Friedrich Wilhelm Bessel

1784-1846

German Astronomer and Mathematician

Friedrich Bessel was one of the founders of modern astronomy. He made the first accurate measurements of the positions of stars and of interstellar distances, providing the data upon which an understanding of the dimensions of the universe could be based. He also made significant contributions to mathematics, especially the development of "Bessel functions."

Bessel was born in Minden, Brandenburg, in 1784. He showed interest in scientific and mathematics in school but ended his formal education in 1799, at age 14, to enter into an apprenticeship with an import-export company. His interest in the naval activities of the company, combined with his natural interest in science,led him to study the mathematics of navigation. This study developed further into an interest in astronomy. In 1804 Bessel published a paper in which he reported his calculation of the orbit of Halley's Comet, based on measurements that had been made in the seventeenth century. Several prominent astronomers recognized Bessel's ability, and he was given a position in the Lilienthal Observatory of astronomer J. H. Schroter soon thereafter.

Bessel's reputation spread rapidly, and in 1809 he was appointed director of Germany's first government observatory which was under construction in Königsberg. Since university professors were required to have doctorates, the University of Königsberg awarded a doctorate to Bessel in 1810 so that he could become the professor of astronomy at that university. He remained in this position until his death in 1846.

At Königsberg Bessel was able to devote his full attention to astronomy and mathematics. His extensive work in developing and refining methods for the accurate instrumental measurement of the positions of stars resulted in the founding of the division of astronomy known as astrometry. Bessel is credited with making the first accurate measurement of the position of a star. In 1818 he published the positions of 3222 stars, providing quantitative astronomy with an accurate data bank for the first time.

Bessel's careful measurements allowed him to observe very small variations in the positions of stars. One of the results was his discovery that stars appear to have minor elliptical motions. Bessel showed that this phenomenon, known as "parallax," results from the earth's motion around the sun, providing final proof of the heliocentric model of the solar system. Bessel predicted companion stars for Sirius and Procyon based on minute periodic deviations in the positions of these bright stars, and predicted that a planet, as yet unobserved, must exist because of slight deviations he detected in the orbit of Uranus. The development of more powerful telescopes led to the discovery of the companion stars of Sirius and Procyon, and the planet Neptune was found where Bessel had predicted it to be.

Bessel's work in related fields included the measurement of a value of 1/299 for the departure of the earth's curvature from a perfect sphere. The best known of his contributions to mathematics is his "Bessel functions." Bessel developed the mathematics of these functions to facilitate calculations involving three bodies—for example, planets or stars—that are simultaneously under the influence of each other's gravitational effects. Bessel also made significant contributions to the reform of university education, even though his own formal education had ended at age 14. His pioneering studies of mental chronometry and reaction times are further indications of his intellectual breadth and curiosity.

Bessel's work brought him considerable recognition within the scientific community. Among other awards, he received a prize from the Berlin Academy in 1815, was made a Fellow of the Royal Society in 1825, and received the Gold Medal of the Royal Astronomical Society.

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