The Royal Observatory, on the Greenwich Meridian sets world time (Greenwich Mean Time) and place (Greenwich Meridian). In 1884 an international convention agreed that all countries would adopt a universal Mean Solar Day, beginning at the Mean Midnight at Greenwich and counted on a 24 hour clock. The Meridian Line, known as zero longitude, is a theoretical line running from the North Pole to the South Pole, through the Observatory at Greenwich. All other lines of longitude are measured from here. Nearby is the elegant Cutty Sark, the last surviving tea-clipper, which was built at Dumbarton in 1869 just as steam and the opening of the Suez Canal were making sailing ships redundant.