A fundamental celestial reference system is a human artifact laid down as a convention. It is merely a snapshot of the current astrometric capabilities employed for connecting observables and their mathematical representation. Accordingly, the associated reference frame and catalogue of celestial objects, which represents the system, are transient products adopted for some interval of time. Typical of this development of a system is the FK5 reference frame that materializes, in principle, a reference system of dynamical definition, and is based essentially on meridian circle astrometry. In 1998 this dynamical system was replaced by the International Celestial Reference System (ICRS), which is a kinematic system represented by the ICRF, the International Celestial Reference Frame, and the Hipparcos Catalogue is its optical realization (IAU 1999). Astrometrically, the ICRF is based on distant extragalactic radio sources that are observed from the Earth by means of VLBI.
KeywordsReference Frame Proper Motion International VLBI Service Hipparcos Catalogue Stellar Proper Motion
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