Rømer

Ole Christensen Roemer, also spelled Roemer or Römer, (1644–1710) was a Danish astronomer who first demonstrated that light travels at a finite speed. His work in the 1670s provided the first quantitative estimate of the speed of light and showed that the propagation of light takes time.

Rømer was born in Ringsted, Denmark. He studied at the University of Copenhagen and later worked at the Paris Observatory, where he collaborated with Jean-Dominique Cassini. While observing Io, one of Jupiter's moons, he compared predicted times of Io's eclipses with actual observations throughout the year 1676–1677.

Rømer found systematic discrepancies in eclipse timings that depended on Earth's position in its orbit. From these, he reasoned that light has a finite speed and that its travel time across the diameter of the Earth's orbit could account for the observed lag. He estimated this travel time to be about 22 minutes, implying a finite speed of light and yielding an order-of-magnitude estimate near 200,000 km/s (the modern value is about 299,792 km/s).

Rømer returned to Denmark and became a professor of astronomy at the University of Copenhagen. He died in Copenhagen in 1710. His work laid the foundation for experimental measurements of light speed and influenced later developments in physics and astronomy.