For most of human history, the smartest people who ever thought about the question believed that light traveled instantaneously — that when you opened your eyes in a dark room and lit a candle, the light simply was everywhere it was going to be, with no travel time at all. Galileo tried to test it with lanterns on hilltops and came away with nothing but a measurement of human reaction time. Then in 1676, a Danish astronomer named Ole Rømer noticed something strange in the moons of Jupiter — a timing discrepancy in their eclipses that varied depending on where Earth was in its orbit — and realized, against everything his era assumed, that he was looking at the finite travel time of light across the solar system. This explainer traces the full journey from Rømer's revolutionary but contested observation, through James Bradley's stellar aberration discovery that confirmed light had a speed without measuring it directly, to the first terrestrial measurements by Hippolyte Fizeau spinning a toothed wheel fast enough to catch light on its return journey, Léon Foucault's refinement that brought the number within a fraction of the modern value, and the Michelson experiments of the late 19th century that measured it with such precision they accidentally dismantled the entire framework of classical physics in the process — handing Einstein the experimental foundation on which special relativity was built. It's the story of how science pinned down the fastest thing in the universe one clever observation at a time — and how the number they found turned out to be not just a measurement but a fundamental constant written into the structure of reality itself.