A light-year or light year (symbol: ly) is a unit of measurement of length, equal to just under ten trillion kilometres. As defined by the International Astronomical Union (which is the body which has the jurisdictional authority to promulgate the definition), a light year is the distance that light travels in a vacuum in one Julian year. Numerical value A light-year is equal to: exactly 9,460,730,472,580.8 km (about 9.461 Pm) about 5,878,625,373,183.61 statute miles about 63,241 astronomica… (More on Light-year)