History of Monitoring Earth Orientation, and Re-analyses of Old Data

The history of monitoring Earth orientation goes back to the end of nineteenth century, when polar motion was discovered. Description of international efforts in organizing coordinated observations to determine Earth’s orientation is presented. The services, such as International Latitude Service (ILS), Bureau International de l’Heure (BIH), International Polar Motion Service (IPMS) and, finally, International Earth Rotation and Reference Systems Service (IERS), are described, as well as the observational techniques used by them. Each improvement of the techniques and their accuracies led to new discoveries and corresponding improvements of the theory of Earth’s rotation. From the very beginning of these efforts, classical astrometric observations were used for almost one century. Later on, as more precise modern data obtained by space geodesy appeared, new and more accurate theories of precession-nutation became necessary. Finally, to explain the differences between the observations and theory, more and more data from other sources (mainly of geophysical origin) became necessary to be considered. These data and their effects are also shortly commented. Here we concentrate on optical astrometry that dominated throughout the twentieth century; namely we present some important re-analyses of these observations. Special attention is devoted to the re-analyses done in 1998–2012 at the Astronomical Institute, Czech Academy of Sciences. Data covering 1899.7–1992.0 were used to derive Earth orientation parameters and to improve star positions and proper motions in the Hipparcos celestial reference frame.