A world lit only by fire_ the medieval m - William Manchester [118]
AT THIS POINT in the history of exploration an eminent fourteenth-century Englishman appears in an unexpected role. He is Geoffrey Chaucer (1342–1400). Like most writers in all ages, Chaucer remained solvent by finding other employment from time to time. In 1368 he became an esquire of the royal household; later he was appointed clerk of the King’s Works. One of his royal admirers was Philippa, daughter of John of Gaunt and granddaughter of King Edward III. Chaucer’s avocation was the study of navigation. He modestly described himself as an “unlearned compiler of the labors of old astrologiens,” and in fact much of his Treatise on the Astrolabe was adapted from a Latin translation of the Composito et operato astrolabii of Messahala, an eighth-century Arabian astronomer. Nevertheless Chaucer was an enthusiast, and his enthusiasm was infectious. Young Philippa caught it. She became intrigued by his lessons in navigation. Later, as queen of Portugal, she taught them to one of her sons, Henry, who, sharing her enthusiasm, grew up to act upon it. He is remembered in history as Prince Henry the Navigator (1394–1460). Although the prince himself did little navigating, he sponsored voyages of discovery, encouraged seaborne commerce, developed the sailing vessel known as the Portuguese caravel, and designed a grand strategy to outflank Islamic power by establishing contact, first with Africa south of the Sahara, and then with the Orient. Islam survived his challenge, but in the process seamen inspired by Henry established the Portuguese overseas empire, which subsequently became the most extensive in the world, dominating European trade with India and the East Indies for 150 years.
In retrospect, their accomplishments seem almost miraculous, for despite the efforts of men like Chaucer and Prince Henry, navigation remained a highly inexact science. The prince is said to have improved the instruments used by navigators. One can only wonder what they were like before him. To be sure, latitude could be measured with any one of several versions of the astrolabe, chiefly the English cross-staff, a forestaff, or, in Magellan’s case, a calibrated backstaff. All, like their Egyptian forerunner, were primitive quadrants that measured the angle between the sun and the horizon. First-rate astronomers could also make an educated guess at longitude—if they were on land. But there was no way a man at sea could determine the longitude of his ship. To do that, he needed to read the position of the stars, which required knowledge of the precise time —an impossibility, since accurate clocks, with balance wheels and hairsprings, would not be invented until the middle of the next century. Of course, every captain had a compass, and all could compute dead reckoning. None, however, knew the difference between magnetic north and true north, or realized that dead reckoning suffered from disastrous errors arising from the drift of the water.
In the days of al-Idrisi, the twelfth-century geographer, Arabs had taught Sicilians how to sail boats, and Sicilians had passed the knowledge along to the Genoese, who had taught the Spaniards and Portuguese. But although the shores washed by the Mediterranean had been mapped, few captains had ventured beyond it. Even where coastlines