Friedrich Bessel

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Bessel, engraving by E. Mandel after a painting by Franz Wolf (credit: The Bettmann Archive)

(born July 22, 1784, Minden, Brandenburg — died March 17, 1846, Königsberg, Prussia) German astronomer. He was the first to measure (by means of parallax) the distance to a star other than the Sun. One of his major discoveries was that the bright stars Sirius and Procyon make tiny motions explainable only by the existence of invisible companions disturbing their motions. His observation of tiny irregularities in the orbit of Uranus, which he concluded were caused by an unknown planet beyond, led to the discovery of Neptune. His mathematical functions for studying planetary motions became widely used in solving a wide range of differential equations.

For more information on Friedrich Wilhelm Bessel, visit Britannica.com.

Friedrich Wilhelm Bessel

Library of Congress

[b. Minden, Prussia (Germany), July 22, 1784, d. Königsberg, Prussia (Kaliningrad, Russia), May 17, 1845]

Mathematicians and physicists often use Bessel functions, developed by Bessel to analyze the motions of planets and stars. In 1838 Bessel was the first to measure the distance to a star (61 Cygni) using parallax and a special instrument he invented known as the heliometer. With the heliometer Bessel also discovered that Sirius has an unseen companion that causes its position to shift slightly as the companion orbits the larger star.

The German astronomer Friedrich Wilhelm Bessel (1784-1846) established the modern ideals and standards of precision in astronomy and obtained the first measurement of the distance to a star.

Friedrich Wilhelm Bessel was born in Minden, North Rhine-Westphalia, on July 22, 1784. After a rather undistinguished schooling, he was apprenticed to a Bremen merchant house at the age of 15. In the course of his bookkeeping work, he acquired a facility in mathematics, and this, together with an interest in astronomy, led him to compute the orbit of Halley's comet from old observations made by T. Harriott of the comet's 1607 circuit. His results were published in a professional journal. This evidence of his ability gained him entry into the profession, starting as an assistant in the private observatory of J. H. Schroter in Lillienthal in 1806. Bessel became the outstanding astronomer of the 19th century and probably the most complete astronomer of all time.

In an era when astronomers had long since given themselves over to specializing in either the observational or the theoretical aspects of the science, Bessel was master of both. He became the director of the newly founded Royal Observatory in Königsberg in 1810. As a worker in, and director of, the observatory for more than 30 years, he complied a catalog of very accurate positions for 75,000 stars.

Observational Precision

By Bessel's day it was already well known that the earth is a difficult place from which to try to map the sky. In addition to being immersed in a dense atmosphere which distorts star positions, the earth was known to be a moving platform whose basic motion (an assumed annual revolution around the sun) was complicated by numerous smaller wiggles and wobbles. Therefore the task of producing precise astronomical information was extremely difficult. Besides getting the best instruments available and then ensuring that intrinsic errors had been eliminated or mitigated as far as possible, astronomers needed to convert their readings into usable data. Prior to Bessel, every astronomer had done it in his own way. Bessel not only reevaluated all of the corrections involved but established a logical, systematic scheme for applying them which has been followed universally ever since. Leaving nothing to chance, he even proved that different observers actually see a given event occur at different times and introduced the "personal equation" to correct for it long before the discipline of psychology arose and took interest in the question of individual differences.

Stellar Parallax

Bessel's enduring concern for both instrumental and mathematical facets of observational precision eventually combined to produce what was undoubtedly the crowning achievement of his life - the detection of stellar parallax. From the time of Copernicus it had been recognized that if the earth were in motion around the sun, the earth's motion would be reflected by annual shifts in the positions of at least some stars against the background sky. Attempts to discover such a parallax had been made by many prominent astronomers from Copernicus's time to Bessel's. But all had failed. Bessel, determined to add his efforts to the cause, decided on a somewhat different approach to the problem. The crucial part of any such project, of course, was the choice of the star: the nearer it was, the more likely the work was to succeed. Previous investigators had invariably chosen bright stars, on the supposition that all stars are about the same size and that the brightest ones are the nearest ones. By Bessel's era, however, this assumption was known to be questionable. Moreover, a new type of information was available, termed "proper motion," which seemed to offer more reliable guidance in guessing which stars were most likely to be nearby. These data signified that some stars have measurable long-term drifts with respect to the rest of the stars. Reasoning that such stars must be closer than the rest, Bessel chose to work with the star that had the swiftest known motion (61 Cygni). After 1 1/2 years of careful observations and laborious calculations, Bessel separated the star's own motion from the various motions of the earth and concluded in 1838 that the star was oscillating back and forth each year by about 3/10 of 1 second of arc.

The discovery of stellar parallax was a landmark in the history of astronomy. On the one hand, it signaled the official end of the dispute over Copernicanism. On the other hand, it constituted the indispensable beginning and foundation of all studies of the stellar system which depend on knowledge of distances.

In addition to his technical astronomical works, Bessel published the widely read series Popular Lectures (1848). He also made an important contribution to applied mathematics by systematizing the functions now known by his name. He died in Königsberg on March 17, 1846.

Further Reading

The most scholarly work on Bessel is in German. Studies in English which discuss him include Henry Smith Williams, The Great Astronomers (1930); Hector MacPhearson, Makers of Astronomy (1933); and Willy Ley, Watchers of the Skies: An Informal History of Astronomy from Babylon to the Space Age (1963).

Additional Sources

Lavrinovich, K. K. (Kazimir Kleofasovich), Friedrich Wilhelm Bessel, 1784-1846, Basel; Boston: Birkhauser, 1995.

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Friedrich Bessel
Friedrich Wilhelm Bessel
Born	22 July 1784(1784-07-22) Minden, Minden-Ravensberg, now Germany
Died	17 March 1846 (aged 61) Königsberg, Prussia, now Kaliningrad, Russia
Residence	Prussia
Nationality	Prussian (German)
Fields	Mathematics and Astronomy
Institutions	University of Berlin
Alma mater	Georg-August University
Doctoral advisor	Carl Friedrich Gauss
Doctoral students	Heinrich Scherk
Known for	Bessel functions Stellar parallax
Notable awards	Gold Medal of the Royal Astronomical Society (1829 & 1841)

Friedrich Wilhelm Bessel (22 July 1784 – 17 March 1846) was a German mathematician, astronomer, and systematizer of the Bessel functions (which were discovered by Daniel Bernoulli). He was a contemporary of Carl Gauss, also a mathematician and astronomer. The asteroid 1552 Bessel was named in his honour.

Life

Bessel was born in Minden in Minden-Ravensberg, the son of a civil servant. At the age of 14 Bessel was apprenticed to the import-export concern Kulenkamp. He soon became the company's accountant. The business's reliance on cargo ships led him to turn his mathematical skills to problems in navigation. This in turn led to an interest in astronomy as a way of determining longitude.

Bessel came to the attention of a major figure of German astronomy at the time, Heinrich Wilhelm Olbers, by producing a refinement on the orbital calculations for Halley's Comet. Within two years Bessel had left Kulenkamp and become an assistant at Lilienthal Observatory near Bremen. There he worked on James Bradley's stellar observations to produce precise positions for some 3,222 stars.

This work attracted considerable attention, and at the age of 26 Bessel was appointed director of the Königsberg Observatory by King Frederick William III of Prussia. There he published tables of atmospheric refraction based on Bradley's observations, which won him the Lalande Prize from the Institut de France. Bessel was able to pin down the position of over 50,000 stars during his time at Königsberg.

With this work under his belt, Bessel was able to achieve the feat for which he is best remembered today: he is credited with being the first to use parallax in calculating the distance to a star. Astronomers had believed for some time that parallax would provide the first accurate measurement of interstellar distances—in fact, in the 1830s there was a fierce competition between astronomers to be the first to measure a stellar parallax accurately. In 1838 Bessel won the race, announcing that 61 Cygni had a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicated that the star was about 3 parsecs (9.8 light years) away. There are more recent results in the article on 61 Cygni. He narrowly beat Friedrich Georg Wilhelm Struve and Thomas Henderson, who measured the parallaxes of Vega and Alpha Centauri in the same year.

As well as helping determine the parallax of 61 Cygni, Bessel's precise measurements allowed him to notice deviations in the motions of Sirius and Procyon, which he deduced must be caused by the gravitational attraction of unseen companions. His announcement of Sirius's "dark companion" in 1844 was the first correct claim of a previously unobserved companion by positional measurement, and eventually led to the discovery of Sirius B.

Despite lacking a university education, Bessel was a major figure in astronomy during his lifetime. He was elected a fellow of the Royal Society, a foreign member of the Royal Swedish Academy of Sciences in 1823, and the largest crater in the Moon's Mare Serenitatis is named Bessel after him.

Bessel won the Gold Medal of the Royal Astronomical Society in 1841. In 1846 he died in Königsberg of cancer.

Further reading

• Fricke, Walter (1970–80). "Bessel, Friedrich Wilhelm". Dictionary of Scientific Biography. 2. New York: Charles Scribner's Sons. pp. 97–102. ISBN 0684101149. 

External links

• O'Connor, John J.; Robertson, Edmund F., "Friedrich Bessel", MacTutor History of Mathematics archive .
• Friedrich Bessel at the Mathematics Genealogy Project

Persondata
NAME	Bessel, Friedrich
ALTERNATIVE NAMES
SHORT DESCRIPTION	Mathematics and Astronomy
DATE OF BIRTH	July 22, 1784(1784-07-22)
PLACE OF BIRTH	Minden, Westphalia, now Germany
DATE OF DEATH	March 17, 1846
PLACE OF DEATH	Königsberg, Prussia, now Kaliningrad, Russia

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