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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