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A marine chronometer is a timekeeper precise enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation. When first developed in the eighteenth century it was a major technical achievement, as accurate knowledge of the time over a long sea voyage is necessary for navigation without electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent trial and error that revolutionized naval (and later aerial) navigation as the Age of Discovery and the Scramble for India waned and Colonialism hit a new gear.

The term chronometer (apparently coined in 1714 by Jeremy Thacker, an early competitor for the prize set by the Longitude Act in the same year) is used more recently to describe wristwatches tested and certified to meet certain precision standards. Timepiece made in Switzerlandmarker may only display the word 'Chronometer' if certified by the COSC.

History

To determine a position on the earth's surface, it is necessary and sufficient to know both the latitude and the longitude. Until the mid 1750s accurate navigation at sea out of sight of land was an unsolved problem due to the difficulty in calculating longitude. Navigators could determine their latitude by measuring the sun's angle at noon (i.e., when it reached its highest point in the sky, or culmination). To find their longitude, however, they needed a time standard that would work aboard a ship. Observation of regular celestial motions, such as Galileo's method based on observing Jupiter's natural satellites, was usually not possible at sea due to the ship's motion. The Lunar Distance Method, initially proposed by Johannes Werner in 1514, was developed in parallel with the marine chronometer.

The purpose of a chronometer is accurately to keep the time of a known fixed location, for example Greenwich Mean Time (GMT). This is particularly important for navigation. Knowing GMT at local noon allows a navigator to use the time difference between the ship's position and the Greenwich Meridian to determine the ship's longitude. As the Earth rotates at a regular rate, the time difference between the chronometer and the ship's local time can be used to calculate the longitude of the ship relative to the Greenwich Meridian (defined as 0°) using spherical trigonometry. In modern practice, a navigational almanac and trigonometric sight-reduction tables permit navigators to measure the Sun, Moon, visible planets, or any of 57 navigational stars at any time that the horizon is visible.

The creation of a timepiece which would work reliably at sea was difficult. Until the 20th century the best timekeepers were pendulum clocks, but the rolling of a ship at sea made a simple 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. His first two sea timepieces used this system, but he realised that they had a fundamental sensitivity to centrifugal force, which meant that they could never be accurate enough at sea. Construction of his third machine, designated H3, included novel circular balances and the invention of the bi-metallic strip and caged roller bearings, inventions which are still widely used. H3's circular balances proved too inaccurate and he eventually abandoned the large machines. Harrison solved the precision problems with his much smaller H4 chronometer design. H4 looked much like a large five-inch (12 cm) diameter pocket watch. In 1761 Harrison submitted H4 for the £20,000 longitude prize that had been offered by the British government in 1714. His design used a fast-beating balance controlled by a temperature-compensated spiral spring. This general layout remained in use until stable electronic oscillators allowed very accurate portable timepieces to be made at affordable cost.

Berthoud chronometer no. 24 (1782)
After Harrison's work proved the feasibility of portable precision timekeepers, work was carried out to develop simpler and lower-cost designs. Pierre Le Roy and Ferdinand Berthoud in France, and Thomas Mudge in Britain successfully produced marine timekeepers. Although none was simple, they proved that Harrison's design was not the only answer to the problem. The greatest strides toward practicality came at the hands of Thomas Earnshaw and John Arnold, who developed simplified, detached, "spring detent" escapements, moved the temperature compensation to the balance, and improved the design and manufacturing of balance springs. This combination of innovations served as the basis of marine chronometers until the electronic era.

The new technology was initially so expensive that not all ships carried chronometers, as illustrated by the fateful last journey of the East Indiaman Arnistonmarker. However by 1825, the Royal Navy had begun routinely supplying its vessels with chronometers.

It was common for ships at the time to observe a time ball, such as the one at Greenwichmarker, to check their chronometers before departing on a long voyage. Every day, ships would anchor briefly in the River Thames at Greenwich, waiting for the ball at the observatory to drop at precisely 1pm. This practice was responsible for the subsequent adoption of Greenwich Mean Time as an international standard. (Time balls became redundant around 1920 with the introduction of radio time signals, which have themselves largely been superseded by GPS time.) In addition to setting their time before departing on a voyage, ship chronometers were also routinely checked for accuracy while at sea by carrying out lunar or solar observations.

Harrison's Chronometer H5
Although industrial production methods began revolutionizing watchmaking in the middle of the 19th century, chronometer manufacture remained craft-based much longer. Around the turn of the 20th century, Swiss makers such as Ulysse Nardin made great strides toward incorporating modern production methods and using fully interchangeable parts, but it was only with the onset of World War II that the Hamilton Watch Company in the USmarker perfected the process of mass production, which enabled them to produce thousands of their superb Hamilton Model 21 chronometers for the US 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 of St. Albans in Britain, for instance, continued to produce high-quality chronometers by traditional production methods well into the 1970s.

Without their accuracy and the accuracy of the feats of navigation that marine chronometers enabled, it is quite likely the ascendancy of the Royal Navy, and by extension that of the British Empire, would not have occurred; the formation of the empire by wars and conquests of colonies abroad took place in a period in which British vessels had reliable navigation due to the chronometer, while their Portuguese, Dutch, and French opponents did not. For example: the French were well established in Indiamarker and other places before Britain colonized or trade, but were defeated by naval forces in the Seven Years' War.

The most complete international collection of marine chronometers, including Harrison's H1 to H4, is at the National Maritime Museum, Greenwich, Britain.

Mechanical chronometers

The crucial problem was to find a resonator that remained unaffected by the changing conditions met by a ship at sea. The balance wheel harnessed to a spring solved most of the problems associated with the ship's motion. Unfortunately, the elasticity of most balance spring materials changes relative to temperature. To compensate for ever-changing spring strength, the majority of chronometer balances used bi-metallic strips to move small weights toward and away from the center of oscillation, thus altering the period of the balance to match the changing force of the spring. The balance spring problem was solved with a nickel-steel named Elinvar for its invariable elasticity at normal temperatures. The inventor was Charles Edouard Guillaume, who won the Nobel Prize for physics in recognition for his metallurgical work (the only Nobel that has been granted for work related to horology).

The escapement serves two purposes. First, it allows the train to advance fractionally and record the balance's oscillations. At the same time, it supplies minute amounts of energy to counter tiny losses from friction, thus maintaining the equilibrium of the oscillating balance. The escapement is the part that ticks. Since the natural resonance of an oscillating balance serves as the heart of a chronometer, chronometer escapements are designed to interfere with the balance as little as possible. There are many constant force and detached escapement designs, but the most common are the spring detent and pivoted detent. In both of these, a small detent locks the escape wheel and allows the balance to swing completely free of interference except for a brief moment at the center of oscillation, when it is least susceptible to outside influences. At the center of oscillation, a roller on the balance staff momentarily displaces the detent, allowing one tooth of the escape wheel to pass. The escape wheel tooth then imparts its energy on a second roller on the balance staff. Since the escape wheel turns in only one direction, the balance receives impulse in only one direction. On the return oscillation, a passing spring on the tip of the detent allows the unlocking roller on the staff to move by without displacing the detent.

Chronometers often included other innovations to increase their efficiency and precision. Hard stones such as ruby and sapphire were often used as jewel bearings to decrease friction and wear of the pivots and escapement. Until the end of mechanical chronometer production in the third quarter of the 20th century, makers continued to experiment with things like ball bearings and chrome-plated pivots.

Marine chronometers always contain a maintaining power which keeps the chronometer going while it is being wound, and a power reserve to indicate how long the chronometer will continue to run without being wound. Marine chronometers are the most accurate portable mechanical clocks ever made, achieving a precision of around a tenth of a second per day. This is accurate enough to locate a ship's position within after a month's sea voyage.

Today

Ships and boats commonly utilize electronic aids to navigation, such as LORAN and Global Navigation Satellite Systems. However celestial navigation, which requires the use of a precise chronometer, is still a requirement for certain international mariner certifications such as Officer in Charge of Navigational Watch, and Master and Chief Mate deck officers,and supplements offshore yachtmasters on long-distance private cruising yachts.Modern marine chronometers can be based on quartz clocks that are corrected periodically by GPS signals or radio time signals (see radio clock). These quartz chronometers are not always the most accurate quartz clocks when no signal is received, and their signals can be lost or blocked. However, there are quartz movements, even in wrist watches, that are accurate to within 10 or 20 seconds per year.At least one quartz chronometer made for advanced navigation utilizes multiple quartz crystals which are corrected by a computer using an average value, in addition to GPS time signal corrections.

See also



References

  1. Pierre Le Roy had developed the detached spring detent escapement around 1748 but abandoned the concept.
  2. Alfred T. Mahan, The Influence of Sea Power on History:


External links




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