The Royal Observatory Greenwich is the home of Greenwich Mean Time and the Prime Meridian of the World. It is also home to London's only planetarium, the Harrison timekeepers and the UK's largest refracting telescope.
Royal Observatory

Entry: Free (some exhibitions may charge an entrance fee)
Address: Blackheath Avenue London SE10 8XJ
Phone: 020 8858 4422
Monday: 10am-5pm
Tuesday: 10am-5pm
Wednesday: 10am-5pm
Thursday: 10am-5pm
Friday: 10am-5pm
Saturday: 10am-5pm
Sunday: 10am-5pm