Editing International Atomic Time (section)

== History ==
Early atomic time scales consisted of [[quartz clock]]s with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory, UK (NPL)]]. It was used as a basis for calibrating the quartz clocks at the [[Royal Greenwich Observatory]] and to establish a time scale, called Greenwich Atomic (GA). The [[United States Naval Observatory]] began the A.1 scale on 13 September 1956, using an [[Atomichron]] commercial atomic clock, followed by the NBS-A scale at the [[National Bureau of Standards]], [[Boulder, Colorado]] on 9 October 1957.{{sfn|McCarthy|Seidelmann|2009|pages=199–200}}

The [[International Time Bureau]] (BIH) began a time scale, T<sub>m</sub> or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of [[VLF]] radio signals. The BIH scale, A.1, and NBS-A were defined by an [[epoch]] at the beginning of 1958{{efn|They were set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding [[UT2]] instant. However, each observatory used its own value of UT2.{{sfn|Guinot|2000|page=181}}}} The procedures used by the BIH evolved, and the name for the time scale changed: ''A3'' in 1964<ref>{{cite web |last1=Allen |first1=Steve |title=The epoch of TAI is 1961-01-01T20:00:00 UT2 |url=https://www.ucolick.org/~sla/leapsecs/taiepoch.html |publisher=UCO/Lick Observatory |quote=By 1964 BIH realized that some atomic chronometers were much better than others, and they constructed A3 based on the best 3 |lang=en|access-date=21 January 2019|archive-date=10 October 2021|archive-url=https://web.archive.org/web/20211010014537/https://www.ucolick.org/~sla/leapsecs/taiepoch.html |url-status=live }}</ref> and ''TA(BIH)'' in 1969.{{sfn|McCarthy|Seidelmann|2009|pages=200–201}}

The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the [[General Conference on Weights and Measures]] and the [[International Committee for Weights and Measures]] made a series of decisions that designated the BIPM time scale International Atomic Time (TAI).{{sfn|McCarthy|Seidelmann|2009|pages=203–204}}

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to [[gravitational time dilation]], and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks. Starting from the Julian Date 2443144.5 (1 January 1977 00:00:00 TAI), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at the [[geoid]] ([[mean sea level]]).  Because the clocks were, on average, well above sea level, this meant that TAI slowed by about one part in a trillion. The former uncorrected time scale continues to be published under the name ''EAL'' (''Échelle Atomique Libre'', meaning ''Free Atomic Scale'').{{sfn|McCarthy|Seidelmann|2009|p=215}}

The instant that the gravitational correction started to be applied serves as the epoch for [[Barycentric Coordinate Time]] (TCB), [[Geocentric Coordinate Time]] (TCG), and [[Terrestrial Time]] (TT), which represent three fundamental time scales in the [[Solar System]].<ref>{{cite journal|last1=Brumberg|first1=V.A.|last2=Kopeikin|first2=S.M.|title=Relativistic time scales in the solar system|journal=[[Celestial Mechanics and Dynamical Astronomy]]|issn=0923-2958|volume=48|issue=1|pages=23–44|date=March 1990|doi=10.1007/BF00050674|bibcode=1990CeMDA..48...23B|s2cid=120112678}}</ref>  All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.{{efn|The 32.184 second offset is to provide continuity with the older [[ephemeris time]].}} TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184&nbsp;s.{{sfn|McCarthy|Seidelmann|2009|p=218&ndash;219}}

The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated, "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."<ref>{{cite web
 |title=CCTF 09-27 
 |url=http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf 
 |lang=en
 |date=3 September 2007
 |publisher=International Bureau of Weights and Measures 
 |access-date=24 September 2018
 |archive-url=https://web.archive.org/web/20120316100907/http://www.bipm.org/cc/CCTF/Allowed/18/CCTF_09-27_note_on_UTC-ITU-R.pdf 
 |archive-date=16 March 2012
 |url-status=dead 
}}</ref>