International Celestial Reference System
The International Celestial Reference System is the current standard celestial reference system adopted by the International Astronomical Union. Its origin is at the barycenter of the Solar System, with axes that are intended to be "fixed" with respect to space. ICRS coordinates are approximately the same as equatorial coordinates: the mean pole at J2000.0 in the ICRS lies at 17.3±0.2 mas in the direction 12 h and 5.1±0.2 mas in the direction 18 h. The mean equinox of J2000.0 is shifted from the ICRS right ascension origin by 78±10 mas.
The defining extragalactic reference frame of the ICRS is the International Celestial Reference Frame based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry. The positions of most are known to 0.001 arcsecond or better. At optical wavelengths, the ICRS is currently realized by the Hipparcos Celestial Reference Frame, a subset of about 100,000 stars in the Hipparcos Catalogue. A more accurate optical realization of the ICRS, based on the observation by the Gaia spacecraft of almost 500,000 extragalactic objects believed to be quasars, is under preparation.
