Editing Clock (section)

==History of time-measuring devices==
{{main|History of timekeeping devices}}
<!--Linked from [[Template:Time topics]]-->

===Sundials===

{{Main|Sundial}}

[[File:Garden sundial MN 2007.JPG|thumb|right|Simple horizontal sundial]]

The apparent position of the Sun in the sky changes over the course of each day, reflecting the rotation of the Earth. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a (usually) flat surface that has markings that correspond to the hours.<ref>{{cite web|title=How Sundials Work|date=November 7, 2013|url=http://sundialsoc.org.uk/discussions/how-do-sundials-work/|publisher=The British Sundial Society|access-date=10 November 2014|archive-date=November 10, 2014|archive-url=https://web.archive.org/web/20141110144533/http://sundialsoc.org.uk/discussions/how-do-sundials-work/|url-status=live}}</ref> Sundials can be horizontal, vertical, or in other orientations. Sundials were widely used in [[ancient times]].<ref>{{cite web|title=Ancient Sundials|url=http://sundials.org/index.php/all-things-sundial/ancient-sundials|publisher=North American Sundial Society|access-date=10 November 2014|archive-date=November 10, 2014|archive-url=https://web.archive.org/web/20141110142221/http://sundials.org/index.php/all-things-sundial/ancient-sundials|url-status=live}}</ref> With knowledge of latitude, a well-constructed sundial can measure local [[solar time]] with reasonable accuracy, within a minute or two. Sundials continued to be used to monitor the performance of clocks until the 1830s, when the use of the telegraph and trains standardized time and time zones between cities.<ref>{{Cite book |last=Lankford |first=John |url=https://books.google.com/books?id=Xev7zOrwLHgC&pg=PA502 |title=History of Astronomy: An Encyclopedia |date=1997 |publisher=Taylor & Francis |isbn=978-0-8153-0322-0 |language=en}}</ref>

===Devices that measure duration, elapsed time and intervals===

[[File:Wooden hourglass 3.jpg|thumb|upright|The flow of sand in an [[hourglass]] can be used to keep track of elapsed time.]]

Many devices can be used to mark the passage of time without respect to reference time (time of day, hours, minutes, etc.) and can be useful for measuring duration or intervals. Examples of such duration timers are [[candle clock]]s, [[incense clock]]s, and the [[hourglass]]. Both the candle clock and the incense clock work on the same principle, wherein the consumption of resources is more or less constant, allowing reasonably precise and repeatable estimates of time passages. In the hourglass, fine [[sand]] pouring through a tiny hole at a constant rate indicates an arbitrary, predetermined passage of time. The resource is not consumed, but re-used.

===Water clocks===
{{main|Water clock}}

[[File:MandalayWaterClock.jpg|thumb|upright|A water clock for [[goldbeating]] [[goldleaf]] in [[Mandalay]] (Myanmar)]]

Water clocks, along with sundials, are possibly the oldest time-measuring instruments, with the only exception being the day-counting [[tally stick]].<ref>{{Harvnb|Turner|1984|p=1}}</ref> Given their great antiquity, where and when they first existed is not known and is perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in [[Babylon]] and Egypt around the 16th century BC. Other regions of the world, including India and China, 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.<ref>{{Harvnb|Cowan|1958|p=58}}</ref>

The [[Ancient Macedonia|Macedonian]] [[ancient Greek astronomy|astronomer]] [[Andronicus of Cyrrhus|Andronicus]] of [[Cyrrhus (Macedonia)|Cyrrhus]] supervised the construction of the [[Tower of the Winds]] in [[Roman Athens|Athens]] in the 1st century BC, which housed a large clepsydra inside as well as multiple prominent sundials outside, allowing it to function as a kind of early [[clocktower]].<ref>{{cite web| url = http://www.sailingissues.com/yachting-guide/tower-of-winds-1.html |website=sailingissues.com | title = Tower of the Winds – Athens| access-date = November 4, 2008| archive-date = December 9, 2008| archive-url = https://web.archive.org/web/20081209015740/http://sailingissues.com/yachting-guide/tower-of-winds-1.html| url-status = live}}</ref> The [[Ancient Greece|Greek]] and [[Ancient Rome|Roman]] civilizations advanced water clock design with improved accuracy. These advances were passed on through [[Byzantine Empire|Byzantine]] and [[Islamic]] times, eventually making their way back to Europe. Independently, the Chinese developed their own advanced water clocks ({{lang|zh|水鐘}}) by 725 AD, passing their ideas on to Korea and Japan.<ref>{{Cite web |title=Water Clock {{!}} China {{!}} Western Han dynasty (206 BCE–9 CE) |url=https://www.metmuseum.org/art/collection/search/696219 |url-status=live |archive-url=https://web.archive.org/web/20230405125014/https://www.metmuseum.org/art/collection/search/696219 |archive-date=Apr 5, 2023 |website=The Metropolitan Museum of Art}}</ref>

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 [[astrology|astrological]] reasons. These early water clocks were calibrated with a sundial. 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]] in 17th-century Europe.

Islamic civilization is credited with further advancing the accuracy of clocks through elaborate engineering. In 797 (or possibly 801), the [[Abbasid]] [[caliph]] of [[Baghdad]], [[Harun al-Rashid]], presented [[Charlemagne]] with an [[Asian elephant]] named [[Abul-Abbas]] together with a "particularly elaborate example" of a water<ref>{{cite book | last = James | first = Peter | title = Ancient Inventions | year = 1995 | location = New York | isbn = 978-0-345-40102-1 | page = [https://archive.org/details/ancientinvention00jame/page/126 126] | publisher = Ballantine Books | url = https://archive.org/details/ancientinvention00jame/page/126 }}</ref> clock. [[Pope Sylvester II]] introduced clocks to northern and western Europe around 1000 AD.<ref>{{cite book |url=https://archive.org/details/livesnecromance04godwgoog|title=Lives of the Necromancers|author=William Godwin|year=1876|page=232|publisher=London, F.J. Mason}}</ref>

=== Mechanical water clocks ===

{{See also|Automaton#Ancient}}

The first known [[gear]]ed clock was invented by the great mathematician, physicist, and engineer [[Archimedes]] during the 3rd century BC. Archimedes created his astronomical clock,<ref>{{Cite book|last=Moussas|first=Xenophon|title=The Antikythera Mechanism, the first mechanical cosmos (in Greek)|publisher=Canto Mediterraneo|year=2018|isbn=978-618-83695-0-4|location=Athens}}</ref>{{fact|reason=I can't find any record of this book, ISBN doesn't even seem to exist|date=April 2024}} which was also a cuckoo clock with birds singing and moving every hour. It is the first carillon clock as it plays music simultaneously with a person blinking his eyes, surprised by the singing birds. The Archimedes clock works with a system of four weights, counterweights, and strings regulated by a system of floats in a water container with siphons that regulate the automatic continuation of the clock. The principles of this type of clock are described by the mathematician and physicist Hero,<ref>{{Cite book|last=Dasypodius|first=K.|title=Heron mechanicus.|year=1580}}</ref> who says that some of them work with a chain that turns a gear in the mechanism.<ref>{{Cite book|last=Hero|first=of Alexandria|title=see Hero's books: Pneumatica (Πνευματικά), Automata, Mechanica, Metrica, Dioptra|orig-date=1st century BC to 1st century AD|location=Alexandria}}</ref> Another Greek clock probably constructed at the time of Alexander was in Gaza, as described by Procopius.<ref>{{Cite book|last=Procopius of Caesarea|first=Προκόπιος ὁ Καισαρεύς|title=Περὶ Κτισμάτων, Perì Ktismáton; Latin: De Aedificiis, On Buildings|date=c. 500s }}</ref> The Gaza clock was probably a Meteoroskopeion, i.e., a building showing celestial phenomena and the time. It had a pointer for the time and some automations similar to the Archimedes clock. There were 12 doors opening one every hour, with Hercules performing his labors, the Lion at one o'clock, etc., and at night a lamp becomes visible every hour, with 12 windows opening to show the time.

[[File:SuSongClock1.JPG|thumb|upright|A [[scale model]] of [[Su Song]]'s [[Astronomical]] Clock Tower, built in 11th-century [[Kaifeng]], China. It was driven by a large [[waterwheel]], [[chain drive]], and [[escapement]] mechanism.]]

The [[Tang dynasty]] Buddhist monk [[Yi Xing]] along with government official [[Liang Lingzan]] made the escapement in 723 (or 725) to the workings of a water-powered [[armillary sphere]] and [[drive wheel|clock drive]], which was the world's first clockwork escapement.<ref name="needham volume 4 part 2 165">Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering''. Taipei: Caves Books Ltd, p. 165.</ref><ref>Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering''. Taipei: Caves Books Ltd, p. 319.</ref> The [[Song dynasty]] [[polymath]] and genius [[Su Song]] (1020–1101) incorporated it into his monumental innovation of the astronomical clock tower of [[Kaifeng]] in 1088.<ref>{{Cite web|title=No. 120: Su-Sung's Clock|url=https://www.uh.edu/engines/epi120.htm|access-date=2021-02-18|website=www.uh.edu|archive-date=February 26, 2021|archive-url=https://web.archive.org/web/20210226194506/https://uh.edu/engines/epi120.htm|url-status=live}}</ref><ref>History of Song 宋史, Vol. 340</ref>{{Page needed|date=July 2011}} His astronomical clock and rotating [[armillary sphere]] still relied on the use of either flowing water during the spring, summer, and autumn seasons or [[mercury (element)|liquid mercury]] during the freezing temperatures of winter (i.e., [[hydraulics]]).
In Su Song's waterwheel linkwork device, the action of the escapement's arrest and release was achieved by gravity exerted periodically as the continuous flow of liquid-filled containers of a limited size. In a single line of evolution, Su Song's clock therefore united the concepts of the clepsydra and the mechanical clock into one device run by mechanics and hydraulics. In his memorial, Su Song wrote about this concept:

<blockquote>According to your servant's opinion there have been many systems and designs for astronomical instruments during past dynasties all differing from one another in minor respects. But the principle of the use of water-power for the driving mechanism has always been the same. The heavens move without ceasing but so also does water flow (and fall). Thus if the water is made to pour with perfect evenness, then the comparison of the rotary movements (of the heavens and the machine) will show no discrepancy or contradiction; for the unresting follows the unceasing.</blockquote>

Song was also strongly influenced by the earlier armillary sphere created by [[Zhang Sixun]] (976 AD), who also employed the escapement mechanism and used liquid [[Mercury (element)|mercury]] instead of water in the waterwheel of his astronomical clock tower. The mechanical clockworks for Su Song's astronomical tower featured a great driving-wheel that was 11 feet in diameter, carrying 36 scoops, into each of which water was poured at a uniform rate from the "constant-level tank". The main driving shaft of iron, with its cylindrical necks supported on iron crescent-shaped bearings, ended in a pinion, which engaged a gear wheel at the lower end of the main vertical transmission shaft. This great astronomical hydromechanical clock tower was about ten metres high (about 30 feet), featured a clock [[escapement]], and was indirectly powered by a rotating wheel either with falling water or [[Mercury (element)|liquid mercury]]. A full-sized working replica of Su Song's clock exists in the [[Republic of China]] (Taiwan)'s [[National Museum of Natural Science]], [[Taichung]] city. This full-scale, fully functional replica, approximately 12 meters (39 feet) in height, was constructed from Su Song's original descriptions and mechanical drawings.<ref>{{Cite web |title=Past Masters: The Astronomical Water Clock Of Su Song |url=https://revolutionwatch.com/past-masters-the-astronomical-water-clock-of-su-song/ |access-date=2022-06-04 |website=revolutionwatch.com |date=August 8, 2014 |archive-date=April 7, 2022 |archive-url=https://web.archive.org/web/20220407055722/https://revolutionwatch.com/past-masters-the-astronomical-water-clock-of-su-song/ |url-status=live }}</ref> The Chinese escapement spread west and was the source for Western escapement technology.<ref name=Derek>[[Derek J. de Solla Price]], [http://www.gutenberg.org/files/30001/30001-h/30001-h.htm On the Origin of Clockwork, Perpetual Motion Devices, and the Compass], p.86</ref>
[[File:Al-jazari elephant clock.png|thumb|upright|An [[elephant clock]] in a manuscript by [[Al-Jazari]] (1206 AD) from ''The Book of Knowledge of Ingenious Mechanical Devices''<ref>[[Ibn al-Razzaz Al-Jazari]] (ed. 1974), ''The Book of Knowledge of Ingenious Mechanical Devices''. Translated and annotated by [[Donald Routledge Hill]], Dordrecht/[[D. Reidel]].</ref>]]
In the 12th century, [[Al-Jazari]], an engineer from Mesopotamia (lived 1136–1206) who worked for the [[Artuqid]] king of Diyar-Bakr, [[Nasrudin|Nasir al-Din]], made numerous clocks of all shapes and sizes. The most reputed clocks included [[Elephant clock|the elephant]], scribe, and [[castle clock]]s, some of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of the status, grandeur, and wealth of the Urtuq State.<ref>{{Cite web |title=Remaking History: Ismail al-Jazari and the Elephant Water Clock - Make |url=https://makezine.com/projects/remaking-history-ismail-al-jazari-and-the-elephant-water-clock/ |access-date=2023-01-11 |website=Make: DIY Projects and Ideas for Makers |date=May 7, 2021 |language=en-US |archive-date=January 11, 2023 |archive-url=https://web.archive.org/web/20230111190253/https://makezine.com/projects/remaking-history-ismail-al-jazari-and-the-elephant-water-clock/ |url-status=live }}</ref> Knowledge of these mercury escapements may have spread through Europe with translations of Arabic and Spanish texts.<ref name="Hassan">[[Ahmad Y Hassan|Hassan, Ahmad Y]], [http://www.history-science-technology.com/Articles/articles%2071.html Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering] {{Webarchive|url=https://web.archive.org/web/20150924030451/http://www.history-science-technology.com/Articles/articles%2071.html |date=September 24, 2015 }}, ''History of Science and Technology in Islam''</ref><ref name=Ajram>{{Cite book|last=Ajram |first=K. |year=1992 |title=Miracle of Islamic Science |chapter=Appendix B |publisher=Knowledge House Publishers |isbn=0-911119-43-4}}</ref>

===Fully mechanical===

{{more citations needed section|date=January 2024}}

The word {{lang|el-Latn|horologia}} (from the Greek {{lang|el|ὥρα}}—'hour', and {{lang|el|λέγειν}}—'to tell') was used to describe early mechanical clocks,<ref name=wmsmith1875>{{cite book |last2=Smith |first2=William |title=A Dictionary of Greek and Roman Antiquities |year=1875 |publisher=John Murray |location=London |pages=615‑617 |url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Horologium.html |author1=Leonhard Schmitz |access-date=February 19, 2021 |archive-date=July 3, 2023 |archive-url=https://web.archive.org/web/20230703112706/https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA%2A/Horologium.html |url-status=live }}</ref> but the use of this word (still used in several [[Romance languages]])<ref>Modern French {{lang|fr|horloge}} is very close; Spanish {{lang|es|reloj}} and Portuguese {{lang|pt|relógio}} drop the first part of the word.</ref> for all timekeepers conceals the true nature of the mechanisms. For example, there is a record that in 1176, [[Cathédrale Saint-Étienne de Sens|Sens Cathedral]] in France installed an '[[horologe]]',<ref>''Bulletin de la société archéologique de Sens'', year 1867, vol. IX, p. 390, available at www.archive.org.</ref><ref>See also [[:fr:Discussion:Horloge]]{{Circular reference|date=April 2024}}</ref> but the mechanism used is unknown. According to [[Jocelyn de 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.<ref>{{cite book | title=The Chronicle of Jocelin of Brakelond, Monk of St. Edmundsbury: A Picture of Monastic and Social Life on the XIIth Century |year=1910 |location=London |publisher=Chatto and Windus. Translated and edited by L.C. Jane}}</ref> The word ''clock'' (via [[Medieval Latin]] {{lang|la-x-medieval|clocca}} from [[Old Irish]] {{lang|sga|clocc}}, both meaning 'bell'), which gradually supersedes "horologe", suggests that it was the sound of bells that also characterized the prototype mechanical clocks that appeared during the 13th century in Europe.

[[File:Läckö slott interior 27.jpg|thumb|upright=1.2|A 17th-century weight-driven clock in [[Läckö Castle]], Sweden]]

In Europe, between 1280 and 1320, there was an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take 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 – marks the beginning of the true mechanical clock, which differed from the previously mentioned cogwheel clocks. The [[verge escapement]] mechanism appeared during the surge of true mechanical clock development, which did not need any kind of fluid power, like water or mercury, to work.

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]]. The former purpose is administrative; the latter arises naturally given the scholarly interests in astronomy, science, and astrology and how these subjects integrated with the religious philosophy of the time. The [[astrolabe]] was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system.

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

In 1283, a large clock was installed at [[Dunstable Priory]] in [[Bedfordshire]] in southern England; its location above the [[rood screen]] suggests that it was not a water clock.<ref name=":0">{{Cite web|url=https://www.crystalinks.com/clocks.html|title=Clocks{{Snd}} Crystalinks|website=www.crystalinks.com|access-date=2019-06-06|archive-date=June 6, 2019|archive-url=https://web.archive.org/web/20190606155715/https://www.crystalinks.com/clocks.html|url-status=live}}</ref> In 1292, [[Canterbury Cathedral]] installed a 'great horloge'. Over the next 30 years, there were mentions of clocks at a number of ecclesiastical institutions in England, Italy, and France. In 1322, a [[Norwich cathedral astronomical clock|new clock was installed in Norwich]], an expensive replacement for an earlier clock installed in 1273. This had a large (2 metre) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two [[clockkeeper]]s for two years.<ref name=":0" />

===Astronomical===

[[File:Abbot Richard Wallingford.jpg|thumb|right|[[Richard of Wallingford]] pointing to a clock, his gift to [[St Albans Abbey]]]]

[[File:Clock machine 16th century-Convent of Christ,Tomar, Portugal.jpg|thumb|16th-century clock machine [[Convent of Christ]], [[Tomar]], Portugal]]

An elaborate water clock, the 'Cosmic Engine', was invented by [[Su Song]], a Chinese [[polymath]], designed and constructed in China in 1092. This great astronomical hydromechanical clock tower was about ten metres high (about 30 feet) and was indirectly powered by a rotating wheel with falling water and [[Mercury (element)|liquid mercury]], which turned an [[armillary sphere]] capable of calculating complex astronomical problems.

In Europe, there were the clocks constructed by [[Richard of Wallingford]] in [[St Albans|Albans]] by 1336, and by Giovanni [[Giovanni Dondi dell'Orologio|de Dondi]] in [[Padua, Italy|Padua]] from 1348 to 1364. They no longer exist, but detailed descriptions of their design and construction survive,<ref name="north">North, John. God's Clockmaker: Richard of Wallingford and the Invention of Time. London: Hambledon and London (2005).</ref><ref name="king">King, Henry "Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks", University of Toronto Press, 1978</ref> and modern reproductions have been made.<ref name="king" /> 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.

The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 in [[Padua]], Italy, by the doctor and clock-maker [[Giovanni Dondi dell'Orologio]]. The Astrarium had seven faces and 107 moving gears; it showed the positions of the Sun, the Moon and the five planets then known, as well as religious feast days. The astrarium stood about 1 metre high, and consisted of a seven-sided brass or iron framework resting on 7 decorative paw-shaped feet. The lower section provided a 24-hour dial and a large calendar drum, showing the fixed feasts of the church, the movable feasts, and the position in the zodiac of the Moon's ascending node. The upper section contained 7 dials, each about 30&nbsp;cm in diameter, showing the positional data for the [[Primum Mobile]], Venus, Mercury, the Moon, Saturn, Jupiter, and Mars. Directly above the 24-hour dial is the dial of the [[Primum Mobile]], so called because it reproduces the diurnal motion of the stars and the annual motion of the Sun against the background of stars. Each of the 'planetary' dials used complex clockwork to produce reasonably accurate models of the planets' motion. These agreed reasonably well both with Ptolemaic theory and with observations.<ref>{{Cite web |title=Giovanni Dondi's Astrarium, 1364 {{!}} cabinet |url=https://www.cabinet.ox.ac.uk/giovanni-dondis-astrarium-1364-0 |access-date=2022-06-05 |website=www.cabinet.ox.ac.uk |archive-date=November 20, 2021 |archive-url=https://web.archive.org/web/20211120141134/https://www.cabinet.ox.ac.uk/giovanni-dondis-astrarium-1364-0 |url-status=live }}</ref><ref>{{Cite news |last=Abrams |first=Melanie |date=2018-02-16 |title='The Beauty of Time' |language=en-US |work=The New York Times |url=https://www.nytimes.com/2018/02/16/style/watches-clocks-mbandf-breuget.html |access-date=2022-06-05 |issn=0362-4331 |archive-date=June 4, 2022 |archive-url=https://web.archive.org/web/20220604212633/https://www.nytimes.com/2018/02/16/style/watches-clocks-mbandf-breuget.html |url-status=live }}</ref>

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 [[Rota Fortunae|wheel of fortune]] and an indicator of the state of the tide at [[London Bridge]]. Bells rang every hour, the number of strokes indicating the time.<ref name="north"/> 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 [[Moveable feast|movable feast]]s, and an eclipse prediction hand rotating once every 18 years.<ref name="king"/> 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, and can be examined in places such as ancient castles and museums. The [[Salisbury Cathedral clock]], built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours.<ref>Singer, Charles, et al. ''Oxford History of Technology: volume II, from the Renaissance to the Industrial Revolution'' (OUP 1957) pp. 650–651</ref>

===Spring-driven===

<gallery mode="packed-hover" heights="150px" caption="Examples of spring-driven clocks">
Matthew Norman carriage clock with winding key.jpg|Matthew Norman carriage clock with winding key
1908 Gilbert mantel clock decorated with Memento Mori decoupage.JPG|Decorated William Gilbert mantel clock
</gallery>

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,<ref name="White2">{{cite book|last=White|first=Lynn Jr.|title=Medieval Technology and Social Change|publisher=Oxford Univ. Press|year=1966|location=New York|isbn=978-0-19-500266-9|pages=[https://archive.org/details/medievaltechnolo00whit/page/126 126–127]|url=https://archive.org/details/medievaltechnolo00whit/page/126}}</ref><ref>{{Cite book|last=Usher|first=Abbot Payson|title=A History of Mechanical Inventions|year=1988|publisher=Courier Dover|isbn=978-0-486-25593-4|url=https://books.google.com/books?id=xuDDqqa8FlwC&pg=PA305|page=305|access-date=June 5, 2020|archive-date=July 3, 2023|archive-url=https://web.archive.org/web/20230703112708/https://books.google.com/books?id=xuDDqqa8FlwC&pg=PA305|url-status=live}}</ref><ref name="Rossum">{{cite book|last=Dohrn-van Rossum|first=Gerhar|title=History of the Hour: Clocks and Modern Temporal Orders|publisher=Univ. of Chicago Press|year=1997|url=https://books.google.com/books?id=53K32RiEigMC&pg=PA121|isbn=978-0-226-15510-4|page=121|access-date=June 5, 2020|archive-date=July 3, 2023|archive-url=https://web.archive.org/web/20230703113720/https://books.google.com/books?id=53K32RiEigMC&pg=PA121|url-status=live}}</ref> although they are often erroneously credited to [[Nuremberg]] watchmaker [[Peter Henlein]] (or Henle, or Hele) around 1511.<ref>{{cite book |last=Milham|first=Willis I.|title=Time and Timekeepers |year=1945 |publisher=MacMillan|location=New York|isbn=978-0-7808-0008-3|page=121}}</ref><ref>{{cite encyclopedia|title=Clock|encyclopedia=The New Encyclopædia Britannica|volume=4|page=747|publisher=Univ. of Chicago|year=1974|url=https://books.google.com/books?id=Eb0qAAAAMAAJ&q=peter+Henlein|isbn=978-0-85229-290-7}}</ref><ref>{{cite book|last=Anzovin|first=Steve|author2=Podell, Janet|title=Famous First Facts: A record of first happenings, discoveries, and inventions in world history|year=2000|publisher=H.W. Wilson|isbn=978-0-8242-0958-2|page=[https://archive.org/details/famousfirstfacts00anzo/page/440 440]|url=https://archive.org/details/famousfirstfacts00anzo/page/440}}</ref> The earliest existing spring driven clock is the chamber clock given to Phillip the Good, Duke of Burgundy, around 1430, now in the [[Germanisches Nationalmuseum]].<ref name="White" /> Spring power presented clockmakers with a new problem: how to keep the clock [[movement (clockwork)|movement]] running at a constant rate as the spring ran down. This resulted in the invention of the ''[[stackfreed]]'' and the [[fusee (horology)|fusee]] in the 15th century, and many other innovations, down to the invention of the modern ''going [[barrel (horology)|barrel]]'' in 1760.

Early clock dials did not indicate minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus,<ref name=haencyc>p. 529, "Time and timekeeping instruments", ''History of astronomy: an encyclopedia'', John Lankford, Taylor & Francis, 1997, {{ISBN|0-8153-0322-X}}.</ref> and some 15th-century clocks in Germany indicated minutes and seconds.<ref>{{Cite book|url=https://books.google.com/books?id=xuDDqqa8FlwC&q=A+history+of+mechanical+inventions,+Abbott+Payson+Usher|page=209|title=A history of mechanical inventions|first=Abbott Payson|last=Usher|publisher=Courier Dover Publications|year=1988|isbn=978-0-486-25593-4|access-date=October 30, 2020|archive-date=July 3, 2023|archive-url=https://web.archive.org/web/20230703113711/https://books.google.com/books?id=xuDDqqa8FlwC&q=A+history+of+mechanical+inventions,+Abbott+Payson+Usher|url-status=live}}</ref>
An early record of a seconds hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection.<ref name=Landes>{{cite book | first1=David S. |last1=Landes |title=Revolution in Time | location=Cambridge, Massachusetts| publisher= Harvard University Press |year= 1983 | isbn = 978-0-674-76802-4 |title-link=Revolution in Time }}</ref>{{rp|417–418}}<ref>{{cite book |first1=Johann |last1=Willsberger |title=Clocks & watches |location=New York |publisher=Dial Press |year=1975 |isbn=978-0-8037-4475-2 |url=https://archive.org/details/clockswatchessix0000will }} full page color photo: 4th caption page, 3rd photo thereafter (neither pages nor photos are numbered).</ref>

During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of [[Nuremberg]] and [[Augsburg]], and in [[Blois]], 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 nearest 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. The [[escapement#Cross-beat escapement|cross-beat escapement]] was invented in 1584 by [[Jost Bürgi]], who also developed the [[remontoire]]. Bürgi's clocks were a great improvement in accuracy as they were correct to within a minute a day.<ref>{{cite book |editor1=Lance Day |editor2=Ian McNeil |title=Biographical dictionary of the history of technology |publisher=[[Routledge]] (Routledge Reference) |year=1996 |page=116 |isbn=978-0-415-06042-4 |url=https://books.google.com/books?id=nqAOAAAAQAAJ&pg=PA116 |access-date=October 30, 2020 |archive-date=July 3, 2023 |archive-url=https://web.archive.org/web/20230703113711/https://books.google.com/books?id=nqAOAAAAQAAJ&pg=PA116 |url-status=live }}</ref><ref>{{citation |title=Table clock c. 1650 attributed to Hans Buschmann that uses technical inventions by Jost Bürgi |publisher=[[The British Museum]] |url=https://www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/t/table_clock_attributed_to_hans.aspx |access-date=2010-04-11 |archive-date=November 6, 2015 |archive-url=https://web.archive.org/web/20151106011107/http://www.britishmuseum.org/explore/highlights/highlight_objects/pe_mla/t/table_clock_attributed_to_hans.aspx |url-status=live }}</ref> These clocks helped the 16th-century astronomer [[Tycho Brahe]] to observe astronomical events with much greater precision than before.{{citation needed|date=November 2014}}{{how|date=November 2014}}

[[File:Renaissance Turret Clock.jpg|thumb|right|Lantern clock, German, {{c.|1570}}]]

=== Pendulum ===

{{Multiple image
| align             = left
| total_width       = 200
| footer            = The first pendulum clock, designed by Christiaan Huygens in 1656
| image1            = Huygens first pendulum clock - front view.png
| width1            = 120
| image2            = Huygens first pendulum clock.png
| width2            = 112
}}

The next development in accuracy occurred after 1656 with the invention of the [[pendulum clock]]. [[Galileo Galilei|Galileo]] 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 (about 99.4&nbsp;cm or 39.1&nbsp;inches for the one second movement) and had the first pendulum-driven clock made. The first model clock was built in 1657 in [[the Hague]], but it was in England that the idea was taken up.<ref>{{cite web|url=http://www.historyworld.net/wrldhis/PlainTextHistories.asp?groupid=2324&HistoryID=ac08&gtrack=pthc|title=History of Clocks|access-date=December 6, 2013|archive-date=December 10, 2013|archive-url=https://web.archive.org/web/20131210183625/http://www.historyworld.net/wrldhis/PlainTextHistories.asp?groupid=2324&HistoryID=ac08&gtrack=pthc|url-status=live}}</ref> The [[longcase clock]] (also known as the ''grandfather clock'') was created to house the pendulum and works by the English clockmaker William Clement in 1670 or 1671. It was also at this time that clock cases began to be made of wood and [[clock face]]s to use [[Vitreous enamel|enamel]] as well as hand-painted ceramics.

In 1670, William Clement created the [[anchor escapement]],<ref>{{cite web |url=http://inventors.about.com/library/weekly/aa072801a.htm |title=The History of Mechanical Pendulum Clocks and Quartz Clocks |work=about.com |year=2012 |access-date=16 June 2012 |archive-date=May 28, 2020 |archive-url=https://web.archive.org/web/20200528165701/https://www.thoughtco.com/history-of-mechanical-pendulum-clocks-4078405|url-status=unfit}}</ref> an improvement over Huygens' crown escapement. Clement also introduced the pendulum suspension spring in 1671. The concentric minute hand was added to the clock by [[Daniel Quare]], a London clockmaker and others, and the second hand was first introduced.

===Hairspring===
In 1675, Huygens and [[Robert Hooke]] invented the [[spiral balance|spiral balance spring]], or the hairspring, designed to control the oscillating speed of the [[balance wheel]]. This crucial advance finally made accurate pocket watches possible. The great English clockmaker [[Thomas Tompion]], was one of the first to use this mechanism successfully in his [[pocket watch]]es, and he adopted the minute hand which, after a variety of designs were trialled, eventually stabilised into the modern-day configuration.<ref>{{cite web|title=History Of Clocks|url=http://www.historyworld.net/wrldhis/PlainTextHistories.asp?groupid=2324&HistoryID=ac08&gtrack=pthc|access-date=December 6, 2013|archive-date=December 10, 2013|archive-url=https://web.archive.org/web/20131210183625/http://www.historyworld.net/wrldhis/PlainTextHistories.asp?groupid=2324&HistoryID=ac08&gtrack=pthc|url-status=live}}</ref> The rack and snail striking mechanism for [[striking clock]]s, was introduced during the 17th century and had distinct advantages over the 'countwheel' (or 'locking plate') mechanism. During the 20th century there was a common misconception that [[Edward Barlow (priest)|Edward Barlow]] invented ''[[rack and snail]]'' striking. In fact, his invention was connected with a repeating mechanism employing the rack and snail.<ref>Horological Journal, September 2011, pp. 408–412.</ref> The [[repeater (horology)|repeating clock]], that chimes the number of hours (or even minutes) on demand was invented by either Quare or Barlow in 1676. [[George Graham (clockmaker)|George Graham]] invented the [[Escapement#Deadbeat escapement|deadbeat escapement]] for clocks in 1720.

===Marine chronometer===

{{main|Marine chronometer}}

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. In 1714, the British government offered large [[longitude prize|financial rewards]] to the value of 20,000 pounds<ref name="Rigden2003">{{cite book|author=John S. Rigden|title=Hydrogen: The Essential Element|url=https://books.google.com/books?id=FhFxn_lUvz0C|year=2003|publisher=Harvard University Press|isbn=978-0-674-01252-3|page=185}}</ref> for anyone who could determine longitude accurately. [[John Harrison]], who dedicated his life to improving the accuracy of his clocks, later received considerable sums under the Longitude Act.

In 1735, Harrison built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship's pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat. The chronometer was tested in 1761 by Harrison's son and by the end of 10 weeks the clock was in error by less than 5 seconds.<ref name=gould>{{cite book|author=Gould, Rupert T.|author-link=Rupert Gould|title=The Marine Chronometer. Its History and Development|location=London|publisher=J.D. Potter|page=66|year=1923|isbn=978-0-907462-05-7}}</ref>

===Mass production===

The British had dominated watch manufacture for much of the 17th and 18th centuries, but maintained a system of production that was geared towards high quality products for the elite.<ref>{{cite book|url=https://books.google.com/books?id=cVUSauNST8EC&q=British+Watch+Company+mass+production|title=Manufacturing Time: Global Competition in the Watch Industry, 1795–2000|author=Glasmeier, Amy|year=2000|publisher=Guilford Press|access-date=2013-02-07|isbn=978-1-57230-589-2|archive-date=July 3, 2023|archive-url=https://web.archive.org/web/20230703113711/https://books.google.com/books?id=cVUSauNST8EC&q=British+Watch+Company+mass+production|url-status=live}}</ref> Although there was an attempt to modernise clock manufacture with mass-production techniques and the application of duplicating tools and machinery by the British Watch Company in 1843, it was in the United States that this system took off. In 1816, [[Eli Terry]] and some other Connecticut clockmakers developed a way of mass-producing clocks by using [[interchangeable parts]].<ref>"Eli Terry Mass-Produced Box Clock." Smithsonian The National Museum of American History. Web. 21 Sep. 2015.</ref> [[Aaron Lufkin Dennison]] started a factory in 1851 in [[Massachusetts]] that also used interchangeable parts, and by 1861 was running a successful enterprise incorporated as the [[Waltham Watch Company]].<ref name="Roe1916">{{citation | last = Roe | first = Joseph Wickham | title = English and American Tool Builders | publisher = Yale University Press | year = 1916 | location = New Haven, Connecticut | url = https://books.google.com/books?id=X-EJAAAAIAAJ | lccn = 16011753 | access-date = November 6, 2015 | archive-date = July 3, 2023 | archive-url = https://web.archive.org/web/20230703113712/https://books.google.com/books?id=X-EJAAAAIAAJ | url-status = live }}. Reprinted by McGraw-Hill, New York and London, 1926 ({{LCCN|27024075}}); and by Lindsay Publications, Inc., Bradley, Illinois, ({{ISBN|978-0-917914-73-7}}).</ref><ref>{{cite book |title=Structures of Change in the Mechanical Age: Technological Invention in the United States 1790–1865 |last=Thomson |first=Ross |year=2009 |publisher=The Johns Hopkins University Press |location=Baltimore, MD |isbn=978-0-8018-9141-0 |page=[https://archive.org/details/structuresofchan0000thom/page/34 34] |url=https://archive.org/details/structuresofchan0000thom/page/34 }}</ref>
{{clear}}

===Early electric===

{{main|Electric clock}}

[[File:Pendule electrique l maitrier 05117.jpg|thumb|upright|Early French electromagnetic clock]]

In 1815, the English scientist [[Francis Ronalds]] published the [[Electric clock#History|first electric clock]] powered by [[Voltaic pile#Dry pile|dry pile]] batteries.<ref>{{Cite book|title=Sir Francis Ronalds: Father of the Electric Telegraph|last=Ronalds|first=B.F.|publisher=Imperial College Press|year=2016|isbn=978-1-78326-917-4|location=London}}</ref> [[Alexander Bain (inventor)|Alexander Bain]], a Scottish clockmaker, patented the [[electric clock]] in 1840. The electric clock's mainspring is wound either with an electric motor or with an [[electromagnet]] and armature. In 1841, he first patented the [[Electromagnetism|electromagnetic]] pendulum. By the end of the nineteenth century, the advent of the dry cell battery made it feasible to use electric power in clocks. Spring or weight-driven clocks that use electricity, either [[alternating current]] (AC) or [[direct current]] (DC), to rewind the spring or raise the weight of a mechanical clock would be classified as an [[electromechanical clock]]. This classification would also apply to clocks that employ an electrical impulse to propel the pendulum. In electromechanical clocks, electricity serves no time-keeping function. These types of clocks were made as individual timepieces but are more commonly used in synchronized time installations in schools, businesses, factories, railroads and government facilities as a [[master clock]] and [[slave clocks]].

Where an [[Alternating current|AC]] electrical supply of stable frequency is available, timekeeping can be maintained very reliably by using a [[synchronous motor]], essentially counting the cycles. The supply current alternates with an accurate frequency of 50&nbsp;[[hertz]] in many countries, and 60&nbsp;hertz in others. While the frequency may vary slightly during the day as the load changes, generators are designed to maintain an accurate number of cycles over a day, so the clock may be a fraction of a second slow or fast at any time, but will be perfectly accurate over a long time. The [[Rotor (electric)|rotor]] of the motor rotates at a speed that is related to the alternation frequency. Appropriate gearing converts this rotation speed to the correct ones for the hands of the analog clock. Time in these cases is measured in several ways, such as by counting the cycles of the AC supply, vibration of a [[tuning fork]], the behaviour of [[quartz]] crystals, or the quantum vibrations of atoms. Electronic circuits divide these high-frequency oscillations into slower ones that drive the time display.

===Quartz===

[[File:Inside QuartzCrystal-Tuningfork.jpg|thumb|Picture of a quartz crystal resonator, used as the timekeeping component in quartz watches and clocks, with the case removed. It is formed in the shape of a tuning fork. Most such quartz clock crystals vibrate at a frequency of {{val|32768|u=Hz}}.]]

The [[piezoelectric]] properties of crystalline [[quartz]] were discovered by [[Jacques Curie|Jacques]] and [[Pierre Curie]] in 1880.<ref name=nistrevolution>{{cite web|url= http://physics.nist.gov/GenInt/Time/revol.html |title= A Revolution in Timekeeping |access-date= 30 April 2008 |publisher= NIST| archive-url = https://web.archive.org/web/20080409174853/http://physics.nist.gov/GenInt/Time/revol.html | archive-date = April 9, 2008}}</ref><ref>{{cite web |url=http://www.aip.org/history/curie/pierre.htm |title=Pierre Curie |access-date=8 April 2008 |publisher=[[American Institute of Physics]] |archive-date=February 16, 2015 |archive-url=https://web.archive.org/web/20150216035509/http://www.aip.org/history/curie/pierre.htm  }}</ref> The first crystal oscillator was invented in 1917 by [[Alexander M. Nicholson]], after which the first quartz crystal oscillator was built by [[Walter Guyton Cady|Walter G. Cady]] in 1921.<ref name=Marrison /> In 1927 the first [[quartz clock]] was built by Warren Marrison and J.W. Horton at [[Bell Telephone Laboratories]] in Canada.<ref name="Marrison2">{{Cite journal|last=Marrison|first=W.A.|author2=Horton, J.W. |title=Precision determination of frequency|journal=I.R.E. Proc.|volume=16|pages=137–154|date=February 1928|doi=10.1109/JRPROC.1928.221372|issue=2|s2cid=51664900}}</ref><ref name="Marrison" /> The following decades saw the development of quartz clocks as precision time measurement devices in laboratory settings—the bulky and delicate counting electronics, built with [[vacuum tube]]s at the time, limited their practical use elsewhere. The National Bureau of Standards (now [[NIST]]) based the time standard of the United States on quartz clocks from late 1929 until the 1960s, when it changed to atomic clocks.<ref name="Sullivan">{{cite web|last=Sullivan|first=D.B.|year=2001|title=Time and frequency measurement at NIST: The first 100&nbsp;years|publisher=Time and Frequency Division, National Institute of Standards and Technology|url=http://tf.nist.gov/timefreq/general/pdf/1485.pdf|page=5|archive-url=https://web.archive.org/web/20110927062444/http://tf.nist.gov/timefreq/general/pdf/1485.pdf|archive-date=September 27, 2011}}</ref> In 1969, [[Seiko]] produced the world's first quartz [[Watch|wristwatch]], the [[Astron (wristwatch)|Astron]].<ref>{{cite web | publisher = IEEE History Center | title = Electronic Quartz Wristwatch, 1969 | url = http://ethw.org/Milestones:Electronic_Quartz_Wristwatch,_1969 | access-date = 11 July 2015 | archive-date = January 22, 2016 | archive-url = https://web.archive.org/web/20160122104239/http://ethw.org/Milestones:Electronic_Quartz_Wristwatch,_1969 | url-status = live }}</ref> Their inherent accuracy and low cost of production resulted in the subsequent proliferation of quartz clocks and watches.<ref name=nistrevolution />

===Atomic===

Currently, [[atomic clock]]s are the most accurate clocks in existence. They are considerably more accurate than [[quartz clock]]s as they can be accurate to within a few seconds over trillions of years.<ref>{{Cite book|url=https://books.google.com/books?id=DNwfG5hQ7-YC|title=Sky and Ocean Joined: The U.S. Naval Observatory, 1830–2000|last=Dick|first=Stephen|page=484|publisher=[[Cambridge University Press]]|isbn=978-0-521-81599-4|year=2002|access-date=June 5, 2020|archive-date=July 3, 2023|archive-url=https://web.archive.org/web/20230703113712/https://books.google.com/books?id=DNwfG5hQ7-YC|url-status=live}}</ref><ref name="auto">{{cite journal |url=https://www.nist.gov/pml/div688/clock-082213.cfm |title=NIST Ytterbium Atomic Clocks Set Record for Stability |first=Laura |last=Ost |date=22 August 2013 |journal=NIST |access-date=30 June 2016 |archive-date=August 23, 2013 |archive-url=https://web.archive.org/web/20130823012832/http://www.nist.gov/pml/div688/clock-082213.cfm |url-status=live }}</ref> Atomic clocks were first theorized by [[Lord Kelvin]] in 1879.<ref>Sir William Thomson (Lord Kelvin) and Peter Guthrie Tait, ''Treatise on Natural Philosophy'', 2nd ed. (Cambridge, England: Cambridge University Press, 1879), vol. 1, part 1, [https://books.google.com/books?id=naXkAAAAMAAJ&dq=atoms&pg=PA227 p. 227] {{Webarchive|url=https://web.archive.org/web/20230404215123/https://books.google.com/books?id=naXkAAAAMAAJ&dq=atoms&pg=PA227 |date=April 4, 2023 }}.</ref> In the 1930s the development of [[Nuclear magnetic resonance|magnetic resonance]] created practical method for doing this.<ref name=Lombardi>{{Cite journal|author1=M.A. Lombardi |author2=T.P. Heavner |author3=S.R. Jefferts |year=2007|title=NIST Primary Frequency Standards and the Realization of the SI Second|url=http://tf.nist.gov/general/pdf/2039.pdf |archive-url=https://web.archive.org/web/20080424174326/http://tf.nist.gov/general/pdf/2039.pdf |archive-date=2008-04-24 |url-status=live|journal=Journal of Measurement Science|volume=2|issue=4|page=74}}</ref> A prototype [[ammonia]] [[maser]] device was built in 1949 at the U.S. [[National Bureau of Standards]] (NBS, now [[National Institute of Standards and Technology|NIST]]). Although it was less accurate than existing [[quartz clock]]s, it served to demonstrate the concept.<ref>{{cite conference|author=Sullivan, D.B.|year=2001|title=Time and frequency measurement at NIST: The first 100 years|url=http://tf.nist.gov/timefreq/general/pdf/1485.pdf|work=2001 IEEE International Frequency Control Symposium|pages=4–17|publisher=[[National Institute of Standards and Technology|NIST]]|archive-url=https://web.archive.org/web/20110927062444/http://tf.nist.gov/timefreq/general/pdf/1485.pdf|archive-date=September 27, 2011}}</ref><ref name="NISTshistorymeasuring">{{cite web|url=http://tf.nist.gov/general/museum/847history.htm|title=Time and Frequency Division|publisher=National Institute of Standards and Technology|access-date=1 April 2008|archive-url=https://web.archive.org/web/20080415135733/http://tf.nist.gov/general/museum/847history.htm|archive-date=April 15, 2008}}</ref><ref name=nistatomic>{{cite web|url=http://physics.nist.gov/GenInt/Time/atomic.html |title=The "Atomic Age" of Time Standards |access-date=2 May 2008 |publisher=National Institute of Standards and Technology| archive-url = https://web.archive.org/web/20080412040352/http://physics.nist.gov/GenInt/Time/atomic.html| archive-date = April 12, 2008}}</ref> The first accurate atomic clock, a [[caesium standard]] based on a certain transition of the [[caesium-133]] atom, was built by [[Louis Essen]] in 1955 at the [[National Physical Laboratory, UK|National Physical Laboratory]] in the UK.<ref>
{{Cite journal | last1 = Essen | first1 = L. | author-link1 = Louis Essen| last2 = Parry | first2 = J.V.L. | doi = 10.1038/176280a0  |bibcode=1955Natur.176..280E| title = An Atomic Standard of Frequency and Time Interval: A Cæsium Resonator | journal = Nature | volume = 176 | issue = 4476 | page = 280 | year = 1955 | s2cid = 4191481 }}</ref> Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ''[[ephemeris time]]'' (ET).<ref>
{{Cite journal|author1=W. Markowitz |author2=R.G. Hall |author3=L. Essen |author4=J.V.L. Parry |year=1958|title=Frequency of cesium in terms of ephemeris time|journal=[[Physical Review Letters]]|volume=1|issue=3 |pages=105–107|doi=10.1103/PhysRevLett.1.105|bibcode=1958PhRvL...1..105M}}</ref> As of 2013, the most stable atomic clocks are [[ytterbium]] clocks, which are stable to within less than two parts in 1 quintillion ({{val|2|e=-18}}).<ref name="auto"/>