International Atomic Time

International Atomic Time (TAI, from the French name temps atomique international[1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.[2] It is a continuous scale of time, without leap seconds. It is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is also the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface. UTC deviates from TAI by a number of whole seconds. As of 1 January 2017, when another leap second was put into effect,[3] UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian days and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth.


TAI is a weighted average of the time kept by over 400 atomic clocks[4] in over 50 national laboratories worldwide.[5] The majority of the clocks involved are caesium clocks; the International System of Units (SI) definition of the second is based on caesium.[6] The clocks are compared using GPS signals and two-way satellite time and frequency transfer.[7] Due to the signal averaging TAI is an order of magnitude more stable than its best constituent clock.

The participating institutions each broadcast, in real time, a frequency signal with timecodes, which is their estimate of TAI. Time codes are usually published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form UTC(NPL) in the UTC form, where NPL identifies the National Physical Laboratory, UK. The TAI form may be denoted TAI(NPL). The latter is not to be confused with TA(NPL), which denotes an independent atomic time scale, not synchronised to TAI or to anything else.

The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible.[5] This combined time scale is published monthly in "Circular T",[7] and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC − UTC(k) (equivalent to TAI − TAI(k)) for each participating institution k. The same circular also gives tables of TAI − TA(k), for the various unsynchronised atomic time scales.

Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T, the TAI scale is not revised. In hindsight, it is possible to discover errors in TAI and to make better estimates of the true proper time scale. Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT).


Early atomic time scales consisted of quartz clocks 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, 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.[8]

The International Time Bureau (BIH) began a time scale, Tm 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[a] The procedures used by the BIH evolved, and the name for the time scale changed: "A3" in 1964[10] and "TA(BIH)" in 1969.[11]

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 which designated the BIPM time scale International Atomic Time (TAI).[12]

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), 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).[13]

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.[14] All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.[b] TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.[15]

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."[16]

Relation to UTC

UTC is a discontinuous time scale. It is occasionally adjusted by leap seconds. Between these adjustments, it is composed of segments that are mapped to atomic time. From its beginning in 1961 through December 1971, the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2. Afterward, these adjustments were made only in whole seconds to approximate UT1. This was a compromise arrangement in order to enable a publicly broadcast time scale. The less frequent whole-second adjustments meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by the public broadcast of UTC.[17]

See also

  • Clock synchronization
  • Time and frequency transfer


  1. ^ 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.[9]
  2. ^ The 32.184 second offset is to provide continuity with the older ephemeris time.


  • "History of TAI−UTC". Time Service Dept., United States Naval Observatory. 2009. Archived from the original on 19 October 2019. Retrieved 4 January 2010.
  • "International Atomic Time". International Bureau of Weights and Measures. Archived from the original on 10 January 2011. Retrieved 24 January 2020.


  1. ^ Temps atomique 1975
  2. ^ Guinot, B. (1986). "Is the International Atomic Time TAI a coordinate time or a proper time?". Celestial Mechanics. 38 (2): 155–161. Bibcode:1986CeMec..38..155G. doi:10.1007/BF01230427. S2CID 120564915.
  3. ^ Bizouard, Christian (6 July 2016). "Bulletin C 52". Paris: IERS. Archived from the original on 13 August 2017. Retrieved 31 December 2016.
  4. ^ "Bureau International des Poids et Mesures (BIPM) Time Department" (PDF). Report of the International Association of Geodesy 2011-2013. Archived (PDF) from the original on 12 April 2017. Retrieved 11 April 2017.
  5. ^ a b "Time". International Bureau of Weights and Measures. Archived from the original on 4 November 2013. Retrieved 22 May 2013.
  6. ^ McCarthy & Seidelmann 2009, p. 207, 214.
  7. ^ a b Circular T, International Bureau of Weights and Measures, retrieved 5 September 2017
  8. ^ McCarthy & Seidelmann 2009, pp. 199–200.
  9. ^ Guinot 2000, p. 181.
  10. ^ Allen, Steve. "The epoch of TAI is 1961-01-01T20:00:00 UT2". UCO/Lick Observatory. Archived from the original on 10 October 2021. Retrieved 21 January 2019. By 1964 BIH realized that some atomic chronometers were much better than others, and they constructed A3 based on the best 3
  11. ^ McCarthy & Seidelmann 2009, pp. 200–201.
  12. ^ McCarthy & Seidelmann 2009, pp. 203–204.
  13. ^ McCarthy & Seidelmann 2009, p. 215.
  14. ^ Brumberg, V.A.; Kopeikin, S.M. (March 1990). "Relativistic time scales in the solar system". Celestial Mechanics and Dynamical Astronomy. 48 (1): 23–44. Bibcode:1990CeMDA..48...23B. doi:10.1007/BF00050674. ISSN 0923-2958. S2CID 120112678.
  15. ^ McCarthy & Seidelmann 2009, p. 218–219.
  16. ^ "CCTF 09-27" (PDF). International Bureau of Weights and Measures. 3 September 2007. Archived (PDF) from the original on 16 March 2012. Retrieved 24 September 2016.
  17. ^ McCarthy & Seidelmann 2009, p. 227–229.


  • Guinot, B (2000). "History of the Bureau International de l'Heure". Polar Motion: Historical and Scientific Problems. ASP Conference Series, Vol. 208. Bibcode:2000pmhs.conf..175G. ISBN 1-58381-039-0.
  • McCarthy, Dennis D.; Seidelmann, Kenneth P. (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 978-3-527-40780-4. Archived from the original on 3 February 2021. Retrieved 12 June 2021.

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