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