Claudius Ptolemy Simulacrum

The Life

Claudius Ptolemaeus — Ptolemy in modern English — was a Greek scholar working in Alexandria in the second century CE. Almost nothing is known about his personal life; the dedications of his works suggest he was active in the reigns of the Roman emperors Hadrian and Antoninus Pius, roughly between 127 and 170 CE. He inherited the library and scholarly traditions of the Alexandrian Mouseion, and he left the two works that defined astronomy and geography for the following fourteen hundred years: the *Almagest* on the heavens, and the *Geographia* on the earth.

The Thought

The *Geographia*, the work that matters here, was a systematic manual of mathematical geography. Its first book explained how to project a spherical earth onto a flat surface — Ptolemy described and compared several projections, each with its own virtues and distortions — and how to locate places on the projection by latitude and longitude. The succeeding seven books were a gazetteer: approximately eight thousand places, each given coordinates drawn from the reports of travellers, merchants, soldiers, and earlier geographers. The eighth book contained the regional maps derived from the gazetteer.

The work was an act of mathematical reconstruction rather than direct survey. Ptolemy could not verify most of the coordinates he tabulated, and the errors in them — his overestimate of the width of Eurasia, his underestimate of the size of the earth — propagated through the maps based on his figures and, fifteen centuries later, encouraged Columbus to believe that Asia lay closer to Spain than it does. But the method was sound, the instinct to reduce the world to coordinates was the foundation of modern cartography, and the framework persisted because nothing better was available.

The Legacy

The *Geographia* was lost to Latin Europe in the early Middle Ages but preserved in the Greek and Arabic scholarly traditions. Its rediscovery in the fifteenth century — the Latin translation by Jacobus Angelus in 1406 was the pivotal event — triggered the European cartographic revolution of the early modern period. The printed atlases of the sixteenth and seventeenth centuries, culminating in Mercator's, began as revisions of Ptolemy and gradually diverged from him as new continental surveys and oceanic voyages corrected his coordinates. His projection methods are still in working use; his gazetteer method is the conceptual ancestor of modern geographic information systems.