SO MANY STORIES, spun by so many chroniclers, eyewitnesses, and ax grinders: The career of Christopher Columbus formed its own Sargasso Sea of weedy narratives, dense and intertwined, thrown together by contrary yet converging currents of belief, justification, revisionism, hagiography, nationalism, condemnation, and early commercial publishing’s sheer love of storytelling. And as a legal battle began to rage in Spain and the New World in the early sixteenth century over his descendants’ heritable rights to his discoveries, depositions accumulated in a cacophony of sworn truths and were piled upon anecdote, memoir, and courtly histories in rival nations. Truth became an interminable feud of opinions, memories, and agendas. One can hack at these narratives all one likes and never clear a single incontrovertible course through their entangled contradictions.

This much appears to be true: By 1478, Columbus was living at least part of the time in Lisbon, and after marrying into a powerful Portuguese family, he was determined to leverage his in-laws’ connections at court—to turn himself into an explorer of an Ocean Sea that could not be boundless, that had to have an end to complement its beginning on the Lusitanian shore.

The Fernando Columbus biography avowed that Columbus inherited through his wife’s dowry a haul of charts and even secret knowledge of lands to the west, across the Ocean Sea. The biography also served up a diverse menu of tales by mariners that supposedly influenced Columbus’s convictions. To be sure, the Atlantic realm was full of stories of landfalls sighted but not quite reached over the western horizon and of evidence carried on ocean currents and washed ashore, ranging from tree branches to strange corpses with wide faces.

The tales of Columbus’s foreknowledge of discovery opportunities accumulated and cross-pollinated within a few years of his death. The sixteenth-century Spanish chronicler of the Indies, Gonzalo Fernández de Oviedo y Valdéz, heard one Columbus tale while he was in Santo Domingo on Española in 1514. Near Columbus’s home on Madeira, a pilot who was the lone survivor of a shipwreck struggled ashore. Columbus took him in, and the dying man confided that he had seen a strange land in the west, but his ship had been driven back by a hurricane before he could reach it. The story was repeated in the Fernando biography as well as by Bartolomé de Las Casas, who was an eyewitness to Columbus’s activities. Las Casas emigrated to Santo Domingo in 1502 and produced the only version of Columbus’s 1492 voyage log we have. Both these later accounts shifted the mysterious landfall far to the north, to the west of Ireland.

Among the men who would offer an opinion on Christopher Columbus’s inspiration was John Cabot’s son Sebastian. Years after both his father and Columbus had died, Sebastian carved an astounding personal trajectory. The renowned chronicler of Columbus’s voyages, Pietro Martire, would write a sentence in his third Decades, published in 1516, that would have seemed inconceivable in the 1480s, while the Cabot family was still in Venice: “I know [Sebastian] Cabot as a familiar friend and sometimes as a guest in my house.” In 1518, Sebastian was hired as pilot-major of Spain, overseeing all navigation to the new lands Columbus had found.

On December 31, 1535, Sebastian Cabot was asked at Seville to provide expert testimony in legal actions surrounding the hereditary rights to Columbus’s discoveries. He stated that no, he had never met Christopher Columbus, but that he had known his son, Diego, a long time. Asked by the crown attorney, Francisco de Aguilar, what he knew of the inspiration for Columbus’s discoveries, he explained what he understood of Julius Solinus, an early third-century geographer who relied on the wisdom of the Roman Pliny, “who says that from the Fortunate Isles, which are [now] called the Canary Islands, sailing westward for thirty days, there were some islands that were named the Hesperides, and that this witness assumes that those Hesperides islands are the islands that were discovered in the time of the Catholic monarchs don Fernando and dona Isabel, of glorious memory, and the witness has heard that don Cristobal Colón discovered them in the times of the Catholic monarchs [Fernando and Isabel].” Asked where he had learned this, Sebastian said: “[F]rom many people in this city of Seville, whose name he does not remember, and that it is public and well known.”

Sebastian did not elaborate, but Oviedo, drawing on Pliny, Solinus, and Isadore of Seville, concluded that the Hesperides had been ruled more than three thousand years earlier by a Spanish king named Hesperus. Columbus thus had rediscovered what Spain already rightfully claimed: “God returned this lordship to Spain after so many centuries.”

For his part, Sebastian Cabot shed no further light for the crown attorney on the impulse to sail west that gripped not only Christopher Columbus but his own father, John. It did not help that Sebastian in his own claims of exploration prowess would give no credence to the accomplishments of John. As far as the son’s version of history was concerned, the father might as well have never lived.

THE DISCUSSIONS OF Columbus’s motivations seemed, for the most part, hearsay, sailor’s tales, and convenient recollection, with the occasional strange label on an enigmatic chart encouraging the idea that someone had gotten somewhere, sooner, only to have the details obscured by the haze of memory and oral tradition or locked away as state secrets for so long that no one still alive was able to recall what actually happened. But amid all of the rumor, conjecture, and discordant personal recollection was one elegantly simple explanation for Columbus’s voyage plan.

In 1474, the Florentine mathematician, astronomer, and physician Paolo dal Pozzo Toscanelli sent a map to Fernan Martins, canon of Lisbon, at the request of Afonso V of Portugal. The map no longer exists, but in the letter, Toscanelli addressed a fundamental question raised by the long-standing Portuguese effort to hunt down more Atlantic islands: Since the world was well understood to be a sphere, what would happen if someone kept sailing west into the Ocean Sea as he sought these elusive landfalls?

Toscanelli proposed that a mariner would encounter Antilla (or Antillia), a quasi-mythical landfall or island group that had materialized on a nautical chart drawn by the Venetian Zuanne Pizzigano in 1424. This archipelago was sometimes equated with another quasi-mystical landfall, the Isle of Seven Cities—so named for the communities supposedly founded by seven Iberian bishops fleeing the expansion of the Muslim empire of the Maghreb in the eighth century. The Portuguese initially thought the Azores were Antilla, and finding the actual Antilla spurred on some of their voyage initiatives. In the Toscanelli scheme, Antilla was a midpassage stepping-stone to spices, gold, and jewels, halfway across the Ocean Sea along the Tropic of Cancer, west of the Canaries. Next came Cipango (Japan), before the mariner reached the Asian mainland.

At the time, the term Atlantic was used only occasionally to describe the ocean beyond the Strait of Gibraltar. For the most part, the name used was the Ocean Sea, and the land on its western side was not Asia per se but the Indies. The plural was used because what we now consider to be Asia was composed of three different forms of India. (The term Asia generally was reserved for the former Roman Empire territories of Asia Minor, although it occasionally cropped up in documents related to Columbus in the broader sense.) India Sinus was China; India Magna (or Mayor) described the Indian subcontinent between the Ganges and Indus rivers; and India Parva-Ethyopis consisted of all the lands and waters from Calicut on India’s southwest coast westward to East Africa, and included Persia, the Arabian Peninsula, and the Indian Ocean.

No one expected North, Central, or South America to stand in the way of a mariner sailing west from Europe, and no one could agree on how far such a voyage might be. The dimensions of Earth were far from settled, and Columbus, who most certainly knew Toscanelli’s map and his voyage proposition, relied on the estimation of the second-century A.D. Greek geographer Ptolemy. His findings cast the planet with a circumference about 18 percent smaller than it actually is, although Columbus advanced a circumference 25 percent too small. But where Ptolemy had squeezed the known and habitable world, or oikoumenē, into half of its total circumference—which meant one would have to sail halfway around the world, across the Ocean Sea, to reach Asia from Europe—Columbus expected to have to sail only about one-third of the way around this smaller globe to reach the Indies, and along the way he would have Antilla as well as Cipango as profitable way stations. Nothing better illustrated the optimism of the scheme than the fact that this version of the planet (drawing on Pierre d’Ailly’s 1410 world map Imago Mundi, which crammed the entire habitable world into the Northern Hemisphere) placed Cipango 1,500 miles east of the Asian mainland, and only 2,400 miles west of the Canaries. Japan in fact is more than 10,000 miles west of the Canaries, with some considerable obstacles lying in the path of any intrepid mariner.

Toscanelli’s ideas may have been known to Cabot even before they reached Columbus. In 1480, four years after Cabot joined the confraternity of San Giovanni Evangelista, Febo Capella became its guardiano grande, or principal official. Capella was a humanist scholar and ducal chancellor who had a lengthy posting as Venice’s ambassador in Florence. Given his movement in cultural circles around the Medici family, Capella could have brought home gleanings from the work of Toscanelli, especially as Capella’s brother Priamo was a celebrated mariner. The idea that you could reach the East by sailing west on the Ocean Sea may have been very old news to Cabot by the time he was in Columbus’s milieu.

THE GENOESE CHRONICLER Antonio Gallo in 1506 credited Columbus’s brother Bartolomé with impressing upon Christopher the logic and feasibility of a westerly route to Asia. Having settled in Lisbon as a cartographer, Bartolomé had absorbed the wisdom of Portuguese mariners in the Atlantic realm. He showed Christopher “that if someone steered a course into the open sea, leaving behind the southern shores of Ethiopia, and directed his course to the right towards the west, he would necessarily run into a continent eventually.”

Bartolomé, Gallo explained, had “dedicated himself to making money by painting maps for the use of sailors, on which seas, harbors, shores, bays, and islands are shown with their correct outlines and proportions maintained.” Bartolomé, in other words, was in the business of making portolans, the coastal charts that provided mariners with distances and bearings but did not yet have a geographic grid of latitude and longitude.

There was a major conceptual leap to be made from a portolan to a chart employing latitude and longitude. It required, for one thing, an actual need, and the average mariner of the fifteenth century concerned with coastal navigation could do fine with the typically regional information of the portolan. He depended as much if not more on the piloting notes he kept in a routier, or rutter, as the English called it. What ocean passages there were—to Iceland, to the Atlantic islands of the Cape Verdes, Azores, Canaries, and Madeiras—left mariners out of sight of land for only a few days. Their destinations provided such a large target (if not in sheer mass like Iceland, then in sprawling archipelagos or towering volcanic landfalls) that it was difficult to go entirely astray, provided the weather cooperated.

The grid of latitude and longitude, proposed by Ptolemy initially in his Almagest and most famously in his Geographia, organized the world into fixed points, whereas piloting based on rutters and portolans was a relativistic exercise, concerned with relationships between locations in distance and compass course. As exploration accelerated in Europe in the fifteenth century, the need to shift to a Ptolemaic system was plain: It was the only effective way to create a coherent picture of the globe as more of it was revealed to far-ranging vessels, and Columbus understood the use of a degree-based grid in explaining and exploiting Toscanelli’s opportunity. The Portuguese in particular drove the necessity, as their voyages southward, down the West African coast, increased the known world as well as the demand for new means of navigation.

The work of Ptolemy was familiar to scholars from manuscripts, but Gutenberg’s press and commercial publishing were required to popularize his views. The first printed version of a two-volume Geographia likely didn’t appear until 1477, in Bologna. There was also little practical way for a seaman to make use of the Ptolemaic grid. The vertical meridians, or lines of longitude, that ran from pole to pole could not be readily determined by any accurate means of celestial observation at sea—or on land, for that matter.

It had long been known that latitude, the lines running east–west parallel to the equator between the poles, could be determined by measuring the angular elevation of the North Star, or polestar. But this was still a difficult exercise, especially at sea. Instruments were crude, and conditions on a rolling deck often made accurate results impossible. No surviving chart with a latitude scale is even known before 1502.

When the Portuguese progressed sufficiently south in their investigation of the West African coast in 1481 that they lost sight of the polestar, another means of determining latitude was needed, and the king, João II, created a commission to find a way to use the noon elevation of the sun. The complication of solar observation (beyond the difficulty of knowing when it is precisely noon wherever one happens to be) is that the height of the sun changes every day, as its trajectory through the heavens shifts with the changing seasons. Solutions were tested off Guinea in 1485, and simple solar tables that could be used to correct the measurement on any given day of a particular year (because every year requires a unique set of tables) were adapted from ones created by the Jewish astronomer Zacuto of Salamanca.

Christopher and Bartolomé Columbus thus were in the Portuguese realm at a critical time in both exploration of the Atlantic and the advancement of scientific navigation and cartography. But practical solutions still lay over the horizon, and the brothers would leave Portugal just as the royal commission was investigating the use of the sun for observations, or “fixes.” The first printed nautical almanac featuring daily correction (declination) tables of sun sights, Regimento do estrolabo e do quadrente (regiment of the astrolabe and quadrant), was still a few years away; it may have first appeared in Portugal in 1495, but the earliest surviving edition is from 1509. Not only was knowledge of the world imperfect; so were the means to explore and describe it.

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