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Concept
Light-second
Summary
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly 299 792 458 metres (approximately 983 571 055 ft).
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 (299.8mm 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 31557600light-seconds, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly 365.25 days, each of exactly 86400 SI seconds.
Communications signals on Earth rarely 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.
The light-second is a convenient unit for measuring distances in the inner Solar System, since it corresponds very closely to the radiometric data used to determine them.
Official source
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