Space-Time Reference Frames
An essential part of astrophysical observation consists in measuring the positions of astronomical objects and in dating events. Obtaining the photometric magnitudes of the emitted radiation is not enough; the position of its source must also be determined to great accuracy, for spatial resolutions of the order of several tenths of a millisecond of arc can now be attained, and this tendency is certain to continue (cf. Chap. 4). 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 are required for space travel and exploration of specific sites in the solar system.
KeywordsProper Motion Practical Realisation Spatial Frame Julian Date Atomic Time
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