Major concepts of recent celestial and terrestrial reference systems

The XXIV IAU General Assembly in Manchester in 2000, recommended to replace on 1 January 2003, the existing celestial reference systems with the International Celestial Reference System (ICRS). The International Terrestrial Reference System (ITRS) has formally been implemented in 1991, on the basis of the Resolution 2 of the XX IUGG General Assembly in Vienna in 1991. The ICRS consists of the ensemble of the Barycentric Celestial Reference System (BCRS) and the Geocentric Celestial Reference System (GCRS) with coordinate times Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG), respectively. New celestial reference systems are built up on the basis of the relativistic theory of astronomical reference systems. Also new definition of the Intermediate Reference System (IRS) that links the celestial systems with the terrestrial system has been introduced. Orientation of the celestial IRS and the terrestrial IRS using CIP, CEO and TEO is discussed. Motion of the IRS with respect to the GCRS is determined by the new IAU 2000 precession-nutation model, while its motion with respect to the ITRS is determined by the Earth rotation parameters provided by the IERS. Presently used time systems and their actual definitions and mutual interrelations are discussed. Comparison of two approaches of computation of apparent places, the classical one and the IAU2000 one, illustrates the use of new reference systems.