At the heart of the UNESCO World Heritage Site of Maritime Greenwich are the four world-class attractions of Royal Museums Greenwich – the National Maritime Museum, Cutty Sark, the Royal Observatory and the Queen’s House.The Royal Observatory, Greenwich, was founded in 1675 to help improve navigation at sea. Since then the Royal Observatory has come to define time and place for the world. The history and the work that has taken place here explain why the historic Prime Meridian of the World passes through Greenwich and why Greenwich Mean Time (GMT) is the basis of the international time-zone system. The Royal Observatory is also home to awe-inspiring astronomy and London's only Planetarium.