... 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!


Jeff wrote:
You mean Variation (sometimes called Declination) of course.

Oops, you're right. I did mean variation.

.... It is
sometimes called deviation, but not by most navigators

Right, only by forgetful ones who are in a hurry & type too
fast.


.... 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.


I think some of the northern European navigators had noticed
variation before that, and one thing Columbus was good at
was gathering material from other places. And easterners had
also noted variation, in fact one of the things Marco Polo
remarked on was that one could tell how far along the Silk
Road one was by the difference between magnetic & celestial
north.

Chinese compasses pointed south, I don't know if they
compiled tables of variation... no reason to think they
wouldn't, really.

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.


I don't think Columbus really was "the first" to do a lot of
the things he claimed (or is claimed for him). He was a good
researcher & synthesist, and not a bad sea captain.

..... 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.


Almost a sure thing... but that people crossed the north
Atlantic to fish was relatively common knowledge even if the
specifics weren't known to many; and it doesn't seem the
sort of thing that would sway a strong willed king & queen
who were not particularly interested in exploration & trade.

The rich trade of Asia, which had propped up Constantinople
for centuries, now *that* was worth gambling for. And there
must have been some reason to believe that Columbus would
bring back a cargo of valuable Oriental goods, silks or
spices, possibly on his first trip. Otherwise they would
have not changed their minds after telling him "No thanks."

Remember that the small cargo (8 tons of cloves IIRC) of
the 'Vittoria', Magellan's only ship to make it all the way
around, paid a handsome profit on Magellan's whole voyage &
fleet.



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


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

Human nature, I guess. Every day, right here, we see
examples of people trying to porve that other people are
stupider than themselves... maybe that's good in a way, if
it weren't so then actually being smart would not be so rare
& valuable!

DSK