International Atomic Time

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Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. It is a weighted average of the time kept by about 300 atomic clocks (including a large number of caesium atomic clocks) in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1, 1972, as decided by the 14th General Conference on Weights and Measures (CGPM). The International Bureau of Weights and Measures is in charge of the realization of TAI.

The highest precision realization of TAI times can only be determined retrospectively, as the timescale is defined by periodic comparisons among its participating atomic clocks. However, these corrections are usually only needed for applications that require nanosecond-scale accuracy. Most time service users use realtime estimates of TAI provided by atomic clocks that have been previously referenced to the composite timescale. GPS is a commonly-used realtime source of time traceable back to TAI.

Coordinated Universal Time (UTC) is the basis for legal time throughout much of the world, and always differs from TAI by an integral number of seconds. From 1 January 2006, UTC was behind TAI by 33 seconds. The difference is due to an initial ten second offset on 1 January 1972 when UTC was established and leap seconds, which have been periodically inserted into UTC since the first on 30 June 1972 due to slight irregularities in Earth's rate of rotation. While TAI is a continuous and stable timescale, UTC has intentional discontinuities to keep it from drifting more than 0.9 second from UT1, a timescale defined by the Earth's rotation. Roughly speaking, solar noon (the time at which the sun is directly overhead) would drift away from 12:00:00 without leap second corrections. UT1 is computed by the International Earth Rotation and Reference Systems Service (IERS). TAI was defined such that TAI = UT2 on January 1 1958.

Because UTC is a discontinuous timescale, it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval. Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead. TAI is also commonly used by systems that can not handle leap seconds.

See also

• Terrestrial Time
• Coordinated Universal Time
• Universal Time
• Sidereal Time
• Time and frequency transfer
• Clock synchronization
• Network Time Protocol

External links

• Bureau International des Poids et Mesures
• IERS website
• NIST Time and Frequency FAQsde:Internationale Atomzeit

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