*IAU(1991) RECOMMENDATION VII*

Definition and realization of the conventional celestial barycentric reference system

The XXIst General Assembly of the International Astronomical Union,

considering,

a) that the new conventional celestial barycentric reference frame should be as close as possible to the existing FK5 equator and equinox and the dynamical equinox which are referred to J2000.0,

b) that it should be accessible to astrometry in visual as well as in radio wavelengths,

recommends,

1. that the principal plane of the new conventional celestial reference system be as near as possible to the mean equator at J2000.0 and that the origin in this principal plane be as near as possible to the dynamical equinox of J2000.0,

2. that the positions of the extragalactic objects selected in accordance with Recommendation VI and representing the reference frame be computed initially for the equator and equinox J2000.0 using the best available values of the celestial pole offset with respect to the IAU expressions for precession and nutation,

3. that a great effort be made to compare reference frames of all types, in particular the FK5, solar system and extragalactic reference frames,

4. that observing programmes be undertaken or continued in order to relate planetary positions to radio and optical objects, and to determine the relationship between catalogues of extragalactic source positions and the best catalogues of star positions, in particular the FK5 and Hipparcos catalogues.

Notes for Recommendation VII

1. This recommendation specifies the choice of the coordinate axes that will be adopted in the final reference frame and describes the work to be done before such a frame can be constructed. Although the considerations call for visual and radio wavelengths for the primary catalogue, other observable wavelengths are not excluded. Positions of objects observed in other wavelengths should also be referred to the same system.

2. The objective set by this recommendation is that there should be no discontinuity in the positions of stars when the present FK5 frame is replaced by the extragalactic reference frame. This means that the position of the extragalactic objects should be in the FK5 system for J2000.0. It is acknowledged that the best values of precession and nutation must be used in order to avoid introducing spurious proper motions into the positions of extragalactic objects. The final transfer to the preferred equinox and principal plane will be done by applying a rotation at J2000.0.

3. The dynamical equinox in this recommendation is defined as the intersection of the mean equator and the ecliptic. The latter is defined as the uniformly rotating plane of the orbit of the Earth Moon barycentre averaged over the entire period for which the ephemerides are valid. Since it is ephemeris dependent, the choice of the equinoctial point will be made using the most accurate and generally available ephemerides of the solar system at the time.

4. The definition given to the reference system by Recommendations I and II implies the stability in time of the system of coordinates realized by the celestial reference frame. The directions of the coordinate axes should not be changed even if at some later date the realizations of the dynamical equinox or the celestrial ephemeris pole are improved. Similarly, modifications to the set of extragalactic objects realizing the reference system should be made in such a way that the directions of the axes are not changed. This means that once the coordinate axes have been specified, in the way described in the first part of the recommendation, the connection between the definition of the conventional reference system and the peculiarities of the Earth's kinematics will have been severed.

5. As long as the relationship between the optical and the extragalactic radio frames is not sufficiently accurately determined, the FK5 catalogue shall be considered as a provisional realization of the celestial reference system in optical wavelengths.