{{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 /><!-- --> {{convert|299792458|m|ly|disp=out|lk=on|sigfig=5}}<br /><!-- --> {{convert|299792458|m|pc|disp=out|lk=on|sigfig=5}} | units3 = [[imperial units|imperial]]/[[US customary units|US]] 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>
== Use in telecommunications == Communications signals on [[Earth]] travel at precisely the [[speed of light]] in free space.{{Citation needed|date=September 2017}} 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 (length)|foot]]<ref>[[David Mermin]] suggested one light-nanosecond might be called a ''phoot'' at page 22 of ''It's About Time'' (2005), [[Princeton University Press]]</ref>), 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 satellite]]s are typically 1.337 light-milliseconds{{Citation needed|date=August 2017}} ([[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.
==Use in astronomy== [[File:1e13m comparison Hale Bopp and smaller - HQ no transparency.png|thumb|upright=1|right|The yellow shell indicating one light-day distance from the Sun compares in size with the positions of ''[[Voyager 1]]'' and ''[[Pioneer 10]]'' as of 2008 (red and green arrows respectively). It is larger than the heliosphere's [[Heliosphere#Termination shock|termination shock]] (blue shell) but smaller than [[Comet Hale-Bopp]]'s orbit (faint orange ellipse below). Click on the image for a larger view and links to other scales.]]
[[File:1e10m comparison Rigel, Aldebaran, and smaller - antialiased no transparency.png|thumb|upright=1|right|The faint yellow sphere centred on the Sun has a radius of one light-minute. For comparison, sizes of [[Rigel]] (the blue star in the top left) and [[Aldebaran]] (the red star in the top right) are shown to scale. The large yellow ellipse represents [[Mercury (planet)|Mercury's]] orbit.]]
The light-second is a convenient unit for measuring distances in the inner [[Solar System]], since it corresponds very closely to the [[radiometry|radiometric]] data used to determine them. (The match is not exact for an Earth-based observer because of a very small correction for [[Time dilation|the effects of relativity]].) The value of the [[astronomical unit]] (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern [[ephemeris|ephemerides]] (tables of planetary positions). It is usually quoted as "light-time for unit distance" in tables of [[astronomical constant]]s, and its currently accepted value is {{gaps|499.004|786|385(20)}} s.<ref name="JPL">{{cite web |last=Standish |first=E. M. |date=1998 |title=JPL Planetary and Lunar Ephemerides, DE405/LE405 |url=http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf |id=JPL IOM 312.F-98-048 |url-status=dead |archiveurl=https://web.archive.org/web/20120220062549/http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf |archivedate=2012-02-20 }}.</ref><ref name="IERS">{{Cite book |editor1=McCarthy, Dennis D. |editor2=Petit, Gérard |date=2004 |contribution=IERS Conventions (2003) |title=IERS Technical Note No. 32 |url=https://iers-conventions.obspm.fr/archive/2003/tn32.pdf |location=Frankfurt |publisher=Bundesamts für Kartographie und Geodäsie |isbn=3-89888-884-3}}</ref>
* The mean diameter of Earth is about 0.0425 light-seconds. * The average distance between Earth and the [[Moon]] (the [[lunar distance (astronomy)|lunar distance]]) is about 1.282 light-seconds. * The diameter of the [[Sun]] is about 4.643 light-seconds. * The average distance between Earth and the Sun (the [[astronomical unit]]) is 499.0 light-seconds.
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 so the average distance between Earth and the Sun is 8.317 light-minutes. * The average distance between [[Pluto]] and the Sun (34.72 AU<ref>{{Cite web |title=Pluto distance from sun - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}}</ref>) is 4.81 light-hours.<ref>{{Cite web |title=Pluto distance from sun in light hours - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}}</ref> * Humanity's most [[List of artificial objects leaving the Solar System|distant artificial object]], ''[[Voyager 1]]'', has an interstellar velocity of 3.57 AU per year,<ref>{{Cite web |title=Voyager - Fast Facts |url=https://voyager.jpl.nasa.gov/frequently-asked-questions/fast-facts/ |access-date=2023-03-07 |website=voyager.jpl.nasa.gov |language=en}}</ref> or 29.7 light-minutes per year.<ref>{{Cite web |title=3.57 au/year in light-minutes/year - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}}</ref> As of 2025 the probe, launched in 1977, is over 23 light-hours from Earth and the Sun<ref>{{Cite web |title=Where are Voyager 1 and Voyager 2 Now?|url=https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/#mission-status|access-date=2025-11-28|website=science.nasa.gov|language=en}}</ref>, and is expected to reach a distance of one light-day around November 2026.<ref>{{Cite web|title=Eyes on the Solar System|url=https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_1/distance?time=2026-11-15T16:21:15.692+00:00&rate=0&to=earth|access-date=2025-11-28|website=eyes.nasa.gov|language=en}}</ref>
{| class="wikitable" !rowspan=2|Unit ! rowspan=2| Definition ! colspan="3" | Equivalent distance in ! rowspan=2 | Example |- !Meters !Kilometers !Miles |- ! light-second | 1 light-second | style="text-align:right" | {{val|299792458|u=m}} || style="text-align:right" |{{val|2.998|e=5|u=km}} || style="text-align:right" |{{val|1.863|e=5|u=miles}} | Average distance from the Earth to the Moon is about 1.282 light-seconds |- ! light-minute | 60 light-seconds<br/>= {{val|1}} light-minute | style="text-align:right" | {{val|17987547480|u=m}} || style="text-align:right" |{{val|1.799|e=7|u=km}} || style="text-align:right" |{{val|1.118|e=7|u=miles}} | Average distance from the Earth to the Sun is 8.317 light-minutes |- ! light-hour | 60 light-minutes<br/>= {{val|3600}} light-seconds | style="text-align:right" | {{val|1079252848800|u=m}} || style="text-align:right" |{{val|1.079|e=9|u=km}} || style="text-align:right" |{{val|6.706|e=8|u=miles}} | The [[Apsis|perihelion]] of [[Saturn]]'s [[orbit]] is about 1.25 light-hours |- ! light-day | 24 light-hours<br/>= {{val|86400}} light-seconds |style="text-align:right" |{{val|25902068371200|u=m}} || style="text-align:right" |{{val|2.590|e=10|u=km}} || style="text-align:right" |{{val|1.609|e=10|u=miles}} | [[Voyager 1]] is about 0.96 light-days from the Sun (as of March 2025) |- ! light-week | 7 light-days<br/>= {{val|604800}} light-seconds | style="text-align:right" |{{val|181314478598400|u=m}} || style="text-align:right" |{{val|1.813|e=11|u=km}} || style="text-align:right" |{{val|1.127|e=11|u=miles}} | The [[Oort cloud]] is thought to extend between 41 and 82 light-weeks out from the Sun |- ! light-year | 365.25 light-days<br/>= {{val|31557600}} light-seconds |style="text-align:right" | {{val|9460730472580800|u=m}} || style="text-align:right" |{{val|9.461|e=12|u=km}} || style="text-align:right" |{{val|5.879|e=12|u=miles}} | [[Proxima Centauri]] is the [[List of nearest stars|nearest star]] to the Sun, about 4.24 light years away |- |}
==See also== * [[100 megametres]] * [[Geometrized unit system]] * [[Light-year]]
== References == {{reflist}}
{{Units of length used in Astronomy}}
[[Category:Units of length]] [[Category:Units of measurement in astronomy]]