Olaus, a celebrated Danish mathematician and astronomer, was born at Arhusen in Jutland in the year 1644, and was sent to the university of Copenhagen at the age of eighteen. By assiduous application to the study of astronomy and mathematics, he became so eminent in those sciences that Picard was astonished and delighted with him, when making observations in the north, by the order of Louis XIV. He was prevailed on to accompany Picard to France, and being presented to the king, he was chosen the Dauphin's tutor in the study of mathematics. He was afterwards united with Picard and J. D. Cassini in making astronomical observations, and became a member of the Academy of Sciences in 1672. His discoveries acquired for him great reputation during his ten years' residence at Paris; and he did not scruple to assert that Picard and Cassini took the merit of many things which belonged exclusively to himself. Roemer was the first person who discovered the velocity with which light moves, by means of the eclipses of Jupiter's satellites, Roermonde determining it to be about seven or eight minutes in coming from the sun to the earth. (See Dissertation Fourth.) This opinion was opposed by many, but it was afterwards demonstrated in a most ingenious manner by Dr Bradley.

Christian V., King of Denmark, recalled Roemer to his native country in the year 1681, when he was appointed professor of astronomy at Copenhagen. He was likewise employed in the reformation of the coin and architecture of the country, in regulating the weights and measures, and in laying out the high roads throughout the kingdom. The consequence was, that the king bestowed many dignities upon him, and among others appointed him chancellor of the exchequer. In fine, he was made councillor of state and burgomaster of Copenhagen under Frederic IV., who succeeded Christian. While Roemer was engaged in preparing to publish the result of his observations, he was cut off on the 19th of September 1710, when about sixty-six years of age. Horrebow, his disciple and successor in the chair of astronomy, made up this loss by publishing, in 4to, in 1753, various observations of Roemer, with his method of observing, under the title of Basis Astronomiae. He had printed various astronomical observations and pieces in several volumes of the Memoirs of the Royal Academy of Sciences at Paris. (See Astronomy, History of; and Micrometer.)