how many years is one lightyear
Look up light year in Wiktionary, the free dictionary. A light-year (or light year, abbreviation: ly) is a unit of length used informally to express astronomical distances. It is approximately 9 petametres, or 9 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical 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.