2010A&A...509A...3V

The astrometric ground-based observations of latitude/universal time variations, covering the interval 1899.7-2003.0, were used in combination with Hipparcos/Tycho positions and some older ground-based catalogs to construct a family of catalogs, tailored to long-term Earth-rotation studies. These catalogs, called Earth Orientation Catalogs (EOC-1 through EOC-3) yielded more accurate proper motions than the original Hipparcos Catalogue, and its latest version, EOC-3, even periodic motions for a large portion of the stars. It appeared that more stars than are contained in EOC-3 are double or multiple and that a better procedure can be used to improve the periodic terms, reflecting the orbital motions of the stars observed in the programs of monitoring Earth orientation. We used about 4.5 million observations of latitude/universal time variations at 33 observatories all over the world, and combined them with the catalogs ARIHIP, TYCHO-2, etc. to obtain the Earth Orientation Catalog (EOC-4). These observations are identical to those used to construct the previous version, EOC-3. Spectral analysis of ground-based data and comparison with the USNO Sixth Catalog of Orbits of Visual Binary Stars was used to discover which of the observed objects display periodic motions. The corresponding amplitudes and phases were then estimated in one-step least-squares solution, together with positions and proper motions. Unlike in EOC-3, where annual averages were used, we use here the individual nightly observations. The fourth version of the catalog, EOC-4, contains 4418 different objects (i.e., stars, components of double stars, photocenters), out of which 599 have significant orbital motions. The catalog will eventually be used for new determination of the Earth orientation parameters during the twentieth century.