The historic Royal Observatory in Greenwich Park was founded in 1675 by King Charles II. With his newly appointed Astronomer Royal, John Flamsteed, the intention was to create the conditions where the stars could be plotted and mapped, and longitude researched, in order to aid navigation.
At the Observatory, you can cross the Prime Meridian line - it is from this point that all time is measured. The Royal Observatory is now part of the National Maritime Museum and is definitely a required visit for anyone coming to Greenwich. The Astronomy Galleries in the Observatory's south building offer free admission and will astound you with its interactive exhibits.
Also at the Observatory, you can enjoy dazzling shows in the state of the art planetarium, including frequent shows on "stargazing for beginners". Don't forget to look at London's only public camera obscura too!