Al-Idrisi and The book of Roger

In the year 1138, the royal palace at Palermo, Sicily was the scene of a long-awaited meeting between an unusual Christian king and a distinguished Muslim scholar. As his visitor entered the hall, the king rose, took his hand and led him across the carpeted marble to a place of honor beside the throne. Almost at once the two men began to discuss the project for which the scholar had been asked to come from North Africa: the creation of the first accurate—and scientific—map of the entire known world.

The monarch was Roger II, King of Sicily; his distinguished guest, the Arab geographer al-Idrisi. Born in Ceuta, Morocco, across the strait from Spain, al-Idrisi was then in his late 30’s. After studying in Cordoba, in Muslim Spain, he had spent some years in travel, covering the length of the Mediterranean, from Lisbon to Damascus. . As a young man with poetic pretensions, he had written, as a student, verse celebrating wine and good company, but in the course of his journeys he had discovered his real passion: geography.

The Mediterranean in the 11th and 12th centuries

Al-Idrisi’s writings tell us less about his own character and personality than about those of the man who became his host and patron. Roger II, son of a Norman-French soldier of fortune who had conquered Sicily at the beginning of the 12th century, was an anomaly among Christian monarchs of his time. . His co-religionists, commenting on his oriental lifestyle, complete with harem and eunuchs, disparagingly referred to him as the “half-heathen king” and “the baptized Sultan of Sicily.” Educated by Greek and Arab tutors, he was an intellectual with a taste for scientific inquiry, and relished the company of Muslim scholars, of whom al-Idrisi was one of the most celebrated.

Such cultural communication at a time when Crusaders and Muslims were battling in the Holy Land and while Mediterranean pirates of both faiths plundered each other’s ships and ports may seem surprising. But Crusades and piracy notwithstanding, medieval merchants did brisk business across the frontiers of religion, and inevitably ideas were exchanged as well as products.

Sicily was a meeting ground for the two civilizations. Captured by the Arabs in 831, the island had remained in Muslim control until the end of the 11th century. Like Muslim Spain was a beacon of prosperity to a Europe caught in the economic slow-down we call the Dark Ages. Like Muslim Spain was a beacon of prosperity to a Europe caught in the economic slow-down we call the Dark Ages. The occupying Arabs had built dams, irrigation systems, reservoirs, and water towers, introduced new crops—oranges and lemons, cotton, date palms, rice—and exploited the island’s mines and fishing grounds.

Early in the 11th century a band of Norman adventurers, the Hautevilles, had ride into southern Italy to wrest it from the Byzantine Greeks and the Muslims, and in 1101 Count Roger d’Hauteville capped his career by conquering Sicily. Four years later, he passed the territory on to his son, Roger, who in 1130 was crowned king as Roger II.

A few practical maps did exist—mariners’ charts showing coastlines, capes, bays, shallows, ports of call and watering and provisioning places—but in a typical medieval divorce of science and technology, these remained in the hands of navigators. Information from travelers, too, filtered only very slowly onto Christian maps. What King Roger had in mind, therefore, was something as factual as the mariners’ charts, but encompassing the whole known world. The mission he entrusted to al-Idrisi was intellectually Herculean: to collect and evaluate all available geographical knowledge —from books and from on-the-spot observers— and to organize it into an accurate and meaningful representation of the world. His purpose was partly practical, but mostly scientific: to produce a work which would sum up all the contemporary knowledge of the physical world.

The geographical charts as an instrument

The reason behind the Muslim domination of the field of geography was simple: economics. While medieval Europe had become fragmented and parochial, both politically and commercially, the Muslim world was unified by a flourishing long-distance commerce as well as by religion and culture. Muslim merchants, pilgrims and officials used so-called “road books,” itineraries that described routes, traveling conditions and cities along the way. Some of the early authors of road books were on al-Idrisi’s list: Ibn Khurdadhbih, an eighth-century Persian who was director of the postal and intelligence service in Iran; al-Yaqubi, an Armenian who in the ninth century wrote a Book of Countries; Qudamah, a 10th-century Christian who had embraced Islam, served as a tax accountant at Baghdad and written a book discussing the postal and tax systems of the Abbasid Caliphate. Others belonged to a later tradition of systematic geography, like the 10th-century scholars Ibn Hawqal and al-Mas’udi, who produced books intended as something more than practical guides for the tax collector or the postman: as additions to the fund of human knowledge.

Al-Idrisi’s two geographers from the pre-Islamic era were Paulus Orosius, a Spaniard whose popular History, written in the fifth century, included a volume of descriptive geography; and Ptolemy, the greatest of the classical geographers, whose Geography written in the second century, had been entirely lost to Europe, but preserved in the Muslim world in an Arabic translation.

After examining at length, the geographical works, they had collected, the king and the geographer observed that they were full of discrepancies and omissions and decided to embark on original research. Sicily’s busy and cosmopolitan ports provided an ideal place for such an inquiry, and for years hardly a ship docked at Palermo, Messina, Catania or Syracuse without its crew and passengers being interrogated about the places they had visited. The commission’s agents haunted the ports, and if they discovered a traveler who had visited any particularly exotic region, he was conducted to the palace at Palermo to be questioned by al-Idrisi or even by Roger. What was the climate of the country, its rivers and lakes, mountains, coastal configurations, and soil? What of its roads, buildings, monuments, crops, crafts, imports, exports, and marvels? What of its roads, buildings, monuments, crops, crafts, imports, exports, and marvels? What, finally, were its culture, religion, customs, and language? In addition, scientific expeditions were dispatched to areas on which information was lacking. These expeditions were accompanied by draftsmen and cartographers so that a visual record of the country could be made.

During this research, al-Idrisi and Roger compared data, keeping the facts on which travelers agreed and throwing out all conflicting information. This process of collecting and assessing material took 15 years, during which, according to al-Idrisi, hardly a day passed when the king did not confer personally with the geographers, studying accounts that disagreed, examining astronomical coordinates, tables, and itineraries, poring over books and weighing divergent opinions.

Al-Idrisi’s great work

Finally, however, the long preliminary study was finished and the task of map making began. First, under al-Idrisi’s direction, a working copy was produced on a drawing board, with places sited on the map with compasses, following the tables that had already been prepared. Then a great disk almost 80 inches in diameter and weighing over 300 pounds was fabricated out of silver, chosen for its malleability and permanence.

Al-Idrisi explained that the disk merely symbolized the shape of the world: “The earth is round like a sphere, and the waters adhere to it and are maintained on it through natural equilibrium which suffers no variation.” It remained “stable in space like the yolk in an egg. Air surrounds it on all sides…. All creatures are stable on the surface of the earth, the air attracting what is light, the earth what is heavy, as the magnet attracts iron.”

As his comment suggests, al-Idrisi thought that the world was round. Nor was he alone. Contrary to a still popular misconception that up to the time of Columbus everyone believed the world was flat, many scholars and astronomers since at least the fifth century B.C. had believed that the earth was a globe.

In the third century B.C. the Alexandrian astronomer Eratosthenes measured a degree of the earth’s circumference with amazing accuracy, arriving at a figure with an error of either 1.7 or 3.1 percent. (The variation in the amount of his error is due to modern uncertainty as to the exact length of the measurement he used.) Ptolemy, four centuries later, estimated the circumference with much less success —at almost 30 percent less than its true extent—. And in the ninth century, 70 Muslim scholars, working under the patronage of Caliph al-Ma’mun, gathered in the Syrian Desert to determine the length of a degree of latitude. Rather than rely on travelers’ guesses of distance, as previous astronomers had done, they used wooden rods to measure the road they traveled until they saw a change of one degree in the elevation of the polestar. Their calculation resulted in a figure for the earth’s circumference equivalent to 22,422 miles, an error of 3.6 percent, almost as accurate as Eratosthenes’ estimate and a considerable improvement over Ptolemy’s.

By al-Idrisi’s time, Muslim astronomers had made great strides in methods of reckoning latitude. (Longitude remained a problem until the 17th century.) Arab geographers had corrected some of the errors of Ptolemy and other Greek scientists. The mathematician al-Khwarizmi reduced Ptolemy’s estimate of the length of the Mediterranean Sea from 62 to 52 degrees; the Spanish Muslim astronomer al-Zarqali further adjusted the figure to the correct 42. Other Muslim scholars, like the Iraqi astronomer al-Battani and the Persian al-Biruni, composed tables giving the latitudes of leading cities.

Al-Idrisi himself gave three figures for the earth’s circumference, without deciding among them: Eratosthenes’ approximately correct estimate, a slightly smaller figure arrived at by Indian astronomers, and a still smaller number—though larger than Ptolemy’s—which was apparently agreed on by Sicilian scholars.

Cartography, nevertheless, remained in a primitive state. Although Ptolemy had discussed several kinds of projection, the problem of flattening out the surface of a sphere so that it could be represented on a map was not solved until the 16th and 17th centuries—the Age of Exploration—and none too satisfactorily even then. The great geographer Gerardus Mercator commented, “If you wish to sail from one port to another, here is a chart… and if you follow it carefully you will certainly arrive at your port of destination… You may get there sooner, or you may not get there as soon as you expected, but you will certainly get there.” Al-Idrisi’s silver disk or “planisphere,” was a form of projection considerably in advance of others of its time.