# Light-second

Light-second
Unit oflength
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 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 299792458 metres by the definition of metre (just over 186282 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 (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 publications. The more commonly used light-year is also presently defined to be equal to precisely 31557600 light-seconds, 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]

## Use in telecommunications

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. 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.
Other Languages
العربية: ثانية ضوئية
español: Segundo luz
فارسی: نور-ثانیه
한국어: 광초
italiano: Secondo luce
Bahasa Melayu: Saat cahaya
Nederlands: Lichtseconde

português: Segundo-luz
sicilianu: Secunnu luci
Simple English: Light-second
slovenčina: Svetelná sekunda
svenska: Ljussekund
Türkçe: Işık saniyesi
українська: Світлова секунда
Tiếng Việt: Giây ánh sáng