It's the birthday of Ole Rømer, who was born in 1644 in Aarhus, Denmark. After graduating from the University of Copenhagen, Rømer worked at the palace of Versailles as the tutor to the heir to the French throne. He returned to Denmark in 1681 to become professor of astronomy at the University of Copenhagen and later Royal Mathematician. Rømer is credited with the first attempt to measure the speed of light. His method entailed timing the passage behind Jupiter of the planet's moon Io. From Rømer's measurements, Christiaan Huygens deduced that light travelled 16 2⁄3 Earth diameters per second, which comes out to be around 300,000 km/s, close to the modern value of 299,792.458 km/s.
