Why the equator is correctly located on Juan de la Cosa’s map of 1500

Detail of Juan de la Cosa's planisphere of 1500. The equator (thick red line) intersects South America (in green) at roughly the correct position.
Detail of Juan de la Cosa’s planisphere of 1500. The equator (thick red line) intersects South America (in green) at roughly the correct place.


The map made by Juan de la Cosa in 1500 is famous for being the oldest extant depiction of the Americas. One of its intriguing details is that it shows the equator at the correct location, close to the mouths of the Amazon river. Last month I sent an email to the ISHMap listserv explaining how, in my opinion, La Cosa was able to get latitude data for the South American coast and therefore draw the equator at the right place. I now publish the email here so that it can be accessed by a wider audience.


Dear all,

It was a pleasure to read Joaquim Gaspar’s article in the latest issue of Terrae Incognitae. I liked how he numerically quantified a phenomenon that had so far been described only qualitatively: the evolution of Caribbean latitudes in early 16th-century Spanish and Portuguese maps.
I had a comment though on one statement found in page 21: “As for the southern part of the region, no historical source mentions the determination of latitudes by astronomical methods along the northern coast of South America, although the position of the equator relative to the mainland is approximately correct, more so on the Cantino planisphere than on the Juan de la Cosa planisphere.” I think there exists at least one such source, and I communicated it to the author by private e-mail. We both then agreed that it would be good to share the discussion with the ISHM list in case somebody else finds it useful and/or wants to chime in with additional information.
The source I am referring to is the De orbe novo by chronicler Pietro Martyr d’Anghiera, better known in English as the Decades of the New World. The first Decade was officially published in Seville in 1511 (it had previously been pirated by several Italian printers), and seven more followed until Martyr’s death in 1526. They narrate the early Spanish expeditions to the Indies mostly based on reports from explorers.

Martyr, an Italian humanist, often discusses recent geographical discoveries under the light of classical cosmography. He pays particular attention to astronomical data reported from the newly found lands, like relative lengths of day and night or the height of the Polar Star, which help pinpoint latitudes. In the first Decade he mentions eight observations of the Pole Star by Spanish explorers, which are detailed in the table at the end of this email. Three of them are of particular importance to Gaspar’s article as they relate to the position of the equator and are prior to La Cosa’s planisphere of 1500.

In a few cases Martyr reports precise latitude values based on the north star: 36º for Seville, 32º for Madeira, 5º at a point of Columbus’s third voyage and again 5º in Paria. As the Spaniards got closer to the equator, however, the method of determining latitude by measuring the position of the Pole Star (and those near it) reached its practical limit. This happened in Peralonso Niño’s expedition of 1499 to South America, which Martyr says got so close to the equator that “the stars the mark the north pole disappeared” and so “it was not possible to calculate precisely the polar degrees” In another trip that took place almost simultaneously, led by Vicente Yáñez Pinzón, Martyr reports the two locations at which the Pole Star was observed to disappear and then appear again. This allows us to know that Pinzón’s expedition entered the southern hemisphere after having sailed around 300 leagues southwest from Santiago, one of the Cape Verde islands, and after exploring the northern coast of today’s Brazil, crossed back into the northern hemisphere at a place slightly to the north of a region they called Marina tombal or Mariatambal. They described that region as a place where “several swift rivers (…) came together (…) and flowed into the sea. A number of islands dotted this sea, which are described as remarkable for their fertility and numerous population”; most likely the mouths of the Amazon.

Both Niño and Pinzón sailed back to Spain in time for their information to be incorporated into the map that Juan de la Cosa drew in 1500. Actually Pinzón is the only explorer mentioned in it by name. The equator on this map crosses the South American coast slightly to the north of the mouth of two rivers, each of which mouths is dotted with islands (see image); very consistent therefore with the place at which Martyr reports Pinzón’s men again sighted the Pole Star. It would be interesting to discuss the other data point reported by Pinzón via Martyr for the equator, 300 leagues SW of Santiago, but this email would then become too long.
In conclusion, the first Decade of Martyr’s De orbe novo supports the idea that latitudes were indeed astronomically determined by early Spanish explorers of America. In particular, the position of the equator was ascertained based on the visibility of the Pole Star. It is also interesting to note that Martyr never mentions the other 16th-century astronomical method for latitude, based on the position of the Sun. This suggests that Spaniards did not master or trust that particular method, which however had the advantage of working well in the southern hemisphere. Martyr mentions one southern latitude in the first Decade (7ºS for the westernmost extremity of South America) but does not say how the value was arrived at.

In the second Decade, first published in 1516, Martyr affirms that later on the Spaniards made several determinations of latitude up until the foundation of Santa María la Antigua del Darién, in 1510. He gives only one numerical data: 8ºN for Veragua, a region that corresponds roughly to today’s Costa Rica, which lies between parallels 8ºN and 11ºN. My question for Joaquim is: Could this quite accurate value be one of the reasons why the anonymous 1519 planisphere shows lower absolute errors in latitude than earlier works?

Best regards,

Luis Robles
Decade, chapter Expedition, date Mentions of latitude

(English text taken from the 1912 edition by MacNutt; Latin text from the 1530 edition)

I, 2
Exploration of the Cannibal islands by Melchior, ca. 1494 “on this voyage the Spaniards never reached the equator, for they constantly beheld on the horizon the polar star, which served them as guide”

In Latin: “ipsi vero aequinoctialem nunquam tetigere, quandoquidem arcticum polum semper habuere ducem: & ab horizonte semper eleuatum.”

I, 6 Columbus’s third voyage, 1498 “at Seville, according to the mariners’ report, the north star rises to the 36th degree”
idem idem “at Madeira it [the north star] is in the 32d [degree]”
idem idem “the Admiral quickly left the [Cape Verde] archipelago behind, and sailed 480 miles towards the west-south-west. (…) The pole star was then at an elevation of five degrees”
idem idem “The Admiral declares that in the whole of that region the day constantly equals the night. The north star is elevated as in Paria to five degrees  above the horizon, and all the coasts of that newly discovered country are on the same parallel.”
I, 8 Peralonso Niño’s exploration of South America’s north coast, 1499/1500 “Each evening the stars which mark the north pole disappeared, so near is that region to the equator; but it was not possible to calculate precisely the polar degrees”
I, 9 Vicente Yáñez Pinzón’s exploration of South America’s north coast, 1499-1500 [from the island of Santiago] “they sailed before the south-west wind for about three hundred leagues, after which they lost sight of the north star”
idem idem “The natives call that entire region Mariatambal. (…) Continuing their march, directly north, but always following the windings of the coast, the Spaniards again sighted the polar star.”
II, 7 Vicente Yáñez Pinzón’s exploration of northern Brazil, 1499-1500 (kind of flash-back by Martyr)
“he reached the extreme point of the continent (…) this point in the New World lies within the seventh degree”
II, 10 not specified “The Spaniards made different calculations up to the time when they were established at Darien”

In Latin the meaning is clearer: “Habent ergo Castellani variā graduū eleuationem, donec ad Darienem statuta earum terrarū sedem primariā veniatur.”

idem idem “they abandoned Veragua, where the north star stood eight degrees above the horizon”

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