The following topics are addressed: space astrometric missions, ground based optical techniques, very long baseline interferometric (VLBI) techniques, impact of these techniques on astrophysics and on geodynamics, and issues related to celestial and terrestrial reference frames. In the area of space optical astrometry, the reduction of one year of data from the Hipparcos mission shows excellent results for more than 40,000 stars. Recent developments in ground-based optical astrometry are concentrated on CCB meridian circles and transit instruments. The VLBI technique is the most powerful method to determine the earth rotation parameters, nutation series, and baseline lengths. More accurate celestial and terrestrial reference frames are needed and recent achievements in this field are presented, for example FK5 catalogues and a comparison of VLA and Hipparcos positions.

In this review talk, I would like to report on the proper motion analysis, which has been recently carried out together with M. Soma and M. Yoshizawa: There has been a persistent demand in astronomy for accurate stellar positions and proper motions, which are represented by an inertial reference system constructed on the basis of a set of consistent astronomical constants. In the reference system the precessional constant plays a primary role. In a series of papers Fricke (1967a,b, 1977a,b) has deter­ mined the luni-solar precessional correction to Newcomb's value and the fictitious motion of the equinox, which have been adopted in the "IAU (1976) System of Astronomical Con­ stants". Based on the precessional correction and the equinoctial motion thus established, the fundamental reference system, the FK5 system (Fricke et al. 1988) for positions and proper motions, has been constructed. However, for several years geodetic VLBI (McCarthy & Luzum 1991) and LLR (Williams et at. 1991) observations have been suggesting an additional correction to the luni-solar precessional constant of the IAU (1976) System. That is, these observations indicate the precessional correction of 6. p ~ -0:'30/cent to the FK5 system. But, the observational period ofthe earth orientation is considered to be still insufficient to separate unambiguously the precessional change of the earth orientation from the nutation with the longest period of 18. 6 years.

This book on reference systems is the first comprehensive review of the problem of celestial and terrestrial reference systems and frames. Over 20 years, the importance of this problem emerged slowly as the accuracy of new observational techniques improved. The topic has already been approached in several symposia such as Stresa (1967), Morioka (1971), Perth (1973), Columbus (1975, 1978 and 1985), Kiev (1977) and San Fernando (1978). Two IAU colloquia held in Turin (1974) and in Warsaw (1980) were exclusively devoted to discuss reference systems. During this time, the problem of terrestrial and celestial reference systems has been discussed also in many astronomical and geodetic symposia, but always among other topics. Thus, a review devoted solely to the definition and practical realization of such systems was needed. It is hoped that this book, containing modern comprehensive reviews of important facets of this problem will contribute not only to a better and wider understanding of the mathematics and the physics that are behind the concepts and the realizations, but also to future development in a field that can only expand with the rapidly increasing accuracy of geodetic and astronomical observations. We are pleased to thank all the authors of the book who have enthusiastically agreed to contribute to the book in their field of competence and have gracefully accepted guidance from the editors in the definition of the subject and of the interfaces with other chapters. We thank Prof. Y.

The lAU Colloquium No. 56, the Second lAU Colloquium, "On Reference Coordinate Systems for Earth Dynamics," co-sponsored by the COSPAR and the International Association of Geodesy of IUGG was held in Warsaw, Po1and, on September 8-12, 1980. The Co11oquium was organized by the Space Research Centre of the Po1ish Academy of Sciences and the Smithsonian Institution Astrophysica1 Observatory. It was sponsored by the Committee of Astronomy, the Committee of Geodesy, and the Committee of Space Research of the Po1ish Academy of Sciences. The first Co11oquium devoted to this subject was held in Torun, Po1and, in 1974. The Scientific Organizing Committee consisted of: Cochairmen Dr. E. M. Gaposchkin USA Dr. B. Ko~aczek Po1and Members of the Program Committee Prof. J. Kova1evsky France Prof. I. I. Mueller USA Prof. M. Rochester Canada Members Dr. M. Bursa Czechos1ovakia Dr. H. K. Eichhorn USA Prof. W. Fri cke FRG Dr. E. Hog Denmark Dr. Y. Kozai Japan Dr. Y. S. Yatskiv USSR The Loca1 Organizing Committee consisted of: Dr. B. Ko~aczek, Chairman Dr. W. Pache1ski, Secretary Dr. W. Dobaczewska Dr. J. Krynski Dr. G. Sitarski Prof. J. Smak Dr. J. B. Zielinski Mr. W. Zarnowiecki ix E. M.Gaposchkin and B. Kotaczek (eds. ), Reference Coordinate Systems for Earth Dynamies, ix-x. Copyright (c) 1981 by D. Reidel Publishing Company. x PREFACE The duties of chairmen of the sessions were performed by Dr. J. Zielinski, Dr. P. Bender, Prof. W. Fricke, Dr. B.

This book on reference systems is the first comprehensive review of the problem of celestial and terrestrial reference systems and frames. Over 20 years, the importance of this problem emerged slowly as the accuracy of new observational techniques improved. The topic has already been approached in several symposia such as Stresa (1967), Morioka (1971), Perth (1973), Columbus (1975, 1978 and 1985), Kiev (1977) and San Fernando (1978). Two IAU colloquia held in Turin (1974) and in Warsaw (1980) were exclusively devoted to discuss reference systems. During this time, the problem of terrestrial and celestial reference systems has been discussed also in many astronomical and geodetic symposia, but always among other topics. Thus, a review devoted solely to the definition and practical realization of such systems was needed. It is hoped that this book, containing modern comprehensive reviews of important facets of this problem will contribute not only to a better and wider understanding of the mathematics and the physics that are behind the concepts and the realizations, but also to future development in a field that can only expand with the rapidly increasing accuracy of geodetic and astronomical observations. We are pleased to thank all the authors of the book who have enthusiastically agreed to contribute to the book in their field of competence and have gracefully accepted guidance from the editors in the definition of the subject and of the interfaces with other chapters. We thank Prof. Y.

The following topics are addressed: space astrometric missions, ground based optical techniques, very long baseline interferometric (VLBI) techniques, impact of these techniques on astrophysics and on geodynamics, and issues related to celestial and terrestrial reference frames. In the area of space optical astrometry, the reduction of one year of data from the Hipparcos mission shows excellent results for more than 40,000 stars. Recent developments in ground-based optical astrometry are concentrated on CCB meridian circles and transit instruments. The VLBI technique is the most powerful method to determine the earth rotation parameters, nutation series, and baseline lengths. More accurate celestial and terrestrial reference frames are needed and recent achievements in this field are presented, for example FK5 catalogues and a comparison of VLA and Hipparcos positions.

In this review talk, I would like to report on the proper motion analysis, which has been recently carried out together with M. Soma and M. Yoshizawa: There has been a persistent demand in astronomy for accurate stellar positions and proper motions, which are represented by an inertial reference system constructed on the basis of a set of consistent astronomical constants. In the reference system the precessional constant plays a primary role. In a series of papers Fricke (1967a,b, 1977a,b) has deter­ mined the luni-solar precessional correction to Newcomb's value and the fictitious motion of the equinox, which have been adopted in the "IAU (1976) System of Astronomical Con­ stants". Based on the precessional correction and the equinoctial motion thus established, the fundamental reference system, the FK5 system (Fricke et al. 1988) for positions and proper motions, has been constructed. However, for several years geodetic VLBI (McCarthy & Luzum 1991) and LLR (Williams et at. 1991) observations have been suggesting an additional correction to the luni-solar precessional constant of the IAU (1976) System. That is, these observations indicate the precessional correction of 6. p ~ -0:'30/cent to the FK5 system. But, the observational period ofthe earth orientation is considered to be still insufficient to separate unambiguously the precessional change of the earth orientation from the nutation with the longest period of 18. 6 years.

This book on reference systems is the first comprehensive review of the problem of celestial and terrestrial reference systems and frames. Over 20 years, the importance of this problem emerged slowly as the accuracy of new observational techniques improved. The topic has already been approached in several symposia such as Stresa (1967), Morioka (1971), Perth (1973), Columbus (1975, 1978 and 1985), Kiev (1977) and San Fernando (1978). Two IAU colloquia held in Turin (1974) and in Warsaw (1980) were exclusively devoted to discuss reference systems. During this time, the problem of terrestrial and celestial reference systems has been discussed also in many astronomical and geodetic symposia, but always among other topics. Thus, a review devoted solely to the definition and practical realization of such systems was needed. It is hoped that this book, containing modern comprehensive reviews of important facets of this problem will contribute not only to a better and wider understanding of the mathematics and the physics that are behind the concepts and the realizations, but also to future development in a field that can only expand with the rapidly increasing accuracy of geodetic and astronomical observations. We are pleased to thank all the authors of the book who have enthusiastically agreed to contribute to the book in their field of competence and have gracefully accepted guidance from the editors in the definition of the subject and of the interfaces with other chapters. We thank Prof. Y.

The lAU Colloquium No. 56, the Second lAU Colloquium, "On Reference Coordinate Systems for Earth Dynamics," co-sponsored by the COSPAR and the International Association of Geodesy of IUGG was held in Warsaw, Po1and, on September 8-12, 1980. The Co11oquium was organized by the Space Research Centre of the Po1ish Academy of Sciences and the Smithsonian Institution Astrophysica1 Observatory. It was sponsored by the Committee of Astronomy, the Committee of Geodesy, and the Committee of Space Research of the Po1ish Academy of Sciences. The first Co11oquium devoted to this subject was held in Torun, Po1and, in 1974. The Scientific Organizing Committee consisted of: Cochairmen Dr. E. M. Gaposchkin USA Dr. B. Ko~aczek Po1and Members of the Program Committee Prof. J. Kova1evsky France Prof. I. I. Mueller USA Prof. M. Rochester Canada Members Dr. M. Bursa Czechos1ovakia Dr. H. K. Eichhorn USA Prof. W. Fri cke FRG Dr. E. Hog Denmark Dr. Y. Kozai Japan Dr. Y. S. Yatskiv USSR The Loca1 Organizing Committee consisted of: Dr. B. Ko~aczek, Chairman Dr. W. Pache1ski, Secretary Dr. W. Dobaczewska Dr. J. Krynski Dr. G. Sitarski Prof. J. Smak Dr. J. B. Zielinski Mr. W. Zarnowiecki ix E. M.Gaposchkin and B. Kotaczek (eds. ), Reference Coordinate Systems for Earth Dynamies, ix-x. Copyright (c) 1981 by D. Reidel Publishing Company. x PREFACE The duties of chairmen of the sessions were performed by Dr. J. Zielinski, Dr. P. Bender, Prof. W. Fricke, Dr. B.

This book on reference systems is the first comprehensive review of the problem of celestial and terrestrial reference systems and frames. Over 20 years, the importance of this problem emerged slowly as the accuracy of new observational techniques improved. The topic has already been approached in several symposia such as Stresa (1967), Morioka (1971), Perth (1973), Columbus (1975, 1978 and 1985), Kiev (1977) and San Fernando (1978). Two IAU colloquia held in Turin (1974) and in Warsaw (1980) were exclusively devoted to discuss reference systems. During this time, the problem of terrestrial and celestial reference systems has been discussed also in many astronomical and geodetic symposia, but always among other topics. Thus, a review devoted solely to the definition and practical realization of such systems was needed. It is hoped that this book, containing modern comprehensive reviews of important facets of this problem will contribute not only to a better and wider understanding of the mathematics and the physics that are behind the concepts and the realizations, but also to future development in a field that can only expand with the rapidly increasing accuracy of geodetic and astronomical observations. We are pleased to thank all the authors of the book who have enthusiastically agreed to contribute to the book in their field of competence and have gracefully accepted guidance from the editors in the definition of the subject and of the interfaces with other chapters. We thank Prof. Y.