In the 1849 Fizeau experiment, what did Hippolyte Fizeau measure using a rotating toothed wheel?

In the 1849 Fizeau experiment, Hippolyte Fizeau measured the speed of light using a rotating toothed wheel.

Fizeau sent a beam of light through a gap in a rapidly spinning wheel toward a distant mirror. The reflected beam returned through the wheel. By increasing the wheel’s rotation until a tooth blocked the returning light, Fizeau could calculate how long the light took to travel to the mirror and back.

The apparatus measured a value close to 315,000 kilometres per second, an important improvement over earlier astronomical estimates. The experiment showed that the speed of light could be measured in a terrestrial laboratory rather than inferred only from observations of planets or moons.

Fizeau’s method was later refined by Léon Foucault and Albert A. Michelson. It is distinct from the Fizeau experiment in moving water, a later study that tested how the motion of a medium affects the measured speed of light.