2. The International Celestial Reference System (ICRS)
The celestial reference system of the International Earth Rotation Service (IERS, Arias et al. 1995) was adopted since January 1998 by the International Astronomical Union (IAU) as the conventional celestial reference system under the name International Celestial Reference System (ICRS). The ICRS follows the IAU recommendations (Bergeron 1992) on the definition of the system. The origin of the system is at the barycentre of the Solar System; the principal plane is close to the mean equator at J2000.0. The shift of the Earth's mean pole at J2000.0 relative to the ICRS celestial pole is in the direction 12h and in the direction 18h. As required by the IAU, and for the sake of continuity with the previous conventional system, the direction of the ICRS pole is consistent with that of the FK5 system within the uncertainty of the latter; assuming that the error in the precession rate is absorbed by the proper motions of stars, the uncertainty of the FK5 pole position relative to the mean pole at J2000.0 is . The IAU recommended that the origin of right ascensions of the new celestial reference system be close to the dynamical equinox at J2000.0. The location of the dynamical equinox in the ICRS was determined within by Folkner et al. (Folkner et al. 1994); they conclude that the x-axis of the ICRS is offset from the mean equinox at epoch J2000.0 by .
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000