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Portrait of the Danish astronomer Ole Christensen Roemer, 1644-1710. In 1675 whilst working on the eclipses of Jupiter's moons, Roemer noticed that the moons reached their predicted eclipse positions later than expected when the Earth was moving away from Jupiter and earlier when it was moving towards it. He realised that this discrepancy must be due to the finite time it took light to reach Earth. He was able to calculate, using Jupiter's distance, that the speed of light was 140,000 miles per second, about 75% of the correct value. This was the first proof of the finite speed of light, later confirmed by Bradley in 1729 and eventually widely accepted.
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