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 | 299,792,460 |
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.
One speed of light in every speed unit
| 9.46073e+18 | millimeter per year (mm/yr) | |
| 2.59021e+16 | millimeter per day (mm/day) | |
| 1.07925e+15 | millimeter per hour (mm/h) | |
| 1.01977e+15 | inch per day (in/day) | |
| 84,980,500,000,000 | foot per day (ft/d) | |
| 42,490,300,000,000 | inch per hour (in/h) | |
| 25,902,100,000,000 | meter per day (m/d) | |
| 17,987,500,000,000 | millimeter per minute (mm/min) | |
| 1,079,250,000,000 | meter per hour (m/h) | |
| 708,171,000,000 | inch per minute (in/min) | |
| 299,792,000,000 | millimeter per second (mm/s) | |
| 59,014,300,000 | foot per minute (ft/min) | |
| 29,979,200,000 | centimeter per second (cm/s) | |
| 17,987,500,000 | meter per minute (m/min) | |
| 11,802,900,000 | inch per second (in/s) | |
| 1,079,250,000 | kilometer per hour (km/h) | |
| 983,571,000 | foot per second (ft/s) | |
| 670,617,000 | mile per hour (mph) | |
| 582,750,000 | knot (kn) | |
| 299,792,000 | meter per second (m/s) | |
| 880,981 | mach (sea level) (Ma) | |
| 299,792 | kilometer per second (km/s) | |
| 1 | speed of light (c)this unit |