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Much has been written about earlier heliocentric theories. Philolaus ( 4th century BC) was one of the first to suppose a movement of the Earth, probably inspired by Pythagoras's theories on a spherical Globe.
Aristarchus of Samos ( 3rd century BC) developed some theories by Heraclides Ponticus (already talking about a revolution of our planet on its axis) to propose what is, to the best of our knowledge, the first serious model of a heliocentric solar system. Unfortunately, his work about his heliocentric hypothesis did not survive, so we can only speculate about what led him to his conclusions. It is notable that, according to Plutarch, a contemporary of Aristarchus accused him of impiety for "putting the Earth in motion".Copernicus cited Aristarchus and Philolaus in an early manuscript of his book which has survived, stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion." For reasons unknown he crossed out this passage before publication of his book.
Copernicus' major theory was published in the book De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres") in the year of his death 1543, even though he had arrived at it several decades earlier.
This book marks the beginning of the shift from a geocentric (and anthropocentric ) universe with the Earth at its centre. Copernicus held that the Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. He arrived at the correct order of the known planets and explained the precession of the equinoxes correctly by a slow change in the position of the Earth's rotational axis. He also gave a clear account of the cause of the seasons: that the Earth's axis is not perpendicular to the plane of its orbit. He added another motion to the Earth, by which the axis is kept pointed throughout the year at the same place in the heavens; from the time of Galileo it has been recognized that for it not to point to the same place would be a motion.
He also replaced Ptolemy's equant circles with epicycles. This is the main source of the statement that his system had even more epicycles than Ptolemy's. With this change his system had only uniform circular motions, correcting what seemed to be a defect in Ptolemy's system. Unfortunately, uniform circular motion is not what happens in the solar system, which runs on elliptical orbits; and this model was no more precise in predicting ephemerides than the then current tables based on Ptolemy's model. Furthermore, he badly underestimated the size of the solar system, like most of the astronomers of the time.
The system nevertheless had a large influence on scientists such as Galileo, Tycho Brahe, and Johannes Kepler, who adopted, championed and (especially in Kepler's case) improved the model. Galileo's observation of the phases of Venus produced, however, the first observational evidence for Copernicus' theory.
The Copernican system can be summarized in seven propositions, as Copernicus himself collected them in a Compendium of De revolutionibus... that was found and published in 1878:
These propositions represent the exact contrary of what the dominant geocentric propositions stated.
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