Liz Bonnin presents a Horizon special about a rare and beautiful event in our solar system, one that we should all be able to see for ourselves - the transit of Venus across the face of the sun. It will start just before midnight of the 5th of June, and won't happen again for more than a century. Liz is joined by Lucie Green and Helen Czerski to show why the transit is such a remarkable event - transforming our understanding of our own solar system as well as helping scientists hunt for alien life on distant planets, hundreds of light years away.