The speed of light was first estimated in 1676 by a Danish Astronomer who was timing the eclipses of Io caused by Jupiter. He noticed the period between eclipses increased with Earth’s distance from Jupiter and guessed it was because light had to travel a longer distance.

Rømer’s determination of the speed of light

Ole Rømer (1644–1710) was already a statesman in his native Denmark some time after his discovery of the speed of light (1676). The engraving is probably posthumous.

Rømer’s determination of the speed of light was the demonstration in 1676 that light has a finite speed and so does not travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710),[note 1] who was working at the Royal Observatory in Paris at the time.

By timing the eclipses of the Jupiter moon Io, Rømer estimated that light would take 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, about 26% lower than the true … Continue Reading (18 minute read)