Royal Observatory, Greenwich

Perched on a hill in Greenwich Park, the Royal Observatory was founded in 1675 to improve navigation and the mapping of the stars. It gives its name to the Prime Meridian of the world, the line of zero longitude from which time zones and global position are measured, marked in the courtyard where visitors straddle two hemispheres. Flamsteed House contains historic telescopes, and a red time ball still drops daily as it has since 1833. A planetarium adds modern astronomy to the historic site.