To try to improve the fit to the positions of the planets, many complications had been added:

Astronomers * had moved the earth slightly off the center of the solar system to avoid needing to have the planets move at varying speeds along their orbits. * had used a different type of epicycle to explain the motions of Mercury and Venus than were used for Mars, Jupiter, and Saturn. * The model got extremely complex for Mercury (and Mars). The figure to the right shows Mercury riding on an epicycle on an epicycle on an epicycle with constant speed measured from a point well off-center! (From Scientific American) Hereafter, when they come to model Heav'n, And calculate the Stars, How will they wield The mighty frame, how build, unbuild, contrive, To save appearances, how grid the Sphere With Centric and Eccentric scribbl'd o'er, Cycle and Epicycle, Orb in Orb. - John Milton, Paradise Lost

Copernicus realized that a few of the complexities would be removed if the earth and planets moved around the sun. This idea was a huge leap philosophically, since he treated the earth as one of the planets rather than as a unique body - previous scientific doctrine had held the earth to be unique.

Essentials of the Copernican Solar System

• The daily motion of the sun and stars is the result of the earth turning on its axis. Copernicus assumed that the air near the surface of the earth shares in its rotation thus preventing any "wind" from arising (this argument had also been made by Nicole Oresme two centuries previously) .
• The sun lies at the center of the solar system with the earth being the third planet orbiting the sun. The moon orbits the earth. Copernicus reconciled placing the sun at the center of the solar system with his religious beliefs by noting that the sun is the source of light and life, a logical choice for the center of the universe.
• All the planets orbit the sun in the same direction with the planets closer to the sun going around faster. A retrograde event then becomes the result of an inner planet overtaking and passing an outer planet.
• Copernicus assumed that the shapes of the orbits are circles and that the planets move at constant speed. (same as Ptolemy)

Copernicus' drawing of his system. He notes that Mars is far from its predicted position. From Marian Zwiercan http://www.bj.uj.edu.pl/bjmanus/revol/intro_e.html

A retrograde event in the Copernican model (from Journey Through Astronomy, http://csep10.phys.utk.edu/):

The dashed line is a comparison of the calculated position of Mars with the Ptolemy/Copernicus systems, as carried out by Origanus, and its true position, as calculated by Kepler (coming soon). The errors were plenty large enough to be obvious: 4 degrees is 8 times the diameter of the moon! (from Owen Gingerich, "Johannes Kepler and the Rudolphine Tables," Sky and Telescope, December, 1971, page 328)

If the earth is moving around the sun, then the nearest stars should exhibit parallax relative to stars. No one had been able to observe any stellar parallaxes. So the model violated a basic observation.

The parallax is smaller if the nearest stars are far from the earth, but there was no compelling reason to put them so far away that it could not be observed. (from Journey Through Astronomy,  http://csep10.phys.utk.edu/).