Every year, thousands of people come to the Royal Observatory in Greenwich to have their photograph taken on the line of the Prime Meridian - longitude 0 Degrees - as they stand in the eastern and western hemispheres at the same time.But what is the purpose and history of the Greenwich Meridian? What other points in the world lie along it? And what links the line with navigation, timekeeping and the stars? Find out on this whistle-stop tour from the North Pole, through Greenwich, to France, Spain, Africa and Antarctica, revealing the Greenwich Meridian's fascinating history along the way.