The sphericity of the Earth was established by Greek astronomy in the 3rd century BC, and the earliest terrestrial globe appeared from that period. The earliest known example is the one constructed by Crates of Mallus in Cilicia (Çukurova in modern-day Turkey), in the mid-2nd century BC.
According to Strabo, Crates devised a globe representing the Earth, which is thus the earliest known globe representing the Earth. Following the theory of five climatic zones, Crates considered that the torrid zone is occupied by the Ocean and that, by analogy, one can imagine people living beyond the torrid zone:
For Crates, following the mere form of mathematical demonstration, says that the torrid zone is "occupied" by Oceanus and that on both sides of this zone are the temperate zones, the one being on our side, while the other is on the other side of it. Now, just as these Ethiopians on our side of Oceanus, who face the south throughout the whole length of the inhabited land, are called the most remote of the one group of peoples, since they dwell on the shores of Oceanus, so too, Crates thinks, we must conceive that on the other side of Oceanus also there are certain Ethiopians, the most remote of the other group of peoples in the temperate zone, since they dwell on the shores of this same Oceanus; and that they are in two groups and are "sundered in twain" by Oceanus.
Early terrestrial globes depicting the entirety of the Old World were constructed in the Islamic world. One such example was the terrestrial globe introduced to Beijing by the Persian astronomer, Jamal ad-Din, in 1267.
The oldest surviving terrestrial globe was made in 1492 by Martin Behaim (1459–1537) with help from the painter Georg Glockendon. Behaim was a German mapmaker, navigator, and merchant. Working in Nuremberg, Germany, he called his globe the "Nürnberg Terrestrial Globe." It is now known as the Erdapfel, in English translation the Earth Apple.
In 1480, trade between Portugal and Flanders attracted Behaim to Lisbon, and he became involved with the mercantile interests and overseas exploration that was occurring in the capital (from Flanders, Germany and the Hanseatic League). He was attracted to the knowledge of the navigators, cosmographers and explorers, acquiring a scientific reputation, and supposedly meeting with Christopher Columbus and Ferdinand Magellan then at the court of King John II of Portugal.
Behaim may or may not have accompanied Diogo Cão during his second expedition (1485–86) along the coast of western Africa, reaching Cabo Negro and Cabo Ledo. Nevertheless it is known that the explorer revisited the Congo and erected two more padrão on land beyond his previous voyage. The first was at Cabo Negro, Angola, the second at Cape Cross. The Cape Cross pillar probably marked the end of his progress southward, some 1,400 kilometers. Cão ascended the Kongo River, which he thought led towards the realm of Prester John, up to the neighborhood of the site of Matadi.
There, in October or November 1485, near the falls of Ielala, he left an inscription engraved on the stone which testifies of its passage and that of his men:
Aqui chegaram os navios do esclarecido rei D.João II de Portugal - Diogo Cão, Pero Anes, Pero da Costa. ("Here reached the ships of the enlightned king John II of Portugal – Diogo Cão, Pero Anes, Pero da Costa").Diogo Cão returned to Portugal by way of the Azores.
If indeed Behaim was present on this second expedition it is still unclear whether Behaim sailed as far as reported, or whether he only reached the coast of Guinea, perhaps as far as the Bight of Benin.
On his return to Lisbon from exploration in western Africa, he was knighted by King John, who afterwards employed him in various capacities. Following his marriage in 1486, he was based on the Portuguese island of Faial in the Azores, where his father-in-law, Josse van Huerter, was Captain-donatário and leader of the Flemish community.
In 1490, he returned to Nuremberg to handle the family business and design his famous globe in collaboration with the painter Georg Glockendon. Martin Behaim constructed his terrestrial globe between 1491 and 1493 which he called the Erdapfel (literally, the earth apple). It conforms to an idea of a globe envisioned in 1475 by Pope Sixtus IV, but has the added improvements of meridians and an equatorial line.
The influence of Ptolemy, who bequeathed the world latitude and longitude, is apparent, but every attempt is made to incorporate the discoveries of the later Middle Ages, including information from the voyages of Marco Polo, among others.
The Earth Apple incorporates numerous errors that do not reflect contemporary geographical understanding. Western Africa is incorrect, though technology at the time made such calculations difficult; the Cape Verde archipelago lies hundreds of miles out of its proper place.
The Atlantic is filled with mythological islands that were psychologically important to isolated Medieval Christendom. Japan is located only 1,500 miles (2,400 km) off the coast, and was just where Marco Polo mentioned it, placing it temptingly within sailing distance of the Canaries.
The Isle of St. Brendan is also located on this globe, but it contains the entire Western Hemisphere in capsule form. There are 16° errors in the location of many of the places, whereas modern maps seldom have more than 1°, because longitude was very difficult to ascertain before the invention of accurate clocks.
The antiquity of this globe and the year of its execution, on the eve of the discovery of the Americas, makes it not just the oldest, but a most historically valuable globe. It corresponds particularly well with Columbus's notion of the Earth, and makes the notion of a jump across that little Ocean Sea to the Far East irresistible; he and Behaim drew their information from the same sources. Though less navigationally accurate than the beautiful Catalan portolani charts of the 14th century, as a scientific work it is of enormous importance; it may be the first terrestrial globe ever built, it is tilted to spin at the correct angle, and represents an encyclopedia of the West's known world in the year 1492.
In Columbus's time, the techniques of celestial navigation, which use the position of the sun and the stars in the sky, together with the understanding that the Earth is a sphere, had long been in use by astronomers and were beginning to be implemented by mariners.
As far back as the 3rd century BC, Eratosthenes had correctly computed the circumference of the Earth by using simple geometry and studying the shadows cast by objects at two remote locations. In the 1st century BC, Posidonius confirmed Eratosthenes's results by comparing stellar observations at two separate locations. These measurements were widely known among scholars, but confusion about the old-fashioned units of distance in which they were expressed led to some debate about the size of the Earth.
For Columbus the size of the Earth mattered because sailing across the Atlantic to China would present an alternative to the land routes to China where increasing difficulties for traders had followed the fall of Constantinople to the Ottoman Turks in 1453.
In 1470, the Florentine astronomer Paolo dal Pozzo Toscanelli suggested to King Afonso V of Portugal that sailing west would be a quicker way to reach the Spice Islands, Cathay, and Cipangu than the route around Africa. Afonso rejected his proposal. Portuguese explorers, under the leadership of King John II, then developed the Cape Route to Asia around Africa.
Meanwhile, in the 1480s, the Columbus brothers had picked up Toscanelli's suggestion and proposed a plan to reach the Indies by sailing west across the "Ocean Sea", i.e., the Atlantic.
From d'Ailly's Imago Mundi Columbus learned of Alfraganus's estimate that a degree of latitude (or a degree of longitude along the equator) spanned 562/3 miles, but did not realize that this was expressed in the Arabic mile rather than the shorter Roman mile with which he was familiar (1,480 m). He therefore estimated the circumference of the Earth to be about 30,200 km, whereas the correct value is 40,000 km (25,000 mi).
Furthermore, most scholars accepted Ptolemy's estimate that Eurasia spanned 180° longitude, rather than the actual 130° (to the Chinese mainland) or 150° (to Japan at the latitude of Spain). Columbus, for his part, believed the even higher estimate of Marinus of Tyre, which put the longitudinal span of the Eurasian landmass at 225°, leaving only 135° of water. He also believed that Japan (which he called "Cipangu", following Marco Polo) was much larger, farther to the east from China ("Cathay"), and closer to the equator than it is, and that there were inhabited islands even farther to the east than Japan, including the mythical Antillia, which he thought might lie not much farther to the west than the Azores. In this, he was influenced by the ideas of Florentine astronomer, Paolo dal Pozzo Toscanelli, who corresponded with Columbus in 1474 and who also defended the feasibility of a westward route to Asia.
Toscanelli's notions of the geography of the Atlantic Ocean (shown superimposed on a modern map), which directly influenced Columbus's plans.
Columbus therefore estimated the distance from the Canary Islands to Japan to be about 3,000 Italian miles (3,700 km, or 2,300 statute miles). The true figure is now known to be vastly larger: about 20,000 km.[35][c] No ship in the 15th century could have carried enough food and fresh water for such a long voyage, and the dangers involved in navigating through the uncharted ocean would have been formidable. Most European navigators reasonably concluded that a westward voyage from Europe to Asia was unfeasible. The Catholic Monarchs, however, having completed an expensive war in the Iberian Peninsula, were eager to obtain a competitive edge over other European countries in the quest for trade with the Indies. Columbus's project, though far-fetched, held the promise of such an advantage.
