FRIEDRICH WILHELM BESSEL (1784-1846), German astronomer, was born at Minden on the 22nd of July 1784. Placed at the age of fifteen in a counting-house at Bremen, he was impelled by his desire to obtain a situation as supercargo on a foreign voyage to study navigation, mathematics and finally astronomy. In 1804 he calculated the orbit of Halley's comet from observations made in 1607 by Thomas Harriot, and communicated his results to H. W. M. Olbers, who procured their publication (Monatliche Correspondenz, x. 425), and recommended the young aspirant in 1805 for the post of assistant in J. H. Schroter's observatory at Lilienthal. A masterly investigation of the comet of 1807 (Konigsberg, 1810) enhanced his reputation, and the king of Prussia summoned him, in 1810, to superintend the erection of a new observatory at Konigsberg, of which he acted as director from its completion in 1813 until his death. In this capacity he inaugurated the modern era of practical astronomy. For the purpose of improving knowledge of star-places he reduced James Bradley's Greenwich observations, and derived from them an invaluable catalogue of 3222 stars, published in the volume rightly named Fundamenta Astronomiae (1818). In Tabulae Regiomontanae (1830), he definitively established the uniform system of reduction still in use. During the years 1821-1833, he observed all stars to the ninth magnitude in zones extending from-15° to +45° dec., and thus raised the number of those accurately determined to about 50,000. He corrected the length of the seconds' pendulum in 1826, in a discussion re-published by H. Bruns in 1889; measured an arc of the meridian in East Prussia in 1831-1832; and deduced for the earth in 1841 an ellipticity of 2 4 9. His ascertainment in 1838 (Astr. Nach., Nos. 365-366) of a parallax of o". 3 i for 61 Cygni was the first authentic result of the kind published. He announced in 1844 the binary character of Sirius and Procyon from their disturbed proper motions; and was preparing to attack the problem solved later by the discovery of Neptune, when fatal illness intervened. He died at Konigsberg on the 17th of March 1846. Modern astronomy of precision is essentially Bessel's creation. Apart from the large scope of his activity, he introduced such important novelties as the effective use of the heliometer, the correction for personal equation (in 1823), and the systematic investigation of instrumental errors. He issued 21 volumes of Astronomische Beobachtungen auf der Sternwarte zu Konigsberg (1815-1844), and a list of his writings drawn up by A. L. Busch appeared in vol. 24 of the same series. Especial attention should be directed to his Astronomische Untersuchungen (2 vols. 1841-1842), Populeire Vorlesungen (1848), edited by H. C. Schumacher, and to the important collection entitled Abhandlungen (4 vols. 1875-1882), issued by R. Engelmann at Leipzig. His minor treatises numbered over 350. In pure mathematics he enlarged the resources of analysis by the invention of Bessel's Functions. He made some preliminary use of these expressions in 1817, in a paper on Kepler's Problem (Transactions Berlin Academy, 1816-1817, p. 49), and fully developed them seven years later, for the purposes of a research into planetary perturbations (Ibid. 1824, pp. 1-52).
See also H. Durege, Bessels Leben and Wirken (Zurich, 1861); J. F. Encke, Geddchtnissrede auf Bessel (Berlin, 1846); C. T. Anger, Erinnerung an Bessels Leben and Wirken (Danzig, 1845); Astronomische Nachrichten, xxiv. 49, 33 1 (1846); Monthly Notices Roy. Astr. Society, vii. 1 99 (1847); Allgemeine deutsche Biographie, ii. 558-567.
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