The Adoption of Crusade Ideology in Mesoamerica

Many motives drove Christopher Columbus to sail west toward the Indies, but one purpose that drove the westward voyage of this complex and often inconsistent man was the dream of converting the Great Khan of Cathay and joining with him in a final, great, and successful crusade against Islam, which in turn would usher in even greater events. In his so-called Book of Prophecies, a compilation of texts prepared essentially between 1501 and 1505 for the Catholic monarchs of Spain, Fernando and Isabel, the master mariner placed the discovery of the Indies into the grand divine plan for the forthcoming salvation of all humanity, the Final Judgment, and the End of Time. He argued that his voyage to the west had been the first step in the process of liberating Jerusalem, itself a necessary step in the unfolding of God’s plan of universal salvation. Leaving aside the issue of whether or not he actually believed these messianic prophecies (and it seems clear that he did), it is clear that Columbus was appealing to a widespread belief that the road to Jerusalem lay through the Indies. After all, he was appealing to two crusaders, the Catholic Monarchs Isabel and Fernando, who had conquered Granada on January 2, 1492, and who saw victory over the last Muslim stronghold in the Iberian Peninsula as another stepping stone in their God-sanctioned struggle against Islam, a struggle that was foreordained to result in triumph.

Even after it became clear that Columbus had not sailed to the Indies, at least some European churchmen continued to harbor the hope that the lands and peoples of the Americas would be the means for the liberation of Jerusalem and the destruction of Islam, and they apparently imparted that dream to at least some of their Indian converts. Festivals celebrating the victory of Christians over Moors became an integral part of Catholic religious culture throughout Latin America wherever Spanish and Portuguese missionaries brought the faith after 1492. Among these were the Tlaxcalans, whose religious pageants offer an important example of how thoroughly Catholic Christianity was assimilated in the New World.

The Tlaxcalans, themselves Nahuatl but nevertheless traditional enemies of the Mexica and their Aztec empire, had been defeated by Hernan Cortés in September 1519, and following that defeat they allied with the Spaniards in their march against the Aztec empire. Thousands of Tlaxcalans participated in the fighting and proved to be a decisive factor in the Spanish victory. A number of Cortés’s lieutenants married Tlaxcalan women of high birth, and despite initial reluctance to give up their ancestral deities, the Tlaxcalan chiefs were baptized as Christians sometime after July 1, 1520.

Mock battle pageants between “Christians and Moors” had been a popular expression of Reconquest realities and ideology in the Iberian Peninsula since at least the late thirteenth century. Now they were translated to the New World and its Christian converts.

We are fortunate to have a description of a Corpus Christi pageant performed by the Christian Indians of Tlaxcala in 1539 to celebrate the peace treaty between Emperor Charles V and the king of France and recorded shortly thereafter by the Franciscan missionary Fray Toribio de Benevente Motolinia in his History of the Indians of New Spain. In this elaborate pageant, which was composed at least in part by the Tlaxcalans but clearly with the help of their Franciscan mentors, the playwrights portrayed the future conquest of Jerusalem by the combined armies of Charles V’s European possessions and New Spain and the consequent baptism of its presumed occupier, the Muslim sultan of Cairo (although, in fact, Jerusalem had passed into Ottoman hands in December 1516).

Significantly, the pageant included several large and spirited mock battles, which apparently served to underscore the fact that holy war and the festivals that celebrated divinely mandated conflict and bloodshed were as much a part of this new religion of the Tlaxcalans as they had been when they and other Mesoamerican tribes conducted “Flower Wars,” preconquest battles fought for the purpose of capturing enemy warriors who were then sacrificed to a local deity. Often the sacrifice was either preceded by or took the form of a mock battle, when the captive was given an ineffective wooden sword with which to battle a fully armed adversary.

The most striking aspect of this pageant and its mock battles is that all of the combined crusader forces, European and Indian, fail to take Jerusalem despite their bravery. The Christians only succeed when the combined Indian forces of New Spain are joined in the fray by a heavenly patron on a brown horse, Hippolytus, a third-century soldier-saint on whose feast day, August 13, 1521, the Spaniards and their Tlaxcalan allies had captured Tenochtitlan. Indeed, just as the Tlaxcalans are led by Saint Hippolytus, the Spaniards, who now sweep to victory alongside these new Christians, are led by Santiago Matamoros- Saint James the Moor-slayer-on a horse “as white as snow.” Significantly, Santiago was the patron-saint of the Reconquista, the crusading wars of reconquest waged by Christian Iberians against the Muslims of Spain from roughly the mid-eleventh century to late fifteenth century.

According to legend, the apostle Saint James the Greater, whose relics were believed to have been miraculously transported to Compostela in northwest Spain, had initially appeared to lead the Christian forces of Asturias to victory at the mythic Battle of Clavijo in 844. He was also the namesake of the Order of Santiago, the most powerful of the Iberian military orders, founded in 1170 in Leon. Significantly, Hernan Cortés, the conqueror of Mexico, belonged to that order, as did many of his subordinates. And we are told by the sources that these conquistadors regularly shouted out the traditional Spanish battle cry, “Señor Santiago,” as they went into battle in the Americas.

The meaning of this Tlaxcalan pageant is clear. There is every reason to conclude that the Tlaxcalans were anxious to present themselves, and fellow Nahuatl converts to Catholicism (including their former enemies, the Mexica), as latter-day crusaders and as having accepted totally the Spanish crusading ethos despite whatever animosities they still harbored against their conquerors.

Moreover, the Tlaxcalans were fully aware, as were the Spaniards, that Cortés’s small army of Spanish soldiers could never have conquered the Aztec empire without the tens of thousands of Indian allies who marched with him, and chief among these were warriors from Tlaxcala. As a consequence, Tlaxcala became a privileged, largely self-governing province under Spanish colonial rule and was showered with honors and privileges.

The ideology of crusade was such a driving force in the Spanish conquest of Mexico that even the conquered and converted felt it necessary to claim identity with it. And what sort of crusade was that? By 1500, indeed well before that date, the crusade had metamorphosed into a struggle of apocalyptic proportions and with deep millenarian overtones. Put simply, it was a global, even cosmological, struggle between Catholic Christendom and Islam, heresy, heathenism, unbelief, and every manner of error, which included the “heathen errors and practices” of preconquest Mesoamerica.

Bibliography Gillespie, Jeanne. Saints and Warriors: Tlaxcalan Perspectives on the Conquest of Tenochtitlan. New Orleans: University Press of the South, 2004. Harris, Max. Aztecs, Moors, and Christians: Festivals of Reconquest in Mexico and Spain. Austin: University of Texas Press, 2000. Matthew, Laura E., and Michel R. Oudijk. Indian Conquistadors: Indigenous Allies in the Conquest of Mesoamerica. Norman: University of Oklahoma Press, 2007. Motolinia, Toribio de Benevente. History of the Indians of New Spain. Edited and translated by Francis Borgia Steck. Washington, DC: Academy of American Franciscan History, 1951.


Portuguese Expansion-sixteenth century


Map of the Arabian Peninsula showing the Red Sea with Socotra Island (red) and the Persian Gulf (blue) with the Strait of Hormuz (Cantino planisphere, 1502).


A replica of Flor do Mar, Maritime Museum of Malacca.

Atlantic European states did not wait until they achieved complete technical predominance at sea to begin their maritime expansion. In the fifteenth century, maneuverable, open-ocean-capable, long ranged sailing merchantmen, armed with enough cannon to fight off galleys, navigated the seas and enormously expanded European commercial horizons.

The Portuguese were among the first to put the new technology to use. 16 Early in the fifteenth century they reached the Madeiras, the Azores, and the Canary Islands. By midcentury they had worked their way down the coast of West Africa as far as Senegal. In 1487 Bartolomeu Dias rounded the southern tip of the African continent-the Cape of Good Hope. Ten years later Vasco da Gama returned to the Cape, entered the Indian Ocean, and sailed as far as Calicut. By the end of the first decade of the sixteenth century, the Portuguese were fighting their way, not always with success, into the enclosed waters of the Red Sea and the Persian Gulf. In 1513 their first ships reached China, and in 1557 the Portuguese established themselves at Macao.

The Portuguese confronted the reality of the maritime revolution as they rounded the African cape and sought first markets and then dominance in the Indian Ocean basin. Portuguese commanders were quick to grasp the relevant geography of the region and identify all the choke points, where ships had to traverse narrow waterways: the Cape of Good Hope, the Bab el Mandeb at the entrance to the Red Sea, the Strait of Hormuz at the entrance to the Persian Gulf, and the Strait of Malacca through which the trade of India, the Indies, and eastern Asia passed. The Portuguese believed that if they could control these points, along with centrally located, fortified bases on the Malabar coast of India, they could destroy and disrupt local commercial traffic, divert the region’s trade to their own routes, and outflank and economically undermine the Islamic powers, especially the Ottoman Empire.

The Portuguese developed and executed a three-phase strategy. First, they quickly disposed of the navies of the Arabs and other regional powers, winning a major naval engagement off Diu in 1509. Second, they conducted a series of raids to destroy local merchant shipping. Third, under the direction of commanders such as Afonso de Albuquerque, captain general and governor of India between 1509 and 1515, they attacked several critical strategic points: they seized the island of Socotra near the entrance to the Strait of Bab el Mandeb (1507), Muscat and the island of Ormuz in the Strait of Hormuz (1508), and the approaches to the Strait of Malacca (1511), but failed to capture Aden (1513) and Jiddah (1517). The ambitious and largely successful plan disrupted, but failed to halt, commercial activity along the Red Sea and Persian Gulf routes, and diverted a great deal of traffic to the new routes opened and controlled by the Portuguese.

The Portuguese were nevertheless unable to consolidate their gains. Economic warfare did undermine the smaller local powers, such as the Mamluk regime in Egypt, but generally to the profit of their larger, more powerful Islamic neighbors, such as the Persians and the Ottomans. Portuguese successes induced the Ottomans to reinforce their southeastern sea flank; they checked the Portuguese at Jiddah in 1517. The Ottomans continued their naval buildup and launched several counterattacks, in the Red Sea in the 1530s and in the Persian Gulf in the 1550s. Despite some successes, the effort failed to break the Portuguese grip on the western basin of the Indian Ocean.

Portugal’s adoption of such an ambitious and bellicose maritime strategy, even though it could send no more than a handful of ships and a few thousand men to the region, demonstrated its faith in cannon-armed sailing ships. D’Albuquerque noted: “At the rumour of our coming the [native] ships all vanished and even the birds ceased to skim over the water.” Another Portuguese governor general advised his king: “Let it be known that if you are strong in ships the commerce of the Indies is yours, and if you are not strong in ships little will avail you of any fortress on land.” The keys, though, were not only ships but also cannon. “The Portuguese could have reached India without gunpowder,” John Francis Guilmartin, Jr., wrote, “but they could never have maintained themselves there or brought their cargoes back.”

There was also a third key to Portuguese success: loyal and able commanders willing to act on their own initiative and monarchs willing to trust those they picked for command. Until the invention of the telegraph in the mid-nineteenth century, states had little choice but to grant commanders operating on distant stations wide powers, often including not only military but also diplomatic and economic responsibilities. Fleets or squadrons dispatched halfway around the world could not be controlled in the same manner as a fleet of galleys operating in concert with an army. Only on-scene commanders, such as d’Albuquerque, could direct the early-sixteenth-century Portuguese effort in the Indian Ocean. Portuguese officials in India were too far from Lisbon to receive or to seek direction or reinforcements. It took months to send correspondence back and forth between India and Portugal, and even longer to outfit and dispatch expeditions. Ships often departed for the Indies without knowing the fate of those which had sailed the previous year. The king had little choice but to appoint commanders he could trust and in whose abilities he had confidence. He would give such men general directions and substantial independent powers, and send them on their way.

In the absence of communications, grand strategic command and control worked best if a state’s political and naval leaders shared an understanding of naval capabilities and long-range national goals. Since no state had ever before conducted sustained naval operations on a global scale, the process of developing this understanding was empirical. Throughout the sixteenth century the Portuguese, and other Europeans, gradually grasped the revolutionary change in the nature of sea power and developed concepts to guide the use of naval forces.

Longitude Problem


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.

The three ancient harbours of the Piraeus

The trireme shipsheds of Zea reconstructed as a model in Piraeus Museum.

Reconstructed view of the three ancient harbours of the Piraeus: Kantharos (today the main harbour), Zea (also called Pashalimani in modern times) and Mounychia (modern Mikrolimano). Note the ship sailing through the fortified harbour mouth of Kantharos.
Illustrator: Ioannis Nakas/© Zea Harbour Project

The compass

It is not known where or when it was discovered that the lodestone (a magnetized mineral composed of an iron oxide) aligns itself in a north-south direction, as does a piece of iron that has been magnetized by contact with a lodestone. Neither is it known where or when marine navigators first utilized these discoveries. Plausible records indicate that the Chinese were using the magnetic compass around 1100, western Europeans by 1187, Arabs by 1220, and Scandinavians by 1300. The device could have originated in each of these groups, or it could have been passed from one to the others. All of them had been making long voyages, relying on steady winds to guide them and sightings of the sun or a familiar star to inform them of any change. When the magnetic compass was introduced, it probably was used merely to check the direction of the wind when clouds obscured the sky.

The first mariner’s compass may have consisted of a magnetized needle attached to a wooden splinter or a reed floating on water in a bowl. In a later version the needle was pivoted near its centre on a pin fixed to the bottom of the bowl. By the 13th century a card bearing a painted wind rose was mounted on the needle; the navigator could then simply read his heading from the card. So familiar has this combination become that it is called the compass, although that word originally signified the division of the horizon. The suspension of the compass bowl in gimbals (originally used to keep lamps upright on tossing ships) was first mentioned in 1537.

On early compass cards the north point was emphasized by a broad spearhead and the letter T for “tramontana ,” the name given to the north wind. About 1490 a combination of these evolved into the fleur-de-lis, still almost universally used. The east point, pointing toward the Holy Land, was marked with a cross; the ornament into which this cross developed continued on British compass cards well into the 19th century. The use of 32 points by sailors of northern Europe, usually attributed to Flemish compass makers, is mentioned by Geoffrey Chaucer in his Treatise on the Astrolabe (1391). It also has been said that the navigators of Amalfi, Italy, first expanded the number of compass points to 32, and they may have been the first to attach the card to the needle.

During the 15th century it became apparent that the compass needle did not point true north from all locations but made an angle with the local meridian. This phenomenon was originally called by seamen the “northeasting” of the needle but is now called the “variation” or “declination.” For a time, compass makers in northern countries mounted the needle askew on the card so that the fleur-de-lis indicated true north when the needle pointed to magnetic north. This practice died out about 1700 because it succeeded only for short voyages near the place where the compass was made. It caused confusion and difficulty on longer trips, especially in crossing the Atlantic to the American coast, where the declination was west instead of east as in Europe. The declination in a given location varies over time. For example, in northern Europe in the 16th century the magnetic north pole was east of true geographic north; in subsequent centuries it has drifted to the west.


Navigation as we know it today was first practiced by Phoenician and Greek seafarers, beginning after the sixth century BCE. They navigated in the Red Sea and the Mediterranean in much the same way as Arabs, but had the advantage of sailing in an enclosed sea where land is never far off.

The first seaman of whom it is told that he steered by stars was Odysseus. At one stage, as he crossed the sea during his imaginary voyage, night after night he had to keep the stars of the Great Bear on his left hand. This was the method of steering by horizon stars. A star was observed shortly after rising (or before setting) and used as a compass direction. After a while the next rising (or setting) star, with more or less the same amplitude, was used to steer by in the same way and thus the ship was held on the same course without the use of a compass. During daytime the position of the sun was observed and used for orientation. Culmination (when the sun reaches its highest daily position in the sky) indicated the south (or north), while in low latitudes sunrise and sunset pointed to two other major compass directions, east and west. In northern latitudes the polestar, although some degrees off the celestial North Pole, observed in combination with the Great Bear, also indicated the north. South of the equator, experienced seafarers could, at least for a while, still use the Great Bear to locate the north. They also estimated their position with the use of phenomena observed around them, such as the direction and force of prevailing winds. At night zenith stars were indicators for their position relative to the northern and southern shores of the Mediterranean, while the altitude of the polestar and the sun at noon provided an indication of latitude. Knowledge of longitude was based on sailing times between the eastern and western shores of the Mediterranean. The magnetic compass was probably introduced on ships in the Mediterranean by crusaders, during the thirteenth century. Sea charts and sailing directions, still in manuscript, came into use in that area in the fourteenth century and after the art of printing was introduced in Europe a century later, these aids were used more generally.

In the early Middle Ages, Norsemen and Irish monks started to cross the North Atlantic Ocean toward Iceland. Because of the long Arctic summer, stars were of less use to navigation here than in southern latitudes. To estimate the position relative to the track to Iceland, the Shetland Islands and the Faeroes were used as “steppingstones.” These navigators also observed migrating birds, which showed them in what direction land was, and there is mention of Norsemen using ravens taken on board for this purpose. Seamen found that a group of clouds over the horizon indicated still-unseen land. When visible, Norsemen used the polestar for orientation at night, and the sun during daytime. They probably also used these celestial bodies for setting out their courses. The magnetic compass came into use in the north of Europe after it was introduced in the Mediterranean.

Navigation with specially developed scientific instruments, and tables with astronomical ephemera, dates from the time of the maritime expansion of Portuguese and Spanish navigators, in the fifteenth century. When the Portuguese embarked on their voyages along the west coast of Africa their courses were generally north-south. They found their latitude by observing the polestar at night, and during daytime from the sun’s altitude at noon. For their observations they used the mariner’s astrolabe, a navigating instrument that was developed out of the astronomical astrolabe, probably around 1445.The cross-staff, inspired on the kamal, and used for the same purpose, was introduced at sea about 1515. Beyond the equator, which was reached by 1474, the polestar is no longer visible. Although the Southern Cross had been seen by Portuguese seamen already in 1454, it was found that there was no southern polestar. It was then decided to try to improve the method of finding latitude from the sun’s altitude at noon. As a result the first day-to-day declination tables of the sun were compiled. When Columbus sailed towards America in 1492, he was provided with such tables in manuscript. Columbus was also the first to observe westerly variation. Variation is the angle between the directions of the magnetic and the geographic North Poles, and depending on the place on the earth, it can be easterly, westerly, or zero. As the position of the magnetic pole changes slightly over a year, the variation increases or decreases. It is important that, when setting a course, the value of variation is taken into account.

After the Atlantic Ocean was crossed, and the Indian Ocean in 1497, east-west courses were sailed more frequently, and the necessity to measure longitude was felt. The difference in longitude between two places can be expressed in the difference in time between the two. Columbus tried to find his longitude in the New World from observing a lunar eclipse, which he knew would also occur in Spain. He compared the local time of his observation with the predicted time it would occur in Spain. The effort failed because of the inaccurate prediction through imperfect knowledge of the moon’s motion.

The Nuremberg astronomer Johann Werner, in 1514, was probably the first to suggest that longitude could be determined by measuring lunar distances, the angle between the moon and a fixed star. The moon moves across the sky relatively rapidly. At a given time the angle between the moon and a particular star is the same for every observer who can see them both. The angle between the moon and a number of stars (called the distance) can be calculated ahead for certain dates and times, for a particular (prime) meridian. An observer measuring the angle would compare the result and his local time with the predicted result and time, and the difference between the two times would provide the difference in longitude. Werner’s method is feasible, but at the time it was technically impossible to develop sufficiently accurate instruments and lunar tables. Fernando de Magallanes’s astrologer in 1519 unsuccessfully tried to find the longitude in South America by conjunction of the moon and Jupiter. Failure was blamed on errors in the predicted astronomical ephemera.

Navigators sometimes used the variation of the magnetic compass for estimating their longitude. Because of the apparent relation between longitude and variation, sixteenth-century scholars designed methods of converting variation into longitude. For this to be successful in practice, however, the method requires a vast amount of data on the magnetic variation, which was not available until the eighteenth century, when Dutch navigators estimated their longitude with variation during crossings of the Atlantic and Indian Oceans.

As the attempts to solve the question of longitude at sea failed, the king of Spain, in 1598, offered a reward for anyone who came up with a practical solution. This example was soon followed by the Dutch States-General, the governments of France and Venice, and eventually in 1714 by the British Parliament, which offered £20,000. The result was that a great many inventors claimed these prizes. Among those responding was Galileo Galilei, who suggested that the eclipse and occultation of Jupiter’s satellites could provide a solution, as they are visible at the same instance for every observer. Lunar eclipse observations to determine longitude were done successfully by astronomer Edmond Halley near the Cape of Good Hope in 1719 and on land by Captain James Cook in Canada in 1766. However, to be successful for navigation, eclipse and occultation observations require great accuracy that, due to the movement of a ship, was almost impossible to accomplish at sea.

For the Royal Society of London for Improving Natural Knowledge (founded 1662) and the Académie Royale des Sciences in Paris (founded 1666), solving the question of how to find longitude at sea was one of the many subjects they engaged. An important step toward this was the founding of the Observatory of Paris in 1667, followed by Greenwich Observatory, near London, in 1675. Both institutions were commissioned to rectify the existing tables of stars in order to find the longitude of places for the use of navigation. Paris Observatory was the first to publish new tables of astronomical data for navigation, such as the daily celestial positions of the sun, moon and planets, and stars, which first appeared in 1678. From 1761 onward the publications also contained lunar tables, which made it possible to find longitude through the aforementioned method of lunar distances. The first accurate lunar tables had been calculated a few years previously by the German mathematician Johann Tobias Mayer. The British astronomer Nevil Maskelyne became the driving force behind the Nautical Almanac, which first appeared for the years 1767–1769 and also contained lunar tables. Another condition for using the lunar distance method was an accurate instrument for measuring the distance. That had been fulfilled in 1731 when John Hadley of London designed the octant (also named Hadley’s quadrant), a double-reflecting-angle measuring instrument far more accurate than the mariner’s astrolabe and cross-staff. About three decades later it was successfully transformed into the sextant, and later adapted as the reflecting circle.

The method of lunar distances (lunars) was soon firmly established as a reliable method to find the longitude at sea. James Cook was among the first to apply it, in New Zealand in 1769, using the Nautical Almanac and a sextant. For many years lunars competed with the method of finding longitude by use of a chronometer. This is a clock that keeps the time on the prime meridian accurately during a voyage, and was developed successfully by the Englishman John Harrison, whose fourth timekeeper was awarded the longitude prize of British Parliament. Until the nineteenth century the astronomical solution by means of lunars and the mechanical method by means of a chronometer were both used at sea. Eventually the chronometer prevailed, because the method to find longitude by that instrument was much simpler than through lunars. An important result of the availability of these methods to find longitude was that sea charts became considerably more accurate.

A clouded sky prevents the taking of a noon observation of the sun, and a weather condition like that can last for days. In the mid-eighteenth century Cornelis Douwes, a teacher of navigation in Amsterdam, developed the first successful method of double altitudes, taken before and after noon. Hence forward latitude could be found without the restriction of the traditional noon observation of the sun. Douwes’s method was improved and remained in use until the nineteenth century, when position line navigation gradually also came into use. This is the name for various methods by which, through an astronomical observation and subsequent calculation and construction in a chart, a ship’s position could be found at almost any time of the day. The first to develop a method by which a single observation of the sun or a star resulted in a position line was the American captain Thomas Sumner, in 1843. The intersection of several “Sumner lines,” taken from stars in different directions, provided a fix. In the 1860s the Sumner method was improved by French astronomers and naval officers, and by then celestial navigation had reached the level at which it remained until long after World War II.