International Atomic Time (TAI, from the French name Temps Atomique International) is a highprecision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. It is the principal realisation of Terrestrial Time, and the basis for Coordinated Universal Time (UTC) which is used for civil timekeeping all over the Earth's surface. As of 1 January 2010 (2010 0101)^{[update]}, 00:00:00 UTC, TAI was exactly 34 seconds ahead of UTC (this is the case since 1 January 2009): an initial difference of 10 seconds at the start of 1972, plus 24 leap seconds in UTC since 1972; the last leap second was added on 31 December 2008.^{[1]}
Time coordinates on the TAI scales are conventionally specified using traditional means of specifying days, carried over from nonuniform 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 at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth.
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Operation
TAI as a time scale is a weighted average of the time kept by over 200 atomic clocks in about 70 national laboratories worldwide. The clocks are compared using satellites.^{[2]} Due to the averaging it is far more stable than any clock would be alone (see signal averaging for a discussion). The majority of the clocks are caesium clocks; the definition of the SI second is written in terms of caesium.^{[3]}
The participating institutions each broadcast, in real time, a frequency signal with time codes, which is their estimate of TAI. Time codes are usually published in the form of UTC. These time scales are denoted in the form TAI(NPL) (UTC(NPL) for the UTC form), where NPL in this case identifies the National Physical Laboratory, UK.
The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM) combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible. This combined time scale is published monthly in Circular T, and is the canonical TAI. This time scale is expressed in the form of tables of differences UTCUTC(x) and TAITA(x), for each participating institution x.
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