The new astrometry

First, the main objectives of astrometry are presented and the most important features or phenomena that intervene in the measurement of positions of celestial objects are shortly described. Then, the two classical astrometric techniques that are still very much used, especially since the invention of CCDs, transit instrument and astrophotography, are described. The third section is devoted to the application of interferometric techniques to astrometry, in optical and in radio wavelengths. In the fourth section, it is shown how much precise time measurements are important in modern astrometry, in particular for ranging to the Moon or planets, and in studying pulsars. Then, astrometry from satellites is presented describing the Hipparcos satellite and its results, and the applications of the Hubble Space Telescope. Finally, after presenting the new needs of astrophysics for more accurate astrometry, a description of two major projects, GAIA and SIM, and of a few other smaller satellites that may be launched during the next decade is given.

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