Copernicus's/Copernicus'

[BLenn]

1. Almost a hundred years later, an Italian, Galileo Galilei, used one of the first telescopes to watch the planets.
His studies gave the first proof of Copernicus's ideas. The Church was very angry. It would not allow anyone to print Copernicus' book and forced Galileo to say his ideas were wrong.
But that did not stop the spread of his ideas. Soon, most of the world believed that the Sun was at the center of the universe.

This question is part of the 'TOFEL Reading Practice' passage and I was wondering if you could explain the difference between ' and 's when they are used?

2. The test was easy enough to finish within an hour.

If this underlined sentence is incorrect when written "to finish", why is it awkward and incorrect when written "to finish"?
Is it correct to write the underlined part as "to be finished"?

 

[Rover_KE]

... I was wondering if you could explain the difference between ' and 's . (It's not a question.)

Welcome to the forum, @BLenn.

There's no difference, though it’s better to be consistent in your own writing. The possessive Copernicus's is mainly used in British English and Copernicus' in American English, though there's considerable overlap. They're pronounced the same.

Please note that your thread title has been changed. Titles must include some or all of the words or phrases you're asking about.

2. The test was easy enough to finish within an hour.

If this underlined sentence is incorrect when written "to finish", why is it awkward and incorrect when written "to finish"?
?

Please start a new thread for this unrelated question, with the title The test was easy enough to finish within an hour.

 

[Tarheel]

I guess it's not a language issue, but as I understand it it took about 200 years for Copernicus's view (the heliocentric model) to become widely accepted. I wouldn't call that "soon".