Since the question of units of measure has come up recently, I have a question... The length of the meter is derived from the speed of light. That is, it is the distance that light travels at speed c in a time period derived from the cesium atom. The speed of light is commonly defined as how many meters it travels in a certain amount of time. 300 kilo meters per second approx. So we determine the lenght of the meter in terms of the speed of light, but then we measure the speed of light by using the meter or.. kilo meter. Why would you measure the speed of something by using a unit of measure that is derived from the speed of the thing you are measuring the speed of ?