The Speed of Light

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The Speed of Light European scientists and philosophers believed that the speed of light was infinite until Danish astronomer Ole Roemer (1644–1710) demonstrated otherwise in 1676. In the early 17th century, Johannes Kepler stated that the speed of light is infinite since empty space presents no obstacle to it. Rene Descartes speculated that if the speed of light were found to be finite, his whole system of philosophy might be demolished. Galileo Galilei proposed to measure the speed of light by observing the delay between uncovering a lantern and its observation some distance away. It is not clear whether Galileo ever actually performed this experiment, and in any case his method would have only been able to determine that light speed was infinite or extraordinarily fast. Galileo proposed the experiment in his final book, Discorsi e dimostrazioni matematiche, intorno à due nuove scienze, published in 1638. The belief in infinite light speed became a problem in maritime celestial navigation when the timing of the occultations[1] of the moons of Jupiter, especially Io, led to errors in navigation. In 1668, Giovanni Cassini observed that the timing of these occultations depended on the position of the earth relative to Jupiter, and considered but ultimately rejected the notion that a finite light speed could be a cause. Ole Roemer, Cassini’s assistant, used Cassini’s data and additional observations of his own to determine the relationship between the earth’s distance from Jupiter and the timing of Io’s occultations. He used his estimate of the time light took to traverse earth’s orbit (22 minutes) to predict the delay in Io’s 1676 eclipse to be approximately 10 minutes. (Roemer did not know the diameter of the earth’s orbit, so he never actually made a prediction of the speed of light.) Roemer presented his results to the French Academy of Sciences,
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