Atomic Time Scales and Their Applications in Astronomy

The unit of time is defined as a multiple of the period of the hyperfine transition of the atom of cesium 133 and realized at the level of a few parts in 10¹⁶ by about a dozen cesium fountains maintained in national metrology institutes. International Atomic Time (TAI) takes its accuracy from the primary frequency standards, but preserves its high stability over intervals of 1 month making use of the largest industrial clock ensemble in the world. Seventy-four institutes worldwide spread or disseminate atomic time for different applications; the International Bureau of Weights and Measures (BIPM) collects their data and integrates them in an algorithm which produces TAI.