Progress in astrometric accuracy
Between the Hipparchus and Hipparcos catalogues two periods of dramatic increase in astrometric accuracy can be identified. In the early 17th Century Tycho Brahe achieved the first significant breakthrough in astronomical accuracy. Almost four hundred years later, Hipparcos, the first space astrometry mission, achieved the next significant improvement in astrometric accuracies. ESA's Gaia mission pushes space-based astrometric measurements to the limits. The Gaia catalogue is expected to be available 3 years after the Gaia mission is completed.