A light-year or light year (symbol: ly)'s a unit of length, equal to just under ten trillion kilometres. As defined by the International Astronomical Union (IAU), a light-year's the distance that light travels in a vacuum in one Julian year. The light-year's often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry's the parsec, because it can be more easily derived from, and… (More on Light-year)