Skip links

Facts about Light-Years for Kids

A light-year, also light year or lightyear, is a unit of length equal to just under 6 trillion miles. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

  • The light-year is mostly used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications.
  • Before 1984, the tropical year and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.
  • A value of 9.460536207×10 m found in some modern sources is the product of a mean Gregorian year of 365.2425 days (31556952 s) and the defined speed of light (299792458 m/s).
  • The first successful measurement of the distance to a star other than our Sun was made by Friedrich Bessel in 1838.
  • The use of this unit in trigonometric calculations based on 61 Cygni’s parallax of 0.314 arcseconds, gave the distance to the star as 660 thousand astronomical units.
  • Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible.
  • Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years.
  • This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec, it is popularly used to gauge the expanses of interstellar and intergalactic space.
  • Distances measured in light-years include distances between nearby stars, such as those in the same spiral arm or globular cluster.
  • Megalight-years are typically used to measure distances between neighboring galaxies and galaxy clusters.
  • For example, the light-second, useful in astronomy, telecommunications and relativistic physics, is exactly 299792458 meters or ⁄31557600 of a light-year.