National Geographic : 1990 Mar
One hop bridges east and west at Greenwich,England,where a brass strip marks zero longitude (right). Firstused by sailors to fix their position, Greenwich mean time was adopted by railroads and, after 1884, worldwide as the standardfor time of day. Time is now set not by earth's rotation but by satel lite and atomic clock. The moon's monthly orbit gives humans another natural unit of time. But on South Caro lina's Stono River, the moon's pull produces a more dramatic timetable:five-foot tides that occur twice a day, captured in a double exposure.