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International Atomic Time
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 the principal realisation of Terrestrial Time (except for 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. As of 31 December 2016[update] when another leap second was added,[3] TAI is exactly 37 seconds ahead of UTC. The 37 seconds results 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 Dates and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time
Universal Time
at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth.

Contents

1 Operation 2 History 3 Relation to UTC 4 See also 5 Notes 6 References 7 Bibliography 8 External links

Operation TAI as a time scale is a weighted average of the time kept by over 400 atomic clocks[4] in over 50 national laboratories worldwide.[5] The clocks are compared using GPS signals and two-way satellite time and frequency transfer.[6] Due to the signal averaging it is an order of magnitude more stable than its best clock alone would be. The majority of the clocks are caesium clocks; the International System of Units (SI) definition of the second is based on caesium.[7] 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 in this case 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
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",[6] 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). History 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
National Physical Laboratory, UK
(NPL). The "Greenwich Atomic" (GA) scale began in 1955 at the Royal Greenwich Observatory.[citation needed] 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
VLF
radio signals. The United States Naval Observatory began the A.1 scale 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado. Both the BIH scale and A.1 were defined by an epoch at the beginning of 1958[8] The procedures used by the BIH evolved, and the name for the time scale changed: "A3" in 1963 and "TA(BIH)" in 1969.[9] This synchronisation was inevitably imperfect, depending as it did on the astronomical realisation of UT2. At the time, UT2
UT2
as published by various observatories differed by several hundredths of a second. The SI second was defined in terms of the caesium atom in 1967, and in 1971 the name International Atomic Time
Time
(TAI) was assigned to a time scale based on SI seconds with no leap seconds.[10] During this time, irregularities in the atomic time were detected and corrected. In 1967 it was suggested that nearby masses caused clocks to operate at different rates, but this was disproven in 1968.[11] 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 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 mean sea level (the geoid). Because the clocks were, on average, well above sea level, this meant that TAI slowed down, by about one part in a trillion. The former uncorrected time scale continues to be published, under the name EAL (Echelle Atomique Libre, meaning Free Atomic Scale).[12] The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time
Time
(TCG), and Terrestrial Time (TT), which represent three fundamental time scales in the solar system.[13] All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.[14] 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 (i.e. regularly adjusted by leap seconds) time scale composed from segments that are linear transformations of atomic time. From its beginning in 1961 through December 1971 the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2. Afterwards 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 post-1971 more linear transformation of the BIH's atomic time meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1
UT1
means that tasks such as navigation which require a source of Universal Time
Universal Time
continue to be well served by the public broadcast of UTC.[17] See also

Clock
Clock
synchronization Network Time
Time
Protocol Precision Time
Time
Protocol Time
Time
and frequency transfer

Notes

^ Temps atomique 1975 ^ "Is the International Atomic Time
Time
TAI a coordinate time or a proper time?". Adsabs.harvard.edu. Retrieved 8 May 2013.  ^ Bizouard, Christian (6 July 2016). "Bulletin C 52". Paris: IERS. Retrieved 31 December 2016.  ^ "Bureau International des Poids et Mesures (BIPM) Time
Time
Department" (PDF). Report of the International Association of Geodesy 2011-2013. Retrieved 11 April 2017.  ^ a b "Time". International Bureau of Weights and Measures. Retrieved 22 May 2013.  ^ a b Circular T, International Bureau of Weights and Measures, retrieved 2017-09-05  ^ McCarthy & Seidelmann 2009, p. 207, 214. ^ It was set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2
UT2
instant. ^ McCarthy & Seidelmann 2009, p. 199–201. ^ McCarthy & Seidelmann 2009, p. 202–204. ^ William Markowitz. "Nondependence of Frequency on Mass: A Differential Experiment" doi:10.1126/science.162.3860.1387 ^ McCarthy & Seidelmann 2009, p. 215. ^ 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.  ^ The offset is to provide continuity with the older Ephemeris Time. ^ McCarthy & Seidelmann 2009, p. 218–219. ^ *"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.  ^ McCarthy & Seidelmann 2009, p. 227–229.

References

" History
History
of TAI−UTC". Time
Time
Service Dept., United States Naval Observatory. 2009. Retrieved 4 January 2010.  "International Atomic Time". International Bureau of Weights and Measures. Retrieved 22 February 2013. 

Bibliography

McCarthy, Dennis D.; Seidelmann, Kenneth P. (2009). Time: From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA. p. 368. ISBN 978-3-527-40780-4. 

External links

Bureau International des Poids et Mesures: TAI Time
Time
and Frequency Section - National Physical Laboratory, UK IERS
IERS
website NIST Web Clock
Clock
FAQs History
History
of time scales NIST-F1 Cesium Fountain Atomic Clock "Optical frequency comb for metrology and timekeeping". Archived from the original on 25 January 2009.  Japan Standard Time
Time
Project, NICT, Japan Time
Time
Dissemation Services (PDF), Bureau International des Poids et Mesures  Standard of time definition: UTC, GPS, LORAN and TAI

v t e

Time

Key concepts

Past

history deep time

Present Future Futures studies Far future in religion Far future in science fiction and popular culture Timeline
Timeline
of the far future Eternity Eternity
Eternity
of the world

Measurement and standards

Chronometry

UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Season Year Decade Century Millennium Tropical year Sidereal year Samvatsara

Measurement systems

Time
Time
zone Six-hour clock 12-hour clock 24-hour clock Daylight saving time Solar time Sidereal time Metric time Decimal time Hexadecimal time

Calendars

Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Horology History
History
of timekeeping devices Main types

astrarium atomic

quantum

marine sundial sundial markup schema watch water-based

Chronology History

Astronomical chronology Big History Calendar
Calendar
era Chronicle Deep time Periodization Regnal year Timeline

Religion Mythology

Dreamtime Kāla Kalachakra Prophecy Time
Time
and fate deities Wheel of time Immortality

Philosophy of time

A-series and B-series B-theory of time Causality Duration Endurantism Eternal return Eternalism Event Multiple time dimensions Perdurantism Presentism Static interpretation of time Temporal finitism Temporal parts The Unreality of Time

Human experience and use of time

Accounting period Chronemics Fiscal year Generation time Mental chronometry Music Procrastination Punctuality Temporal database Term Time
Time
discipline Time
Time
management Time
Time
perception

Specious present

Time-tracking software Time-use research Time-based currency
Time-based currency
(time banking) Time
Time
value of money Time
Time
clock Timesheet Yesterday – Today – Tomorrow

Time
Time
in

Geology

Geological time

age chron eon epoch era period

Geochronology Geological history of Earth

Physics

Absolute time and space Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time
Time
dilation

gravitational

Time
Time
domain Time
Time
translation symmetry Time
Time
reversal symmetry

other subject areas

Chronological dating Chronobiology Circadian rhythms Dating methodologies in archaeology Time
Time
geography

Related topics

Carpe diem Clock
Clock
position Space System time Tempus fugit Time
Time
capsule Time
Time
complexity Time
Time
signature Time
Time
travel

Time
Time
portal Category

v t e

Time
Time
measurement and standards

Chronometry Orders of magnitude Metrology

International standards

Coordinated Universal Time

offset

UT ΔT DUT1 International Earth
Earth
Rotation and Reference Systems Service ISO 31-1 ISO 8601 International Atomic Time 6-hour clock 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second Solar time Terrestrial Time Time
Time
zone 180th meridian

Obsolete standards

Ephemeris time Greenwich Mean Time Prime meridian

Time
Time
in physics

Absolute time and space Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Planck time Proper time Theory of relativity Time
Time
dilation Gravitational time dilation Time
Time
domain Time
Time
translation symmetry T-symmetry

Horology

Clock Astrarium Atomic clock Complication History
History
of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History
History
of sundials Sundial
Sundial
markup schema

Calendar

Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology

Chronological dating Geologic time scale International Commission on Stratigraphy

Astronomical chronology

Galactic year Nuclear timescale Precession Sidereal time

Other units of time

Flick Shake Jiffy Second Minute Moment Hour Day Week Fortnight Month Year Olympiad Lustrum Decade Century Saeculum Millennium

Related topics

Chronology Duration

music

Mental chronometry Metric time System time Time
Time
value o

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