A light-year is a unit of length used to express astronomical distances. It is about 9.5 trillion kilometres or 5.9 trillion miles.[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).[1] The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in nonspecialist and popular science publications. The unit usually used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).[1]


  1. One trillion is 1012 (one million million, or billion in long scale).


  1. 1.0 1.1 International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units,, retrieved 10 November 2013