Navigation at sea was a real problem in the Eighteenth Century. Although ships could work out their latitude from the position of the sun, it was difficult to know how far east or west they were. In 1714 a Longitude Act was passed, offering rewards of up to £20,000 for anyone who could solve the problem of finding longitude at sea.
The National Maritime Museum and Cambridge University have put the archives relating to this period of exploration and invention online - do watch the film and explore the website. If you live near enought to visit Greenwich, you may enjoy one of the Longitude Season events.