[Etymology: Temps Atomique International, ‘International Atomic Time’] time A scheme for measuring time with a fixed-size second derived from atomic clocks, rather than the variable natural day or other astronomical period, and the underlying reference for Universal Time (UT) and for the special scheme used for astronomical observations, Terrestrial Time. In TAI every day has exactly 86 400 seconds, so it does not stay in exact step with the real world, i.e. with the rotational realities of Earth; UT is an adjusted form addressing that problem by allowing leap seconds, so does not always have 86 400 seconds in a day.
Modern precision demands a statistical derivation from a multiplicity of atomic clocks, coordinated through the US Naval Observatory.
| 1970 | CIPM: ‘International Atomic Time (TAI) is the time reference coordinate established by the Bureau International de l'Heure on the basis of the readings of atomic clocks operating in various establishments in accordance with the definition of the second, the unit of time of the International System of Units.’ |
| 1971 | 14th CGPM: ‘considering that the second, unit of time of the International System of Units, has since 1967 been defined in terms of a natural atomic frequency, and no longer in terms of the time scales provided by astronomical motions, that the need for an International Atomic Time (TAI) scale is a consequence of the atomic definition of the second, that several international organizations have ensured and are still successfully ensuring the establishment of the time scales based on astronomical motions, particularly thanks to the permanent services of the Bureau International de l'Heure (BIH)… that the BIH has started to establish an atomic time scale of recognized quality and proven usefulness, that the atomic frequency standards for realizing the second have been considered and must continue to be considered by the CIPM helped by a Consultative Committee, and that the unit interval of the International Atomic Time scale must be the second realized according to its atomic definition, that all the competent international scientific organizations and the national laboratories active in this field have expressed the wish that the CIPM and CGPM should give a definition of International Atomic Time, and should contribute to the establishment of the International Atomic Time scale, that the usefulness of International Atomic Time entails close coordination with scales based on astronomical motions, requests the CIPM |
| 1. to give a definition of International Atomic Time [already done, see above], |
| 2. to take the necessary steps, in agreement with the international organizations concerned, to ensure that the available scientific competence and existing facilities are used in the best possible way to realize the International Atomic Time scale and to satisfy the requirements of users of International Atomic Time.’see note below |
[
Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)]
