By your life, had it not been for Sulbār, the pilots
Of the fig, the date and the betel would never have been guided.
No instrument which they use over Madwara is like it
As a guide . . .26
Ibn Mājid’s praise of naked-eye observations notwithstanding, specialized instruments have long proved valuable in navigation. Some, including the quadrant and the astrolabe, began life as land-based aids, devised by astronomers and mathematicians, and were later simplified to make them seaworthy.
Foremost among readily available length-measuring devices, of course, are the independently movable parts of the human body: fingers, hands, arms, the landlubber’s striding foot. In the 1150s an Icelander who had just visited the Holy Land declared that there a man could determine the altitude of the Pole Star by lying on the ground, putting his fist upon his raised knee, and raising his thumb from his fist. A Venetian sailing in the 1450s for the Portuguese crown described the Pole Star’s altitude at a certain location along the coast of West Africa as “the height of a man above the sea.” In the 1950s a commodore in Britain’s Royal Navy still felt free to declare that even a modern navigator might approximate the altitude of a star by consulting the span of his wrist (eight degrees) or his hand (eighteen degrees) held at arm’s length.27 Today, too, any amateur astronomer knows that a fist held at arm’s length spans ten degrees on the sky. This system works because people with big hands tend to have longer arms, preserving the standardized angles of measurement.
Early Indian Ocean navigators consulted the width of a typical knuckle but also the kamal. In its most streamlined form the kamal is a card-shaped piece of wood through whose center passes a piece of string, knotted at equal intervals representing units of latitude. One end of the string is held between your teeth and the other in a hand. You mark the altitude by pulling the string taut, parallel to the ground, and moving the card with the other hand until its top aligns with the target star, Polaris, and its bottom aligns with the horizon; the resulting number of knots sitting between your teeth and the card translates into the latitude. Used across the Indian Ocean until well into the nineteenth century, the kamal was deployed again in the late twentieth century on a recreated voyage from Oman to China, and its effectiveness reconfirmed. Marco Polo mentions that Chinese navigators relied on a similar instrument, the qianxingban, “polar star–aiming plates,” which is a series of different-sized plates held at arm’s length in such a way as to align top and bottom with star and horizon, respectively. The choice of plate depended on the altitude of the star. A millennium earlier, the Chinese estimated latitude with the liangtianchi, “star-measuring ruler.”28
The real sea change in navigation, however, came with the rapid spread of the seemingly magical magnetic compass. Now one could have an instant sense of direction, clouds or no clouds, stars or Sun, day or night.
Many countries have claimed authorship or at least awareness of crucial components of the compass. Both the ancient Greeks and the ancient Chinese saw that certain brownish stones drew iron to them; the “lode” in lodestone, the magnetic form of the iron-rich mineral magnetite, is Old English for “way.” Some scholars confidently apply the term “compass” to objects that began to be used by Chinese navigators around AD 500 as sea routes to Japan were established, and the first mention of a south-pointing shipboard needle appears in a Chinese navigational text written in AD 1100. A resident of Amalfi, a southern Italian maritime power in the twelfth century, has traditionally been credited with the invention of the north-pointing mariner’s compass, and a contemporary chronicler described medieval Amalfi itself as famous for showing sailors the paths of the sea and sky. The first Arab text to mention a compass, written in the thirteenth century, calls the instrument by its Italian name. To some historians, the fact that the Chinese referred to south-seeking needles and the Italians to north-seeking ones suggests the likelihood of independent invention.29
Whatever the origins, compasses worked, and the way they worked was well understood in the Mediterranean by 1200, when a French writer described in detail how to rely on a compass to navigate by “the star that never moves”:
This is the star that the sailors watch whenever they can, for by it they keep course. [W]hile all the other stars wheel round, this stands fixed and motionless. By the virtue of the magnet-stone they practice an art which cannot lie. Taking this ugly dark stone, to which iron will attach itself of its own accord, they find the right point on it which they touch with a needle. Then they lay the needle in a straw and simply place it in water, where the straw makes it float. Its point then turns exactly to the star. There is never any doubt about it; it will never deceive. When the sea is dark and misty, so that neither star nor Moon can be seen, they put a light beside the needle, and then they know their way. Its point is toward the star, so that the sailor knows how to steer. It is an art that never fails.30
In other words, float a magnetic iron needle by attaching it to something buoyant, and it will invariably come to rest along Earth’s north–south magnetic axis, with its point aimed north.
Soon came the pivoting compass needle and the compass’s essential partner: a radial diagram called a compass card or wind rose, divided into as many as sixty-four directions. At sea, a direction meant a wind, and each wind bore a name. With the aid of this new technology, the literate, numerate ship’s pilot sailing the Mediterranean or the Black Sea in the early thirteenth century could confidently determine not only when he was heading Tramontane (north) or Ostro (south), Greco (northeast) or Sirocco (southeast), but also when he was heading Tramontane quarter Greco—and he could keep to his course by dead reckoning, a mostly reliable technique based on knowing the relative positions of one’s departure point and specific or general destination, as well as the direction of the vessel’s movement and the distance traveled, both of which are determined at regular intervals. The pilot’s sons and grandsons, continuing in the family profession, would have even more aids: scaled marine charts and a pilot book filled with detailed sailing directions.
Imagine you’re the captain of a Venetian vessel in the year 1320. You’ve just brought in a load of grain from Egypt, and you’re now heading to the east coast of Spain with some prized Sardinian cheeses and another shipload of grain, this one picked up at Constantinople. On the return trip you’ll carry Spanish wool. Your masters and the navies they support have thus far kept Portuguese ships out of your path, and neither the Black Death nor the mounted Ottoman Turks have yet arrived on European soil. Your exceptional nephew at the University of Bologna has told you of two groundbreaking books that he says are relevant to your profession, though you have no intention of reading them: Fibonacci’s Book of Calculation and Sacrobosco’s On the Sphere of the World. The former includes a reader-friendly introduction to Hindu-Arabic numerals, including the indispensable 0; the latter is your era’s preferred text for Astronomy 101. What you do read closely, and keep with you on board, is a hand-lettered copy of Lo Compasso da Navigare, which directs you on a clockwise circuit of the Mediterranean, and a Toleta de Marteloio, a series of tables showing you how to correct your course as you tack with the wind. You also have an exquisite portolan chart of the whole sea, showing distances, harbors, and major landmarks; it is carefully scaled and even signed by an illustrious Jewish cartographer from Majorca. On your oak table sit a pair of silver dividers and a silver ruler to work the chart. Your ship’s compass, with its freely pivoting needle and its attached compass card, is safely housed inside a circular metal box; your sandglasses (you keep some spares) were blown in Venice.31 Thanks to all this state-of-the-art equipment, you can figure out which direction your ship is heading; you can monitor the advancing and waning of night and thus the hours of the watch; and you know the distance to the nearest port, how many days’ sailing it will take to get there, and what to look for as you approach it. Unlike your Arab, Indian, Polynesian, and Chinese counterparts, you extract almost no information from the stars,
and since you stay within the confines of your home sea, the Mediterranean, you find little cause to heed latitudes and even less to ponder longitude.
But the known world had already reached well beyond the Mediterranean, and change was coming fast. Centuries had elapsed since the Vikings had begun to ship dried cod to Britain, since Icelanders had sojourned on Vinland (Newfoundland), since Polynesians had settled on New Zealand, and since the Chinese had crossed the Arabian Sea and learned that certain inhabitants of East Africa drank fresh ox blood mixed with milk. A handful of recent European-drawn maps had begun to show the southern half of Africa, which Ptolemy had hinted, more than a millennium earlier, extended well below the equator. Plutarch knew Africa was circumnavigable, and Alexander the Great knew it could be reached by sea from the mouth of the Euphrates. But in the interim it had, so to speak, gone missing. By the end of the thirteenth century, a Venetian had escorted a Mongol princess from the South China Sea to the Persian Gulf, and a Genoese had built a castle in the Canary Islands. By the end of the fourteenth century, Arab and Indian traders had established themselves along the East African coast as far south as present-day Mozambique. At the beginning of the fifteenth century, a heavily armed fleet of more than three hundred Chinese vessels, commanded by the formidable eunuch admiral Zheng He and carrying nearly 28,000 soldiers and half a dozen astrologers, had sailed forth to impress and intimidate China’s southern neighbors by a lavish display of both Ming treasures and military might.32 Last but certainly not least from the perspective of Western Europe, the Portuguese had begun to sail the Atlantic far and wide.
Portugal’s Prince Henry the Navigator, born in 1394, dedicated himself to the discovery of Africa’s “River of Gold,” to the erasure of Islam, to the gathering of slaves and pepper, and, according to the court chronicler of his day, to the fulfillment of his horoscope—the “inclination of the heavenly wheels” that inclined him to conquer new lands:
His ascendant was Aries, which is the house of Mars and exaltation of the sun, and his lord is in the XIth house, in the company of the sun. And because the said Mars was in Aquarius, which is the house of Saturn and in the mansion of Hope, it is signified that the Lord [Henry] should toil at high and mighty conquests, especially in seeking out things that were hidden from other men and secret, according to the nature of Saturn, in whose house he is. And the fact of his being accompanied by the sun, as I said, and the sun being in the house of Jupiter, signified that all his traffic and his conquests would be loyally carried out.33
There are many rational, strategic reasons why one might invoke the universe in the name of conquest. You might want to stage a nighttime attack at the time of the new Moon, giving you maximum darkness, as was done at the start of Operation Desert Storm in 1991. You might need to carefully monitor lunar tides during a naval invasion, to ensure that your ships don’t run aground in shallow waters. You might decide to invade during a phase of high auroral activity, which will muck up the other side’s radio communication. The reasons ascribed to Prince Henry, anchored in the pseudoscience of astrology, are neither rational nor strategic.
In his own day, it was well understood that Prince Henry—governor of the wealthy Order of Christ, which had replaced the wealthy Knights Templar of earlier centuries—was undertaking a crusade, the quintessential fusion of war, profit-seeking, exploration, and the imposition of foreign ideas. Writing in the twentieth century about space exploration, the American journalist William E. Burrows called it a “drive that was defined and tempered by politics and competition on every level,” which could easily be said as well of Prince Henry’s program. As Burrows says, “Exploration was always done for the wrong reasons. But it was done.”34 What he means, of course, is that whether or not the explorer is an explorer, exploration is hardly ever motivated by the desire to explore. Part the curtains of curiosity, and you’ll find individuals hungry for political, cultural, or economic dominion funding the expedition.
Henry and his actual navigators could not have done their venturing without astronomy, a fact made explicit in the florid decoration of noble works of Portuguese architecture executed during and shortly after his lifetime. The exuberantly sculptured windowframes and archways, the floor mosaics and ceiling paintings of the vast Convento de Cristo and elsewhere repeatedly present the astronomer’s armillary sphere linked with the Crusader’s cross as well as with the vegetation of exotic lands. Historian Jorge Cañizares-Esguerra captures the association between astronomical knowledge and conquest embodied by Prince Henry and succeeding generations of Iberian colonizers: “[T]he cosmographer as knight, or the knight as cosmographer, was a hallmark of the Portuguese and Spanish fifteenth- and sixteenth-century colonial expansion.” The gathering of knowledge, he argues, was “an expansion of crusading virtues.” An influential mid-sixteenth-century book by a royal cosmographer, Arte de Navegar, presented ships’ pilots as “the new knights, whose horses were their vessels and whose swords and shields were their compasses, charts, cross-staffs, and astrolabes.”35
Prince Henry’s first conquest was Ceuta, a Mediterranean town in what is now Morocco identified with the southerly Pillar of Hercules and piled high with African goods of great beauty and value. Under Henry’s sponsorship and direction, many Atlantic islands, including the Azores, Canaries, and Madeiras, were tilled and grazed. Navigators in the prince’s employ mastered a route around fearsome Cape Bojador, far out to sea and down past the cape’s winds and currents, and rounded the westernmost point of Africa, reaching as far as Sierra Leone. Along the way, the captains recorded star altitudes at notable capes, islands, and river mouths, which astronomers back in Portugal translated into tables of latitude. During Henry’s fiftieth year of fulfilling destiny, his brother, the king of Portugal, granted him monopolistic rights over the lands discovered and all enslavable persons therein. Henry’s death in 1460 barely interrupted the travels of the Portuguese. In 1473 Lopes Gonçalves crossed the equator; in 1488 Bartolomeu Dias rounded the “cape of storms” at the southern tip of Africa; in 1498 Vasco da Gama reached southern India by sea; in 1500, eight years after Columbus’s first crossing of the Atlantic, Pedro Álvares Cabral arrived in Brazil. Their aims, like those of Henry and of the conquistadors who followed them, were “to serve God and His Majesty, to give light to those who were in darkness, and to grow rich, as all men desire to do.”36 In short, they were laying the cornerstones of empire.
Not that trade hadn’t already made many people rich, as well as building a global economy across the Old World. Consider this: the Muslim soldiers who fought in the Middle East against the Crusaders wore chain mail from the Caucasus and wielded steel swords smelted in South Asia from sub-Saharan iron. As the Ottoman caliphs extracted taxes and the Chinese emperors extracted tribute (and invented paper money), merchants transported product from market to port and port to market. Much intercontinental trade was pan-Asian, private, and conducted by diasporas of blood, dialect, or faith: Jews, Hindus, Muslims, Armenians, Lebanese, Fujianese, Gujaratis. The Indian Ocean was the crossroads of a network of trading centers that stretched for thousands of miles, and local lords from the East China Sea to the eastern shores of Africa generally permitted merchants of every stripe and origin to enter their ports. But medieval Middle Eastern and Asian trade networks, however sprawling, were not colonial empires. The Muslim caliphate settled for the collection of taxes hefty enough to fund the army that safeguarded the highways, which allowed for the commerce that yielded life’s luxuries. And the Chinese state, which grew plenty of sugar and other tropical delights within its own borders, had little cause to put money, personnel, and effort into creating colonies overseas.37
The Portuguese, on the other hand, representing king, country, and God, sought both control and colonies. Good ships and newfangled guns gave them the advantage as they revived the practices of building forts, blocking trade routes, claiming trade monopolies, boarding foreign ships, and generally seeking to rule the waves and harbors. A key part of their p
rogram was to find routes free of Ottoman control and thus free of Ottoman tax collectors.38
Venturing far across the Atlantic and Indian oceans in the fifteenth century, the Portuguese had need of knowledge and instruments more elaborate than those used by the average captain crisscrossing the Mediterranean or exploring the east coast of Africa or taking soundings for depth, sampling bottom silt, and monitoring the tides in the fogbound English Channel or the Baltic. Portugal’s sailors may have been wary of unfamiliar concepts and new techniques, but Portugal’s ocean-bound navigators had little choice other than to use the mounting numbers of charts, pilot books, and mathematical rules. Far more than their predecessors, they consulted the stars as well as the compass. Having mastered the art of dead reckoning, they continually checked their bearing against the height of the Sun or the Pole Star as measured with a quadrant or mariner’s astrolabe and used arithmetic and geometry to recalculate that bearing when winds and currents threatened to send the ship off course. Instructions for the use of the quadrant cautioned that the Pole Star was not entirely stationary and should be observed only when its two Guards lay east–west. Compilations of tables, known as ephemerides, listed the predicted daily position of the sky’s major occupants. Tables of midday solar altitude in various cities helped navigators sail to the correct latitude and then keep to it as their ship headed due east or west—known as running down the latitude.39
The drive to amass navigational information that would grant an advantage over one’s seagoing adversaries kept accelerating, and the stakes kept rising. Faith, glory, and commerce, writes the historian Emilia Viotti da Costa, were the driving motives. The pope himself pronounced Portugal’s African project to be a “just war,” and three papal bulls concerning it were issued during Prince Henry’s last ten years of life. The first, in 1452, proclaimed the right of the Portuguese king to attack and enslave non-Christians and to confiscate their goods and lands. The second, in 1455, specified that this right applied to Africans from Morocco down to Cape Bojador, who
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