A light-second's a unit of length useful in astronomy, telecommunications and relativistic physics. It's defined as the distance that light travels in free space in one second, and's equal to exactly metres. It's just over 186,000 miles and almost 109 feet. 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… (More on Light-second)