Until the mid 1750s, navigation at sea was an unsolved problem due to the difficulty in calculating longitudinal position. Navigators could determine their latitude by measuring the sun’s angle at noon (i.e., when it reached its highest point in the sky). To find their longitude, however, they needed a portable time standard that would work aboard a ship. Chronometer provided such time. By comparing local high noon to the chronometer’s time, a navigator could use the time difference to determine the ship’s present longitude. Since the Earth rotates 360 degrees every day (that is, 24 hours or 1,440 minutes), the time difference between the two points reveals how many degrees separate them.
The creation of a seaworthy timepiece was difficult. Until the 20th century, the best timekeepers were pendulum clocks, but the rolling of a ship at sea rendered the ordinary, gravity-based pendulum useless. John Harrison, a Yorkshire carpenter, invented a clock based on a pair of counter-oscillating weighted beams connected by springs whose motion was not influenced by gravity or the motion of a ship. In 1761 Harrison submitted his invention for the £20,000 longitude prize that had been offered by the British government in 1714. By 1825, the British Navy had begun routinely supplying its vessels with chronometers. Till the 2nd World War, chronometer manufacture was hand made and craft based. With the onset of the war, the Hamilton Watch Company in the US perfected the process of mass production, which enabled them to produce thousands of their superb Hamilton Model 21 chronometers for the US Navy and other Allied navies. Despite Hamilton’s success, chronometers made in the old way never disappeared from the marketplace during the era of mechanical timekeepers. Mercer, in St. Albans, England, for instance, continued to produce high-quality chronometers by traditional production methods well into the 1970s.