speed of light
Speedexact by definition
The speed of light in vacuum is exactly 299,792,458 metres per second. The value is exact by definition rather than by measurement: since 1983 the metre has been defined by fixing c, so measuring the speed of light now means calibrating your metre.
Watch out: The exact value applies in vacuum. Light in glass travels at about two thirds of it, and in water three quarters, which is what makes refraction happen. As a unit of speed, c is used for particle velocities (β = v/c) and for relativistic bookkeeping rather than for anything an object with mass can reach.
| 1 c | 299,792,460 m/s |
Ole Rømer got the first finite value in 1676 from the timing of Jupiter's moons, about 25% low. Fizeau's toothed wheel, Foucault's rotating mirror and Michelson's decades of work narrowed it, and by the 1970s laser measurements were more precise than the krypton-86 definition of the metre itself. Fixing c and letting the metre follow was the only way forward.