Ptolemy's Geography

About 1,850 years ago, a scholar in Egypt sat down to solve a giant puzzle: how do you draw the entire known world on a flat piece of paper without getting the shapes all wrong? His name was Claudius Ptolemy, and the book he wrote, called Geographia, listed the coordinates of over 8,000 places — from Britain to India — using a grid system that is the ancestor of the latitude and longitude lines you see on maps today.

In this lesson you will: - Learn who Ptolemy was and when he lived - Understand what a coordinate grid is and why it matters - See how Ptolemy's maps influenced explorers more than 1,300 years later - Find out where Ptolemy's map made mistakes

Claudius Ptolemy lived from about 100 to 170 CE in Alexandria, Egypt, home to the famous Great Library. He was an astronomer, mathematician, and geographer who wrote Geographia around 150 CE — essentially an eight-book instruction manual for drawing an accurate map of the entire known world.

Ptolemy's big idea was a coordinate grid. He divided the world using degrees of latitude (distance north or south of the equator) and degrees of longitude (distance east or west of a starting line, which he placed through the Canary Islands). Then he listed coordinates for roughly 8,000 cities, rivers, and mountains, so that any mapmaker, anywhere, could use his numbers to redraw a consistent map of the world.

In the 1400s, Geographia was translated into Latin and copies spread across Europe as printed maps. But Ptolemy had made a major error: he underestimated Earth's circumference by about 30% and overestimated how far Asia stretched eastward. In 1492, Christopher Columbus relied on calculations shaped by this error, which is one reason he believed he had reached Asia when he had actually reached the Americas.