The light-year, alternatively spelled lightyear, is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km) or 5.88 trillion miles (5.88×1012 mi).[note 1] As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days).[2] Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.[3]

Map showing the stars that lie within 12.5 light-years of the Sun[1]
General information
Unit systemastronomy units
Unit oflength
1 ly[2] in ...... is equal to ...
   metric (SI) units   9.4607×1015 m
   9.46075 Pm
   imperial & 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.[3] 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 subtends an angle of one second of arc.[2]