German astronomer and mathematician who was made director of the Königsberg Observatory. He developed methods for treating what are now called Bessel functions. Bessel was the first to measure the parallax of a star. Using an instrument of his own invention, the heliometer, he measured the parallax of 61 Cygni, measuring a distance of 10.1 light-years (the currently accepted value is 11.2 light-years ). He was the first to systematically investigate Bessel functions, which had been originally encountered by Daniel Bernoulli.

Additional biographies: MacTutor (St. Andrews), Bonn