Commenting on Arthur's question and responses by Patrick and Fernando,I agree with the comments and recommendation for the book "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time" by Dava Sobel (Penguin Books ISBN 0 14 02-5879 5). It demonstrates difficulty of the problem and teaches that the phrase used by lawyers also applies to navigators solving the problem of longitude. "Time is of the essence".
A personal hero of mine who solved the longitude problem is David Thompson, a fur trader, explorer and cartographer who explored and mapped my part of the world from 1797 to 1812. He taught himself celestial navigation and surveying while working for the Hudsons Bay Company. On his travels and trading missions in Western Canada, he took sextant sights reflected in a small pool of mercury and determined the time by observing the eclipse of moons of Jupiter. His map accurately fixed the position (Latitude and Longitude) of the major features of western Canada. This formed the basis for the later suryeying for the railroads and farm settlement. His original map hung in the meeting room of the North West Company. A 16" x 20" photo reproduction now hangs in my den. Roger Bailey Walking Shadow Designs 51 N 115 W
