The British Royal Observatory is home of Greenwich Mean Time and the Prime Meridian line, making it one of the most important historic scientific sites in the world. It was founded by Charles II in 1675 and is, by international decree, the official starting point for each new day, year and millennium (at the stroke of midnight GMT as measured from the Prime Meridian).
Museums - Science, Scientific - Astronomy
The Prime Meridian (0 Longitued) passes right through the building.
Links: en.wikipedia.org, www.world-heritage-tour.org