THE HIPPARCOS CATALOG

The principal observational characteristics of the Hipparcos Catalogue , and a summary of its main astrometric and photometric properties, ar e presented. Median astrometric standard errors (in position, parallax , and annual proper motion) are in the range 0.7-0.9 milliarcsec (mas) for stars brighter than 9 mag at the catalogue epoch (J1991.25). The catalogue is a materialisation of the ICRS reference system, coincidin g with its principal axes at the level of +/-0.6 mas, and with proper motions consistent with an inertial system at the level of +/-0.25 mas /yr. The 118218 constituent stars provide a mean sky density of simila r to 3 stars deg(-2). The catalogue is available in printed and machin e-readable forms.