Marine chronometer
A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time and the time at the current location found from observations of celestial bodies. When first developed in the 18th century, it was a major technical achievement, as accurate knowledge of the time over a long sea voyage was vital for effective navigation, lacking electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and testing that revolutionized naval navigation.
The term chronometer was coined from the Greek words wikt:χρόνος and . The 1713 book Physico-Theology by the English cleric and scientist William Derham includes one of the earliest theoretical descriptions of a marine chronometer. It has recently become more commonly used to describe watches tested and certified to meet certain precision standards.
History
To determine a position on the Earth's surface, using classical models, it is necessary and sufficient to know the latitude, longitude, and altitude. Altitude considerations can naturally be ignored for vessels operating at sea level. 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 or, in the Northern Hemisphere, by measuring the angle of Polaris from the horizon. 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 distances method, initially proposed by Johannes Werner in 1514, was developed in parallel with the marine chronometer. The Dutch scientist Gemma Frisius was the first to propose the use of a chronometer to determine longitude in 1530.The purpose of a chronometer is to measure accurately the time of a known fixed location. This is particularly important for navigation. As the Earth rotates at a regular predictable 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 Prime Meridian if accurately enough known, using spherical trigonometry. Practical celestial navigation usually requires a marine chronometer to measure time, a sextant to measure the angles, an almanac giving schedules of the coordinates of celestial objects, a set of sight reduction tables to help perform the height and azimuth computations, and a chart of the region. With sight reduction tables, the only calculations required are addition and subtraction. Most people can master simpler celestial navigation procedures after a day or two of instruction and practice, even using manual calculation methods. The use of a marine chronometer to determine longitude by chronometer permits navigators to obtain a reasonably accurate position fix. For every four seconds that the time source is in error, the east–west position may be off by up to just over one nautical mile as the angular speed of Earth is latitude dependent.
The creation of a timepiece which would work reliably at sea was difficult. Until the 20th century, the best timekeepers were pendulum clocks, but both the rolling of a ship at sea and the up to 0.2% variations in the gravity of Earth made a simple gravity-based pendulum useless both in theory and in practice.
First examples
, following his invention of the pendulum clock in 1656, made the first attempt at a marine chronometer in 1673 in France, under the sponsorship of Jean-Baptiste Colbert. In 1675, Huygens, who was receiving a pension from Louis XIV, invented a chronometer that employed a balance wheel and a spiral spring for regulation, instead of a pendulum, opening the way to marine chronometers and modern pocket watches and wristwatches. He obtained a patent for his invention from Colbert, but his clock remained imprecise at sea. Huygens' attempt in 1675 to obtain an English patent from Charles II stimulated Robert Hooke, who claimed to have conceived of a spring-driven clock years earlier, to attempt to produce one and patent it. During 1675 Huygens and Hooke each delivered two such devices to Charles, but none worked well and neither Huygens nor Hooke received an English patent. It was during this work that Hooke formulated Hooke's law.file:H1 low 250.jpg|thumb|left|John Harrison's H1 marine chronometer of 1735
The first published use of the term chronometer was in 1684 in Arcanum Navarchicum, a theoretical work by Kiel professor Matthias Wasmuth. This was followed by a further theoretical description of a chronometer in works published by English scientist William Derham in 1713. Derham's principal work, Physico-theology, or a demonstration of the being and attributes of God from his works of creation, also proposed the use of vacuum sealing to ensure greater accuracy in the operation of clocks. Attempts to construct a working marine chronometer were begun by Jeremy Thacker in England in 1714, and by Henry Sully in France two years later. Sully published his work in 1726 with Une Horloge inventée et executée par M. Sulli, but neither his nor Thacker's models were able to resist the rolling of the seas and keep precise time while in shipboard conditions.
file:Harrison H4 clock in The principles of Mr Harrison's time-keeper 1767.jpg|thumb|Drawings of Harrison's H4 chronometer of 1761, published in The principles of Mr Harrison's time-keeper, 1767
In 1714, the British government offered a longitude prize for a method of determining longitude at sea, with the awards ranging from £10,000 to £20,000 depending on accuracy. John Harrison, a Yorkshire carpenter, submitted a project in 1730, and in 1735 completed 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 H1 and H2 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, in 1759 included novel circular balances and the invention of the bi-metallic strip and caged roller bearings, inventions which are still widely used. However, H3's circular balances still proved too inaccurate and he eventually abandoned the large machines.
file:Marine watch no 3-CnAM 1388-IMG 1522-black.jpg|thumb|left|Ferdinand Berthoud's marine chronometer no. 3, 1763
Harrison solved the precision problems with his much smaller H4 chronometer design in 1761. H4 looked much like a large five-inch diameter pocket watch. In 1761, Harrison submitted H4 for the £20,000 longitude prize. His design used a fast-beating balance wheel controlled by a temperature-compensated spiral spring. These features remained in use until stable electronic oscillators allowed very accurate portable timepieces to be made at affordable cost. In 1767, the Board of Longitude published a description of his work in The Principles of Mr. Harrison's time-keeper. A French expedition under Charles-François-César Le Tellier de Montmirail performed the first measurement of longitude using marine chronometers aboard Aurore in 1767.
Further development
In France, 1748, Pierre Le Roy invented the detent escapement characteristic of modern chronometers. In 1766, he created a revolutionary chronometer that incorporated a detent escapement, the temperature-compensated balance and the isochronous balance spring: Harrison showed the possibility of having a reliable chronometer at sea, but these developments by Le Roy are considered by Rupert Gould to be the foundation of the modern chronometer. Le Roy's innovations made the chronometer a much more accurate piece than had been anticipated.file:Harrison's Chronometer H5.JPG|thumb|left|Harrison's Chronometer H5 of 1772, now on display at the Science Museum, London
Ferdinand Berthoud in France, as well as Thomas Mudge in Britain also successfully produced marine timekeepers. Although none were 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 in 1780 developed and patented 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.
file:Berthoud clock 24 p1040260.jpg|thumb|Ferdinand Berthoud chronometer no. 24, on display at the Musée des Arts et Métiers, Paris
The new technology was initially so expensive that not all ships carried chronometers, as illustrated by the fateful last journey of the East Indiaman Arniston, shipwrecked with the loss of 372 lives. However, by 1825, the Royal Navy had begun routinely supplying its vessels with chronometers.
Beginning in 1820, the British Royal Observatory in Greenwich tested marine chronometers in an Admiralty-instigated trial or "chronometer competition" program intended to encourage the improvement of chronometers. In 1840 a new series of trials in a different format was begun by the seventh Astronomer Royal George Biddell Airy. These trials continued in much the same format until the outbreak of World War I in 1914, at which point they were suspended. Although the formal trials ceased, the testing of chronometers for the Royal Navy did not.
Marine chronometer makers looked to a phalanx of astronomical observatories located in Western Europe to conduct accuracy assessments of their timepieces. Once mechanical timepiece movements developed sufficient precision to allow for adequately accurate marine navigation, these third party independent assessments also developed into what became known as "chronometer competitions" at the astronomical observatories located in Western Europe. The Neuchâtel Observatory, Geneva Observatory, Besançon Observatory, Kew Observatory, German Naval Observatory Hamburg and Glashütte Observatory are prominent examples of observatories that certified the accuracy of mechanical timepieces. The observatory testing regime typically lasted for 30 to 50 days and contained accuracy standards that were far more stringent and difficult than modern standards such as those set by the Contrôle Officiel Suisse des Chronomètres. When a movement passed the observatory test, it became certified as an observatory chronometer and received a Bulletin de Marche from the observatory, stipulating the performance of the movement.
It was common for ships at the time to observe a time ball, such as the one at the Royal Observatory, Greenwich, 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 in small part responsible for the subsequent adoption of Greenwich Mean Time as an international standard. 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. In typical use, the chronometer would be mounted in a sheltered location below decks to avoid damage and exposure to the elements. Mariners would use the chronometer to set a so-called hack watch, which would be carried on deck to make the astronomical observations. Though much less accurate than the chronometer, the hack watch would be satisfactory for a short period of time after setting it.