It is well known that the Hipparcos catalog is characterized by a high degree of internal consistency, but its global orientation and a slow rotation, defined by six parameters, is left free (Froeschlé & Kovalevsky 1982 , Kovalevsky 1995 ). To determine these parameters, an external link to other objects with known positions in an existing reference frame is needed. The natural choice is the reference frame tied to extragalactic objects since it is the closest approximation of the ideal barycentric, non-rotating system. In addition, its realization as maintained by the International Earth Rotation Service (IERS) is very close to the mean equator and dynamical equinox of J2000.0 (Arias & Feissel 1990 ). This very system is now under consideration to be adopted by the IAU as the International Celestial Reference System (ICRS).
To link the Hipparcos catalog to this system, several methods are used, their overview being given by Lindegren & Kovalevsky (1995 ), the results obtained by these methods and their combination are discussed by Kovalevsky et al. (1996 ). Most of the methods used consist in more or less direct observations of relative positions of the Hipparcos stars with respect to extragalactic objects.
The method presented here belongs to the ones that have been considered only very recently. It qualitatively differs from all the others, mainly because it is indirect; it uses the rotating Earth as an intermediary reference system. We measure the rapidly changing orientation of the Earth with respect to Hipparcos stars (by optical astrometry) and with respect to extragalactic objects (by VLBI). The principal idea of the method and a numerical example of linking FK5 to extragalactic system has recently been published by one of us (Vondrák 1996 ).
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998