# Light-second

Light-second | |
---|---|

Unit of | length |

Unit conversions | |

1 light-second in ... |
... is equal to ... |

SI units | 792458 m 299 |

astronomical units | 0.0020040 AU 3.1688×10 ^{−8} ly9.7156×10 ^{−9} pc |

imperial/US units | 186282 mi |

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 792458 299metres by the definition of metre (just over 282 186miles).

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 (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 presently defined to be equal to precisely 557600 light-seconds, since the definition of a year is based on a 31Julian year (not Gregorian year) of exactly 365.25 days, each of exactly 400 86SI seconds.^{[1]}

## Use in telecommunications[edit]

Communications signals on Earth rarely travel at precisely the speed of light in free space^{[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
^{[2]}), which limits the speed of data transfer between different parts of a large 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 are typically 1.337 light-milliseconds
^{[citation needed]}(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 (119.4ms times 2); this delay is just perceptible in a transoceanic telephone conversation routed by satellite.

## Use in astronomy[edit]

The light-second 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 Earth-based observer because of a very small correction for the effects of relativity), the value of the astronomical unit (roughly the distance from the Earth to the Sun) in light seconds is a fundamental measurement for the calculation of modern ephemerides (tables of planetary positions): it is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is 499.004786385(20) s.^{[3]}^{[4]}

- The mean diameter of the Earth is about 0.0425 light-seconds.
- The average distance from the Earth to the Moon is about 1.282 light-seconds.
- The diameter of the Sun is about 4.643 light-seconds.
- The average distance from the Earth to the Sun 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 the average distance from the Earth to the Sun is 8.317 light-minutes.

Unit |
Definition |
Distance |
Example |
||

m (exact values) | km | miles | |||

light-second | 792458 m 299 | ×10^{5} km 2.998 |
×10^{5} mi 1.863 |
average distance from the Earth to the Moon is about 1.282 light-seconds | |
---|---|---|---|---|---|

light-minute | 60 light-seconds | 754748×10^{10} m 1.798 |
×10^{7} km 1.799 |
×10^{7} mi 1.118 |
average distance from the Earth to the Sun is 8.317 light-minutes |

light-hour | 60 light-minutes = light-seconds 3600 |
2528488×10^{12} m 1.079 |
×10^{9} km 1.079 |
×10^{8} mi 6.706 |
semi-major axis of Pluto's orbit is about 5.473 light-hours |

light-day | 24 light-hours = 400 light-seconds 86 |
20683712×10^{13} m 2.590 |
×10^{10} km 2.590 |
×10^{10} mi 1.609 |
Sedna is currently 0.52 light-days from the Sun; on an orbit that varies from a perihelion of 0.44 light-days to an aphelion of 5.41 light-days |

light-week | 7 light-days = 800 light-seconds 604 |
144785984×10^{14} m 1.813 |
×10^{11} km 1.813 |
×10^{11} mi 1.127 |
The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun |

light-year | 365.25 light-days = 557600 light-seconds 31 |
7304725808×10^{15} m 9.460 |
×10^{12} km 9.461 |
×10^{12} mi 5.879 |
Proxima Centauri is the nearest star to the Sun, about 4.24 light years away |

## See also[edit]

## References[edit]

**^**IAU Recommendations concerning Units Archived 2007-02-16 at the Wayback Machine.**^**David Mermin suggested one light-nanosecond might be called a*phoot*at page 22 of*It's About Time*(2005), Princeton University Press**^**Standish, E. M. (1998),*JPL Planetary and Lunar Ephemerides, DE405/LE405*(PDF), JPL IOM 312.F-98-048, archived from the original (PDF) on 2012-02-20.**^**McCarthy, Dennis D.; Petit, Gérard, eds. (2004), "IERS Conventions (2003)",*IERS Technical Note No. 32*, Frankfurt: Bundesamts für Kartographie und Geodäsie, ISBN 3-89888-884-3.