Zero degrees in Alexandria

Eratosthanes figured out how to mark places on a map using latitude and longitude.

I know what you’re thinking: “Eratosthanes marked Benghazi 9 degrees west on a map. Nine degrees west from where?”

That’s a terrific question and I’m glad you asked it. When you measure longitude, you have to start at zero degrees. Everything else on that 360° circle is either east or west of 0°. Eratosthanes was the head librarian of the Great Library in Alexandria, so he used his home town as that starting point. It’s the Alexandrian Prime Meridian. The Prime Meridian is an imaginary line that extends from the North Pole to the South Pole. The other meridians—lines of longitude—are east or west of it.