Ole Roemer (1644-1710) was a Danish astronomer who made the first measurement of the speed of light. As the great explorers of the 15th Century shrank the globe and made long ocean voyages possible, the need arose for a way to determine longitude at sea. Galileo's observation of the moons of Jupiter orbiting the large planet in stately and regular procession suggested a way to solve the "longitude problem." At the invitation of Cassini, who directed the Paris Observatory, Roemer (or Rømer) noticed that the timing of the eclipses of the Galilean satellites shifted as Earth journeyed closer to or farther from Jupiter, and from the estimated radius of Earth's orbit and the amount of these shifts, Roemer obtained a value for the speed of light of140,000 miles per second, which is about 25% low.
Updated 10/2/00 by Peter N. Saeta .