Early mariners had no accurate or reliable way of finding longitude at sea. In 1714 the British Parliament offered a reward of £20,000 to anyone who could solve the longitude problem by any means. While astronomers continued to develop the "lunar distance method" by observing and attempting to predict the motion of the moon relative to fixed stars, English carpenter and clockmaker John Harrison began work on a chronometer that would be accurate and sturdy enough to withstand long sea voyages.
Harrison submitted his first chronometer to the Board of Longitude in 1736, and built two more large, heavy chronometers over the course of the next 19 years. When these failed the Board’s tests, Harrison realized that he needed an entirely different design. The result of several more years of work was H4, a chronometer that resembled a pocket-watch and weighed only 1.45 kg. H4 was tested on voyages to Jamaica and Barbados and found to have an average error of a mere 39.25 seconds after 47 days at sea. The success of H4 was met with disbelief by Nevil Maskelyne, the Astronomer Royal and a great proponent of the lunar method. The Board of Longitude withheld the monetary reward until Harrison agreed to submit a written explanation of H4’s design, dissemble it for a group of inquisitive inspectors, and give the Board all four of his instruments. Harrison eventually complied and in 1773 George III forced the Board to relinquish most of the promised £20,000 prize.
Captain James Cook took an exact copy of H4 on his voyage to the Antarctic and the South Sea Islands in 1772, and praised its performance even under drastic temperature and climate fluctuations. The cost of manufacture eventually decreased, and by 1825, each ship of the Royal Navy had with at least one chronometer. By mid-century, chronometers were considered indispensable devices on most sea voyages.
Americans were introduced to chronometers by Nathaniel Bowditch, who gave a brief account of their use in his New American Practical Navigator (Salem, 1802). The U.S. Navy was using chronometers by the 1820s, and established a Depot of Charts and Instruments in order to determine the “rate” of these instruments in 1830. This Depot would later become the U.S. Naval Observatory. Dealers in several American port cities were selling chronometers by the 1830s.
Chronometers were also used to determine longitude on land. Lewis and Clark had one on their voyage of discovery to the American northwest in 1804. It cost of $250.75 and was by far the most expensive instrument in their inventory. By mid-century, chronometers were carried by many of the Army engineers who explored and mapped the country.
In use, a chronometer would be set to the time of a place of known longitude, such as Paris, London, or Greenwich, which was established as the Prime Meridian in 1884. Standard time would then be compared with local time, where one hour of difference is equivalent to 15º longitude.
Ref: William J.H. Andrewes, ed. The Quest for Longitude (Cambridge: 1996), p. 165-254.
Carlene E. Stephens, On Time: How America Has Learned to Live by the Clock
(Boston: 2002), p. 32-34.
Marvin E. Whitney, The Ship’s Chronometer (Cincinnati: 1986), p.