Ole Christensen RÝmer (September 25, 1644 - September 19, 1710) was a Danish astronomer who made the first, somewhat, successful measurements of the speed of light (1676). RÝmer was born in Aarhus and died in Copenhagen.
After studies in Copenhagen, RÝmer in 1672 went to Paris. It was there that he demonstrated that light propagates at a finite speed. RÝmer observed the 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 16.6 minutes, 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. This discovery lent support to the heliocentric theory of the solar system. Romer's measurements had an error of 10-25%, due to a lack of understanding regarding the actual distances involved.
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.
He was employed by the French government: King Louis XIV made him teacher for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.
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.
In the year 1700, he managed to get the government to introduce the Gregorian calendar in Denmark-Norway -- something which Tycho Brahe had argued for in vain a hundred years earlier.