As far as appearance, if that were a significance, I would choose Nick Nolte: http://en.wikipedia.org/wiki/File:Nikolaus_Kopernikus.jpg He certainly has an intensity, but I'm not sure that kind of intensity would work with a possible movie (Whether it's in the works or not, I don't know). I would choose Richard Dreyfuss or Robin Williams. They would/add humor to the movie, and they both are good at personally driven roles. They're both good at playing the role of someone with a purpose.
Just about any good actor would be a good pick. It's not like Copernicus brings an associated image to mind like if you ask someone who should play Superman.
Nick Nolte has the enormous chin, I guess. Robin Williams - god, no. But what's exciting about Copernicus? De revolutionibus orbium coelestium wasn't even published until about the time of his death, so the Inquisition couldn't get their hands on him. In fact, the Catholic church showed very little interest in undermining the heliocentric theory until the time of Galileo, 60 years or so later.
This is why the role would be so hard to cast. Actors don't believe in heliocentrism. Most leading men view themselves as the center of the universe.
Unfortunately a movie like this could turn out to be along the lines of "Inherit The Wind". The Scopes Monkey Trial apparently wasn't viewed as interesting enough, so a very loosely related movie took it's place.