<Table> <Tr> <Td> </Td> <Td> Look up light year in Wiktionary, the free dictionary . </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> Look up light year in Wiktionary, the free dictionary . </Td> </Tr> <P> The light - year is a unit of length used to express astronomical distances . It is about 9.5 trillion kilometres or 5.9 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 . </P> <P> 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 usually used in professional astrometry is the parsec (symbol: pc, about 3.26 light - years; the distance at which one astronomical unit subtends an angle of one second of arc). </P>

How many miles in a light year wiki