Rømer, Ole (1644–1710)

Ole Rømer, also called Olaus, was a Danish astronomer who first showed that light has a finite velocity (see speed of light). He noticed that Jupiter eclipsed its moons at times differing from those predicted and correctly concluded that this was due to the finite nature of light's velocity, which he calculated as 227,000 kilometers per second (the modern value is 299,800 kilometers per second).