On this day in 1762, the great mystery of longitude was solved when a chronometer successfully guided the HMS Deptford from Portsmouth to Jamaica.
Throughout much of history, some of the greatest scientific minds spent their life’s work finding a method of determining exact longitude. The precise measurement was critical both to cartography and ocean navigation. In the 3rd century BC, Greek astronomer Eratosthenes proposed the first system of latitude and longitude for a map of the world. By the 2nd century BC, Greek astrologer and astronomer Hipparchus proposed a system of determining longitude by comparing local and absolute time. By the 11th century, Muslim scholar and polymath Al-Biruni established the modern notion that time and longitude are related by postulating that the Earth rotated on an axis. From there, scores of scientists, from Galileo to Edmund Halley to Nevil Maskelyne, struggled to find an accurate method for determining longitude.
Latitude was easy–it was calculated by using a quadrant or astrolabe to determine the inclination of the Sun or charted stars. And determining longitude on land was fairly easy compared with the task at sea–on land, scientists had a stable surface, comfortable location, and the ability to repeat experiments. But at sea, navigators were hard-pressed to know their exact longitudinal location, which oftentimes led to disaster. When a navigation error led to the Scilly naval disaster of 1707, the British government established the Board of Longitude in 1714, promising a £20,000 prize (equivalent to £2.87 million in modern currency) for the solution.
Enter John Harrison, a self-educated English clockmaker. Harrison believed the solution lay in a mechanical device that could be taken on ships when they went to sea. Harrison designed and built a cumbersome marine clock with two interconnected bar balances designed to withstand the lurching of a ship at sea. In 1737 the device, a marine chronometer, was tested on a voyage to Lisbon. Harrison’s clock correctly indicated the ship’s position, but he wasn’t satisfied. He went on to make more marine chronometers, each with increased accuracy. An exact copy of his invention was even taken on Captain James Cook’s voyage between 1772-1774, and allowed the explorer to make the first accurate charts of the South Sea Islands.
Despite accomplishing the requirements of the Longitude Prize, the Astronomer Royal, Nevil Maskelyne, vehemently refused to accept that longitude could be solved by mechanical means. He convinced the Board to not award the prize. Harrison’s son William petitioned to Parliament and even wrote to King George III, who examined the timepiece for himself. In the end, Harrison was granted a total of £14,250 throughout his longitude-solving career.
On 24 March 1776, John Harrison died in London at age 83. Though his marine chronometers never became standard devices in nautical navigation, they marked a major achievement in navigation and paved the way for modern devices.