Light-second

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 (299.8 mm or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also currently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly 365.25 d, each of exactly 86400 SI seconds.

Use in telecommunications

Communications signals on Earth travel at precisely the speed of light in free space. Distances in fractions of a light-second are useful for planning telecommunications networks.

• One light-nanosecond is almost 300 millimetres (299.8 mm, 5 mm less than one foot), which limits the speed of data transfer between different parts of a computer.
• One light-microsecond is about 300 metres.
• The mean distance, over land, between opposite sides of the Earth is 66.8 light-milliseconds.
• Communications satellites are typically 1.337 light-milliseconds (low Earth orbit) to 119.4 light-milliseconds (geostationary orbit) from the surface of the Earth. Hence there will always be a delay of at least a quarter of a second in a communication via geostationary satellite (119.4 ms times 2); this delay is just perceptible in a transoceanic telephone conversation routed by satellite. The answer will also be delayed with a quarter of a second and this is clearly noticeable during interviews or discussions on TV when sent over satellite.