Designing the World

Claudius Ptolemy (ca. 100-ca. 178) lived in Alexandria of Egypt in the second century CE, in the age of the Roman Emperor Hadrian (76-138). He was an accomplished astronomer and geographer. One of his major works was specifically devoted to geography. The book consisted largely of a list of localities with their geographic coordinates and a brief description of their topographic characteristics. The list was certainly accompanied by maps, but these have not survived. Ptolemy foresaw that his maps would inevitably be replicated with imperfections by copyists. He therefore decided to provide the method for reproducing them accurately and to describe his geometric system for representing the Earth’s sphere on a flat surface.

In the first book of the Geography, Ptolemy proposed two geometric methods for accurately drawing the general map of inhabited lands; in Book 8, he supplied instructions for a perspective representation of the globe surrounded by an armillary sphere. Ptolemy claimed that the oikumene—the Greek term for the inhabited world—extended 180 degrees in longitude from the zero meridian passing through the Fortunate Isles, the modern-day Canaries. In latitude, the oikumene ran from the Island of Thule—probably the Shetlands—at 63 degrees north, all the way to the parallel called anti-Meroë, 16°25’ south of the Equator.

Ptolemy’s geographic coordinates, largely derived from Hipparchus (ca. 190 BCE.-ca. 127 BCE), were, however, relatively inaccurate. This was especially true for the remotest regions, whose representation was largely based on travel narratives. While latitude was easy to measure, longitudinal coordinates required extreme accuracy. The scientific method for calculating them relied on contemporaneous observations of lunar eclipses in different places, but this required special arrangements that appear to have been seldom applied in practice. In any event, the scope for observation errors led to inaccurate geographic results. Ptolemy had reduced the longitudinal extension of the oikumene from the 225 degrees defined by Eratosthenes (ca.276-ca.195 a.C.) to 180 degrees, but even this was an overestimate relative to the actual 115 degrees that separate the Canary Islands from Indochina—in other words, the western and eastern bounds of the ancient world.