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International Celestial Reference Frame


The International Celestial Reference Frame (ICRF) is a quasi-inertial frame of reference centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources (mainly quasars). Although general relativity implies that there are no true inertial frames around gravitating bodies, the ICRF is important because it definitely does not exhibit any measurable angular motion since the extragalactic sources used to define the ICRF are so far away. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects. It has been adopted by International Astronomical Union since 1 January 1998. ICRF had a noise floor of approximately 250 microarcseconds (µas) and an axis stability of approximately 20 µas; this was an order-of-magnitude improvement over the previous Fifth Fundamental Catalog (FK5).

The ICRF also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue.

Note that, in astrometry, a reference frame is the physical realization of a reference system, i.e. the reference frame is the reported coordinates of data points. The ICRF is the realization of the International Celestial Reference System (ICRS), and agrees with the orientation of the Fifth Fundamental Catalog (FK5) "J2000.0" frame to within the (lower) precision of the latter.


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