The heliocentric system ? already sketched out by Aristarchus of Samos (3rd century BC) ? is based on both the immobility and centrality of the Sun, around which all planets move. Among them was the Earth, rotating on its own axis every 24 hours. Only the Moon, as in the ancient geocentric conception, moved around the Earth.
By "downgrading' the Earth to a planet, many phenomena are resolved that the Ptolemaic system could not give a reasonable explanation for. The diurnal motion of celestial bodies from east to west, is due, in reality, to the rotation of the Earth in the opposite direction. The annual motion of the Sun along the Zodiac is due to the revolution of the Earth. Finally, the retrograde motion of the planets are due to their own motion around the Sun combined with the Earth's revolution.
The Copernican system, however, was even more complicated than the Ptolemaic one. Copernicus wanted to maintain the uniformity of planetary motion, which had characterised the Greek astronomy of the ancients. Therefore, he rejected the Ptolemaic equant, which violated this principle. Moreover, Copernicus gave credit to some fictitious phenomena, which were due solely to the observational errors of his predecessors. These circumstances compelled him to introduce a lot of additional circles.