Light-second


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.
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 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 31,557,600 light-seconds, since the definition of a year is based on a Julian year of exactly 365.25 days, each of exactly SI seconds.

Use in telecommunications

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.
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. The value of the astronomical unit in light-seconds is a fundamental measurement for the calculation of modern ephemerides. It is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is s.
Multiples of the light-second can be defined, although apart from the light-year, they are more used in popular science publications than in research works. For example, a light-minute is 60 light-seconds, and the average distance between Earth and the Sun is 8.317 light-minutes.