The Speed of Light

What is a light-year?

A year is a unit of time, but a light-year is a unit of distance. Specifically, it is the distance that light travels in one year. Since light travels at 299,792 km/s and a year consists of 31.557 x 106 s, a light-year is 9.4605 x 1012 km or about 6 trillion miles.

The light-year is used to express distances to stars and galaxies.


This is a web preview of the "The Handy Physics Answer Book" app. Many features only work on your mobile device. If you like what you see, we hope you will consider buying. Get the App