also known as Longomontanus
(b. Longberg, Jutland, Denmark, 4 October 1562;d. Copenhagen, Denmark, 8 October 1647)
Severin was the son of Søren Poulsen and Maren Christensdatter, both of whom were humble peasants. Finding education an uncertain and intermittent luxury, especially after the early death of his father, Severin did not complete his basic education until 1588. At that time he entered the service of Tycho Brahe and stayed with him until 1597, when Tycho left Denmark. After his Wanderjahre in Germany, Severin received the M.A. at the University of Rostock and then returned home to begin his career. By 1607 he was professor of mathematics and astronomy at the University of Copenhagen, where he remained until his death.
When Tycho died in 1601, his program for the restoration of astronomy was unfinished. The observational aspects were complete, but two important tasks remained: the selection and integration of the data into accounts of the motions of the planets, and the presentation of the results of the entire program in the form of a systematic treatise.
Severin, Tycho’s sole disciple, assumed the responsibility and fulfilled both tasks in his voluminous Astronomia danica (1622). Regarded as the testament of Tycho, the work was eagerly received and quickly won a place in seventeenth-century astronomical literature. Even after the appearance of Kepler’s Tabulae Rudolphinae (1627), a rival work that bore the imprimatur of Tycho, Severin’s Astronomia danica retained sufficient prestige (despite its staidness) to warrant reprinting in 1640 and 1663.
Unfortunately Severin found himself looking backward to Tycho, instead of forward into the seventeenth century. Although Severin worked and wrote in the era of Kepler and Galileo, he denounced ellipses, denied heliocentrism, denigrated the telescope, and ignored logarithms. Severin departed from Tycho in only one significant respect–he assumed diurnal rotation of the earth.
Because Severin’s career was virtually determined by his unique status as the literary heir of Tycho, it is impossible to form, to form an independent estimate of his contemporary reputation. He was highly esteemed by Tycho for his skill at manipulating observational data, and he may have played an important role in Tycho’s remarkable research on the lunar theory. Regardless of his competence as a planetary theorist, Severin’s reputation will always suffer in comparison with Kepler’s achievements in the same task. In addition to his astronomical interests, Severin also also displayed considerable enthusiasm for pure mathematics, but with notabley less success. Concerned principally with the quadrature of the circle, he believed that he had solved the problem with a precise evaluation of π as equal to 78/43 √ 3.
A complete bibliography of Severin’s works is in H. Ehreneron-Möller, Forfatterlexicon, V (Copenhagen, 1929), 181–185. His major works are Cyclometria exlunulis reciproce demonstrata (Copenhagen, 1612); and Inventio quadraturae circuit (Copenhagen, 1634): and Introduction in theatrum astronomicum (Copenhagen, 1639).
On Severin and his work, see J.-B.-J. Delambre. Histoire de I’astronomie moderne, I (Paris, 1821), 262–287.
Victor E. Thoren