Editing Light-second (section)

{{short description|Unit of length}}
{{Infobox unit
| image = Earth and Moon Size and Distance scale - with real-time light speed!.webm
| caption = The distance between the Earth and the Moon is approximately 1.3 light-seconds
| quantity = [[length]]
| units1 = [[SI units]]
| inunits1 = {{val|299792458|ul=m}}
| units2 = [[Astronomical system of units|astronomical units]]
| inunits2 = {{convert|299792458|m|AU|disp=out|lk=on|sigfig=5|comma=gaps}}<br /><!--
-->&emsp;{{convert|299792458|m|ly|disp=out|lk=on|sigfig=5}}<br /><!--
-->&emsp;{{convert|299792458|m|pc|disp=out|lk=on|sigfig=5}}
| units3 = [[imperial units|imperial]]/[[US customary units|US]]&nbsp;units
| inunits3 = {{convert|299792458|m|mi|disp=out|lk=on|0|comma=gaps}}
}}
The '''light-second''' is a [[unit of length]] useful in [[astronomy]], [[telecommunications]] and [[Theory of relativity|relativistic physics]]. It is defined as the [[distance]] that [[light]] travels in [[free space]] in one [[second]], and is equal to exactly {{val|299792458|ul=m}} (approximately {{Val|983571055|u=ft}} or {{val|186282 |ul=miles}}).

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 ({{val|299.8|u=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 {{val|31557600|u=light-seconds}}, since the definition of a year is based on a [[Julian year (astronomy)|Julian year]] (not the [[Gregorian calendar|Gregorian year]]) of exactly {{Val|365.25|u=days}}, each of exactly {{val|86400|u=[[SI]] seconds}}.<ref>[http://www.iau.org/Units.234.0.html IAU Recommendations concerning Units] {{webarchive|url=https://web.archive.org/web/20070216041250/http://www.iau.org/Units.234.0.html |date=2007-02-16 }}</ref>