Redirected from Ole RÝmer
After studies in Copenhagen, RÝmer in 1672 went to Paris. It was there that he demonstrated that light propagates at a finite speed.
Galileo Galilei had earlier in the century attempted to measure the speed of light using two people with lanterns standing a mile apart. When one person uncovered his lantern, the other would uncover his as fast as possible. This of course, failed miserably. Galileo could only conclude that light traveled at least 10 times faster than sound.
Ole RÝmer, on the other hand, observed the frequent eclipses of Jupiter's moon Io, and developed a model that allowed him to predict the next eclipse. He observed that, over the course of a year, the range of variation in the timing of these eclipses was about 16.6min and concluded that this was the length of time required for light to travel across the diameter of the Earth's orbit, a distance of 2 AU.
His discovery was published in a short paper, "Dťmonstration touchant le mouvement de la lumiŤre trouvť par M. Roemer de l'Acadťmie des Sciences[?]", in Journal des savants, 7. December 1676.
In 1681, RÝmer returned to Denmark and was appointed professor of Astronomy at Copenhagen University. He was active also as an observer, both at the University Observatory at the Round Tower[?] and in his home, using improved instruments of his own construction. Unfortunately, his observations have not survived: they were lost at the great fire of Copenhagen in 1728.