The length of a metre and its relation to the speed of light
The metre was first defined as 1 10-millionth the distance between the equator and the north pole, however in 1983 it was redefined.
The metre was changed so that the EXACT speed of light is a whole number (no decimals) when expressed in metres per second. so now 1 metre is exactly the distance light travels in 1/299,792,458 seconds, where 299,792,458 is the exact speed of light in metres per second.
This changed the length of a metre by 2 nanometers (in todays definition.)