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The first recognised astrometric catalogue was compiled in 129 B.C. by the Greek astronomer Hipparchus. His catalogue, of a thousand stars, specified their relative brightness and position with an accuracy of about one degree. Progress in the accuracy of angular measurements was slight until the 16th century when Tycho Brahe (1546-1601), a Danish astronomer, achieved the next leap in accuracy and fixed star positions to about a minute of arc.
During the next four centuries slow but steady progress was made in improving the accuracy of astrometric catalogues. The next breakthrough in astrometric accuracy came with ESA's Hipparcos satellite - the first space experiment dedicated to astrometry. The results from this mission - The Hipparcos and Tycho Catalogues - gave the astronomical community access to astrometric catalogues with an accuracy which would have only been obtained, for such a large number of stars, by the middle of the 23rd Century had the earlier trend been continued!
The next significant breakthrough in astrometry is expected with the Gaia catalogue of stellar positions, parallaxes and proper motions which is expected to be completed about 3 years after the Gaia mission ends. Accuracies of the order of 10µas in position and parallax measurements will be available for of order 1 billion objects.
And after Gaia? Who knows what the future will bring, but it is a challenge to foresee how the dramatic improvements in astrometric catalogues brought about by space-based experiments can be improved upon.
For more about The Hipparcos and Tycho Catalogues see the catalogue page on the Hipparcos web site.
Image based on an original figure by Erik Høg.
High-resolution version of the above image available here.
[Published: 18/08/2003]
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