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This patristic dismissal of so elementary a fact was a sign of how deep the wisdom of the ancient world had been buried. More than three hundred years before the birth of Christ, Aristotle had determined that the planet must be a sphere; after an eclipse he had pointed out that only an orb could throw a circular shadow on the moon. The existence of India and Spain was known in Athens. However, few other geographical or scientific facts were available to Aristotle, and this led him into error. Holding that land was heavier than water, and that the masses of each must balance, he had inferred that the distance between the Iberian Peninsula and the Indian subcontinent could not be great, and that, consequently, there was no land between them — that is, no North or South America. Therein lay the origin of Columbus’s error, which others would challenge and which Magellan, ultimately, would discredit.

Aristotle’s spherical theory of the globe had been the cornerstone of classical geography. The Greeks arbitrarily divided the planet into five zones, two of them polar, too cold to be inhabitable; two others temperate; and one an equatorial region. Albertus Magnus and Roger Bacon, their medieval successors, later concluded that the equator, because of its great heat, must be incapable of sustaining life. Their conviction that man could not survive in the tropics, widely accepted, persisted until the fifteenth century.

With the exceptions of Pliny, Macrobius, and Agrippa, whose contributions were slight, the Romans added nothing to geographical knowledge. However, in Alexandria, on the outskirts of the empire, a school of Egyptian astronomers led by Ptolemy and Hipparchus flourished four centuries after Aristotle. Their calculation of the earth’s circumference (twenty-five thousand geographical miles) was surprisingly accurate. They also partitioned the globe into 360 degrees, lined its surface with parallels of latitude and meridians of longitude, and, with their invention of the astrolabe, which measured latitude by “shooting the sun,” provided an instrument which was to be used by mariners, including Magellan, until the Elizabethan Age.

But the Alexandrians, like the Greeks, erred. They concluded that the earth was both immovable and the center of the universe. Furthermore, Ptolemy’s Geógraphiké hyphégésis (Guide to Geography), which greatly influenced medieval geographers, inferred that Asia extended much farther east than it actually does. Here again, those who were misled included Columbus, whose belief that Asia could be reached by sailing westward was thereby strengthened. Any doubts in his mind were resolved by Imago mundi, a comprehensive world geography by Pierre d’Ailly, a fourteenth-century cardinal and master of the College of Navarre. D’Ailly took the Aristotelian view that Europeans could reach India by sailing westward. Imago mundi became Columbus’s favorite bedside book. His copy, with heavy marginal scribblings, is preserved in Seville’s Biblioteca Colombina.

During the long medieval night, Hellenic and Egyptian learning was preserved by Muslim scholars in the Middle East, where it was discovered by early Renaissance humanists. After poring over it, Pope Pius II, Cardinal Borgia’s early critic, wrote his influential Historia rerum ubique gestarum. Though largely a rehash of Ptolemy, Pius’s Historia was by no means uncritical; earlier works notwithstanding, he reached the startling conclusion that Africa could be circumnavigated. His premise that an equator existed, even though it was invisible, went unchallenged; by then the spherical shape of the planet, and the Greek partition of it into climatic zones, was accepted except by those who insisted upon literal interpretation of the Scriptures.

THAT IS, it was accepted by scholars. Average people still assumed that the earth was flat, and their knowledge of the world beyond the horizon was largely derived from mythical lore. The sources of these fables were protean. Some could be traced to Homer. Others derived from romantic yarns told by wanderers;