The Hipparcos Catalogue characteristics are described in full detail in the documentation accompanying the catalogue. 1997a, Lindegren et al. 1997, Kovalevsky et al. Observations were made from 1989.85 to 1993.21, with a mean epoch of close to J1991.25, adopted as the catalogue epoch.
Standard errors are functions of magnitude and ecliptic latitude.
As a result of this high accuracy, investigations for variability were made possible for all programme stars.