clock
    Longcase clock
    A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

     (or tall-case clock)
  • Mantel clock
    Mantel clock
    Mantel clocks — or shelf clocks — are relatively small house clocks traditionally placed on the shelf, or mantel, above the fireplace. The form, first developed in France in the 1750s, can be distinguished from earlier chamber clocks of similar size due to a lack of carrying handles.These clocks...

  • Skeleton clock
    Skeleton clock
    A skeleton clock is any clock or wristwatch, though typically mechanical in nature, in which the parts that usually conceal the inner workings of the mechanism have been removed or significantly modified so as to display these inner parts....

  • Tower clock
    Clock tower
    A clock tower is a tower specifically built with one or more clock faces. Clock towers can be either freestanding or part of a church or municipal building such as a town hall. Some clock towers are not true clock towers having had their clock faces added to an already existing building...

  • Turret clock
    Turret clock
    A Turret clock is a clock mounted in a tower or turret, usually to show the current time on a dial with hand or to announce the time by strike, or both. It can also have more than one dial to show days, moon phases, and other astronomical data.-Sundials:...

  • Watch
    Watch
    A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...


  • See also



    • Allan variance
      Allan variance
      The Allan variance , also known as two-sample variance, is a measure of frequency stability in clocks, oscillators and amplifiers. It is named after David W. Allan. It is expressed mathematically as\sigma_y^2. \,...

    • American Watchmakers-Clockmakers Institute
      American Watchmakers-Clockmakers Institute
      The American Watchmakers-Clockmakers Institute is a not-for-profit trade association based in the United States that is dedicated to the advancement of the modern watch industry, from which it receives a significant portion of its funding. While the AWCI is an American organization, it also has...

    • BaselWorld
      BaselWorld
      Baselworld Watch and Jewellery Show is a trade show for the watch and jewellery industry organized annually in the city of Basel, Switzerland. The international show unites about 2,100 exhibitors from over 45 countries, including the leading watch and jewelry brands, as well as companies...

    • Biological clock
      Biological clock
      Biological clock may refer to:* Circadian rhythm, living organisms' adaptations to solar related rhythms* Age, as a general factor of female infertility...

    • Castle clock
      Castle clock
      A castle clock is a clock whose face is mounted outside a castle or perhaps a church tower. They may be elaborate or simple.Al-Jazari constructed an elaborate clock and described it in his Book of Knowledge of Ingenious Mechanical Devices in 1206. It was about 3,3 metres high , and had multiple...

    • Clock as herald of the Industrial Revolution (Lewis Mumford)
    • Clock face
      Clock face
      A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

    • Clock network
      Clock network
      A clock network or clock system is a set of clocks designed to always show exactly the same time by communicating with each other. Clock networks usually include a central master clock kept in sync with an official time source, and one or more slave clocks which receive and display the time from...

    • Clock of the Long Now
      Clock of the Long Now
      The Clock of the Long Now, also called the 10,000-year clock, is a proposed mechanical clock designed to keep time for 10,000 years. The project to build it is part of the Long Now Foundation....

    • Clock signal
      Clock signal
      In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

       (digital circuits)
    • Clockkeeper
      Clockkeeper
      A clockkeeper, sometimes seen as clock keeper, refers to a form of employment seen prevalently during Middle Age Europe involving the tracking of time and the maintaining of clocks and other timekeeping devices...

    • Clockmaker
      Clockmaker
      A clockmaker is an artisan who makes and repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique shops, and places devoted strictly to repairing clocks and watches...

    • Colgate Clock (Indiana)
      Colgate Clock (Indiana)
      The Colgate Clock, located at a Colgate-Palmolive factory in Clarksville, Indiana, is one of the largest clocks in the world. It has a diameter of 40 feet . It was first illuminated in Clarksville on November 17, 1924...

    • Colgate Clock (New Jersey), largest clock in USA

    • Corpus Clock
      Corpus Clock
      The Corpus Clock is a large sculptural clock at street level on the outside of the Taylor Library at Corpus Christi College, Cambridge University, at the junction of Bene't Street and Trumpington Street, looking out over King's Parade. It was conceived and funded by John C...

    • Cosmo Clock 21
      Cosmo Clock 21
      Cosmo Clock 21 is a giant Ferris wheel located in Yokohama, Japan. It is also the world's largest clock.Built for the YES '89 Yokohama Exposition at Minato Mirai 21 in 1989, Cosmo Clock 21 was originally constructed with a height of ....

      , world's largest clock
    • Cox's timepiece
      Cox's timepiece
      Cox's timepiece is a clock developed in the 1760s by James Cox. It was developed in collaboration with John Joseph Merlin . Cox claimed that his design was a true perpetual motion machine, but as the device is powered from changes in atmospheric pressure via a mercury barometer, this is not the case...

    • Cuckooland Museum
      Cuckooland Museum
      The Cuckooland Museum, previously known as the Cuckoo Clock Museum, is a museum that exhibits mainly cuckoo clocks, located in Tabley, co. Cheshire, England...

    • Death Clock
    • Le Défenseur du Temps
      Le Défenseur du Temps
      Le Défenseur du Temps is a large mechanical work of art in the form of a clock created by the French artist Jacques Monestier.-Description:...

       (automata)
    • Department of Defense master clock
      Department of Defense master clock
      The Department of Defense master clock is the master clock to which time and frequency measurements for the United States Department of Defense are referenced....

       (U.S.)
    • Doomsday Clock
      Doomsday Clock
      The Doomsday Clock is a symbolic clock face, maintained since 1947 by the board of directors of the Bulletin of the Atomic Scientists at the University of Chicago. The closer the clock is to midnight, the closer the world is estimated to be to global disaster. , the Doomsday Clock now stands at six...

    • Earth clock
      Earth clock
      An Earth Clock is a computer program or maybe separate device that will display a map of the Earth showing the zones where is day and where is night....

    • Federation of the Swiss Watch Industry FH
      Federation of the Swiss Watch Industry FH
      The Federation of the Swiss Watch Industry FH is the Swiss watch industry's leading trade association.-History:...

    • Guard tour patrol system
      Guard tour patrol system
      A Guard tour patrol system is a system for logging the rounds of employees in a variety of situations such as Security guards patrolling property, technicians monitoring climate-controlled environments, and correctional officers checking prisoner living areas...

       (watchclocks)
    • Iron Ring Clock
      Iron Ring Clock
      The Iron Ring Clock is a clock of unusual design created by four Mechanical Engineering students at McMaster University in Hamilton, Ontario, Canada. The clock was designed and built as a thesis project by Patrick Burton, Braden Kurczak, Michael Paddags, and Peter Whitred...

    • Jens Olsen's World Clock
      Jens Olsen's World Clock
      Jens Olsen's World Clock or Verdensur is an advanced astronomical clock which is displayed in Copenhagen City Hall.The clock was designed and calculated by Jens Olsen who was a skilled locksmith, but later learned the trade of clockmaking...

    • Jewel bearing
      Jewel bearing
      A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewel material is usually synthetic sapphire...


    • List of biggest clock faces
    • List of clocks
    • List of international common standards
    • List of world's largest cuckoo clocks
    • Metrology
      Metrology
      Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...

    • Mora clock
    • National Association of Watch and Clock Collectors
      National Association of Watch and Clock Collectors
      The National Association of Watch and Clock Collectors is an American non-profit organization with about 18,000 members.The NAWCC was founded in 1943 by members of the Horological Society of New York and the Philadelphia Watchmakers' Guild who wished to create a national organization...

    • Replica watch
    • Star clock
      Star clock
      A star clock is a method of using the stars to determine the time. This is accomplished by measure the Big Dipper's position in the sky based on a standard clock, and then employing simple addition and subtraction. This method requires no tools; others use an astrolabe and a planisphere.A clock's...

    • System time
    • Time to digital converter
      Time to digital converter
      In electronic instrumentation and signal processing, a time to digital converter is a device for converting a signal of sporadic pulses into a digital representation of their time indices. In other words, a TDC outputs the time of arrival for each incoming pulse...

    • Timeline of time measurement technology
      Timeline of time measurement technology
      Timeline of time measurement technology* 270 BC - Ctesibius builds a popular water clock, called a clepsydra* 46 BC - Julius Caesar and Sosigenes develop a solar calendar with leap years...

    • Timer
      Timer
      A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

    • Watchmaker
      Watchmaker
      A watchmaker is an artisan who makes and repairs watches. Since virtually all watches are now factory made, most modern watchmakers solely repair watches. However, originally they were master craftsmen who built watches, including all their parts, by hand...



    External links