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CHRONOMETER. The design, construction, and successful replication of marine chronometers, or precision timekeepers, was one of the great scientific triumphs of the early modern period. This scientific instrument was crucial to the accurate determination of longitude (or east-west direction from a given meridian on the globe) to vessels at sea. Hence the development of marine chronometers was a pivotal factor in early modern European navigation, transport, trade, cartography, and colonial enterprise.


Determining longitude remained the most persistent problem facing oceangoing vessels in the early modern world. With the discovery of the New World, the expansion of trade, and the conquest of new territories, there soon followed an increased movement of men, precious metals, manufactured goods, and raw commodities. Hence, more and more was at stake for European ships traveling on the oceans. While scientists and mathematicians had proposed several methods to determine longitude at sea, none of these methods had yet proved practical. The best trained navigators relied on dead reckoning, a crude estimate of the speed and distance traveled, to learn their ship's longitude. In practicality, they could only hope for propitious winds and currents to get them to their ports or destinations safely. In 1707 four British warships under the command of Admiral Clowdisley Shovell crashed into the jagged rocks of the Scilly Isles off the southwest tip of England. As the warships sank, almost 2,000 men perished because of the navigational error. Less dramatic results of erroneous readings of longitude often resulted in protracted voyages, a not inconsiderable danger when scurvy and other disease could break out after ninety days of vitamin C deprivation. Sagging shipboard morale, exhausted food supplies, and even mutiny resulted from unexpected delays at sea.


While a ship's latitude could be easily established at sea by measuring the height of the sun (or stars, particularly the North Star, above the horizon) with the aid of a good sextant, determining longitude proved a more stubborn problem. The best scientific minds of Europe wrestled with the problem. In 1530 the Flemish astronomer and mathematician Gemma Frisius (15081555) published a solution. He predicated that since the Earth rotates 360 degrees in 24 hours, or 15 degrees of longitude per hour, the mechanical clock might be the answer to the longitude problem. He suggested that if an accurate timekeeper were to record the local time of the ship's departure port, and if this were compared to the local time of the ship at noon, (determined by measuring the highest point of the sun in the sky), the difference could indicate longitude. Obviously the difference in hours would be multiplied by 15 degrees, with further refinements for minutes and seconds of time to get correct readings for minutes and seconds of arc. Frisius's solution would ultimately prove the basis of the solution, but innumerable practical problems intervened. How to build a clock that would keep accurate time in a rough sea or a pitching and rolling ship? Since ordinary clocks often became erroneous over time, the challenge of accuracy was paramount. Constructing a clock that would be unaffected by changes in humidity, gravity, and temperature presented further obstacles.

Although the clock method would ultimately prove the winner, the logic of this was not at all apparent to many talented scientific and mathematical minds of the early modern era. Rival theories abounded. Among those offered were Galileo Galilei's (15641642) proposal of measuring and using the motions of Jupiter's four moons as celestial clocks, and comparing the times when these moons eclipsed one another with the same astronomical event at his local time. While Galileo's method was theoretically correct, and proved useful for finding longitude on land once accurate predictive tables of the positions of Jupiter moons could be drawn up, it was ultimately useless at sea. While some astronomers, including the Danish Ole Roemer (16441710) and the Frenchman Jean Dominque Cassini (16251712) continued to refine Galileo's method after his death, other astronomers proposed alternate solutions. John Flamsteed (16461719) toiled in Greenwich to construct star tables to aid in the determination of longitude. Christiaan Huygens (16291695), an accomplished astronomer, worked on both the mechanical and astronomical methods simultaneously. However, not all solutions offered were high-minded. One, proposed by Humphry Ditton in 1713, suggested a series of anchored boats spaced 600 miles apart that would fire cannons to alert nearby vessels of their proximity to known positions of the great guns.


In 1598 King Philip III of Spain (ruled 15981621) offered a considerable life pension to the discoverer of longitude. Louis XIV (ruled 16431715) of France spent considerable money and energy on the problem by erecting the Royal Observatory at Paris and attracting (and paying handsomely) the best minds of Europe to work there. In 1714 the English parliament offered a reward of 20,000 pounds for a solution that would prove no more than one-half degree of error after a six-week voyage at sea. The prize offered in 1714 did exactly what its authors had hopedit induced a wide array of talented men to labor doggedly at a new solution. The Longitude Act of 1714 established a committee to judge submissions and authorized the award of partial funds to stimulate further investigation of promising proposals. Members of the committee included the most outstanding astronomers and mathematicians of the time, including Edmund Halley, James Bradley, and Neville Maskelyne.


The production of the precision marine mechanical timekeeper was the accomplishment of a self-educated English clockmaker of modest origins, John Harrison (16931776). Starting his career by working on wooden clocks, in about 1720 Harrison designed and built a tower clock in Brocklesby Park. As early as 1722 he hit upon three solutions that he would incorporate in his later clocks. He used lignum vitae, a tropical wood that required no oiling since the hardwood naturally secreted its own grease. Eliminating lubricants eliminated the friction and errors introduced by changing viscosity. He also invented the gridiron pendulum, which used strips of two metalssteel and brassto compensate for the shrinkage in metals caused by temperature changes in the atmosphere. He subsequently designed a new escapement to eliminate friction and wear on the teeth connecting the wheels and the oscillator and referred to his design as a "grasshopper escapement." In his efforts to produce a winning precision scientific instrument, Harrison worked for thirty years and produced four prototypes, known to scientists as H-1, H-2, H-3, and H-4. Each model contained significant technical improvements. Each model earned him the grudging and slow respect of a series of influential friends, if not the commissioners of the Board of Longitude, who alone could award the prize money. Ever his own harshest critic, Harrison continued to scrutinize the defects of his own solutions and to correct them. He completed his final masterpiece, H-4, in 1759: His final solution was a large pocket watch, five inches in diameter, and weighing only three pounds.


The acid test for the Board of Longitude was the accuracy of a timekeeper at sea over time. Harrison's son and assistant, William Harrison, set forth in November 1761, with H-4, aboard the H.M.S. Deptford from the English port of Plymouth for Jamaica. William was expected to guard the watch, to wind it daily, and with astronomer John Robison to keep careful records and make astronomical observations of the longitude in Jamaica. During the three-month journey, the ship's captain several times chose to value Harrison's estimation of longitude over the ship's official navigator. Despite rough seas on the return voyage, the watch had lost just under two minutes outbound and homebound combined. Having met the margin of error specified in 1714, Harrison fully deserved the prize. However, machinations of opponents favoring the lunar distance method delayed his receiving the reward. Nathaniel Bliss, the presiding astronomer royal of 1763, declared that the accuracy of H-4 was a chance occurrence and demanded a second trial voyage. In 1764 William Harrison set forth on yet another trial voyage, this time to Barbados. Again the H-4 proved successful: Since it had an error of only 54 seconds over a period of 156 days, it had far exceeded the standards demanded. Delays, favoritism of the lunar distance method, and constant amending of the rules help explain why Harrison was so slow to be recognized the rightful winner of the prize. Required in 1765 to dismantle his watch piece by piece and to explain the function of each part, the board next asked Harrison to reassemble the watch, to surrender H-4 to the judges, and to build two replicas of the H-4 without using the original as a model. Finally awarded one half of the prize money, Harrison had precious little leverage to get the whole prize out of the committee.


In 1767, the Board, still reluctant to award John Harrison the full prize, hired the respected watchmaker Larcum Kendall to replicate H-4. The attempt to replicate the intricate timekeeper consumed two-and-a-half years of work from Kendall, who named his model K-1. By 1770, the aging Harrison had not yet finished building the first of the two watches the Board had ordered him to make (subsequently called H-5.) Eager for yet another opportunity to test the precious instrument on a long sea voyage, the Board entrusted Captain James Cook to take the K-1 with him on his voyage to Tahiti to observe the transit of Venus. Cook also took with him three other timekeepers made by clockmaker John Arnold. By the time Cook returned to England in July 1775, the famous sea captain was full of praise for Kendall's replica of Harrison's H-4. Cook set an example for other ship captains when he prominently chose to carry the K-1 on his third expedition. Soon other watchmakers were producing accurate imitations of Harrison's H-4 and even improving on the design. John Arnold, Thomas Mudge, and an increasing number of nautical instrument makers were soon offering marine chronometers for sale. Increased precision in mapmaking, navigation, and ocean crossings resulted. Despite the widespread use today of satellite-informed Global Positioning Systems to give ships instant knowledge of their positions at sea, ships still carry chronometers as backup systems. They have proved reliable, simple, and astonishingly accurate.

See also Cartography and Geography ; Clocks and Watches ; Communication and Transportation ; Exploration ; Scientific Instruments ; Shipbuilding and Navigation ; Shipping .


Andrewes, William J. H., ed. The Quest for Longitude. Cambridge, Mass., 1996.

Bedini, Silvio A. The Pulse of Time: Galileo Galilei, the Determination of Longitude, and the Pendulum Clock. Florence, 1991.

Gould, Rupert T. John Harrison and His Timekeepers. London, 1978.

Howse, Derek. Greenwich Time and the Discovery of Longitude. London, 1997.

Landes, David. Revolution in Time: Clocks and the Making of the Modern World. Cambridge, Mass., 1983.

Quill, Humphrey. John Harrison, the Man Who Found Longitude. London, 1966.

Sobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York, 1995.

Sobel, Dava, and William J. H. Andrewes. The Illustrated Longitude. New York, 1998.

Taylor, E. G. R. The Haven-Finding Art: A History of Navigation from Odysseus to Captain Cook. London, 1971.

Martha Baldwin

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"Chronometer." Europe, 1450 to 1789: Encyclopedia of the Early Modern World. . 20 Jul. 2017 <>.

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chronometer (krənŏm´ətər), instrument for keeping highly accurate time, used especially in navigation. Before the advent of radio time signals it was the only device that provided the time accurately enough for a ship at sea to determine its longitude. A mechanical chronometer is a spring-driven escapement timekeeper, like a watch, but its parts are more massively built. Changes in the tension of the spring caused by variations in temperature are compensated for by devices built into it. Modern chronometers are electronic, using the vibrations of a quartz crystal to regulate the rate at which a time-indicating display moves.

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"chronometer." The Columbia Encyclopedia, 6th ed.. . 20 Jul. 2017 <>.

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chro·nom·e·ter / krəˈnämətər/ • n. an instrument for measuring time, esp. one designed to keep accurate time in spite of motion or variations in temperature, humidity, and air pressure. Chronometers were first developed for marine navigation.

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"chronometer." The Oxford Pocket Dictionary of Current English. . 20 Jul. 2017 <>.

"chronometer." The Oxford Pocket Dictionary of Current English. . (July 20, 2017).

"chronometer." The Oxford Pocket Dictionary of Current English. . Retrieved July 20, 2017 from