ABSTRACT
As early as the fourth century bc it was realized that it would be possible in theory to sail west from the Pillars of Herakles to India. Ptolemy had determined the width of the known world at 180 degrees, so it was obvious that the remainder was the same distance. Even after he started his voyages, Ptolemy continued to be a factor in Columbus’s thoughts. In 1494, when he reached what is now the Bahía de Cortes in western Cuba, where the coast turns sharply to the south, he believed that he was on the eastern side of Ptolemy’s Golden Chersonesos. The discovery of the source of the Nile and, assumedly, the Selene Mountains and the lakes on Ptolemy’s map, was a classic example of Victorian exploration and perhaps one of the last major uses of the data supplied by the Geographical Guide about remote places on the earth.
