 Until early in the 18th century, it was generally believed that the speed of light was infinite. This view was held by Aristotle in ancient Greece and vigorously argued by the French philosopher Descartes and agreed to by almost all the major thinkers for over 2,000 years. Galileo was an exception. But when he tried to measure the speed of light, he failed. But Galileo did set the stage for the first measurement. After he discovered the first four moons of Jupiter, he suggested that the eclipse of the moon Io would make a good celestial clock that navigators could use to help determine their location. In 1676, the Danish astronomer Ole Romer was compiling extensive observations of the orbit of Jupiter's moon Io to see if Galileo was correct. The satellite is eclipsed by Jupiter once every orbit. Timing these eclipses over many years, Romer noticed something peculiar. The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved towards Jupiter and became steadily longer as the Earth moved away from Jupiter. In a brilliant insight, he realized that the time difference must be due to the finite speed of light. That is, light from the Jupiter system has to travel further to reach the Earth when the two planets are on opposite sides of the Sun than when they are closer together. Using what he knew about planetary orbits from Kepler, he estimated that light required 22 minutes to cross the diameter of the Earth's orbit. The speed of light could then be found by dividing the diameter of the Earth's orbit by the time difference. The actual math was done by others after Romer's death in the early 1700s. Those who did the first arithmetic found a value for the speed of light to be 227,000 kilometers per second or 141,000 miles per second. Not too bad for instruments of the 18th century. The modern value is 300,000 kilometers per second or 186,000 miles per second.