More than two centuries before Einstein,using a crude telescope and a mechanical timepiece, Dutch astronomer Ole Romermeasured the speed of light with astounding accuracy. How was he able to dothis when most scientists didn’t even believe that light traveled? Like manyparadigm-shattering discoveries, Romer’s was accidental. Night after night hewas timing the disappearance and reappearance of Jupiter’s moon Io behind thehuge, distant planet. Eventually he realized that the discrepancies in hismeasurements could have only one explanation: Light had a speed, and it took longerto reach Earth when Earth was farther from Jupiter. All he needed then tocalculate light’s speed was some fancy geometry.