Theory and practice of terrestrial and celestial reference systems. Part 1: the concepts, with a bias towards Geodesy

Abstract The analysis of physical phenomena presiding to the evolution of the Earth as well as that of the planets requires the existence of a common reference system in which the spatial and temporal relation between such phenomena is described. The problem of how to define a reference system is an old one in both Geodesy and Astronomy. Such a problem has undergone a substantial greening since spatial techniques of Earth observation and astrometric satellite missions like Hipparcos and Gaia have become so widely used in both disciplines, so much so that the International Association of Geodesy (IAG) and the International Astronomical Union (IAU) have created a common international service, called International Earth Rotation and Reference Systems Service (IERS), in charge of maintaining terrestrial and celestial reference systems and continuously providing the relation between the two. This paper tries to present in a quite elementary way concepts and definitions leading to the implementation of a reference system, also called reference frame, in a general fashion. Of course, given the author’s life experience, the presentation suffers from a certain bias towards Geodesy. In a subsequent paper (Part 2), a group of scientists of the Italian Space Agency in charge of contributing to the international effort of the IERS function, will describe the activity and results of such a service.