Rising south of Queen's House, idyllic Greenwich Park climbs up the hill, affording stunning views of London from the Royal Observatory, which Charles II had built in 1675 to help solve the riddle of longitude. To the north is lovely Flamsteed House and the Meridian Courtyard, where you can stand with your feet straddling the western and eastern hemispheres; admission is by ticket. The southern half contains the highly informative and free Weller Astronomy Galleries and the Peter Harrison Planetarium.
In 1884 Greenwich was designated as the prime meridian of the world, and Greenwich Mean Time (GMT) became the universal measurement of standard time. Becoming a Royal Museums Greenwich Member (from £39 per year) allows you unlimited access to the Royal Observatory, the Peter Harrison Planetarium, the Cutty Sark as well as exhibitions at the National Maritime Museum; see the website for details.