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

As the Age of Exploration got underway in the late 1400s, ships ventured far out to sea: a vast expanse with no landmarks to give sailors their position. Though they could determine their distance north or south of the Equator by observing the sun, moon, and stars, they had no way of knowing how far east or west they were. This was the “longitude problem,” named for the vertical lines of longitude on maps of the Earth. Without knowing a sailing vessel’s position relative to one of those lines, it and its crew effectively were lost. Although methods existed by which captains could estimate their longitude, being even slightly off could prove disastrous as errors compounded over time and distance. A ship might wander for weeks while its crew starved or, as it neared known land, crash onto shores hundreds of miles east or west of its destination. This was what happened in 1707, when four British naval vessels and nearly everyone on board sank after striking rocks miles away from their expected location.

In 1714, the English Parliament passed the Longitude Act, which provided a large cash prize to anyone who could develop a method of determining a ship’s longitude to within half a degree (roughly 34 miles at the Equator), with lesser prizes for methods that neared that level of accuracy: “The fact that the government was willing to award such huge sums for ‘Practicable and Useful’ methods that could miss the mark by many miles eloquently expresses the nation’s desperation over navigation’s sorry state” (54).

A clock might solve the problem if it were accurate to within three seconds a day, but most timepieces of the era were off by minutes a day. In theory, a super-precise clock could keep home-port time reliably, and by comparing that time to local time (as determined by the location of the sun, moon, or stars), a ship could locate itself on the globe to within a few tens of miles—a margin of error significantly smaller than the hundreds of miles possible with guesswork.

By the 1730s, John Harrison had invented a large clock with a mechanism accurate enough to pass the Longitude Act’s strictest tests. Decades later, improved and miniaturized versions called chronometers would lead to general adoption of Harrison’s sea clock method, but not before another equally accurate system, the lunar method, would compete for the prize. This method used books called ephemerides that contained timetables for the motion of the moon against the background of stars. Sailors who used a quadrant or sextant to observe the moon’s position relative to a star and compared the local time with the listed time for the same event in London or Paris could reckon their ship’s longitude.

The English Board of Longitude, charged with finding the best method, recognized the value of both timepieces and time charts, but the sea clock method stood out as more practical. The lunar method could calculate longitude, but it was complicated, took more than a half-hour (and up to four hours) to complete, and its many steps increased the chance for error. Clocks, on the other hand, themselves contained the correct time in London or Paris, eliminating the need for all those painful sky-watching calculations; all that was necessary was a quick estimate of local time, and the time difference between ship and home port was known. The first marine clocks, however, were very expensive; it wasn’t until watchmakers mastered the techniques of mass-producing precision chronometers that the sea clock method of determining longitude finally was adopted worldwide.