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International Atomic Time (TAI, from the French name フランス語:''Temps Atomique International''〔Temps atomique 1975〕) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface, and for Terrestrial Time, which is used for astronomical calculations. when the last leap second was added, TAI is exactly 36 seconds ahead of UTC. The 36 seconds results from the initial difference of 10 seconds at the start of 1972, plus 26 leap seconds in UTC since 1972. Time coordinates on the TAI scales are conventionally specified 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 at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth. ==Operation== TAI as a time scale is a weighted average of the time kept by over 400 atomic clocks in over 50 national laboratories worldwide. The clocks are compared using GPS signals and two-way satellite time and frequency transfer.〔Circular T 2009.〕 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.〔McCarthy &Seidelmann 2009, 207, 214〕 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 (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible.〔"Time" n.d.〕 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 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. Doing so does not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「International Atomic Time」の詳細全文を読む スポンサード リンク
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