Backlund, Jöns Oskar

(b. Långhem, Sweden, 28 April 1846; d. Pulkovo, Russia, 29 August 1916)

astronomy.

The main object of Backlund’s research was comet 1786 I, known as Encke’s comet or (in the U.S.S.R.) comet Encke-Backlund. Despite its forty-eight observed returns to perihelion between 1786 and 1967, this comet still puzzles astronomers; Backlund was the third man, following Johann Franz Encke and Friedrich Emil von Asten, to devote a major portion of his life to studying it, and the first to show that the long-term acceleration in its motion is subject to irregular changes, attributable to nongravitational forces.

Born in poverty, Backlund left school at an early age but nevertheless managed to prepare himself for entrance to the University of Uppsala, where he received a Ph.D. in 1875. After three years at the observatory of Dorpat, he went to Pulkovo Observatory, located about ten miles south of Petrograd (now Leningrad), to work under Otto von Struve. In 1886 he left Pulkovo for the Imperial Academy of Sciences of Petrograd, to which he had been elected in 1883, but returned in 1895 to serve for the rest of his life as director at Pulkovo. He also traveled throughout Europe and the United States to attend scientific meetings. He received worldwide recognition for his work on Encke’s comet, including the award in 1909 of the Gold Medal of the Royal Astronomical Society.

Although Backlund favored Encke’s idea that the comet’s accelerated motion was due to a thin interplanetary medium, he nevertheless wanted to make sure no gravitational accelerations had been overlooked. He therefore decided to recalculate all planetary perturbations since 1819 the year Encke first obtained an orbit for the comet. By 1886 Backlund had got the financial backing necessary to carry out this tremendous task and had hired a number of people as computers.

The results, published between 1892 and 1898, were impressive, but the comet continued to depart from predictions based upon them in a way that Backlund was unable to explain, although he considered two different possibilities: first, a patchy stream of meteoroids lying along the inner part of the comet’s orbit, then electrical forces related to the sunspot cycle. Until the advent of electronic computers the efforts of later workers served mainly to confirm Backlund’s suspicion that no single explanation would suffice. Current thinking tends toward the idea, first suggested in 1836 by Friedrich Wilhelm Bessel, that loss of mass by the comet itself is responsible.

BIBLIOGRAPHY

I. Original Works. Backlund’s first paper on Encke’s comet was written with A. Bonnsdorf and appeared as “Allmänna störingar, som of jorden förorsakas i Enckeska Kometens rörelse i en viss del af dess bana,” in Bihang till Kongliga Svenska Vetenskaps-Akademiens Handlingar, 4th series, 3 , no. 16 (1876), 39 pp.; another early work on the subject was “Ueber die Berechnung der allgemeinen Jupiterstörung des Encke’schen Cometen,” in Vierteljahrsschrift der Astronomischen Gesellschaft (Leipzig), 12 (1877), 313–323. His recalculation of the perturbations affecting Encke’s comet between 1819 and 1891 was published in six parts in Mémoires de l’Académie Impériale des Sciences de Saint-Pétersbourg under the general title “Calculs et Recherches sur la Cométe d’Encke”: 7th series, 38 , no. 8 (1892); 7th series, 41 , nos. 3, 7 (1893); 7th series, 42 , nos. 7, 8 (1894); 8th series, 6 , no. 13 (1898).

Three papers by Backlund summarize his attempts to reconcile the observed motion of Encke’s comet with theory: “Sur la Masse de la Planète Mercure et sur l’Accélération du Mouvement Moyen de la Comète d’Encke,” in Bulletin astronomique (Paris), 11 (1894), 473–485; “Vergleichung der Theorie des Encke’schen Cometen mit den Beobachtungen 1894–95,” in Mémoires de l’Académie Impériale des Sciences de Saint-Pétersbourg, 8th series, 16 , no. 3 (1904); and “Encke’s Comet, 1895–1908,” in Monthly Notices of the Royal Astronomical Society (London), 70 (1910), 429–442.

Approximately fifty other papers that Backlund wrote between 1874 and 1900 are listed in the Royal Society of London’s Catalogue of Scientific Papers, IX (London, 1891), 93, and XIII (Cambridge, 1914), 228–229. After 1900 he wrote about thirty more, mainly for various publications of the Pulkovo Observatory and of the St. Petersburg Academy. His last paper, “On Chandler’s Period in the Latitude Variation,” appeared posthumously in Monthly Notices of the Royal Astronomical Society (London), 77 (1917), 2–2.

II. Secondary Literature. Further details about Backlund’s life can be found in an obituary notice, signed H. H. T. [Herbert Hall Taylor], in Proceedings of the Royal Society (London), 94A (1918), xx-xxiv. The citation delivered by Hugh Frank Newall when Backlund received the Gold Medal of the Royal Astronomical Society provides an assessment of Backlund’s work by one of his contemporaries; it appeared in Monthly Notices of the Royal Astronomical Society (London), 69 (1909), 324–331. A retrospective evaluation, pointing out the shortcomings of Backlund’s methods, appeared in a paper by S. G. Makover and N. A. Bokhan, of the Leningrad Institute of Theoretical Astronomy, entitled “The Motion of Comet Encke-Backlund during 1898–1911 and a New Determination of the Mass of Mercury,” in Soviet Physics-Doklady, 5 (1961), 923–925.

For other recent material on Encke’s comet, sec; K. Wurm, “Die Kometen,” in Handbuch der Physik, Vol. 52, ed. S. Flügge (Berlin, 1959), p. 476; F. L. Whipple, “On the Structure of the Cometary Nucleus,” chap. 19 of The Moon, Meteorites and Comets, ed. B. M. Middlehurst and G. P. Kuiper (Chicago, 1963), pp. 648 and 659; and B. G. Marsden, “Comets and Nongravitational Forces,” in Astronomical Journal, 73 (1968), 367–379.

Sally H. Dieke