Editing Clock (section)

===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"/>