Olaus Rømer (ōlä´ŏŏs, ō´lə röm´ər), 1644–1710, Danish astronomer. He is noted for his discovery that light travels at a definite speed and does not move through space instantaneously. While assistant (1672–79) at the Royal Observatory, Paris, he estimated the approximate velocity of light through observations of the eclipses of satellites of Jupiter. From 1681 he was professor of astronomy at the Univ. of Copenhagen and royal mathematician. He constructed the first practical transit instrument (1690) and the earliest transit circle (1704) and supervised the erection of an observatory near Copenhagen. The name also appears as Roemer.