A light year is a distance, not a time: it is how far light travels in one year, about 9.46 trillion kilometers. Light covers roughly 300,000 kilometers every second, and a year gives it a very long run.

Astronomers use light years because space is so empty. The nearest star beyond the Sun, Proxima Centauri, is about 4.2 light years away, and our galaxy is roughly 100,000 light years across. Seeing a star ten light years away means seeing light that left it ten years ago.

In writing, the light year is usually shortened to "ly". One light year is 9.460730e+15 meters, and that ratio is what every figure on this page is built from.