International Atomic Time
0International Atomic Time, by Wikipedia https://en.wikipedia.org/wiki?curid=334 / CC BY SA 3.0
#Time_scales
International Atomic Time （TAI, from the French name temps atomique international） is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.
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, 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 in over 50 national laboratories worldwide.
The majority of the clocks involved are caesium clocks; the International System of Units （SI） definition of the second is based on caesium.
The clocks are compared using GPS signals and two-way satellite time and frequency transfer.
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...
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