Friedrich Wilhelm Bessel

German astronomer

• Born: July 22, 1784
• Birthplace: Minden, Brandenburg (now in Germany)
• Died: March 17, 1846
• Place of death: Königsberg, Prussia (now Kaliningrad, Russia)

Bessel’s contribution to astronomy was to increase greatly the accuracy of measuring the positions of stars by using more advanced instruments and developing methods to account for instrument and observer error. The most famous discovery resulting from these observations was the first accurate determination of the distance to a star.

Early Life

Friedrich Wilhelm Bessel (BEHS-ehl) was born to a civil servant and a minister’s daughter. One of nine children, he went to the local gymnasium but left after only four years to become a merchant’s apprentice. He showed no particular talent at school. At the age of fifteen, he began his unpaid seven-year apprenticeship to a merchant firm in Bremen. He excelled at his accounting job and received a small salary after one year. He spent his spare time teaching himself geography and languages because of his interest in foreign trade. He also learned about ships and practical navigation through self-study. Determining the position of a ship at sea—a long-standing problem in navigation—intrigued him. He therefore began to study astronomy and mathematics to understand the theory behind the existing methods.

Bessel began to make observations of stars on his own, which he was able to compare to observations reported in numerous professional journals of astronomy. One of his earliest tasks was to determine the orbit of Halley’s Comet based on several observations of its position. He studied existing methods to determine the easiest way to do this, and he used observations made in 1607 to supplement his own. The precision of his observations and the scrupulous care given to minimizing or correcting for observational errors would characterize his professional work throughout his life. He made the observations, adjusted (or reduced) the 1607 data to make them directly comparable to his own, and submitted the results to Wilhelm Olbers in 1804.

Olbers, a physician and highly esteemed amateur astronomer, was impressed with the agreement between Bessel’s observations and Edmond Halley’s calculation of the orbit. He urged Bessel to improve it further with more observations. Olbers was impressed enough to recommend Bessel in 1806 for a position as assistant at a private observatory near Bremen. Bessel made further observations of the comet and published the results in 1807 to wide acclaim.

Life’s Work

In 1809, Bessel was appointed director of the new observatory at Königsberg, where he remained for the rest of his life. His early fame came from his reduction of the earlier observations of James Bradley. Bradley’s measurement of the apparent position of stars had to be corrected for the motion of Earth, the bending of starlight as it passes through the air, and instrument errors. With sufficient care, any observation can be reduced to a universal coordinate system.

While waiting for the construction of the observatory, Bessel worked on reducing Bradley’s observations of more than thirty-two hundred stars with the goal of producing a reference system for measuring the positions of other stars. Bessel received the Lalande Prize of the Institute of France for his production of tables of refraction based on these observations. In 1818, he completed the reduction and published the results in his work Fundamenta astronomiae (1818; fundamental astronomy). This work provided the most accurate positions of a chosen set of stars. Accurate positions of a few stars are required to form the basis for extremely accurate measurements of positions for all other stars. This work has been said to mark the birth of modern astrometry. Bessel also provided accurate proper motions of many stars. (The so-called fixed stars actually move a very small amount over the centuries. When all perturbing effects are removed, the motion that is left to the star is called its proper motion.)

Bessel’s next important contribution was to increase the accuracy of the measurement of stellar positions and motions. In 1820, he determined the position of the vernal equinox with great accuracy. The equinox is employed as the origin of the coordinate system used to record a star’s position. He further improved accuracy in his work Tabulae regiomontanae (1830; Refraction Tables , 1855), in which he published the mean positions for thirty-eight stars for the period 1750-1850. In 1821, he noticed a systematic error in observation that was peculiar to each observer and called it the personal equation. This systematic error was reduced as each observer became more experienced, but it never disappeared. Bessel devised a method to remove the error.

Identifying and measuring the proper motion of stars was crucial in producing an important contribution to astronomy. The slight but periodic variation in proper motion of a few stars was not accountable by considering the motion of Earth or instrumental factors. In making the observations that later appeared in the Refraction Tables, Bessel suggested that the variations in proper motion of the stars Sirius and Procyon could be explained by the existence of an as-yet-unseen companion star. More than a century later, the companions to these stars were observed, as well as companions to many others.

Bessel also made important contributions to mathematics. Prior to Bessel, it was common for observers of the heavens to record their data and only later, if ever, reduce that data. Bradley, whose observations Bessel used extensively, carefully noted any possible perturbing effects in his observations. Nevertheless, reduction of the data for the positions of the stars was put aside in favor of recording lunar data. Bessel emphasized the need for the data reduction to be done immediately by the observer. Such reduction required extensive manipulation of complicated equations. In the process of developing ways to remove errors, Bessel noticed that he could use a class of functions that solved problems involving the perturbing influence of one planet on the orbit of another. He systematically investigated and described this class of functions in 1824. These functions, which bear Bessel’s name, are not restricted to astronomy: They are used in the solution of a wide variety of problems in physics, mathematics, and engineering.

Another direct benefit of increased accuracy in stellar positions was Bessel’s determination of the distance to a star, which is his most important contribution to astronomy. Although many astronomers had earlier claimed to have measured stellar distance using methods based on questionable assumptions, and although two of Bessel’s contemporaries also correctly determined such distances, Bessel’s comprehensive treatment of the data and his high accuracy of observations were convincing to his contemporaries.

As Earth moves in its annual orbit around the sun, the stars appear to move across the dome of the sky. By determining the location of a star at opposite ends of Earth’s orbit and using some simple trigonometry, the distance to a star can be measured. However, this so-called parallax (the apparent motion of an object caused by the motion of the observer) was small because the stars are very far away. The parallax had therefore never been measured. Indeed, some opponents of the heliocentric theory, according to which Earth revolves around the sun, used this failure to measure parallax as an argument against the theory. Astronomers had a rough figure for the radius of Earth’s orbit and some idea of the extent of the solar system, but there did not seem to be a way to determine stellar distances that did not require the assumption that all stars had the same intrinsic brightness.

Earlier attempts at measuring parallax involved circumventing the problem of the immeasurably small parallax by looking at two stars that appeared to be very near to each other but were of different brightness. It was thought that the dimmer star would be farther away and that observing the relative change of position could lead to a determination of stellar distances. This method did not work, because not all stars are of the same intrinsic brightness. Most stars that appear near to each other are in fact binary stars and really are near each other.

Bessel used a different approach: He assumed that stars with large proper motions are closer than stars with small proper motions. He chose the star known as 61 Cygni, because it had the largest proper motion known. He used a new measuring device called the Fraunhofer heliometer (after Joseph von Fraunhofer, a nineteenth century optician), which was designed to measure the angular diameter of the sun and the planets. Its manner of comparing the images from two objects to determine angular diameter was more accurate than earlier instruments. Using nearby stars for comparison and observing for eighteen months, Bessel was able to measure a parallax of slightly under one-third of a second of arc, which is equal to the width of a dime viewed from twenty miles. From this amount of parallax, Bessel calculated that 61 Cygni was 10.9 light years or seventy trillion miles away. He completed his calculations and published the results in 1838.

The last six years of Bessel’s life were marked by deteriorating health, but he managed to complete a number of works before his death from cancer in March, 1846.

Significance

Friedrich Wilhelm Bessel made important contributions to astronomy, mathematics, and geodesy. His work marks the turning point from a concern with planetary, solar, and lunar observations to investigations of the stars. His measurement of the distance to a star is noteworthy because it settled the centuries-old question of whether stars exhibited parallax. The care he took in his observations set much higher standards for the science of astronomy. Bessel’s goal was to observe the stars accurately enough to predict their motion and to establish a reference system for their positions. As part of that plan, he developed methods for the careful determination of instrument and observer error, conducted years of observations himself, and developed the mathematical techniques to reduce the data. Bessel’s lasting achievement was to raise the science of observing, reducing, and correcting astronomical data to an art.

Bibliography

Clerke, Agnes. A Popular History of Astronomy During the Nineteenth Century. New York: Macmillan, 1887. Although written during the period it was supposed to cover, this work has several redeeming qualities. Expressly written for a general audience, the book’s language is clear and precise. A valuable record of what near-contemporaries thought of Bessel.

“The Deep Sky.” Astronomy 31, no. 8 (August, 2003): 65. Describes the special place the star system Cygnus holds in the history of astronomy because of Bessel’s measurements of 61 Cygni.

Herrmann, Dieter B. The History of Astronomy from Herschel to Hertzsprung. Translated by Kevin Krisciunos. Rev. ed. New York: Cambridge University Press, 1984. Traces the history of astronomy from 1780 to 1930. Written from a Marxist perspective.

Hirshfeld, Alan. Parallax: The Race to Measure the Cosmos. New York: W. H. Freeman, 2001. Bessel’s measurement of 61 Cygni is recounted in this history of astronomers’ attempts to measure the distances to stars and plants by parallax observation.

Hoskin, Michael A. Stellar Astronomy: Historical Studies. Chalfont, England: Science History, 1982. A collection of material published in Journal for the History of Astronomy, with the addition of some new material. Attempts a synthesis of existing scholarship on the history of stellar (as opposed to planetary) astronomy as of the early 1980’s.

Pannekoek, Anton. A History of Astronomy. New York: Interscience, 1961. Traces the history of astronomy from antiquity to the present. Part 3, “Astronomy Surveying the Universe,” contains information on Bessel and places him in the historical context of nineteenth century astronomy.

Williams, Henry Smith. The Great Astronomers. Westport, Conn.: Greenwood Press, 1930. Reprint. New York: Newton, 1932. Book 5 deals with Bessel, among other subjects. Concerns parallax and Bessel’s contributions to measuring the distance to 61 Cygni. Describes the work of Bessel’s contemporaries who measured the parallax of other stars at about the same time.