The Hipparcos, Tycho, TRC, and ACT catalogues - A whole sky comparison of the proper motions

We present a whole sky comparison of the proper motions contained in the Hipparcos Catalogue, the Tycho Cat- alogue, the Tycho Reference Catalogue (TRC), and the Astro- graphic Catalogue plus Tycho Reference Catalogue (ACT). The catalogues are compared in the 20 declination zones defined by the original Carte du Ciel project. We find that (1) the proper- motion errors in the ACT Catalogue are underestimated in all zones, sometimes by as much as 30%. (2) There are systematic differences, as large as 1.2 mas yr 1 , between the proper mo- tions in the TRC and ACT depending on the zone. We confirm the known underestimation of the proper-motion error (by as much as 40%) of the faint stars (BT > 10 mag) in the Tycho Catalogue.