P.L. Tassaert’s half-tone print of Thomas King’s original 1767 portrait of John Harrison, located at the Science and Society Picture Library, London.
The problem of the longitude, locating the east-west position of a ship on the open sea, was the classic technological problem of the early modern period, assaulted by many of the greatest scientists of the scientific revolution, including Galileo Galilei (1564–1642), Christiaan Huygens (1629–1695), and Edmond Halley (1656–1742). The great astronomical observatories founded in the period, most notably the Paris Observatory and the English Royal Observatory at Greenwich, had the solution of the longitude problem high on their agendas. All failed, leaving the problem for the eighteenth century. If anything, it was of increasing urgency, given the expansion of the territory covered by European vessels. As the celestial bodies seemed to rotate around Earth from east to west, they did not seem to offer a way to know one’s position on it. Existing methods, based on observation of the Moon, or simply estimating the speed one had been traveling for a given time, were maddeningly and even dangerously imprecise. Most approaches to the longitude reduced the problem to that of finding the difference between the time on the ship, set by observation of the Sun’s meridian at noon, and the time at a fixed point, usually that of the home port. The difference in time could be translated into spatial terms as the difference in longitude between the two points. There were all sorts of bizarre schemes for this, but the two main approaches were using astronomical events to give the correct time and creating a clock able to give accurate time on a ship. If the home-port time of a celestial occurrence were known, all that would be necessary would be to compare the ship’s own time on observation of the occurrence. For example, if the time when an eclipse would occur at a fixed point were known, all that would be necessary would be to compare the time that the ship’s navigator saw the eclipse, and the distance between the two points would be known. This method was limited in its uses, however, as eclipses were quite rare. Galileo’s idea of using the frequent eclipses of the moons of Jupiter became dominant in geography and cartography on land, but the difficulty of observing Jupiter’s moons from a moving ship made it difficult if not impossible at sea.
The greatest eighteenth-century sea power, Great Britain took the lead in most eighteenth- century longitude schemes, although its colonial rival, France, was not far behind. Two unsuccessful longitude solvers, William Whiston and Humphrey Ditton (1675–1715), set forth a project in 1713 for the creation of a network of stationary ships over the seas, whose crews would fire guns at designated times, enabling passing ships to set their distances by factoring in their knowledge of the speed of sound. This idea was impractical on many levels, and never seriously considered. Whiston and Ditton’s lobbying of the British Parliament for a more active approach to the problem along with London’s maritime leaders whom they had organized resulted in the Longitude Act of 1714. This act established a prize of 20,000 pounds for a solution accurate to half a degree of a great circle around Earth; 15,000 pounds for a solution accurate within two-thirds of a degree; and 10,000 for a solution accurate within a degree. It also set up the Longitude Board whose ex officio members included the astronomer royal, the president of the Royal Society, and the first lord of the Admiralty, among others. The board disposed of funds to encourage promising ideas and was the first great institutional patron of science. It was deluged with solutions, most of them crackpot, and for the first decade and a half of its existence never met and concerned itself with little beyond sending out rejection letters. The French Royal Academy of Sciences meanwhile had used a bequest from the magistrate Rouille de Meslay to set up a prize of 125,000 livres for the longitude and other improvements in navigation, and were considerably more active, awarding 2,000 livres in 1720.
Serious eighteenth-century longitude ideas divided into two categories: the creation of an accurate shipboard clock and the astronomical method known as “lunar distances.” Lunar distances rested on the invention of a new astronomical instrument, the octant. This happened twice in 1731, with the independent work of the Englishman John Hadley (1682–1744) and the Philadelphian Thomas Godfrey (1704–1749). An arrangement of mirrors enabled a navigator to hold the distances between two celestial objects steady, even on the deck of a rolling ship. By observing the angular separation of the Moon and a given star, then comparing the time of observation with a table giving the times when that angular separation would appear from a fixed point such as London or Paris, the navigator could get the time differential and thus the longitude. All this plan required were accurate, mathematically skilled navigators and accurate tables of the extremely complex lunar motion, and legions of astronomers all over Europe set to work to provide the latter. Although the English and French scientific establishments poured effort and money into the project, the most accurate tables were the work of a German, Johann Tobias Mayer. Mayer’s death prevented him from claiming the prize, although his widow received 3,000 pounds from the board.
By comparison, the clock idea was somewhat old-fashioned. The leader in the creation of a navigational clock was a self-taught English clock maker of genius named John Harrison (1693–1776), who worked outside the London-based English clock-making establishment. Harrison had contacted Astronomer Royal Edmond Halley early in the project and enjoyed some support from the Longitude Board and the Royal Society. But he also faced opposition from a series of royal astronomers, including James Bradley and Nevil Maskelyne, who strongly favored lunar distances and were ex officio members of the Longitude Board, often supervising the trials. Harrison received several thousand pounds from the Longitude Board, at one point benefiting from the personal intervention of King George III (r. 1760–1820), but never won the prize he sought. The French, meanwhile, were also investigating the possibility of an accurate watch, led by the royal clock maker Ferdinand Berthoud (d. 1807). After shipboard watches were tested on voyages to Saint Domingue in 1769 and 1771, their use became common in the French marine.
In England Harrison was pitted against Maskelyne, the greatest exponent of lunar distances, who Harrison believed applied unnecessarily stringent conditions to the tests of the clocks and did not care for them properly when they were in his custody. Maskelyne’s annual Nautical Almanac and Nautical Ephemeris, first published in 1767, with its associated lunar tables, was the best available and put the lunar-distance method on a sound footing. This idea originated in the work of the Frenchman Nicolas-Louis de Lacaille (1713–1762) in the 1750s, but the French had never followed up Lacaille’s work. They did publish a French translation of Maskelyne’s almanac, beginning in 1772, a project with which Maskelyne cooperated even while the two countries were at war. The British navy required its navigators to be certified as proficient in Maskelyne’s method, although this was not consistently enforced at first. Updated, Maskelyne’s works served the international navigational community into the early twentieth century. It is due to Maskelyne’s lunar tables that the meridian of the Royal Observatory at Greenwich became the determining point for world time.
The lunar-distance method had the disadvantages of not being possible on moonless nights, and of requiring several observations and much tedious and difficult calculation. The chronometric method using timepieces eventually became the most common way to find the longitude. The problem was not the accuracy of the watches, particularly after Captain James Cook used a timekeeper based on Harrison’s on his second voyage, from 1772 to 1775, and enthusiastically testified to its merits (although he also praised Maskelyne’s almanacs).The difficulty was the cost of reproducing accurate timepieces. Late-eighteenth-century London watchmakers, most notably John Arnold (1736–1799) and Thomas Earnshaw (1749–1829), simplified Harrison’s designs and began mass production of accurate shipboard watches, which became the dominant way of finding the longitude by the 1820s. The Longitude Board itself was disbanded in the new Longitude Act of 1828. Its greatest prize was never awarded.
References Howse, Derek. Nevil Maskelyne: The Seaman’s Astronomer. Cambridge: Cambridge University Press, 1989. Sobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York:Walker, 1995.