Jacob K.E. Halm, mathematician and astronomer, was the eldest son of Karl Joseph Halm and his wife Sabinal, born Dietrich. He studied at the universities of Giessen, Berlin and Kiel, and was awarded the degree Doctor of Philosopy in mathematics at the latter institution in 1890. His mathematical interests related mainly to differential equations, the mathematical theory of the tides in lakes, and the structure of atomic spectra. He started his professional career in 1889 as assistant at the Strasbourg Observatory (then the Imperial University Observatory in Strassburg, Germany, now in France), where he made observations with the meridian circle and heliometer, but also took part in the reduction of the observations. His first scientific paper (1889, in German) dealt with the elements of the orbit of Barnard's Comet. On 4 August 1894 he married Hanna Bader of Basle, with whom he had a son and two daughters.
In 1895 Halm published a theoretical account of the daily variations in air temperature (in German). That same year he moved to Scotland to take up the post of first-class assistant at the newly erected Royal Observatory in Edinburgh, a post he held for 12 years. There he truly established himself as an astronomer, rather than a mathematician, publishing a number of papers on his observations of comets and planets, and on the relationship between solar activity, earth magnetism, and variations in latitude. Other research by him dealt with the theory of star atmospheres and the spectroscopy of Fraunhofer lines. He was one of a small group of astronomers who first realised that the spectroscope could be used to determine accurately the speed of rotation of the sun and the radial velocities of stars, and from 1901 carried out a thorough study of the rotation periods of different regions of the sun. This work was published in a paper on "Spectroscopic observations of the rotation of the sun" in the Transactions of the Royal Society of Edinburgh (1904). It led to his being awarded the society's Brisbane Gold Medal, having already been elected a Fellow. He was elected a Fellow also of the Royal Astronomical Society. When the director of the observatory, Professor Copeland, became ill in about 1901 Halm took over many of his administrative duties and lectured on his behalf at Edinburgh University.
In April 1907, following the promotion of S.S. Hough* to His Majesty's Astronomer at the Royal Observatory, Cape of Good Hope, Halm was appointed chief assistant at the observatory. He assumed duty on 1 July that year and remained until his retirement twenty years later. During this period he was responsible for several advances in astronomical knowledge. At first he participated in a project initiated earlier by Sir David Gill* to determine the mean distance between the earth and the sun by spectroscopically measuring the variations in radial velocity of stars caused by the motion of the earth in its orbit. This work was completed in May 1908. Thereafter he used the McClean telescope to systematically determine the radial velocities of the brightest southern stars. The results were published in 1915 and allowed Halm to identify a third star stream (of very hot and luminous stars) in addition to the two streams described by J.C. Kapteyn* in 1905. He played a leading role in proving the existence of an interstellar medium; was the first to suggest a mass-luminosity relationship for stars; and made significant contributions to the determination of the photographic magnitudes of stars.
In addition to his professional work Halm played an important role in the advancement of astronomy in South Africa. In 1912 he became a foundation member of the Cape Astronomical Association and was elected its first president (with Hough as honorary president). His presidential address was followed by an illustrated lecture on spectroscopy - the first of many lectures. When the society was revived in 1916 after a lapse of two years he was elected honorary joint vice-president. And when the association amalgamated with the Johannesburg Astronomical Association to form the Astronomical Society of South Africa in 1922 he served as joint vice-president of the latter, then as president for 1924/5, and as a member of its council until 1935. During this period and up to 1937 he contributed several papers to the society's Journal. In 1929 he was furthermore a member of the first South African National Committee in Astronomy. He became well known for his popular pamphlet, A universal sundial: description of an instrument which can be constructed and set up by anyone so as to give correct time in all countries (Cape Town, 1923). This relatively simple instrument could provide local solar time at any latitude, which was most useful in the country districts before radio time signals became available.
In the year of his arrival Halm became a member of the South African Philosophical Society. Soon thereafter he was elected a Fellow of its successor, the Royal Society of South Africa, and served on its council for some time. In 1910 he joined the South African Association for the Advancement of Science, serving as president of Section A (which included astronomy) for 1923/4 and delivering his presidential address "On the luminosities of the stars" (South African Journal of Science, 1924, Vol. 21, pp. 39-51). In 1908 he was an examiner in mathematics and applied mathematics for the BA Honours examinations of the University of the Cape of Good Hope.
Halm was a modest man of great personal charm. He was an accomplished violinist, a member of the Chamber Music Union, and became well known in Cape Town for his performances on the viola. Upon his retirement in 1927 he settled on a farm at Stellenbosch and for some years lectured on astronomy at the University of Stellenbosch. From May to October 1935 he returned temporarily to Cape Town to take charge of the Royal Observatory when the director went overseas.