1. Historical background
1.1. The quasi-inertial frame
Adopting extragalactic directions as primary celestial references instead of star positions has been envisioned for over a decade by the International Astronomical Union (IAU). A series of Working Groups was organized jointly by several IAU commissions mainly to establish lists of objects appropriate for observation by Very Long Baseline radio Interferometry (VLBI) at wavelengths of 13 and 3.6 cm. VLBI is the technique that measures most efficiently directions of compact extragalactic radio sources, reaching sub-milliarsecond (mas) accuracy.
A decisive breakthrough was the formulation by the Working Group on Reference Systems (1988-1991) of a comprehensive group of recommendations that dealt with both the fundamental concepts of space-time references and their actual realizations. During the 1991-1994 term, the IAU Working Group on Reference Frames (WGRF) produced a list of about 600 radio sources that could be used to realize an extragalactic celestial reference system, provided that careful observations could tie them together into a consistent system. Meanwhile the International Earth Rotation Service (IERS) had initiated and developed operational procedures to maintain the celestial reference system. During the 1994-1997 term, the WGRF concluded this preparatory phase by recommending that the 1997 IAU General Assembly adopt the IERS celestial reference system under the name International Celestial Reference System (ICRS) and to charge the IERS to maintain its realization, the ICRF (International Celestial Reference Frame), in cooperation with an IAU working group on reference frames.
The fixed directions underlying the FK5 (Fricke et al. 1988) and the preceding fundamental star catalogues of positions and proper motions were defined by the mean equator and equinox at a reference epoch (J2000.0 for the FK5, B1950 for the FK4), which implied dynamical modelling of the Earth's orbital motion (ecliptic, equinox) for the realization of the reference frame and its subsequent access. In the past, these fundamental directions were considered conventional for some decades; they were changed from time to time, in particular to take advantage of advances in modelling of the motion of solar system objects. As was originally proposed by Guinot (1979), the IAU, in its 1991 Recommendations on Reference Systems (Bergeron 1992), decided to select distant extragalactic objects as the basis of its new celestial reference system and to adopt directions that would remain fixed with respect to a selected set of these objects. Continuity between the old and new system was mandatory so that these directions would be consistent with their previous realizations, i.e., the pole and origin of right ascensions in the new reference system should be close to the FK5 pole and right ascension origin, within the uncertainties of the FK5. A fundamental advantage of selecting extragalactic objects is that they are so distant that their proper motions are currently undetectable, even by the most precise techniques. More quantitatively, if we assume that the tangential velocity of a distant extragalactic object is comparable to its recession speed, the angular motion would be given by the Hubble constant, or about 0.01 mas/a.
According to the new rules adopted in the 1991 IAU Recommendations, the fundamental directions of the celestial reference system will remain fixed in space, as long as the directions of the distant extragalactic sources can be regarded as fixed directions. They will no longer be dependent on precise modelling of the motion of solar system objects and on the definition of the equinox by means of the celestial equator and the ecliptic.
The adopted positions of the defining sources may be re-estimated when improved information is available, but the direction of the coordinate axes will be maintained by implementing the statistical condition that the new coordinates of selected sources show no global rotation with respect to the old set. It is also foreseen that some sources may be deleted or new ones could be added in the future. This will be done by adhering strictly to a predefined procedure.
In order that the use of this new reference system should not be detrimental to the analysis of the highly accurate astrometric techniques, the IAU explicitly introduced the Theory of General Relativity as the background for all theoretical and data analysis problems related to time and space.
1.2. The IAU Resolutions
At its 23rd General Assembly in August 1997, the IAU decided (Appenzeller 1998),
In the next two sections we briefly describe the ICRS, the ICRF and the Hipparcos catalogue, summarize the basic principles of the maintenance of the ICRF, and list the main consequences of the change.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998