Among the 5,000 scientific instruments at the National Maritime Museum at Greenwich in London, UK, is a remarkable collection of sundials and related instruments, ranging in origin from the Far East and Islamic countries to London itself. It includes dipleidoscopes, like the one shown here, which were used to provide an accurate time check at noon, when the Sun passed over the meridian of the observer. The collection is catalogued in extraordinary detail in Sundials at Greenwich: A Catalogue of the Sundials, Horary Quadrants and Nocturnals in the National Maritime Museum, Greenwich (Oxford University Press, £99.50), edited by Hester Higton. The book also includes chapters that set the instruments in a cultural and historical context.