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Friedrich Bessel. Portrait of Friedrich Wilhelm Bessel (1784-1846), German astronomer. He studied the perturbations of planetary and stellar motions, inventing the Bessel mathematical func- tions to aid his analyses. In 1838 he was the first to establish the distance to a star by measuring its apparent angular motion (parallax), using 61 Cygni as his first object. He predicted that a small companion star formed a binary system with Sirius. He computed the mass of Jupiter by analysing the orbits of its satellites, and found that it had a relatively low density. He suggested the existence of the planet beyond Uranus, but died a few months before the discovery of Neptune.
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