Have you ever wondered how we know the distance between Earth and Proxima Centauri? What about the furthest reaches of the universe — how do we know they're the furthest? The Royal Observatory in Greenwich, London currently has an exhibition on the subject and has uploaded a video that gives a simple explanation of how astronomers deal with a very complex problem.
Measuring the Universe: from the transit of Venus to the edge of the cosmos will run until September 2nd 2012, excluding July 7th - August 5th. Admission is free.