Space–Time Reference Frames

An essential part of astrophysical observation and data analysis consists in measuring the positions of astronomical objects and in dating events. Obtaining the photometric magnitudes of the emitted radiation is not enough. The source must be identified in catalogues or data bases, and the position must be determined to great accuracy, since spatial resolutions of the order of several tenths of a millisecond of arc can now be attained in images, and this trend is certain to continue. Concerning the absolution positioning accuracies of celestial objects, over the past 20 years or so, these have gone from a few hundredths of a second of arc before the Hipparcos satellite was launched to about ten microseconds of arc with the Gaia mission. In terms of sensitivity, detectors can now measure the characteristics of phenomena whose time of variation is less than one millisecond. And further, extremely accurate time scales, taking into account the effects of general relativity, are required for space travel and exploration of specific sites in the Solar System.