Greenwich is the home of Greenwich Mean Time (GMT), and the Prime Meridian passes directly through the Observatory.  All times in the world are measured from here (i.e., New York is GMT minus 5hrs).  This is also where the Eastern and Western Hemispheres meet.  On display in the Observatory are some of the most important time pieces in history.  John Harrison's clocks H1, H2, H3 and H4 are all on display.  Harrison was the man who determined how to use time pieces to determine longitude (as opposed to the stars).  This may have been the most important finding in the history of navigation.

Royal Observatory, the Prime Meridian and Harrison's Clocks

The Millennium Dome:

The Cutty Sark:  Launched on November 22, 1869, the Cutty Sark was a tea clipper and became the most famous of the ship of its type.  She was operational until 1954.