This small, beautiful building was designed by Christopher Wren - the same genuis who designed St Paul's Cathedral and many other gems of churches in London. Since the light pollution in London grew, not much astronomy has taken place here, as the sky is often obscured.
Parts of it are now open to the public as a museum. It contains sky maps, antique instruments, and a very interesting display of early watches - necessary to calculate longitude accurately. The top room, octaganol in shape, contains a fascintating camera obscura.
Entrance costs £4, less for concessions, and you can buy a combined ticket for this, the Queen's House, and the National Maritime Museum, for £5, which is a bargain!