Editing Time standard (section)

== Definitions of the second ==
{{Main|Second#History of definition}}

There have only ever been three definitions of the second: as a fraction of the day, as a fraction of an extrapolated year, and as the microwave frequency of a caesium atomic clock.<ref>{{cite web |author1=U.S. Naval Observatory |title=Leap Seconds |url=https://tycho.usno.navy.mil/leapsec.html |archive-url=https://web.archive.org/web/20191019051714/https://tycho.usno.navy.mil/leapsec.html |access-date=19 October 2019|archive-date=2019-10-19 }}</ref>

In early history, clocks were not accurate enough to track seconds. After the invention of mechanical clocks, the [[CGS|CGS system]] and [[MKS system of units]] both defined the second as {{frac|86,400}} of a [[mean solar day]]. MKS was adopted internationally during the 1940s.

In the late 1940s, quartz crystal oscillator clocks could measure time more accurately than the rotation of the Earth. [[Time metrology|Metrologists]] also knew that Earth's orbit around the Sun (a year) was much more stable than Earth's rotation. This led to the definition of [[ephemeris time]] and the [[tropical year]], and the ephemeris second was defined as "the fraction {{frac|31,556,925.9747}} of the tropical year for 1900 [[January 0]] at 12 hours ephemeris time".<ref>''Whitaker's Almanac 2013'' (ed. Ruth Northey), London 2012, p. 1131, {{isbn|978-1-4081-7207-0}}.</ref><ref name="USNO">{{cite web
 | title=Leap Seconds
 | publisher=Time Service Department, [[United States Naval Observatory]]
 | url=http://tycho.usno.navy.mil/leapsec.html
 | access-date=November 22, 2015
 | archive-url=https://web.archive.org/web/20150312003149/http://tycho.usno.navy.mil/leapsec.html
 | archive-date=March 12, 2015
 | url-status=dead
 }}</ref> This definition was adopted as part of the [[International System of Units]] in 1960.<ref>{{cite web
 |title=SI Brochure (2006)
 |work=SI Brochure 8th Edition
 |url=https://www.bipm.org/utils/common/pdf/si_brochure_8.pdf
 |page=112
 |publisher=[[BIPM]]
 |access-date=May 23, 2019
 |archive-url=https://web.archive.org/web/20190503133741/https://www1.bipm.org/utils/common/pdf/si_brochure_8.pdf
 |archive-date=May 3, 2019
 |url-status=live
 }}</ref>

Most recently, atomic clocks have been developed that offer improved accuracy. Since 1967, the [[SI base unit]] for time is the [[SI]] second, defined as exactly "the duration of 9,192,631,770 [[frequency|periods]] of the radiation corresponding to the transition between the two [[Hyperfine structure|hyperfine levels]] of the ground state of the [[caesium-133]] atom" (at a temperature of [[Absolute zero|0&nbsp;K]] and at mean [[sea level]]).<ref>{{cite book |last1=McCarthy |first1=Dennis D. |author-link1=Dennis McCarthy (scientist) |last2=Seidelmann |first2=P. Kenneth |title=Time: From Earth Rotation to Atomic Physics |pages=231&ndash;232 |year=2009 |location=Weinheim |publisher=Wiley}}</ref><ref name="second">{{cite web |title=Base unit definitions: Second |url=http://physics.nist.gov/cuu/Units/second.html |publisher=[[NIST]] |access-date=9 April 2011 |url-status=live |archive-url=https://web.archive.org/web/20110417135428/http://physics.nist.gov/cuu/Units/second.html |archive-date=17 April 2011}}</ref> The SI second is the basis of all atomic timescales, e.g. coordinated universal time, GPS time, International Atomic Time, etc.