# Describe methods for measuring the speed of light.

Until the seventeenth century, it was believed that light spreads instantly. This was confirmed by observations of the lunar eclipse. At a finite speed of light, there should be a delay between the position of the Earth relative to the Moon and the position of the Earth’s shadow on the surface of the Moon, but no such delay was found. We now know that the speed of light is too fast to notice a delay. Galileo doubted the infinity of the speed of light. He proposed a way to measure it by closing and opening a lantern several miles away. It is not known whether he tried to carry out such an experiment, but due to the very high speed of light, the measurement could not be successful. The first successful measurement of c was made by Olaf Roemer in 1676. He noticed that the time between eclipses of Jupiter’s moons is shorter when the distance from Earth to Jupiter decreases, and longer when this distance increases. He realized that this is due to a change in the time it takes for light to travel from Jupiter to Earth as the distance between them changes. He calculated that the speed of light is 214,000 km / s. The inaccuracy is due to the fact that the distances between the planets at that time were not yet well defined. In 1728, James Bradley estimated the magnitude of the speed of light by observing the aberration of stars (a change in the apparent position of a star caused by the movement of the Earth around the Sun). He observed one of the stars in the constellation Draco, and found that its apparent position changes throughout the year. This effect works for all stars, as opposed to parallax, which is more noticeable for nearby stars. Aberration is similar to the effect of motion on the angle of incidence of raindrops. If you are standing and there is no wind, then the drops fall vertically on your head. If you run, it turns out that the rain comes at an angle and hits your face. Bradley measured this angle for starlight. Knowing the speed of the Earth’s movement around the Sun, he determined that the speed of light is 301,000 km / s The first measurement of c on Earth was performed by Armand Fizeau in 1849. He used the reflection of light from a mirror 8 km away. A beam of light passed through the gap between the teeth of the rapidly rotating wheel. The speed of rotation was increased until the reflected beam became visible in the next gap. The calculated value of c turned out to be 315,000 km / s. A year later, Leon Foucault improved this method using a rotating mirror and obtained a much more accurate value of 298,000 km / s. The improved method was accurate enough to determine that the speed of light in water is slower than in air.

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