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On the occasion of the 600th birthday of Ulugbek (1394 - 1449) we review his work in astronomy. His catalog of 1018 star positions is the second observational catalog in the history of astronomy, published 12 centuries after the Hipparcos-Ptolemy catalog. Ulugbek's observatory in Samarkand, a meridian instrument, was the largest before the development of optical telescopes. We review theories for the uses of this observatory for astrometry and the determination of astronomical constants.