· Astrophysics
How Did Rømer Measure the Speed of Light with Io?
In 1676, at the Paris Observatory, Ole Rømer noticed that eclipses of Jupiter’s moon Io came late when Earth was moving away from Jupiter and early when it was approaching. His explanation: light is not instantaneous. Here is how the timing argument works, what Rømer actually published, and why the first number for the speed of light came from Huygens.