Daily Archives: May 2, 2020

Good news for canaries

Ptolemy published a book titled Geography which gave the latitude and longitude for over 8,000 places.

Ptolemy’s map* shows all of Earth he knew back then: Europe, the north coast of Africa, India and most of Asia. The Prime Meridian of Ptolemy’s map runs through the Canary Islands just west of Africa. Everything else is east.

As mapmaking took off as a business in the Age of Exploration, mapmakers in every little country, state and principality used a different Prime Meridian. You can see how this might get confusing.



* Maybe Ptolemy drew this map, maybe he didn’t. The locations on the map, though, are based on their coordinates given in Ptolemy’s book.

Back to the beginning of The Western Civ User’s Guide to Time & Space

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.