Ole Rømer

Ole Christensen Rømer (1644–1710), recruited from Copenhagen to the French Royal Academy of Sciences in Paris, worked closely with Cassini studying the moons of Jupiter. Throughout history, philosophers had assumed that light travels instantaneously across infinite space.

Rømer observed that the eclipses of Jupiter's moon Io occurred systematically earlier when Earth was approaching Jupiter and up to 22 minutes later when Earth was receding. In 1676, he deduced that this delay is caused by the finite time light takes to cross the diameter of Earth's orbit. Returning to Denmark, he became Royal Astronomer, designed street lighting, and invented the transit instrument and the Rømer temperature scale.