Light's speed was first clocked using the moons of Jupiter

In 1676, Ole Rømer noticed that eclipses of Jupiter's moon Io ran late when Earth was farther from Jupiter in its orbit, and early when it was closer. He correctly reasoned that light itself takes time to cross the extra distance — the first evidence that light's speed, though enormous, is finite.

— Ole Rømer, A Demonstration Concerning the Motion of Light — Presented to the French Academy of Sciences, 1676