Light-year

Also called: ly, parsec, pc.

The distance light travels in a year: about 9.5 trillion kilometres. A parsec is 3.26 light-years.

Distances in space are so vast that astronomers measure them by light's travel time. Light takes 1.3 seconds from the Moon, 8 minutes from the Sun, 4.2 years from the nearest star, 1,300 years from the Orion Nebula and 2.5 million years from the Andromeda Galaxy — so you see each as it was when that light set off.