A licht-year, an aa licht year or lichtyear (seembol: ly), is an astronomical unit o length equal tae juist unner 10 trillion kilometres (or aboot 6 trillion miles).[note 1] As defined bi the Internaitional Astronomical Union (IAU), a licht-year is the distance that licht travels in vacuum in ane Julian year.
The licht-year is maist eften used when expressin distances tae starns an ither distances on a galactic scale, especially in non-specialist an popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 licht-years), as it can be mair easily derived frae, an compared wi, observational data.
The licht-year is a measur o distance, nae o time.
- Ane trillion here is taken tae be 1012 (ane million million).