MöLLER, DIDRIK MAGNUS AXEL

(b. Sjörup, Sweden, 16 February 1830; d. Lund, Sweden, 26 October 1896)

astronomy.

Möller studied at the University of Lund, where in 1853 he became a docent, in 1855 associate professor, and in 1863 full professor. He was also director of the observatory, which he founded, until his resignation in 1895.

Möller’s predecessor, John Mortimer Agardh, had sought to establish an observatory at Lund, but his efforts bore fruit only after his death. The government granted money for an observatory in 1863, and during the following years Möller devoted much of his time to its completion. The main building, still in use, was dedicated in 1867. Among the instruments Möller ordered were a refractor with a nine-inch objective and a thirteen-foot focal length, installed in 1867, and a meridian circle with a six-inch aperture and a seven-foot focal length, mounted in 1874. He intended these two instruments to be used simultaneously. Differential measurements of moving objects (comets and planets) and stars were made with the refractor, and accurate positions of the stars were determined with the meridian circle. Positions of about 11,000 stars in the declination zone + 35° to +40° were measured with the meridian circle, and observations of planets, comets, and double stars were made with the refractor. The recently modernized meridian circle is still in use.

Möller’s most important contributions to astronomy concern the motion of the comet discovered in 1843 by the French astronomer H. Faye and that of the asteroid Pandora. His interest in Faye’s comet was aroused by J. F. Encke, who, on the basis of Newton’s law, had computed the orbit of a comet of very short period (3.3 years) first seen in 1786 (later called Encke’s comet) and found that the comet showed a retardation in relation to the computed positions. His explanation was that in interplanetary space there is a low-density medium which slows the motion of the comet. Möller started to test Encke’s hypothesis by using Faye’s comet, which has a period of 7.5 years. He first concluded that the observations of this comet indeed indicated a retardation in comparison with theory. But through new, laborious, and careful calculations he was able to show that full agreement between theory and observations was obtained on the basis of Newton’s theory: when the comet was observed in 1865, 1873, and 1880, the agreement was perfect. Encke’s hypothesis could be rejected. For this brilliant work Möller was awarded the gold medal of the Royal Astronomical Society in 1881.

In several papers Möller studied the motion of the asteroid Pandora. He first calculated the special perturbations and later, according to Hansen’s method, the general perturbations, including certain second-order perturbations depending on the masses of Jupiter and Saturn. In this case too Möller, through his skillful and accurate calculations, reached extremely good agreement with the observations.

Besides his theoretical work Möller performed extensive series of observations of planets and comets.

Möller was an exceptionally able person, and was frequently called on for special commissions by the university and other agencies. He served as rector of the university in 1874–1875 and 1891–1895.

BIBLIOGRAPHY

Möller contributed many articles on Faye’s comet and Planet 55 Pandora to Astronomische Nachrichten and Viertelijahrsschrift der Astronomischen Gesellschaft. Poggendorff, III, 924, gives a list of these and other publications.

In addition see Erik Holmberg, “Lundensisk astronomi under ett sekel,” in Cassiopeia (1949), 21–27; Anders Lindstedt, “Didrik Magnus Axel Möller,” in Minnesteckning, Kungliga Svenska Vetenskapakademiens lefnadsteckningar, 4 (1899–1912), no. 79; and Carl Schalén, Nils Hansson, and Arvid Leide, “Astronomiska Observatoriet vid Lunds Universitet in Lands Universitets historia, 4 (1968), 52–72.

C. Schalen