# Light-second

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Unit of length

Light-second The distance between the Earth and the Moon is approximately 1.3 light-seconds General information Unit of length Conversions 1 light-second in ... ... is equal to ... SI units 299792458 m astronomical units 0.0020040 AU 3.1688×10−8 ly 9.7156×10−9 pc imperial/US units 186282 mi

The **light-second** is a [unit of length](/source/Unit_of_length) useful in [astronomy](/source/Astronomy), [telecommunications](/source/Telecommunications) and [relativistic physics](/source/Theory_of_relativity). It is defined as the [distance](/source/Distance) that [light](/source/Light) travels in [free space](/source/Free_space) in one [second](/source/Second), and is equal to exactly 299792458 [m](/source/Metre) (approximately 983571055 ft or 186282 [miles](/source/Mile)).

Just as the second forms the basis for other units of [time](/source/Time), the light-second can form the basis for other units of [length](/source/Length), ranging from the light-nanosecond (299.8 mm or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in [popular science](/source/Popular_science) publications. The more commonly used [light-year](/source/Light-year) is also currently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a [Julian year](/source/Julian_year_(astronomy)) (not the [Gregorian year](/source/Gregorian_calendar)) of exactly 365.25 d, each of exactly 86400 [SI](/source/SI) seconds.[1]

## Use in telecommunications

Communications signals on [Earth](/source/Earth) travel at precisely the [speed of light](/source/Speed_of_light) in free space.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] 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](/source/Foot_(length))[2]), 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](/source/Communications_satellite) are typically 1.337 light-milliseconds[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] ([low Earth orbit](/source/Low_Earth_orbit)) to 119.4 light-milliseconds ([geostationary orbit](/source/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

The yellow shell indicating one light-day distance from the Sun compares in size with the positions of *[Voyager 1](/source/Voyager_1)* and *[Pioneer 10](/source/Pioneer_10)* as of 2008 (red and green arrows respectively). It is larger than the heliosphere's [termination shock](/source/Heliosphere#Termination_shock) (blue shell) but smaller than [Comet Hale-Bopp](/source/Comet_Hale-Bopp)'s orbit (faint orange ellipse below). Click on the image for a larger view and links to other scales.

The faint yellow sphere centred on the Sun has a radius of one light-minute. For comparison, sizes of [Rigel](/source/Rigel) (the blue star in the top left) and [Aldebaran](/source/Aldebaran) (the red star in the top right) are shown to scale. The large yellow ellipse represents [Mercury's](/source/Mercury_(planet)) orbit.

The light-second is a convenient unit for measuring distances in the inner [Solar System](/source/Solar_System), since it corresponds very closely to the [radiometric](/source/Radiometry) data used to determine them. (The match is not exact for an Earth-based observer because of a very small correction for [the effects of relativity](/source/Time_dilation).) The value of the [astronomical unit](/source/Astronomical_unit) (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern [ephemerides](/source/Ephemeris) (tables of planetary positions). It is usually quoted as "light-time for unit distance" in tables of [astronomical constants](/source/Astronomical_constant), and its currently accepted value is 499.004786385(20) s.[3][4]

- The mean diameter of Earth is about 0.0425 light-seconds.

- The average distance between Earth and the [Moon](/source/Moon) (the [lunar distance](/source/Lunar_distance_(astronomy))) is about 1.282 light-seconds.

- The diameter of the [Sun](/source/Sun) is about 4.643 light-seconds.

- The average distance between Earth and the Sun (the [astronomical unit](/source/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](/source/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](/source/Pluto) and the Sun (34.72 AU[5]) is 4.81 light-hours.[6]

- Humanity's most [distant artificial object](/source/List_of_artificial_objects_leaving_the_Solar_System), *[Voyager 1](/source/Voyager_1)*, has an interstellar velocity of 3.57 AU per year,[7] or 29.7 light-minutes per year.[8] As of 2025 the probe, launched in 1977, is over 23 light-hours from Earth and the Sun[9], and is expected to reach a distance of one light-day around November 2026.[10]

Unit Definition Equivalent distance in Example Meters Kilometers Miles light-second 1 light-second 299792458 m 2.998×105 km 1.863×105 miles Average distance from the Earth to the Moon is about 1.282 light-seconds light-minute 60 light-seconds = 1 light-minute 17987547480 m 1.799×107 km 1.118×107 miles Average distance from the Earth to the Sun is 8.317 light-minutes light-hour 60 light-minutes = 3600 light-seconds 1079252848800 m 1.079×109 km 6.706×108 miles The perihelion of Saturn's orbit is about 1.25 light-hours light-day 24 light-hours = 86400 light-seconds 25902068371200 m 2.590×1010 km 1.609×1010 miles Voyager 1 is about 0.96 light-days from the Sun (as of March 2025) light-week 7 light-days = 604800 light-seconds 181314478598400 m 1.813×1011 km 1.127×1011 miles The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun light-year 365.25 light-days = 31557600 light-seconds 9460730472580800 m 9.461×1012 km 5.879×1012 miles Proxima Centauri is the nearest star to the Sun, about 4.24 light years away

## See also

- [100 megametres](/source/100_megametres)

- [Geometrized unit system](/source/Geometrized_unit_system)

- [Light-year](/source/Light-year)

## References

1. **[^](#cite_ref-1)** [IAU Recommendations concerning Units](http://www.iau.org/Units.234.0.html) [Archived](https://web.archive.org/web/20070216041250/http://www.iau.org/Units.234.0.html) 2007-02-16 at the [Wayback Machine](/source/Wayback_Machine)

1. **[^](#cite_ref-2)** [David Mermin](/source/David_Mermin) suggested one light-nanosecond might be called a *phoot* at page 22 of *It's About Time* (2005), [Princeton University Press](/source/Princeton_University_Press)

1. **[^](#cite_ref-JPL_3-0)** Standish, E. M. (1998). ["JPL Planetary and Lunar Ephemerides, DE405/LE405"](https://web.archive.org/web/20120220062549/http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf) (PDF). JPL IOM 312.F-98-048. Archived from [the original](https://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf) (PDF) on 2012-02-20..

1. **[^](#cite_ref-IERS_4-0)** McCarthy, Dennis D.; Petit, Gérard, eds. (2004). "IERS Conventions (2003)". [*IERS Technical Note No. 32*](https://iers-conventions.obspm.fr/archive/2003/tn32.pdf) (PDF). Frankfurt: Bundesamts für Kartographie und Geodäsie. [ISBN](/source/ISBN_(identifier)) [3-89888-884-3](https://en.wikipedia.org/wiki/Special:BookSources/3-89888-884-3).

1. **[^](#cite_ref-5)** ["Pluto distance from sun - Wolfram|Alpha"](https://www.wolframalpha.com/). *www.wolframalpha.com*. Retrieved 2023-03-07.

1. **[^](#cite_ref-6)** ["Pluto distance from sun in light hours - Wolfram|Alpha"](https://www.wolframalpha.com/). *www.wolframalpha.com*. Retrieved 2023-03-07.

1. **[^](#cite_ref-7)** ["Voyager - Fast Facts"](https://voyager.jpl.nasa.gov/frequently-asked-questions/fast-facts/). *voyager.jpl.nasa.gov*. Retrieved 2023-03-07.

1. **[^](#cite_ref-8)** ["3.57 au/year in light-minutes/year - Wolfram|Alpha"](https://www.wolframalpha.com/). *www.wolframalpha.com*. Retrieved 2023-03-07.

1. **[^](#cite_ref-9)** ["Where are Voyager 1 and Voyager 2 Now?"](https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/#mission-status). *science.nasa.gov*. Retrieved 2025-11-28.

1. **[^](#cite_ref-10)** ["Eyes on the Solar System"](https://eyes.nasa.gov/apps/solar-system/#/sc_voyager_1/distance?time=2026-11-15T16:21:15.692+00:00&rate=0&to=earth). *eyes.nasa.gov*. Retrieved 2025-11-28.

v t e Units of length used in Astronomy Astronomical system of units Earth radius (R🜨 or RE) Jupiter radius (RJ) Light-second (ls) Lunar distance (LD or Δ⊕L) Solar radius (R☉) Astronomical unit (au) light-year (ly) parsec (pc) kiloparsec (kpc) megaparsec (Mpc) gigaparsec (Gpc) See also Cosmic distance ladder Orders of magnitude (length) Conversion of units

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Adapted from the Wikipedia article [Light-second](https://en.wikipedia.org/wiki/Light-second) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Light-second?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.