Modern Astrometry

Abstract

The difficult but fundamental task of measuring stellar distances and space motions has preoccupied astronomers for centuries. One hundred years ago, J.C. Kapteyn collaborated in the production of the Cape Photographic Durchmusterung, exploiting the new medium of the photographic plate to create a state-of-the-art stellar catalogue of the southern sky, with positional accuracies of about 2–3 arcsec. A series of substantial stellar positional catalogue programmes has been undertaken since then, improving positional accuracies significantly, and the astrophysical relevance of the catalogues correspondingly. Achieving further improvements in positional accuracy remains a challenging problem. The possibility of carrying out these measurements from space, above the perturbing effects of the Earth’s atmosphere, became reality with the launch of the European Space Agency’s HIPPARCOS satellite in 1989, yielding position, parallax, and proper motion accuracies at the level of 0.001 arcsec. I give some examples of the astrophysical applications of this catalogue, and discuss the prospects and expected scientific value of measurements at even higher accuracy, as typified by the SIM and GAIA space experiments, in the future. I exclude from the discussions details of recent developments in ground-based optical astrometry, as well as the substantial progress that is taking place in radio and infrared/sub-millimeter astrometry.