Friedrich Bessel was a self-taught astronomer. Born in Minden, Germany, on July 22, 1784, he became an accountant in Bremen, but his true interests were astronomy and mathematics. In fact, in 1806, at the age of twenty, he recalculated the orbit of Halley's Comet, which was due to reappear in 1835. This so impressed astronomer Heinrich Olbers (1758-1840) that Olbers helped Bessel obtain a post at the observatory.

Bessel worked laboriously. He produced a new star catalogue of over 50,000 stars and introduced improvements to astronomical calculations, developing a method of mathematical analysis along the way that can be applied to many problems not related to astronomy. He oversaw construction of the first large German observatory and served as its director from 1813 until his death in 1846.

Bessel's greatest achievement was in determining the parallax of a star. As the Earth orbits the sun, our position relative to any star shifts by a maximum...