# ARMAND FIZEAU (1819- 96)

1849 – France

‘The first successful experiment to determine the speed of light’

Prior to this experiment it was believed that light had an infinite speed – although in 1676 the Danish astronomer Ole Rømer (1644-1710) had used GALILEO’s 1610 discovery of the four largest moons of Jupiter to describe the way of measuring the speed of light by measuring the times at which the moons were eclipsed by Jupiter itself.
The timing of the eclipses is affected by whether the Earth is on the same side of the Sun as Jupiter, or on the opposite side. Rømer explained the differences in the eclipse timings as due to the extra time required for light from the moons to reach the Earth when it is on the opposite side of the Sun.

Using modern measurements, it is calculated that it takes light more than eight minutes, traveling at 300,000km per second, to reach us from the Sun, across half the diameter of the Earth’s orbit; so the maximum delay in observing an eclipse of one of the moons of Jupiter is twice that – more than a quarter of an hour.

Fizeau carried out his experiment in Paris between the belvedere of a house at Montmartre and a hill at Suresnes – a distance of 8.67 kilometres.
He placed a rotating toothed wheel with 720 gaps at Montmartre and a mirror at Suresnes. When the wheel was at rest, light passed through one gap and was reflected. When the wheel was rotated slowly the light was completely eclipsed from the observer. When the wheel was turned rapidly the reflected light passed through the next gap. Fizeau observed this at a maximum speed of 25 revolutions per second. Therefore the time required by light to travel a distance of 8.67 × 2 kilometres was 1/25 × 1/720 of a second. This gave a speed of 312,320 kilometres per second (the correct value is 299,792 kilometres per second).

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