Longitude and latitude are generally considered two sides of the same coin, but when it came to navigation in the 1700s, they were nothing alike. Latitude was easy: use a sextant to measure the angle of the sun at its highest point, and you knew how far north or south you were from the equator. But longitude was a whole other ballgame. Because the Earth rotates about 15 degrees every hour, traveling east and west changes the time of sunset: every 15 degrees you travel changes sunset by an hour. If you know the time where you are and the time in another known location, you can use the difference between them to work out your longitude: every four minutes means one degree difference in longitude.

The problem was that clocks weren't very accurate, especially not on ships. The state-of-the-art clocks of the time used pendulums, and when you expose the steady beat of a pendulum to the chaotic churn of the sea, well, you don't have a steady beat anymore. One solution was to avoid using clocks altogether, and instead relying on the motion of the moon relative to the stars. But that took a lot of astronomical prediction and even more advanced instruments, so that was also a problem.

In 1714, the British government put up a prize of £20,000 (about £3.5 million today) that would go to anyone who could find longitude to within half a degree. Nobody had claimed the prize a decade later when John Harrison came to London, hoping to claim it himself. The first clock he developed used two double-ended pendulums, and despite the waves, it actually did fairly well on a trial journey to Lisbon in 1736. Britain's Commissioners of Longitude weren't satisfied with the results, but they agreed to fund Harrison's future efforts in hopes he might create a better timekeeper.

He created two more models until he hit upon the invention that would change the world: H4, a sort of oversized pocket watch but with much more powerful internal components that made it tick five times per second. John was pushing 70 by this point, so the Commissioners agreed to let his son William test it out on a journey to Jamaica. It succeeded with flying colors, and despite some political drama, Harrison eventually got the £20,000 prize. Harrison's marine chronometer eventually combined with more accurate celestial measurements to help sailors pinpoint their place on the planet. What's more, it made a successful end to what some call the world's first crowdsourcing event.

The whole story is worth reading in Dava Sobel's "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time." We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.