Measuring the speed of light

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Measuring the speed of light

The first real attempt at measuring the speed of light was made by Ole Christensen Rømer in 1676. He was studying Jupiter’s moon Lo motions with a telescope. He observed that Io went round Jupiter once every 42,5 hour when Earth was at it’s closest to Jupiter. When Earth moved away from Jupiter Los its motions took slower than Rømer predicted. The signals from Lo took longer to reach Earth when Jupiter moved away from Earth. The signals from took longer to reach earth when the two planets moved away from each other. This was because of the extra time it took for the light to cross the extra distance. He calculated that it would take about 22 minutes to cross the diameter of Earths orbit. Using these calculations Rømer found the speed of light. He measured the distance between the sun and Earth and found out that the speed of light was 125,000 miles per second. James Bradley (1728) was not satisfied, he said that starlight could cause an “aberration of light” and could cause speed of light measurements go wrong.

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Rømer used an Astronomical method.

Hippolyte Fizeau in 1849 used an earthbound method to measure the speed of light. He used a beam of light reflected from a mirror 8 km away.  The beam was passed through the gaps between the teeth of a rapidly rotating wheel.  The speed of the wheel was increased until the beam of light passed though the next hole and could be seen.  Then c was calculated to be 315,000 km/s.  Leon Foucault improved on this a year later by using rotating mirrors and got a more accurate answer of 298,000.

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