<Tr> <Th> imperial & US units </Th> <Td> 7015946068963840000 ♠ 5.8786 × 10 mi </Td> </Tr> <Tr> <Th> astronomical units </Th> <Td> 7015946071894093870 ♠ 63 241 au 7015946068746477840 ♠ 0.3066 pc </Td> </Tr> <P> The light - year is a unit of length used to express astronomical distances and measures 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 most commonly 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>

One light year is equal to how much