Ole Rømer

Ole Christensen Rømer (; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, first demonstrated that light travels at a finite speed. Rømer also invented the modern thermometer showing the temperature between two fixed points, namely the points at which water respectively boils and freezes.

Rømer made his discovery regarding the speed of light while working at the Royal Observatory in Paris and studying Jupiter's moon Io. He estimated that light takes about 11 minutes to travel from the Sun to Earth. Using today's knowledge of the Sun-Earth distance, this would amount to a speed of light of approximately 220,000 kilometers per second, compared to today's accepted value of just under 300,000 kilometers per second.

In scientific literature, alternative spellings such as "Roemer", "Römer", or "Romer" are common. Provided by Wikipedia