Aristotle was the first to discuss the speed of light, where he quotes Empedocles as saying the light from the sun must take some time to reach the earth. Almost 2000 years later, Galileo became the first to try to measure the speed of light. Galileo claims he actually tried the experiment at distances less than a mile, and couldn’t detect a time lag.
It wasn’t until 1676 did Danish astronomer Ole Römer actually measure the speed of light through the observation of Jupiter’s moon Io.
Now we know the speed of light to be 299 792 458 m/s
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