German astronomer and mathematician, the first person to find the approximate distance to a star by direct methods when he measured the parallax (annual displacement) of the star 61 Cygni in 1838. In mathematics, he introduced the series of functions now known as Bessel functions.

Bessel was born in Minden, NW Germany. As an amateur, he wrote a paper on Halley's comet 1804 which got him a post as an assistant at Lilienthal Observatory. After only four years the Prussian government commissioned Bessel to construct the first large German observatory at Königsberg; this was completed 1813 and Bessel spent his life as its director.
Bessel's work laid the foundations of a more accurate calculation of the scale of the universe and the sizes of stars, galaxies, and clusters of galaxies. In addition, he made a fundamental contribution to positional astronomy (the exact measurement of the position of celestial bodies), to celestial mechanics (the movements of stars), and to geodesy (the study of the Earth's size and shape). In 1840 he predicted the existence of Neptune.