Atomic Time Scales and Their Applications in Astronomy

Abstract

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.

For practical reasons the time scale which has been recommended as the world’s time reference is Coordinated Universal Time (UTC), derived from TAI by applying a procedure defined by the International Telecommunication Union (ITU). The institutes and observatories contributing clock data to the BIPM maintain local representations of UTC, indicated by UTC(k), and provide time for all sorts of applications, including those linked to Earth and space science.

For some applications in astronomy, long-term stability is a requisite and cannot be satisfied by the “quasi-real-time TAI.” For these users, the BIPM computes yearly a representation of terrestrial time, TT(BIPM), supported by all the primary frequency standards submitted measurements.

This paper will describe the characteristics of the atomic time scales and present the link between international time metrology and the astronomical science.