Ole Rømer

Ole Rømer was a Danish astronomer who made the first quantitative measurement of the speed of light. While working at the Paris Observatory in 1676, he noticed that eclipses of Jupiter's moon Io occurred earlier when Earth was closer to Jupiter and later when farther away. He correctly attributed this to the finite speed of light traveling different distances, estimating it at about 220,000 km/s — remarkably close to the true value of ~300,000 km/s.

First measurement of the speed of light using timing of Io's eclipses by Jupiter. Demonstrated that light travels at a finite speed, overturning the prevailing view that light propagation was instantaneous. His estimate of ~220,000 km/s was within 26% of the modern value.