The Long Search for Longitude

[A] A ship's latitude can be read from the height of the sun at noon, and sailors could do it reasonably well by the fifteenth century. Longitude was another matter. [B] Because the earth turns, knowing how far east or west you are amounts to knowing what time it is at home while you observe local noon: every hour of difference is fifteen degrees. The problem was therefore not astronomical but mechanical. Somebody had to build a clock that would keep the time of a home port on a rolling, damp, temperature-swinging deck for months.

[C] The astronomers' alternative was to use the moon, whose position against the stars changes fast enough to serve as a clock in the sky. That method worked, but it demanded a clear night, a trained observer and four hours of calculation. [D] John Harrison's sea watch, finished in 1759, gave the answer in seconds. The Board of Longitude, staffed largely by astronomers, took some years to concede the point, and Harrison's supporters have never quite forgiven them. It is worth noting that the two methods were used side by side for decades afterwards.