How compelling is evidence that Portuguese explorers found America years or decades in advance of Columbus?

by VardarOS

I recently encountered this claim in an article from the Belfast Center:

Considering how little knowledge anyone had of the geography of the Americas at the time, King John’s adamant stance against the pope’s line of demarcation can be hard to understand. By 1494, neither power knew definitively that the Americas existed at all, let alone that a part of modern-day Brazil lay to the east of the 46th meridian. However, Christopher Bell hypothesizes that Portuguese explorers in the Atlantic prior to 1494 may have actually sighted land when accidentally blown off course on an African voyage, and returned to tell the king about it. Therefore, when King John disputed the 1493 papal bulls, it is possible that “he already knew that there was land across the Atlantic in the southern hemisphere; that, whether it was islands or terra firma, it was suitable for colonization and that it was to be found in the neighbourhood of 36' W.”

I also encountered this answer which asserts that the Portuguese may have landed in North America prior to Columbus, potentially as early as the 1470s.

Could anyone shed light on the state of current scholarship on the matter? It seems that, if the Portuguese made such a discovery, there would be some surviving documentary evidence.

terminus-trantor

I've written about it in this question

in short, there is no concrete evidence whatsoever to support this thesis. The few weak and vagues claims of concrete travels have been debunked by now, although that doesn't stop the repetition of the claims. The entire theory basically hinges on modern speculation of why Joao II moved the demarcation line.

Some sources that deal with this:

  • Portuguese voyages to America in the fifteenth century by Samuel Eliot Morison (available to be borrowed on archive.org) which is solely dedicated to those myths and claims of discovery.

  • Foundations of the Portuguese Empire 1415-1580 by Diffie and Winius has a short appendix that draws on work of Morison and others and gives brief descriptions of each claimed discovery and evidence for and against.