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 m (approximately 983571055 ftor186282 mi).
| Light-second | |
|---|---|
| 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 |
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 currently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly 365.25 d, each of exactly 86400 SI seconds.[1]
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.
The light-second is a convenient unit for measuring distances in the inner Solar System, since 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 between Earth and 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]
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:
| Unit | Definition | Equivalent distance in | Example | ||
|---|---|---|---|---|---|
| Meters | Kilometers | Miles | |||
| light-second | 1 light-second | 299792458 m | 2.998×105 km | 1.863×105 mi | 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 mi | 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 mi | The perihelionofSaturn'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 mi | Voyager 1 is about 0.94 light-days from the Sun (as of April 2024) |
| light-week | 7 light-days = 604800 light-seconds |
181314478598400 m | 1.813×1011 km | 1.127×1011 mi | 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 mi | Proxima Centauri is the nearest star to the Sun, about 4.24 light years away |