Astrometry, the measurement of the positions and motions of stars, is at the foundation of much of our knowledge of cosmology and astrophysics, and parallax measurements, the measurement of the apparent motion of the stars due to the motion of the Earth around the Sun, is the most fundamental astrometric technique. These motions are tiny, even for the nearest stars. The parallax of Proxima Centauri, the nearest star, and consequently the star with the largest parallax is only about 0.78 arc seconds, equivalent to less than 4 mm at a distance of 1 kilometer. Before the development of space based astrometry, the most distant stars whose parallaxes could be measured were about 100 light years away. The new GAIA satellite of the ESA should extend that the center of the Galaxy, some 27000 light years away.

GAIA has some other tricks too: finding near Earth asteroids, Quasars, and exoplanets, to name a few.


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