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Friedrich Wilhelm Bessel[frEd´rikh vil´helm bes´ul] Pronunciation Key, 1784–1846, German astronomer and mathematician. He became (1810) director of the new observatory at KOnigsberg and professor of astronomy at the Univ. of KOnigsberg. Among his many achievements the most noted is his discovery of the parallax of the fixed star 61 Cygni. Announced in 1838, it was officially recognized in 1841 as the first fully authenticated measurement of the distance of a star. His observations had, by 1833, increased the number of accurately cataloged stars to 50,000. This work was continued and extended by his pupil Argelander. Through observing the variations of the proper motions of Sirius and Procyon, he concluded that they possessed dimmer companions, which was verified a century later by astronomers. Bessel's works on astronomy include Fundamenta Astronomiae (1818) and Astronomische Untersuchungen (1841–42). Bessel also introduced a class of mathematical functions, named for him, which he established as a result of work on perturbation of the planets and which are widely used in applied mathematics, physics, and engineering.
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