Speed of light
Use in unit systems

Since 1983, the constant c has been defined in the International System of Units (SI) as exactly 299 792 458 m/s; this relationship is used to define the metre as exactly the distance that light travels in vacuum in 1 ⁄ 299 792 458 of a second. By using the value of c, as well as an accurate measurement of the second, one can thus establish a standard for the metre. As a dimensional physical constant, the numerical value of c is different for different unit systems. For example, in imperial units, the speed of light is approximately 186 282 miles per second, or roughly 1 foot per nanosecond.

In branches of physics in which c appears often, such as in relativity, it is common to use systems of natural units of measurement or the geometrized unit system where c = 1. Using these units, c does not appear explicitly because multiplication or division by 1 does not affect the result. Its unit of light-second per second is still relevant, even if omitted.
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