1676: Ole Rømer Measures the Speed of Light

Light travels so amazingly fast that it was very difficult to measure its speed. The first person to succeed was Danish astronomer Ole Christensen Rømer. His accomplishment was an essential step in developing an understanding of the universe. Eventually it would figure in the development of Einsteins theory of relativity, but the speed of light also defines the light year, which is the yardstick that astronomers use to measure the vast distances between stars and galaxies. 

Rømer spent many years observing Jupiter’s moons— the same four moons that Galileo reported having seen in 1610. By 1676 the exact orbital periods had been worked out and timed quite precisely. Using telescopes at Uraniborg on the island of Hveen, near Copenhagen, and the Paris Observatory, Rømer noticed that the time between eclipses of a given moon were not always the same. Furthermore, he noticed a pattern to these discrepancies and suspected that the difference of about 16.7 minutes over a year’s time was due to the additional time it took light to reach Earth when it was far from Jupiter than when it was closer. Given the known diameter of Earth’s orbit at the time he made a fairly good estimate of the speed of light, now known to be approximately 186,000 miles per second.