Google doodle celebrates 340th anniversary of Olaus Roemer’s determining the speed of light

Ole Roemer, portrait by Jacob Coning from 1700 (Source: Wikimedia commons)

Though Roemer is attributed to the success of the determination, many say without Galileo’s methods and studies, he could not have done it. In his quest, Galileo had deduced two things — one, speed of light was too fast; two, it was 10 times faster than the speed of sound. Based on these observations, Roemer began his research few decades later in 1673 when he had noticed the time lapse between the eclipses of a Jupiter moon called Io. Even Io was discovered by Galileo in 1610.

By timing eclipses of Io, he estimated that light takes about 22 minutes to travel a distance equal to the diameter of Earth’s orbit around the Sun. This would give light a velocity of about 220,000 kilometres per second in SI units, about 26 per cent lower than the accurate value. Roemer had been working at the Royal Observatory in Paris at the time, where Giovanni Domenico Cassini was his director and boss whom he could not convince as his methods were quite controversial. However, many other supported him including Sir Issac Newton and it gain recognition.

It was following these important observations that the accurate speed was determined much later in 1975.