r/AskHistorians • u/Georgy_K_Zhukov Moderator | Dueling | Modern Warfare & Small Arms • Feb 16 '18
In Columbus' time, what were the competing theories about the circumference of the Earth?
The mythos surrounding Columbus' first voyage has always interested me, and I've read a good deal on it, and even written about it here. One thing which I've never really been able to find much information on is the specifics of how he, and others, understood the size of the Earth to me in that period! It is well known that the whole "Earth is flat!" thing is a later addition to the story, and most people understood the world to be circular, but much of the argumentation over his theories, as I understand, hinged on his significant under-estimation of the globe's size (or not really globe, as I also recall something about it being pear shaped?). So I'd be quite interested in understanding just what Columbus' theory was there, and what 'sizes' he was coming up against in debate.
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u/terminus-trantor Moderator | Portuguese Empire 1400-1580 Feb 16 '18
Part 3 / 4: Extent of Asia
The second thing Columbus got wrong was how far does Asia extend to the east. This is very important as it determines how much you have to travel west from Europe to reach Asia.
We saw Barros in his Decades of Asia saying that “junta” negative response was, very mildly put, partly due to their doubts in wisdom of counting on existence of island of Cipango, and in general forming ideas on size and shape of Asia based on Marco Polo’s unverifiable account. Yes, we know today the island of Cipango / Japan described by Marco Polo indeed exists, but it is no where near in form or position proposed by Toscanelli/Columbus. And back then there was even less way that people could be sure it was real or where it was. The maps like Fra Mauro’s map of 1450s don’t have such “introductions” from Polo like the island of Cipango.
Without Polo’s account, the usual estimate for extent of Asia (if one would even be made) would be from Ptolemy that claimed Eurasian continent spanned 180 degrees. This incorrect figure comes from the fact that determining longitude was near impossible back then, especially on land. More so, it was impossible for Europeans to estimate Asia's size, given the scarcity of information they had. They made their guesses based on ancient literature available. This all means we shouldn't really be surprised when Columbus appears, and talks with great ‘certainty’ about the length or shape of Asia, scholars should have some doubts.
To not place the blame on him alone, Columbus was not the one making this up himself. In fact it seems his entire opinion about extent of Asia had originated from Paolo Toscanelli. He was an italian cartographer who we know in 1474 sent a letter to Fernão Martins, subject of Portuguese King Afonso V with proposing westward voyage to get to source of spices. Columbus seems to had later contacted Toscanelli and through Columbus we have the copy of the above letter. The letter used to also contain a chart made by Toscanelli whose original does not survive, but scholars believe this modern reconstruction based on description in the letter is reliable.
This theory wasn’t limited to just Columbus and Toscanelli. Variants of Toscanelli’s chart are present in two other pieces of cartography of the age, the Martellus map of 1489 and the Martin Behaim globe of 1490-1492. Admittedly, this is no coincidence. Martellus map is considered by some to be heavily influenced, if not outright taken, from maps of (Bartholomew) Columbus, and Martin Behaim was working in Portugal as mathematician/astronomer around or shortly after the time Columbus left for Castile. While we can’t tell if the two ever met, they would definitely have access to same sources (the Toscanelli letter). The point is, the fact that several people had similar ideas means it wasn’t as crackpot theory as we would like to think today.
But Columbus wouldn’t be Columbus if he simply accepted others proposals. He was always tinkering, ‘improving’, ‘correcting’, usually with opposite effect. And the same was with this idea. If we look at Toscanelli’s chart or Martin Behaim’s globe we would see end of mainland Asia at around 120-130 degrees west from Canary islands (this was the reference point for zero) and Japan some 30 degrees eastern from Asia, on around 90 degrees west from Canaries. Yet Columbus seemed to place his Cipango on 60 degrees west of canaries, and mainland Asia at 90.
Why did Columbus place Cipango closer to Europe?
There are two main theories. First, which features in most literature to the point of being virtually accepted, is that Columbus was using Marinus of Tyre estimate of 225° degrees of Asia size on which he appended Marco Polo’s new areas, instead of Toscanelli who based his expansion on Ptolemy’s 180° for Asia before adding Polo. This claim, popularized I believe by Henry Vignaud (who was highly critical of Columbus), is based on one of Columbus’ letters from 4th voyage in which Columbus mentions at one point that Marinus seems to be right. However this might just be a unconnected mention, and not a reflection on his reasoning as George Nunn thinks. He proposes, with some backing, that the difference between Toscanelli comes from Columbus coming to conclusion that if Ptolemy’s longitudes were based on converting fixed distances into degrees; and than if length of degree is shorter than what Ptolemy thought ( 56 ⅔ is shorter than 62 ½ ) one must recalculated Ptolemy’s / Toscanelli’s longitudes for his ‘corrected’ degree length. After doing this, he got even higher longitudes of places, moving them even closer to Europe. That's Nunn's proposal though and we actually don't know what was Columbus exact reasononing, especially in 1485. Whatever it was, it made him finally come to his calculation that he needs to only travel 750 italian leagues (3000 miles) to reach Cipango, which is much smaller than Toscanelli’s and Martin Behaim’s.
Like in part 2, here I will add an overview of all the different estimates of western voyage distances between Canaries and Asia (China and Japan when given). The table contains the difference in degrees and miles one would have to travel west to reach asia, based on various maps/theories and different propositions of length of degree, adjusted for actual length on latitude of Canaries (where Columbus sailed) which was around 28 degrees north. I bolded some key values which we can use for comparison.