A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (6 trillion miles). As defined by the IAU, a light year is the distance light travels in a vacuum in one Julian year.
The light-year is 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 astrometry is the parsec, because it can be more easily derived from, and compared with observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond or parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light years.
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