The home of Greenwich Mean Time and the Prime Meridian line, this is 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). Now a detached part of the National Maritime Museum, it houses an impressive display and a planetarium built in 2007. There are several different star shows per day that are well worth the money, especially as it's now the only celestial performance of its kind around, after the London Planetarium completely converted to Madame Tussauds.