Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (just under one U.S. or imperial foot) to the light-minute, light-hour and light day that are sometimes used in popular science publications. Definition of the metre This definition fixes the speed of light at exactly m/s, and hence the light-second at exactly m. Use in telecommunications Communications signals on Earth rarely travel at… (More on Light-second)