Title of an excellent non-fiction book by Dava Sobel describing the final solution to the problem of the accurate determination of longitude by navigators at sea. Explains some of the politics and skulduggery behind the eventual development of reliable chronometers.

Why is it necessary to be able to tell the time? If you know that the earth rotates on its axis once every 24 hours, and you know that at noon the sun is at its zenith in the sky, the difference between the time when it reaches the zenith in Greenwich and the time it reaches its zenith where you are allows you to determine how far east or west you are of the prime meridian.