Claudius Ptolemy, who lived from approximately 85 to 168 AD, was an ancient mapmaker whose works were rediscovered in Europe after being lost until the fifteenth century. He lived in Alexandria, Egypt, where he used Alexandria's famous library to compile existing knowledge of astronomy, geography, and astrology into three treatises. The astronomy and geography treatises had a long-lasting influence, but they both presented serious errors that went uncorrected for about 1,300 years. Ptolemy's astronomy treatise, Almagest, rejected the theory earlier proposed by Aristarchus (approximately 230 BC) that Earth revolves around the Sun. Ptolemy's geocentric idea-that Earth was the center of the universe -accepted the ideas of Aristotle and formed the main thesis of his treatise. When Ptolemy's works resurfaced in the fifteenth century, they were accepted as gems of ancient wisdom, and few had the nerve or the authority to challenge them. Likewise, any sixteenth century maps that altered the Ptolemy map were regarded with suspicion.

In his other influential treatise, Geographia, Ptolemy rejected the nearly correct computation of the distance around Earth-Earth's circumference-made by Eratosthenes in approximately 240 BC Rather, he chose an erroneous and much smaller distance (about 75 percent of the actual size). Ptolemy did not make any measurements himself, as Eratosthenes had done, but selectively compiled other information that was known at the time. The estimate he chose came from the Greek astronomer Poseidonius. Subsequently, however, his choices became known as Ptolemaic ideas and were considered irrefutable. Also, Ptolemy assumed that the known world's land surface covered 180 degrees of longitude ranging from the Canary Islands in the west to the easternmost part of Asia (about 20 degrees of longitude too much). This error on his map showed the Atlantic Ocean much too narrow and connecting western Europe and east Asia, without the American continents in between. This remained the understanding of the world for 1.300 years.

The combination of these two errors-Earth's circumference too small and land area too large-encouraged mariners of the fifteenth century to assume that a relatively short voyage across the Atlantic Ocean would take them to Asia. Columbus was the first to promote an expedition on the basis of these errors. When Columbus reached land, he had traveled as far as he expected to travel to reach Asia and logically assumed that he had succeeded.

To his credit, Ptolemy's map introduced some excellent standard to mapmaking, despite the errors. Though he was not the first to use he idea of a gridded coordinate system, his method of showing attitude and longitude became a standard for future maps. Also Ptolemy insisted that maps should be drawn to scale. Many maps of his time were distorted by enlarging the better known places in order to include all the known information. Unfortunately, many mapmakers of his time failed to adopt his practical approach to scale and location.

Maps began to proliferate in the sixteenth century. Each voyage of exploration and discovery provided new information that had to be mapped. In 1507, a German mapmaker, Martin Waldseemoler, produced a map of the world, Universalis Cosmographia, which was the first to show Columbus's discovery as a separate continent. But he cautiously made the new continent very narrow-just a long, skinny island-rather than contradict Ptolemy's erroneous circumference of Earth By the middle of the sixteenth century, enough voyages had been made, including Magelan's trip around the world, that most mapmakers recognized that Earth's circumference as shown on Ptolemy's map was wrong.

In 1569, the Flemish mapmaker, Gerardus Mercator, made a map of the world showing all the known lands using his now famous innovative grid system of latitude and longitude. In 1570, Abraham Ortelus, a Belgian mapmaker, made the first known atlas of the world in an effort to compile the rapidly accumulating geographic knowledge. Although these maps still had some remaining traits of the Ptolemy map, they showed great improvements in detail and accuracy. A major feature retained from the Ptolemy map was the presence of a very large continent in the Antarctic-large enough to counterbalance the weight of the Northern Hemisphere land. This belief was based on the Greek concept of symmetry, as well as the idea that Earth needed to be balanced to turn smoothly.