DSK wrote:
....


Nobody was able to reliably determine longitude with any available
technology in 1492. One thing Colombus did have, which relatively few
navigators of his era knew about, was a table of compass deviation. It
was thought by many (and Columbus may have believed this too) that the
magnetic deviation from true north varied directly with longitude. This
actually is true for large section of earth & sea but you have to know
where the deviation begins to swing back again!


You mean Variation (sometimes called Declination) of course. It is
sometimes called deviation, but not by most navigators, who use the
term for local affects, mainly the ship itself. Variation in the
Mediterranean is fairly small, so Columbus was one of the first
Europeans to notice a large Variation. In had been well understood in
China for a very long time.

Its also been said the Columbus was one of the first to notice the
difference between true north and the postion of Polaris, which was
about 2.5 degrees back then. I'm skeptical about that, since the
Portuguese had been very interested in documenting Latitude for 50 years.

There are some accounts which credit Columus with having a chart or map
or pilot book from China, this would explain why he kept thinking he
knew where he was in the Caribbean and also how he got financing from
the monarchy of Spain. Ferdinand and Isabella were more interested in
uniting Spain & driving out the Moors than in exploration. They had
already turned down other similar explorers and in fact turned Columbus
down, but then reconsidered. It's likely that when they turned him down,
he revealed just enough of his secret book or chart to convince them to
change their minds.

I'd say its more likely he knew about the fishing grounds to the north.



Personally, I lean towards the theory that there were numerous
European voyages to America long before Columbus.

Very possible. There are extant records of Basque whaling voyages which
made temporary bases at land that sounds a lot like Cape Cod &
Nantucket. And of course there were some records of the Viking voyages,
long disbelieved.

For some reason, everybody wants to think that previous generations had
no clue.

History is what the writers of history want it to be.