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SI units  

299.79×10 ^{3} km  299.79×10 ^{6} m 
Astronomical units  
2.0040×10 ^{−3} AU  31.688×10 ^{−9} ly 
US customary / Imperial units  
186.28×10 ^{3} mi  983.57×10 ^{6} ft 
SI units  

299.79×10 ^{3} km  299.79×10 ^{6} m 
Astronomical units  
2.0040×10 ^{−3} AU  31.688×10 ^{−9} ly 
US customary / Imperial units  
186.28×10 ^{3} mi  983.57×10 ^{6} ft 
A lightsecond 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. It is just over 186,000 miles and almost 9.84×10^{8} feet.
Just as the second forms the basis for other units of time, the lightsecond can form the basis for other units of length, ranging from the lightnanosecond (just under one U.S. or imperial foot) to the lightminute, lighthour and lightday, which are sometimes used in popular science publications. The more commonlyused lightyear is also presently defined to be equal to precisely 31557600 lightseconds, since the definition of a year is based on a Julian year (not Gregorian year) of exactly 365.25 days, each of exactly 86400 SI seconds.^{[1]}
Contents 
“  The metre is the length of the path traveled by light in vacuum during a time interval of ^{1}⁄_{299792458} of a second.  ” 
This definition fixes the speed of light in vacuum at exactly 299792458 m/s, and hence the lightsecond at exactly 299,792,458 m.^{[2]}
Communications signals on Earth rarely travel at precisely the speed of light in free space, but distances in fractions of a lightsecond are still useful for planning telecommunications networks as they indicate the minimum possible delay between sender and receiver.
The lightsecond is a convenient unit for measuring distances in the inner Solar System, because it corresponds very closely to the radiometric data used to determine them (the match is not exact for an Earthbased observer because of a very small correction for the effects of relativity). The value of the astronomical unit in light seconds is a fundamental measurement for the calculation of modern ephemerides (tables of plantary positions): it is usually quoted as "lighttime for unit distance" in tables of astronomical constants, and its currently accepted value is 499.004786385(20) s.^{[3]}^{[4]}
Multiples of the lightsecond can be defined, although apart from the lightyear they are more used in popular science publications than in research works. For example, a lightminute is 60 lightseconds and the average distance from the Earth to the Sun is 8.317 lightminutes.
Unit  Definition  Distance  Example  
m  km  miles  
lightsecond  2.997924580×10^{8} m  2.998×10^{5} km  1.863×10^{5} mi  average distance from the Earth to the Moon is about 1.282 lightseconds  
lightminute  60 lightseconds  1.798754748×10^{10} m  1.799×10^{7} km  1.118×10^{7} mi  average distance from the Earth to the Sun is 8.317 lightminutes 
lighthour  60 lightminutes = 3600 lightseconds  1.079252849×10^{12} m  1.079×10^{9} km  6.706×10^{8} mi  semimajor axis of Pluto's orbit is about 5.473 lighthours 
lightday  24 lighthours = 86400 lightseconds  2.590206837×10^{13} m  2.590×10^{10} km  1.609×10^{10} mi  Sedna is currently 0.52 lightdays from the Sun; on an orbit that varies from a perihelion of 0.44 lightdays to an aphelion of 5.41 lightdays 
lightweek  7 lightdays = 604800 lightseconds  1.813144786×10^{14} m  1.813×10^{11} km  1.127×10^{11} mi  The Oort cloud is thought to extend between 41 and 82 lightweeks out from the Sun 
lightyear  365.25 lightdays = 31557600 lightseconds  9.460730473×10^{15} m  9.461×10^{12} km  5.879×10^{12} mi  Proxima Centauri is the nearest star to the Sun, about 4.24 lightyears away 
