Finding the Speed of Light

More than two centuries before Einstein, using a crude telescope and a mechanical timepiece, Danish astronomer Ole Romer measured the speed of light with astounding accuracy. How was he able to do this when most scientists didnt even believe that light traveled? Like many paradigm-shattering discoveries, Romers was accidental. Night after night he was timing the disappearance and reappearance of Jupiters moon Io behind the huge, distant planet. Eventually he realized that the discrepancies in his measurements could have only one explanation: Light had a speed, and it took longer to reach Earth when Earth was farther from Jupiter. All he needed then to calculate lights speed was some fancy geometry.