Why was Copernicus' theory considered revolutionary?

Answer 1

The revolutionary theory Nicolaus Copernicus published in 1543 was the Heliocentric model of the solar system. Published in his book On the Revolutions of the Heavenly Bodies, Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. The book wasn't published until Copernicus was on his deathbed because of the outrage he expected his theory to cause.

Answer 2

It was revolutionary in both the literal and the figurative sense. Nicholas Copernicus 1473-1543 was a Polish priest and astronomer who created an alternative model of the planets which put the Sun at the centre, instead of the Earth as generally accepted at that time.
He knew it would get him into trouble with the church, and his book 'De Revolutionibus' was not published until the year he died. The theory with the Sun at the centre was similar to the old Ptolemaic system with its collection of circles and epicycles allowing for the changes in distance and the changes in planets' speeds, and their departure from the ecliptic.
Later Tycho Brahe made new more accurate observations of the planets from Denmark, and these were used by Johannes Kepler to produce a new model published in 1609.
Kepler's model is the one we use today, and it has the Sun at the centre, like the Copernican model, but all the other details of Copernicus's theory were rejected in favour of elliptical orbits.