The generation of time scales left the astronomical community more than 40 years ago when, in 1967, the second became defined by an atomic transition in the International System of units SI.

But time still interacts with astronomy in many ways: as the independent variable for the description of all dynamical systems, its stability allows one to study these systems and their perturbations (see Earth rotation, ephemerides, pulsar timing, binary pulsar ...); also many observation techniques actually rely on the measurement of the time of propagation of electromagnetic signals; then general relativity theory yields a consistent picture of the astronomical systems under study; finally atomic time scales and modern synchronization methods enable all astronomical events to be dated with adequate uncertainty.

These goals are summarized in our Commission's Terms of Reference