The light year is a unit of distance that comes from the knowledge that light (or electromagnetic radiation in general) takes a finite length of time to travel through space. It is defined as the distance that light travels in a year through a vacuum.The speed of light in a vacuum = 299,792,458 metres/second The number of seconds in a year =60 × 60 x 24 × 365.25 1 light year or ly = 60 × 60 x 24 × 365.25 × 299,792,458 = 9.46 × 10 ^{15}m= 9.46 × 10 ^{12}km= 63,240 AU = 0.306 pc Remember that a light year

*is not*a measure of time!