How did Ptolemy turn the world into a list of coordinates?
A 2nd-century book of numbers let a Byzantine monk redraw maps for which he had no model, and Columbus used its geography too.
▶ Start the storyPtolemy turned the world into a list. In his Geography, written in Greek in Alexandria around 150 AD, he revised an older atlas by Marinus of Tyre. From Book II to the start of Book VII, a gazetteer gives a longitude and a latitude for each place in the ecumene, the world known to the Romans, some 8000 names in all.
Latitude was counted in degrees from the equator, the same system used now. Longitude needed a zero line, and Ptolemy's prime meridian ran through the Fortunate Isles, near El Hierro in the Canary Islands. Book VII then gives three projections for drawing the world map from those numbers. One chapter title in Book I states that astronomical observations should take priority over data from records of travel.
Step 1: Pick a place
A latitude from the equator
Step 2: Add a longitude
Counted from the Fortunate Isles
Step 3: Choose a projection
Book VII gives three
Step 4: Plot every point
Some 8000 names make the map
The clearest sign that the method worked comes from a monk. When the Byzantine monk Maximus Planudes searched for a copy of the Geography in the summer of 1295, he and his assistants probably had no pictorial models. Their success in recreating the maps suggests that Ptolemy had managed to encode the map in words and numbers.
The numbers were not perfect. A sample of 80 well-known cities shows Ptolemy overestimated longitude by a factor of 1.428, which stretched his map, for instance the profile of Italy. He also rejected Eratosthenes's Earth circumference of 252,000 stadia and shrank it to 180,000. The first printed edition with maps appeared in Bologna in 1477. Columbus, who modified this geography further, reasoned that eastern Asia lay about 90 degrees west of the Canary Islands.
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Recap
Ptolemy encoded the world as coordinates, so it could be redrawn from numbers, including their errors.
💡 A trick to remember it · Numbers outlive pictures: give a mapmaker the coordinates and the world comes back.
Surprising fact · Planudes and his assistants probably had no pictorial models when they recreated Ptolemy's maps.
Sources (1)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.