ROEMER (Olaus)

, a noted Danish astronomer and mathematician, was born at Arhusen in Jutland, 1644; and at 18 years of age was sent to the university of Copenhagen. He applied assiduously to the study of the mathematics and astronomy, and became so expert in those sciences, that when Picard was sent by Lewis the XIVth in 1671, to make observations in the north, he was greatly surprised and pleased with him. He engaged him to return with him to France, and had him presented to the king, who honoured him with the dauphin as a pupil in mathematics, and settled a pension upon him. He was joined with Picard and Cassini, in making astronomical observations; and in 1672 he was admitted a member of the academy of sciences.

During the ten years he resided at Paris, he gained great reputation by his discoveries; yet it is said he complained afterwards, that his coadjutors ran away with the honour of many things which belonged to him. Here it was that Roemer, first of any one, found out the velocity with which light moves, by means of the eclipses of Jupiter's satellites. He had observed for many years that, when Jupiter was at his greatest distance from the earth, where he could be observed, the emersions of his first satellite happened constantly 15 or 16 minutes later than the calculation gave them. Hence he concluded that the light reflected by Jupiter took up this time in running over the excess of distance, and consequently that it took up 16 or 18 minutes in running over the diameter of the earth's orbit, and 8 or 9 in coming from the sun to us, provided its velocity was nearly uniform. This discovery had at first many opposers; but it was afterwards confirmed by Dr. Bradley in the most ingenious and beautiful manner.

In 1681 Roemer was recalled back to his own country by Christian the Vth, king of Denmark, who made him professor of astronomy at Copenhagen. The king employed him also in reforming the coin and the architecture, in regulating the weights and measures, and in measuring and laying out the high roads throughout the kingdom; offices which he discharged with the greatest credit and satisfaction. In consequence he was honoured by the king with the appointment of chancellor of the exchequer and other dignities. Finally he became counsellor of state and burgomaster of, Copen- | hagen, under Frederic the IVth, the successor of Christian. Roemer was preparing to publish the result of his observations, when he died the 19th of September 1710, at 66 years of age: but this loss was supplied by Horrebow, his disciple, then professor of astronomy at Copenhagen, who published, in 4to, 1753, various observations of Roemer, with his method of observing, under the title of Basis Astronomiæ.—He had also printed various astronomical observations and pieces, in several volumes of the Memoirs of the Royal Academy of Sciences at Paris, of the institution of 1666, particularly vol. 1 and 10 of that collection.