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This television documentary follows the search for the key to navigating the seas. It is based on the novel by Dava Sobel, Lost at Sea -- The Search for Longitude. It begins with Will Andrewes, as he joins a crew to try out a replica of a typical log and line of the early 1700s. It was a crude method, using a triangle of wood (called a log) with a knotted line tied to it. They would count the number of knots that passed through the navigator's fingers in the time it took a 28-second sand glass to flow through. This would give the speed of the ship in knots and was the only way to tell the distance a ship traveled at sea. These ideas were advanced by astronomers and by accurate timepieces made by John Harrison in the 1700s. The rest of the episode shows the continued quest for accurate longitude through astronomy and accurately tracking time at sea.