An Etymological Dictionary of Astronomy and Astrophysics

The experiment which provided the first accurate measurement of the speed of light. The experiment, conducted by the French physicist Armand H. L. Fizeau (1819-1896) in 1849,
used a rotating wheel containing 720 teeth. The function of the wheel was to cut a light beam into short pulses and to measure the time required for these pulses to travel to a distant mirror and back (17.34 km). The round-trip time for each pulse could be calculated to be about 1/18,000 sec, which yielded the value of 315,300 km/sec for the speed of light. Leon Foucault (1819-1868) improved on Fizeau’s method by replacing the cogwheel with a rotating mirror. Foucault’s estimate, published in 1862, was 298,000 km/s.

Etymology (EN): From tooth; M.E.; O.E. toth (cf. O.S., Dan., Swed., Du. tand, O.N. tönn, O.Fris. toth, O.H.G. zand, Ger. Zahn, Goth. tunthus), cognate with Pers. dandân, as below;
→ wheel; → experiment.

Etymology (PE): Âzmâyeš, → experiment; carx→ wheel; dandâne-dâr “toothed,” from dandân “tooth,” Mid.Pers. dandân; Av. dantan-; cf. Skt. dánta-; Gk. odontos; L. dens (Fr. dent); Lith. dantis, O.Ir. det, Welsh dent; PIE base *dont-/*dent- “tooth.”