Editing Caesium (section)

===Atomic clocks===
[[File:Usno-mc.jpg|thumb|Atomic clock ensemble at the U.S. Naval Observatory|alt=A room with a black box in the foreground and six control cabinets with space for five to six racks each. Most, but not all, of the cabinets are filled with white boxes.]]
Caesium-based [[atomic clock]]s use the [[electromagnetic radiation|electromagnetic transitions]] in the [[hyperfine structure]] of caesium-133 atoms as a reference point. The first accurate caesium clock was built by [[Louis Essen]] in 1955 at the [[National Physical Laboratory, UK|National Physical Laboratory]] in the UK.<ref>{{cite journal |first1=L. |last1=Essen |first2=J. V. L. |last2=Parry |date=1955 |title=An Atomic Standard of Frequency and Time Interval: A Caesium Resonator |journal=[[Nature (journal) |Nature]] |volume=176 |pages=280–282 |doi=10.1038/176280a0 |bibcode=1955Natur.176..280E |issue=4476 |s2cid=4191481}}</ref> Caesium clocks have improved over the past half-century and are regarded as "the most accurate realization of a unit that mankind has yet achieved."<ref name="USNO"/> These clocks measure frequency with an error of 2 to 3&nbsp;parts in 10<sup>14</sup>, which corresponds to an accuracy of 2&nbsp;[[nanosecond]]s per day, or one second in 1.4&nbsp;million&nbsp;years. The latest versions are more accurate than 1 part in 10<sup>15</sup>, about 1 second in 20&nbsp;million years.<ref name="USGS"/> The [[caesium standard]] is the primary standard for standards-compliant time and frequency measurements.<ref>{{cite journal |last1=Markowitz |first1=W. |last2=Hall |first2=R. |last3=Essen |first3=L. |last4=Parry |first4=J. |title=Frequency of Cesium in Terms of Ephemeris Time |doi=10.1103/PhysRevLett.1.105 |journal=Physical Review Letters |volume=1 |issue=3 |pages=105–107 |year=1958 |bibcode=1958PhRvL...1..105M}}</ref> Caesium clocks regulate the timing of cell phone networks and the Internet.<ref>{{cite news |first=Monte |last=Reel |date=22 July 2003 |title=Where timing truly is everything |newspaper=The Washington Post |page=B1 |url=http://www.highbeam.com/doc/1P2-284155.html |access-date=26 January 2010 |archive-url=https://web.archive.org/web/20130429044454/http://www.highbeam.com/doc/1P2-284155.html |archive-date=29 April 2013 |url-status=dead}}</ref>

====Definition of the second====
The second, symbol ''s'', is the SI unit of time. The [[BIPM]] restated its definition at its 26th conference in 2018: "[The second] is defined by taking the fixed numerical value of the caesium frequency {{math|Δ''ν''<sub>Cs</sub>}}, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be {{val|9192631770}} when expressed in the unit [[Hz]], which is equal to s<sup>−1</sup>."<ref>{{cite web |title=Resolution 1 of the 26th CGPM |url=https://www.bipm.org/en/CGPM/db/26/1/ |publisher=Bureau International des Poids et Mesures |location=Paris |pages=472 of the official French publication |language=FR,EN |date=2018 |access-date=2019-12-29 |archive-date=2021-02-04 |archive-url=https://web.archive.org/web/20210204120336/https://www.bipm.org/en/CGPM/db/26/1/ |url-status=dead }}</ref>