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.