3.17. Astrometry

The Hipparcos catalog contains 118,218 astrometric entries (milliarcsecond position, parallax and proper motion) with a median error of 1mas. The Tycho catalog on the other hand includes more than a million objects with a median error of 7mas for the brighter sources and 25mas for the fainter ones. More than 100 epochs were acquired for the photometry of the objects in the Hipparcos catalog. The photometric error is between 0.0004 - 0.0007 magnitudes for the brighter stars.

These catalogs are now the standards for astrometry. The catalog epoch is J1991.25; the equinox J2000.0. The European Space Agency distributes theses catalogs as a series of 16 books and several CD-ROMs. The catalogs can also be queried in the WWW (http://astro.estec.esa.nl/Hipparcos)