The contribution of astrolabes to the establishment of reference systems

At present there are several types of modern astrolabes being able to observe objects from fundamental and other stars to the brightest object of the solar system, the sun itself. The paper reviews the programs, the sujects of investigation, the adjustement of the reference system, the rôle of the IAU and the place of the astrolabes. As in many other fields, an intensive cooperation is needed.George Teleki invited me to write this review paper in a letter which was to be the last one he wrote to me. I dedicate it to the memory of this man whose life was devoted to astrometry, the fundamental science which is the basis of all astronomy.