Light Year

Light year is a unit of astronomical distance defined as the distance that light travels in vacuum during one Julian year.  It is equivalent to approximately 9.46073 trillion kilometers, or 5.878625 trillion miles.  This value is derived from the internationally accepted speed of light in vacuum, which is exactly 299,792,458 meters per second, multiplied by the number of seconds in a Julian year of 365.25 days.  The light-year is used primarily in astronomy to express vast interstellar and intergalactic distances in a more manageable way than kilometers or miles, since the scales involved make those units cumbersome.  This definition and its precise numerical value are established by organizations such as the International Astronomical Union (IAU) and are based on fundamental physical constants and conventions for time measurement.