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Friedrich Wilhelm Bessel (22 July 1784-17 March 1846) was a German astronomer, mathematician, physicist, and geodesist.
He was the first astronomer who determined reliable values for the distance from the Sun to another star by the method of parallax. Certain important mathematical functions were first studied systematically by Bessel and were named Bessel functions in his honour.
Bessel was born in Minden, Westphalia, then capital of the Prussian administrative region Minden-Ravensberg, as second son of a civil servant into a large family. At the age of 14 he left the school, because he did not like the education in Latin language, and apprenticed in the import-export concern Kulenkamp at Bremen. 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 Heinrich Wilhelm Olbers, a practising physician of Bremen and well-known astronomer, by producing a refinement on the orbital calculations for Halley's Comet in 1804, using old observation data taken from Thomas Harriot and Nathaniel Torporley in 1607. Franz Xaver von Zach edited the results in his journal Monatliche Correspondenz.
Having finished his commercial education, Bessel left Kulenkamp in 1806 and became assistant at Johann Hieronymus Schröter's private observatory in Lilienthal near Bremen as successor of Karl Ludwig Harding. There he worked on James Bradley's stellar observation data to produce precise positions for some 3,222 stars.
Despite lacking any higher education, especially at university, Bessel was appointed director of the newly founded Königsberg Observatory by King Frederick William III of Prussia in January 1810, at the age of 25, and remained in that position until his death. Some elder professors of the Philosophical Faculty disputed Bessel's right to teach mathematics without any academic degree. Therefore, he turned to his fellow Carl Friedrich Gauss, who provided the award of an honorary doctor degree from the University of Göttingen in March 1811. Both scientists were in correspondence from 1804 to 1843. In 1837 they got in quarrel about Gauss's habit of very slow publication.
In 1842, Bessel took part in the annual meeting of the British Association for the Advancement of Science in Manchester, accompanied by the geophysicist Georg Adolf Erman and the mathematician Carl Gustav Jacob Jacobi, where he gave a report on astronomical clocks.
Bessel married Johanna Hagen, the daughter of the chemist and pharmacist Karl Gottfried Hagen who was the uncle of the physician and biologist Hermann August Hagen and the hydraulic engineer Gotthilf Hagen, the latter also Bessel's student and assistant from 1816 to 1818. The physicist Franz Ernst Neumann, Bessel's close companion and colleague, was married to Johanna Hagen's sister Florentine.
Neumann introduced Bessel's exacting methods of measurement and data reduction into his mathematico-physical seminar, which he co-directed with Carl Gustav Jacob Jacobi at Königsberg. These exacting methods had a lasting impact upon the work of Neumann's students and upon the Prussian conception of precision in measurement.
While the observatory was still in construction Bessel elaborated the Fundamenta Astronomiae based on Bradley's observations. As a preliminary result he produced tables of atmospheric refraction that won him the Lalande Prize from the French Academy of Sciences in 1811. The Königsberg Observatory began operation in 1813.
Starting in 1819, Bessel determined the position of over 50,000 stars with a meridian circle from Reichenbach, assisted by some of his qualified students. The most prominent of them was Friedrich Wilhelm Argelander, his successors were Otto August Rosenberger and August Ludwig Busch.
Bessel determined the first reliable value for the distance between a star and the Solar System with a heliometer from Fraunhofer using the method of stellar parallax.
In 1838 he published a parallax of 0.314 arcseconds for 61 Cygni, which indicated that the star is 10.3 ly away.
Compared with the current measurement of 11.4 ly, Bessel's figure had an error of 9.6%. Thanks to these results astronomers had not only enlarged the vision of the universe well beyond the cosmic magnitude, but after the discovery in 1728 by James Bradley of the aberration of light a second empirical evidence of the Earth's relative movement was produced. A short time later Friedrich Georg Wilhelm Struve and Thomas Henderson reported the parallaxes of Vega and Alpha Centauri.
Precise measurements with a new meridian circle from Adolf Repsold allowed Bessel to notice deviations in the motions of Sirius and Procyon, which 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 by Alvan Graham Clark in 1862, the first discovery of a white dwarf. John Martin Schaeberle discovered Procyon B in 1896.
Bessel was the first scientist who realized the effect later called personal equation, that several simultaneously observing persons determine slightly different values, especially recording the transit time of stars.
In 1824, Bessel developed a new method for calculating the circumstances of eclipses using the so-called Besselian elements. His method simplified the calculation to such an extent, without sacrificing accuracy, that it is still in use.
More information: Maths History
We must admit with humility that,
while number is purely a product of our minds,
space has a reality outside our minds,
so that we cannot completely prescribe its properties a priori.
Carl Friedrich Gauss
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