KinexiaLEARN PHYSICS

Light-year (ly)

Quantity: Distance · Category: Non-SI

The light-year is a unit of distance equal to how far light travels in one Julian year in vacuum — about 9.46 trillion kilometres. It is the standard yardstick for interstellar distances.

Relation to SI

1 ly = c × 1 Julian year = 9.4607304725808×10¹⁵ m

Examples

  • Proxima Centauri, the nearest star, is 4.25 light-years away.
  • The Milky Way is about 100,000 light-years across.
  • Light from the Sun takes 8.3 minutes — 8.3 light-minutes — to reach Earth.