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Long before people knew about electromagnetic waves and their propagation according to the wave equations (see the page "What is light?"), they realized that something must bring to their eyes the information from surrounding objects. They called it light. They knew that light propagates at an extremely high speed, if not with an infinite speed, but measuring it was a tough task! The first proof that the speed of light is finite came from a Danish astronomer, Olaf Romer. He was watching the motion of one of Jupiter's moons. He noticed some irregularities in his observations when he compared data taken in the summer (when Earth was closer to Jupiter) and in the winter (when Earth was farther from Jupiter). In winter, everything seemed to be delayed by roughly 16 minutes compared to in summer! His explanation was that this time delay comes from the fact that the speed of light is finite and, in winter, light needs more time to get to his telescope because Earth is farther from Jupiter. He was even able to predict the speed of light, and got a remarkably good value for it. For more details read a little story I wrote as an answer to one of our reader's question. So the bottom line is that already in 1650 there was a proof that the speed of light is finite. On the next page, I will give a modern view of the speed of light. 

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last modified 1/5/2001 email Fermilab 