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OLE (Latinized Olaus) (1644-1710), Danish astronomer, was born at Aarhuus in Jutland on the - 25th of September 1644. He became in 1662 the pupil and amanuensis of Erasmus Bartholinus at Copenhagen, and assisted J. Picard in 1671 to determine the geographical position of Tycho Brahe's observatory (Uraniborg on the island of Hveen). In 1672 he accompanied Picard to Paris, where he remained nine years, occupied with observations at the new royal observatory and hydraulic works at Versailles and Manly. On the 22nd of November 1675 he read a paper before the Academy on the successive propagation of light as revealed by a certain inequality in the motion of the first of Jupiter's satellites. A scientific mission to England in 1679 made him acquainted with Newton, Halley and Flamsteed. In 168r, on the summons of Christian V., king of Denmark, he returned to Copenhagen as royal mathematician and professor of astronomy in the university; and from 1688 he discharged, besides, many important admini strative functions, including those of mayor (1705), chief of police and privy councillor. He died at Copenhagen on the 23rd of September 1710.

Roemer will always be remembered as the discoverer of the finite velocity of light. He showed besides wonderful ingenuity in the improvement of astronomical apparatus. The first transit instrument worthy the name was in 1690 erected in his house. In the same year he set up in the university observatory an instrument with altitude and azimuth circles (for observing equal altitudes on both sides of the meridian) and an equatorial telescope. In 1704 he built, at his own cost, the so-called "Tusculan" observatory at Vridlosemagle, a few miles west of Copenhagen, and equipped it with a meridian circle (the transit instrument and vertical circle combined) and a transit moving in the prime vertical. Roemer thus effectively realized nearly all our modern instruments of precision, and accumulated with them a large mass of observations, all of which unfortunately perished in the great conflagration of the 21st of October 1728, except the three nights' work discussed by J. G. Galle (0. Roemeri triduum observationum astronomicarum a. 1706 institutarum, Berlin, 1845).

See E. Philipsen, Nordisk Universitets Tidskrift, v. I 1 (1860); P. Horrebow, Basis Astronomiae (Copenhagen, 1 735); J. B. J. Delambre, Hist. de l'astr. moderne, ii. 632; J. F. Montucla, Hist. des mathematiques, ii. 4 8 7, 579; R. Grant, Hist. of Phys. Astronomy, p. 461; R. Wolf, Gesch. der Astronomie, pp. 45 2, 4 8 9, 57 6; J. F. Weidler, Historia Astronomiae, p. 538; W. Doberck, Nature,  :vii. 105; C. Huygens, Ouvres completes, t. viii. pp. 30-58; L. Ambronn, Handbuch der astr. Instrumentenkunde, ii. 552, 966;T. J. J. See, Pop. Astronomy, No. 105, May 1903.