In the early 18th century, sailors could measure latitude using the sun and stars. However, there were no accurate means to determine the longitudinal coordinate of a ship's location. It was known that the coordinates could be calculated mathematically by comparing two accurate times—one at a known longitude and the other at an unknown point. Unfortunately, the pendulum clocks of the day could not be counted on to keep reliable measures when subjected to the rolling of a ship on ocean waves. After several naval disasters resulted from poor navigation, the British government offered a reward for technical solutions to the problem of longitude. The Longitude Act of 1714 offered £20,000 ($26,302) to anyone who could devise an accurate method of calculating the location of a ship within one half degree.Clockmakers set out to win the prize money by inventing a marine chronometer which could keep accurate time aboard a vessel. The English carpenter and clockmaker John Harrison eventually won the prize, and he is credited with the invention of the first accurate marine chronometer. In 1728, Harrison began to build his first prototype clock, known as H1. By 1736, the administrators of the prize—the Board of Longitude—were intrigued enough to order a sea trial of his first model. Pleased with the initial results, Harrison was awarded grants to continue refining his mechanism for improved accuracy. The clockmaker built models H2 and H3 over the next two decades. These models used springs and balances rather than the classic pendulum; however, the accuracy of each chronometer was still slightly lacking.
Harrison's big break came with his fourth model, H4. In the mid-1750s the inventor decided to craft his next sea clock as a watch, rather than the earlier bulky models. In 1761, the Board tested H4 on a trans-Atlantic voyage. The results were shocking—the sea watch kept almost perfect time. Arriving in Jamaica, the watch showed a local time only five seconds behind the true time. The calculations of longitude made from the time kept on H4 were accurate to within one nautical mile. Unfortunately, the Board and the English Parliament were not immediately convinced of the watch's accuracy. It took a fifth model, H5, and personal intervention by King George III before Harrison received the prize money for his life's work.
The impact of the sea watch—or marine chronometer as it came to be known—was enormous. Navigation capabilities were greatly improved for explorers of the 18th and 19th centuries. The infamous explorer Captain James Cook used a replica of the H4 watch on his travels, finding it exceptionally accurate. While electronic GPS systems have mostly replaced the marine chronometer today, some sailors are still required to learn the method during naval service or certification processes. Chronometers today are encountered particularly in the tradition of Swiss watches, where the term denotes precision and accuracy.
If you would like to learn more about this clock which changed the world, scroll down to watch a video from the BBC.