Friedrich Wilhelm Bessel (July 22, 1784 – March 17, 1846) was a German mathematician, astronomer, and systematizer of the Bessel functions (which, despite their name, were discovered by Daniel Bernoulli). He was born in Minden, Westphalia and died of cancer in Königsberg (now Kaliningrad, Russia). Bessel was a contemporary of Carl Gauss, also a mathematician and astronomer.
Bessel was the son of a civil servant, and at the age of 14 he was apprenticed to the import-export concern Kulenkamp . He shortly became an accountant for them, and the business' reliance on cargo ships led him to turn his mathematical skills to problems in navigation. This in turn led to an interest in astronomy as a way of determining longitude.
He came to the attention of a major figure of German astronomy at the time, Heinrich Wilhelm Olbers, by producing a refinement on the orbital calculations for Halley's Comet. Within two years he had left Kulenkamp and become an assistant at Lilienthal Observatory near Bremen, Germany. There he worked on James Bradley's stellar observations to produce precise positions for some 3222 stars.
This work attracted considerable attention, and at the age of 26 he was appointed director of the Königsberg Observatory by Frederick William III of Prussia. There he published tables of atmospheric refraction based on Bradley's observations, which won him the Lalande Prize from the Institut de France. On this base, he was able to pin down the position of over 50,000 stars during his time at Konigsberg.
With this work under his belt, Bessel was able to achieve the feat for which he is best remembered today: he is credited with being the first to use parallax in calculating the distance to a star. Astronomers had believed for some time that parallax would provide the first accurate measurement of interstellar distances -- in fact, the 1830s housed a fierce competition between astronomers to be the first to accurately measure a stellar parallax. In 1838 Bessel won the "race", announcing that 61 Cygni had a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicated that the star was ~3 parsecs away. Hipparcos has calculated the parallax at 0.28547 arcseconds. He narrowly beat Friedrich Georg Wilhelm Struve and Thomas Henderson, who measured the parallaxes of Vega and Alpha Centauri in the same year.
As well as helping determine the parallax of 61 Cygni, Bessel's precise measurements allowed him to notice deviations in the motions of Sirius and Procyon, which he deduced must be caused by the gravitational attraction of unseen companions. His announcement of Sirius' "dark companion" in 1841 was the first correct claim of a previously unobserved companion by positional measurement, and eventually led to the discovery of Sirius B.
Despite lacking a university education, Bessel was a major figure in astronomy during his lifetime. He was elected a fellow of the Royal Society, and the largest crater in the moon's Mare Serenitatis was named after him.