Origin of Ptolemy's Ideas
Travelers Inform Ptolemy
Ptolemy writes in the Geographia: "But now as we propose to describe our habitable earth, and in order that the description may correspond as far as possible with the earth itself, we consider it fitting at the outset to put forth that which is the first essential, namely, a reference to the history of travel, and to the great store of knowledge obtained from the reports of those who have diligently explored certain regions. . . ." (Ptolemy 1932, 26).
Although Ptolemy describes instruments for making astronomical measurements, few travelers could supply the astronomical observations (height of the pole, observations of eclipses of the moon) that were necessary for locating places precisely. Nevertheless, Ptolemy collected reports of travelers, since so much of the earth was still unknown. He writes: "We consider it necessary therefore for us to pay more attention to the newer records of our own time, weighing, however, in our description of these new records and those of former times and deciding what is credible and what is incredible" (Ptolemy 1932, 29). Travelers provided much of the raw material for Ptolemy’s Geographia.
The reports of travelers to the East like Marco Polo (13th-14th centuries) gave birth to the idea that Asia extended further east from Europe than Ptolemy had thought. According to Polo and other travelers, many islands were located in the seas east of Asia.