Blackheath Avenue Greenwich London SE10 8XJ
Founded by Charles II in 1675, the home of the so-called prime meridian, at zero degrees longitude, and of course of Greenwich Mean Time, the observatory is Greenwich's most historic building, perched at the top of the hill in a wonderful spot at the peak of Greenwich Park overlooking the river that was not surprisingly originally the site of a castle. It was also the site of what was probably Britain's first terrorist incident in 1894 when a French anarchist blew himself up on his way to plant a bomb here (an incident Joseph Conrad used for this book, The Secret Agent), and nowadays is probably best-known as the home of John Harrison's four marine chronometers (the stars of Dava Sobel's runaway 1990s bestseller Longitude), which are on display in the Wren-designed original building, known as Flamsteed House after the first Astronomer Royal. You can also visit the Astronomer Royal's apartments and the so-called Octagon Room, used for celestial observations, although the onion-domed building next door has the observatory's main 28-inch telescope (which remains the largest of its kind in the UK and is sometimes open for guided tours). Outside, join the tourists in straddling the meridian line and inspecting the standard units of imperial measurement displayed by the gate before checking out the exhibition in the Victorian observatory and its statue of first man in space Yuri Gagarin.