Bessel, Friedrich Wilhelm (1784–1846)

The parallax of a star, the apparent change of its position during the year, can be used to find the star's distance from the Earth. The star appears to change its position because it is viewed at a different angle in July and January. By measuring the angle of parallax, and knowing the diameter of the Earth's orbit, simple geometry can be used to calculate the distance to the star. The technique was developed by the German astronomer Friedrich Bessel.

German astronomer and mathematician. He was 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's work laid the foundations for a more accurate calculation of the scale of the universe and the sizes of stars, galaxies, and clusters of galaxies. He was a pioneer of very precise observation and reduction in astronomy, and published a catalogue of 3,222 star positions under the title Fundamenta Astronomiae (1818). From the gravitational disturbance of the motions of Sirius and Procyon he deduced the existence of invisible companions of these stars. In 1840 he predicted the existence of Neptune.

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).

Bessel was born in Minden, northwestern Germany. As an amateur, he wrote a paper on Halley's Comet in 1804 which got him a post as an assistant at Lilienthal Observatory. After only four years the Prussian government commissioned him to construct the first large German observatory at Königsberg; this was completed in 1813 and Bessel spent his life as its director.