The light-year is a unit of length used toexpress astronomical distances and measuresabout 9.5 trillion kilometres or 5.9 trillionmiles. As defined by the InternationalAstronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word “year”, the term light-year is sometimes misinterpreted as a unit of time, as a year is a unit of time equivalent to approximately 365 days. 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 most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 lightyears; the distance at which one astronomical unit subtendsan angle of one second of arc).