This museum of time and space was built by Sir Christopher Wren in 1675. It marks the site of the Greenwich or Prime Meridian - the base point for World Time and the site of Longitude 0. If you time it right, you can watch the timeball drop at 13:00 each day. Between them, the National Maritime Museum, at this site since 1937, and the Royal Observatory have accumulated over 2 million objects about the sea, ships, astronomy and time. Visitors can also see the Astronomer Royal's apartments, the famous John Harrison timekeepers (1735-60) with computer simulations, intricate instruments, Wren's Octagon Room and the largest refracting telescope in Britain. In 2007 the £15 million Peter Harrison Planetarium opened, a beautiful cone-shaped addition to the Royal Observatory designed by architects Allies and Morrison. This 120-seat, state-of-the-art facility replaces the small and out of date planetarium in the dome of the South Building.