Born July 22, 1784, in Minden; died Mar. 17, 1846, in Königsberg. German astronomer; member of the Berlin Academy of Sciences (1812).
When he was 20 years old, Bessel calculated the orbit of Halley’s comet. In 1806, he obtained the position of assistant in a private observatory in Lilienthal. There he reinterpreted the data of J. Bradley’s observations, from which he determined constants of refraction, precession, and nutation that surpassed in accuracy all previous determinations. In 1810 he became a professor at the University of Konigsberg and built an observatory there, which he directed until his death. On the meridian circle of this observatory, Bessel performed observations of 75,011 stars between +47° and-16° declination. Bessel developed a theory of errors of astronomical instruments and discovered the presence of personal bias—that is, the systematic error introduced by a given observer. In the treatment of his observations, Bessel applied the theory of probability and the method of least squares. In 1838, using a heliometer, he determined the parallax of the star 61 Cygni, having measured the distance to fixed stars in this manner. He developed a theory of solar eclipses and determined the masses of planets and the constants of Saturn’s satellites.
Bessel’s work in geodesy was also of great value. In particular, together with J. Bayer he carried out triangulation in East Prussia and, on the basis of the ten best degree measurements, determined the elements of the terrestrial spheroid. He invented an instrument for measuring the base line.
In mathematics, Bessel’s name is given to the so-called cylinder functions of the first kind and the differential equation which they satisfy, an inequality for the coefficients of the Fourier series, and also an interpolation formula.