Rising like a beacon of time atop Greenwich Park, the Royal Observatory is home to the prime meridian (longitude 0° 0' 0''). Tickets include access to the Christopher Wren–designed Flamsteed House (named after the first Royal Astronomer) and the Meridian Courtyard, where you can stand with your feet straddling the eastern and western hemispheres. You can also see the Great Equatorial Telescope (1893) inside the onion-domed observatory and explore space and time in the Weller Astronomy Galleries.

　　In a small brick structure next to the Meridian Courtyard, the astonishing camera obscura projects a live image of Queen's House – as well as the people moving around it and the boats on the Thames behind – onto a table. Enter through the thick, light-dimming curtains and close them behind you to keep the room as dark as possible.

　　Night-sky shows are projected daily on the inside of the roof of the Peter Harrison Planetarium.

　　The Royal Observatory was built by order of Charles II in 1675 to help solve the riddle of longitude. In 1884, Greenwich was designated as the prime meridian of the world, and Greenwich Mean Time (GMT) became the universal measurement of standard time.