Danish astronomer and mathematician. His primary discovery was the finite velocity of light. Observations of Jupiter's satellites had disagreed with Cassini's tables, and Roemer demonstrated in 1675 that the error was due to the time taken by light to cross the changing distance between Jupiter and the Earth. He announced that light took 11 minutes to travel the distance between the Earth and the Sun.
Roemer also developed the transit instrument, which was to become the fundamental instrument of positional astronomy.
Römer was born in Århus, Jutland, and went to the University of Copenhagen in 1662, where he studied mathematics and astronomy. In 1671 Jean Picard, who had been sent by the French Academy to verify the exact position of Tycho Brahe's observatory, was impressed by Römer's work and invited him back to Paris with him. In Paris, Römer was made a member of the Academy and tutor to the crown prince. He conducted observations, designed and improved scientific instruments, and submitted various papers to the Academy. He returned to Denmark in 1681 to take up the dual post of Astronomer Royal to Christian V and director of the Royal Observatory in Copenhagen. He was mayor of Copenhagen and prefect of the police, and was also a senator and head of the state council.
It was through the precision of both his observations and his calculations that Römer not only demonstrated that light travels at a finite speed but also put a rate to it. Noticing that the length of time between eclipses of the satellite Io by Jupiter was not constant, he realized that it depended on the varying distance between the Earth and Jupiter. He was able to announce in September 1679 that the eclipse of Io by Jupiter predicted for 9 November would occur ten minutes later than expected. Römer's prediction was borne out; his interpretation of the delay provoked a sensation. He said that the delay was caused by the time it took for the light to traverse the extra distance across the Earth's orbit.