Reference frames in astronomy

Advances In wide-angle astrometric measurements of three to four orders of magnitude in the last thirty years have resulted in a redefinition of the f undamental astronomical reference frame. This new frame, the International Celestial Reference Frame (ICRF), is based on the radio positions of 212 co mpact extragalactic radio sources. The ICRF defines the direction of the ax es of the International Celestial Reference System (ICRS) with a precision of approximately 20 mu as. At optical wavelengths, the Hipparcos catalog is the realization of this frame. The precision with which the ICRF is now de termined requires that the ICRS models for precession, nutation, and others , be revised. Increases in the precision of measurements from astrometric s pace missions will further improve the celestial reference frame and may re quire its redefinition within the next ten years. These improvements will a gain challenge the models for the celestial reference system.