Römer , Ole Christensen

(1644–1710) Danish astronomer

Born at Aarhus, Denmark, Ole (or Olaus) Römer was professor of astronomy at the University of Copenhagen when Jean Picard visited Denmark to inspect Tycho Brahe's observatory at Uraniborg. Picard recruited him and Römer joined the Paris Observatory in 1671. In Paris, working on Giovanni Cassini's table of movements of the satellites of Jupiter, he noticed that whether the eclipses happened earlier or later than Cassini had predicted depended on whether the Earth was moving toward or away from Jupiter. Römer realized that this anomaly could be explained by assuming that the light from the satellite had a longer (Earth moving away from Jupiter) or shorter (Earth moving toward Jupiter) distance to travel. As Cassini had recently established the distance between the Earth and Jupiter, Römer realized that he had all the information needed to calculate one of the fundamental constants of nature – the speed of light. In 1676 he announced to the French Academy of Sciences that the speed of light was, in modern figures, 140,000 miles (225,000 km) per second. This value is too small but was an excellent first approximation. In 1681 Römer was made Astronomer Royal and returned to Copenhagen where he designed and developed the transit telescope.