Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of his Time

by Dava Sobel

Pope Alexander VI issued the Bull of Demarcation in settlement. With aloof equanimity, His Holiness drew a meridian line from north to south on a chart of the great ocean, one hundred leagues west of the Azores. He assigned all lands west of the line, discovered or undiscovered, to Spain and all lands east of it to Portugal. It was masterful diplomacy, particularly when no one knew where the line fell.

I learned the trick for remembering the difference between latitude and longitude. The latitude lines, the parallels, really do stay parallel to each other as they girdle the globe from the Equator to the poles in a series of shrinking concentric rings. The meridians of longitude go the other way: They loop from the North Pole to the South and back again in great circles of the same size, so they all converge at the ends of the Earth.

A common misconception of his day held that anyone living below the Equator would melt into deformity from the horrible heat.

The zero-degree parallel of latitude is fixed by the laws of nature, while the zero-degree meridian of longitude shifts like the sands of time.

The two clock times enable the navigator to convert the hour difference into a geographical separation. Since the Earth takes twenty-four hours to complete one full revolution of three hundred sixty degrees, one hour marks one twenty-fourth of a spin, or fifteen degrees. And so each hour’s time difference between the ship and the starting point marks a progress of fifteen degrees of longitude to the east or west.

English clockmaker John Harrison, a mechanical genius who pioneered the science of portable precision timekeeping, devoted his life to this quest. He accomplished what Newton had feared was impossible: He invented a clock that would carry the true time from the home port, like an eternal flame, to any remote corner of the world.

Cornered, Harrison led Maskelyne into the room where he kept the clocks, which had been his close companions for thirty years. They were all running, each in its own characteristic way, like a gathering of old friends in animated conversation. Little did they care that time had rendered them obsolete.