Liz Bonnin explores the transit of Venus, an event which takes place just after 11pm tonight and offers a rare opportunity to see the planet passing across the face of the sun. She explains why the orbits of Venus and Earth mean they are only aligned twice every century, and investigates what studies of Venus have revealed about life on Earth. Solar physicist Lucie Green investigates James Cook's 1769 voyage to Tahiti to observe the transit, and oceanographer Helen Czerski explores how Earth and Venus came to be so different.