Roemer

, or Rømer (Olaus), a Danish astronomer and mathematician, was born at Arhusen in Jutland in 1644; and, at eighteen, was sent to the university of Copenhagen. He applied himself assiduously to the study of mathematics and astronomy, and became such an adept in those sciences, that, when Picard was sent by Lewis XIV. in 1671, | to make observations in the North, he was so pleased with him, that he engaged him to return with him to France, and had him presented to the king, who ordered him to teach the dauphin mathematics, and settled a pension on him. He was joined with Picard and Cassini, in making astronomical observations; and, in 1672, was admitted a member of the academy of sciences. During the ten years he resided at Paris, he gained a prodigious reputation by his discoveries; yet is said. to have complained afterwards that his coadjutors ran away with the honour of many things which belonged to him. In 1681, Christian V. king of Denmark called him back to his own country, and made him professor of astronomy at Copenhagen. He employed him also in reforming the coin and the architecture, in regulating the weights and measures, and in measuring the high roads throughout the kingdom. Frederic IV. the successor of Christian, shewed the same favour to Roemer, and conferred new dignities on him. He was preparing to publish the result of his observations, when he died Sept. 19, 1710, aged 66; but some of his observations, with his manner of making those observations, were published in 1735, under the title of “Basis Astronomise,” by his scholar Peter Horrebow, then professor of astronomy at Copenhagen. Roemer was the first who 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 J 6 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 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. ¹