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Ptolemy of Alexandria / TAHL-uh-mee /

or Claudius Ptolemaeus (Latin)

c. 100 – c. 178
Greek Astronomer, Mathematician and Geographer
Exploration Ranking 9th of 26
Ptolemy with globe - Sierra Leone Book.j
Ptolemy (with globe, back to viewer), detail from Raphael's painting, School of Athens. Sierra Leone stamp from 1983.

Ptolemy’s book, The Guide to Geography, is the most comprehensive and scientific work of antiquity (the time up to about 476 C.E.) on geography. It represented the beginning of modern mapmaking and is arguably the most influential book in the history of mapmaking.(1) The translation of The Guide to Geography into Arabic in the ninth century, and then into Latin in 1406, ensured that Ptolemy’s influence lasted for a thousand years. His ideas about constructing maps and map projections were not superseded until the sixteenth century.(2) Ptolemaic maps acted as a matrix for the development of Renaissance world maps.(3) Ptolemy’s geographical writings, largely disregarded in the Christian Middle Ages, became the basis for Renaissance geography and were crucial to the cartography of the Great Discoveries.(4) Together with the invention of the printing press in 1440, Ptolemy's Guide sparked a revival in scientific cartography in western Europe, accelerated by voyages by the Spanish and Portuguese to Africa, the Americas, and the Spice Islands in Indonesia.

Ptolemy refined the grid system for maps by including degrees, minutes and seconds; brought the terms “latitude” and “longitude” into popular usage; and established the custom of orienting maps with north at the top. He calculated the circumference of the Earth at 18,000 miles (actual is 24,901) and portrayed Asia as bigger than it really is, which people (most significantly explorers) believed, until disproven in the 1500s.

Ptolemy’s depiction of the globe contained significant gaps and errors, which inadvertently encouraged exploration, because he made the world seem smaller and more navigable than it really was. If he had correctly estimated the size of the world, the Age of Discovery might never have occurred. This smaller sized earth, combined with an elongated Eurasian landmass and longer Mediterranean Sea, helped to convince Columbus that if he sailed westward he could reach the Far East. Columbus estimated the voyage was 2,400 miles to reach Asia when the actual distance is about 10,600 miles. On his first voyage he arrived in the West Indies in the time he had judged it would take him to reach Japan. Had there been no “New World” dividing the Ocean Sea and blocking his way to the Far East, however, Columbus’s voyage would have ended in disaster on the open sea.

The Guide to Geography was a complete mapmaking kit, as conceived by the ancient world. Ptolemy’s treatise included a tabulation of more than 8,000 places in Europe, Asia and Africa with their latitude and longitude; an explanation of the role of astronomy in geography; a detailed mathematical guide for making maps of the Earth and its regions; and for the Western geographical tradition established an enduring definition of geography. No text before or since Ptolemy’s provided such a comprehensive account of the Earth and how to describe it.(5)

Ptolemy’s innovation in mapmaking established a repeatable methodology for mapping the known world according to recognized mathematical principles. He was the first person to consciously formulate a method to “project” Earth, a three-dimensional globe on a flat two-dimensional surface employing a grid of geometrical lines of latitude and longitude. However, Ptolemy accepted the fact that Earth cannot be comprehensively mapped onto a flat surface.

His map projections allowed anyone with a basic understanding of Euclidean geometry to create a map of the world. His innovation of tables of latitude and longitude, drawn from the Alexandrian maps, established the coordinates of locations throughout the Greek inhabited world. These tables enabled mapmakers to plot the positions of every known location upon a map with utter simplicity, and by refusing to place explicit boundaries upon his Greek world, Ptolemy encouraged future mapmakers to plot ever more locations upon the surface of their world maps. Ptolemy’s Geography “is arguably the most influential in the history of mapmaking. . .”(1)

The Guide came to be for geography what his Almagest was for astronomy, and until well into the Renaissance, Ptolemy was hardly less celebrated as a geographer than as an astronomer. Ptolemy created a map of the sky as a land-based explorer through his diligent and accurate observations of the positions of the stars and planets. Ptolemy’s Almagest or Mathematiki Syntaxis, a work in thirteen books, contained a complete mathematical description of the Greek model of the universe, with parameters for the various motions of the sun, moon and planets. The book was the culmination of Greek astronomy.(6) His book identified 1,022 stars. Forty-seven of the eighty-eight star constellations we know today were names chosen by Ptolemy, including Orion, Capricorn, Andromeda, Cancer (crab), Gemini (twins), Ursa Major (great bear) and Leo (lion).




(1) and (5) Jerry Brotton, A History of the World in 12 Maps (New York, 2012), p. 21.

(2) Andrew Humphreys, lead contributor, R. G. Grant, and Simon Adams, Journey: An Illustrated History of Travel (New York, 2017), p. 53.

(3) Kenneth Nebenzahl, Atlas of Columbus and The Great Discoveries (New York, 1990), p. 4.

(4) Ibid., pgs. 4, 15.

(5) Brotton, p. 20.

(6) Ronald Brashear and Daniel Lewis, Star Struck: One Thousand Years of the Art and Science of Astronomy (Seattle, 2001), p. 24.


Key References


1. There is no biography of Ptolemy. Ptolemy’s geographical work is discussed by J. O. Thomson in History of Ancient Geography, 1948, and Jerry Brotton in A History of the World in 12 Maps, 2012.

2. A full English text of Almagest is the translation by R. Catesby Taliaferro published in Great Books of the Western World, v. 16, 1952.

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