Science Explained: What is a light year?
13 August 2010 Last updated at 12:21 BST
Distances in space are enormous - so big that our standard units of measurement, kilometres and miles, are just too small to be useful.
So scientists use something else - the light year. But what is it and how does it work?
Dr Tim O'Brien from the Jodrell Bank Centre for Astrophysics at the University of Manchester explains.
Please note: The graphics used in this video are not to scale
Images courtesy of Nasa