Friedrich Wilhelm Bessel
As a fifteen year old boy Bessel became an apprentice to a mercantile firm; he became especially interested in foreign trade which in turn led to an interest in navigation. He therefore began to study astronomy and mathematics. He was self-taught, but awarded a doctorate from Göttingen through the efforts of Gauss. He was one of the few (Gauss did not suffer fools) lifelong correspondents of Gauss. He was the director of the observatory at Königsberg from 1810 until his death in 1846. Bessel determined the properties of the family of solutions of the equation
(now known respectively as Bessel functions and Bessel's equation) while studying perturbations of the planetary system. Undergraduate mathematics majors see it in the section on regular singular points in the elementary differential equations course.