Halm, Jacob Karl Ernst

(b. Bingen, Germany, 30 November 1866; d. Stellenbosch, Union [now Republic] of South Africa, 17 July 1944)

astronomy.

Halm attended the Gymnasium at Bingen, then studied mathematics at the universities of Giessen (1884–1886), Berlin (1886–1887), and Kiel (1887–1889), where he took his doctorate with a dissertation on homogeneous linear differential equations. From 1889 he was assistant at the astronomical observatory in Strasbourg. In 1895 Halm was invited to become assistant astronomer of the Royal Observatory at Edinburgh; in 1907 he became chief assistant at the Cape Observatory, Cape Town, from which post he resigned in 1926. He was married in 1894 to Hanna Bader; they had one son and two daughters.

Halm’s method of attacking astronomical questions was to make observations and then try to find a convincing explanation for them. A good example of this was his work at Edinburgh on the rotation of the sun. He used spectroscopic measurements of the radial velocity of the solar limb to determine the velocity of rotation in various heliographic latitudes. His results agreed fairly well with observations made previously at Uppsala—and with those made later at Mt. Wilson in California. But a careful analysis of his results led Halm to the hypothesis that the velocity of the solar rotation is slightly variable in the course of the eleven-year cycle of the variation of sunspots. The minuteness of the postulated variation makes it impossible even now to decide whether his hypothesis is correct.

At the Cape Observatory, Halm conducted extensive research on stellar statistics. He examined a great number of radial velocities of fixed stars and concluded that there was a systematic streaming of those stars which astronomers usually call B-type stars. By studying the distribution of the fixed stars in space he found indications that there was absorbing matter in the galactic system. The relation between a star’s mass and luminosity that he suggested was later well established by A. S. Eddington. Halm also did very intensive work to create good standard sequences to be used for photographic photometry in the southern sky.

Halm did much to make astronomy popular in South Africa, assisting amateur astronomers in their work and publishing A Universal Sundial (1924), a little book on the construction of sundials. He was also an enthusiastic musician.

BIBLIOGRAPHY

I. Original Works. Halm’s writings include “Further Considerations Relating to the Systematic Motions of the Stars,” in Monthly Notices of the Royal Astronomical Society, 71 (1911), 610–639; “On the Rotation of the Sun’s Reversing Layer,” ibid., 82 (1922), 479–483; and Magnitudes of Stars Contained in the Cape Zone Catalogue of 20,843 Stars (London, 1927).

II. Secondary Literature. See E. von der Pahlen, Lehrbuch der Stellarstatistik (Leipzig, 1937), pp. 261–267, 728–729; and M. Waldmeier, Ergebnisse und Probleme der Sonnenforschung (Leipzig, 1941), pp. 43–49, 112, 123.

F. Schmeidler