Light-year
A light-year, alternatively spelled light year (ly), is a unit of length used to express astronomical distances and is equal to exactly 9,460,730,472,580.8 km, which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the word "year", the term is sometimes misinterpreted as a unit of time.
| Light-year | |
|---|---|
Map showing the stars that lie within 12.5 light-years of the Sun | |
| General information | |
| Unit system | astronomy units |
| Unit of | length |
| Symbol | ly |
| Conversions | |
| 1 ly in ... | ... is equal to ... |
| metric (SI) units | 9.4607×1015 m 9.46073 Pm |
| imperial and US units | 5.8786×1012 mi |
| astronomical units | 63241 au 0.3066 pc |
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit (au) subtends an angle of one second of arc.