Atomic Time
From the Systeme Internationale (SI) comes the second as defined by the duration of 9 192 631 770 cycles of a particular hyperfine structure transition in the ground state of 133Cesium. For practical usability a device is required that attempts to produce the SI second (s) such as an atomic clock. But not all such clocks agree. The weighted mean of many clocks distributed over the whole Earth defines the Temps Atomique International; i.e., the Atomic Time TAI. From the General theory of relativity the time measured depends on the altitude on Earth and the spatial velocity of the clock so that TAI refers to a location on sea level that rotates with the Earth.
Read more about this topic: Theoretical Astronomy, Theory of Astronomical Time Keeping
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