Earlier thoughts: How fast is the speed of light?
Hundreds of year ago people thought light travelled instantaneously. They thought so because after a military artillery fired at a large distance, they saw the flash immediately, but sound took a noticeable delay before you heard it.
Even in 1600 A.D. the famous Johann Kepler believed that the speed of last was instantaneous. According to him the vacuum of space did not slow the speed of light down.
Galileo was correct in saying that all this proved was that light moved faster than sound - not necessarily instantaneously.
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