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