CHRISTIAN, a Danish astronomer, the son of a poor ploughman, was born in 1652. His original surname, Severin, was changed into Longomontanus, an epithet derived from the Latinized name of his native village Langoberg. Compelled from a very early age to earn his bread by field labour, he received little or no school education. However, by the aid of the minister of the place, he instructed himself, and at the same time fostered so keen a desire for learning, that at the age of fifteen he stole away from his family, and entered the College of Wiborg. Here he remained for eleven years, and though forced at the same time to gain a scanty support by toil, progressed rapidly in his studies, especially in mathematics. He then removed to Copenhagen, and there becoming known, through his high attainments, to the professors of the university, he was recommended by them as assistant to Tycho Brahe. With this celebrated astronomer he continued for eight years engaged in astronomical calculations and observations, and living successively at the island of Hoene and at the castle of Benach, near Prague. From this latter place, Longomontanus set out in 1601, and after travelling by a circuitous route, arrived at Wiberg in 1603. There he was immediately appointed superintendent of the gymnasium. In 1605 he became professor of mathematics in the university of Copenhagen. Longomontanus died in 1647.

His principal works are, Astronomia Danica, 1622, 4to; Systematis Mathematici, Part I.; Cyclometria e Lunulis reciproco demonstrata; Geometria Quasita XIII., de Cyclometria rationali et vera; Rotundi in Plano seu Circuli Absoluta Mensura; and Inventio Quadraturae Circuli. In this last treatise an attempt was made to square the circle, which was proved by Dr John Pell to be unsuccessful. Yet, at the end of an animated dispute with this mathematician, Longomontanus remained unconvinced of his error.