Hippolyte Fizeau measures the speed of light on Earth

Background. Before Fizeau's work, the speed of light had been estimated through astronomical observation: Ole Rømer made the first quantitative estimate in 1676. In the 19th century, Fizeau developed a different approach, determining the speed of light through time-of-flight measurements on Earth rather than astronomical setups. His 1849 result achieved better precision than the earlier Rømer value.

Methodology. Fizeau employed a toothed-wheel apparatus. Light passed through a gap in the wheel, traveled to a mirror, and returned. The speed of the wheel was increased until the returning light hit the next tooth, blocking the view. By knowing the distance, number of teeth, and rotation rate, the speed of light could be calculated. This working principle was distinct from later rotating mirror methods used by Foucault.

Results. Fizeau's apparatus yielded a computed value of 313,274,304 m/s, which is within 5% of the correct value of 299,792,458 meters per second. He also reported a value of 315,000 km/s from his Earth-based time-of-flight method. His 1849 calculation achieved better precision than the previous value determined by Ole Rømer in 1676, and his measurement was accurate to within 5%.