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Before the seventeenth century it was generally thought that light is transmitted instantaneously. This was supported by the observation that there is no noticeable lag in the position of the Earth's shadow on the moon during a lunar eclipse as would be expected if c is finite. Now we know that light is just too fast for the lag to be noticeable. Galileo doubted that light speed is infinite and described an experiment to measure its speed by covering and uncovering lanterns observed at a distance of a few miles. We don't know if he really attempted the experiment, but again c is too high for such a method to work.
The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that the time between the eclipses of the moons of Jupiter was less as the distance away from Earth is decreasing than when it is increasing. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance changes. He obtained a value equivalent to 214,000 km/s which was very approximate because planetary distances were not accurately known at that time.
The first measurement of c on Earth was by Armand Fizeau in 1849. He used a beam of light reflected from a mirror 8km away. The beam passed through the gaps between teeth of a rapidly rotating wheel. The speed of the wheel was increased until the returning light passed through the next gap 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 the much more accurate answer of 298,000 km/s. His technique was good enough to confirm that light travels slower in water than in air.
This table gives some of the best measurements according to Froome and Essen:
| Date | Author | Method | Result (km/s) | Error |
|---|---|---|---|---|
| 1676 | Olaus Roemer | Jupiter's satellites | 214,000 | |
| 1726 | James Bradley | Stellar Aberration | 301,000 | |
| 1849 | Armand Fizeau | Toothed Wheel | 315,000 | |
| 1862 | Leon Foucault | Rotating Mirror | 298,000 | +-500 |
| 1879 | Albert Michelson | Rotating Mirror | 299,910 | +-50 |
| 1907 | Rosa, Dorsay | Electromagnetic constants | 299,788 | +-30 |
| 1926 | Albert Michelson | Rotating Mirror | 299,796 | +-4 |
| 1947 | Essen, Gorden-Smith | Cavity Resonator | 299,792 | +-3 |
| 1958 | K. D. Froome | Radio Interferometer | 299,792.5 | +-0.1 |
| 1973 | Evanson et al | Lasers | 299,792.4574 | +-0.001 |
| 1983 | Adopted Value | 299,792.458 |
The speed of light in vacuum has been measured to be 300 million meters per second. This is the fastest that anything has been observed to move. At that speed it would take light a mere one ten thousandth of a second to circumvent the globe. When light enters any material, it slows down. The amount that it slows down depends on the nature of the material. The more dense the material, the slower the speed light. For example, in water, light is approximately 30% slower than in vacuum, while in glass it is 50% slower. In diamand, one of of the most dense material known, light travels at less than 150 million meters per second. Remarkably, in the past year research scientists at MIT have been able to make microscopic quantities of a very strange, and very dense state of matter in which the speed of light is (theoretically) calculated to be just a few meters per second.
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