Ole Rømer

Ole Christensen Rømer (1644 to 1710) was a Danish astronomer who in 1676 first demonstrated that light travels at a finite speed. Working at the Royal Observatory in Paris and studying eclipses of Jupiter's moon Io, he saw that the intervals between eclipses shortened as the Earth approached Jupiter and lengthened as it moved away, and he explained this by the time light needs to cross the distance. Born in Aarhus and educated at the University of Copenhagen under Rasmus Bartholin, he returned to Denmark in 1681 as professor of astronomy. He introduced a national system of weights and measures, built improved instruments including the meridian circle, devised a temperature scale that influenced Fahrenheit, and as head of the Copenhagen police installed the city's first street lights.