Ole Romer Timed Light’s Delay

In 1676, Danish astronomer Ole Rømer timed the eclipses of Io, a moon of Jupiter, while working at the Paris Observatory. He noticed the eclipses seemed to run late when Earth moved farther from Jupiter and early when Earth moved closer. Rømer argued the pattern made sense if light traveled at a finite speed, because the signal had to cross a longer distance when Earth was farther away. Using that idea, he estimated light needed about 22 minutes to cross a distance equal to the diameter of Earth’s orbit, which is in the right ballpark but about a quarter too low.

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