How the world map changed

In 2003 the Library of Congress paid $10 million for one map: the only known copy of a 1507 world map, the first to use the name America.

Maps before the voyages

A well-read European of 1450 pictured a world of three parts: Europe, Asia and Africa. In the early 1400s the Geography of Ptolemy, a Greek scholar of 2nd-century Alexandria, was translated into Latin, and mapmakers began to copy its method of placing every town by latitude and longitude on a grid. Ptolemy’s world had no Americas and no Pacific, and it made Asia stretch much farther east than it really does. That error helped convince Columbus that Asia was only a few weeks’ sail west of Spain. Sea charts used by Mediterranean sailors, called portolan charts, were far more accurate for coasts, but they covered only the waters that sailors already knew.

Dividing the world on paper

Columbus’s return in 1493 set off a quarrel between Spain and Portugal over who could claim the new lands. On 7 June 1494 the two kingdoms signed the Treaty of Tordesillas, which drew a line from pole to pole 370 leagues west of the Cape Verde islands. Lands found west of the line would go to Spain and lands east of it to Portugal. Nobody could yet measure longitude at sea, so neither side could say exactly where the line ran. When Pedro Álvares Cabral reached the coast of Brazil in 1500, it lay east of the line, which is why Brazilians speak Portuguese today. The people who already lived in these lands were not asked.

A new continent with a new name

Amerigo Vespucci, a Florentine merchant, sailed along the coast of South America in 1501 and 1502 and wrote that these lands were a new world, not part of Asia. In the small town of Saint-Dié, near Strasbourg, a group of scholars led by Martin Waldseemüller was updating Ptolemy with the latest discoveries. Their large world map of 1507, printed from twelve woodblocks, showed the new lands as a separate continent with an ocean on its far side, the first map to show a separate Western Hemisphere with the Pacific as its own ocean. Across what is now Brazil they printed a new name, America, in honour of Vespucci. About 1,000 copies are thought to have been printed. Only one survives.

Mercator’s map for sailors

Magellan’s voyage showed how vast the Pacific was, and through the 16th century mapmakers kept redrawing the coasts. In 1569 the Flemish mapmaker Gerardus Mercator published a world map with a title that explained its purpose: a new and more complete description of the earth, adapted for the use of navigators. On his projection a straight line between two points is a course of constant compass bearing, so a navigator could lay a ruler on the chart and read off one heading to steer. To make that work, Mercator stretched the map north and south by the same amount that the meridians are spread apart east and west, and that stretch grows toward the poles. At 60 degrees north or south every length is doubled, so every area appears four times larger than it is. Greenland looks about the size of Africa on a Mercator map, but Africa is about 14 times larger.