Throughout the time he spent in Lidzbark-Warminski, Copernicus continued to study astronomy. Among the sources that he consulted was Regiomontanus's 15th-century work Epitome of the Almagest, which presented an alternative to Ptolemy's model of the universe and significantly influenced Copernicus's research.
Scholars believe that by around 1508, Copernicus had begun developing his own celestial model, a heliocentric planetary system. During the second century A.D., Ptolemy had invented a geometric planetary model with eccentric circular motions and epicycles, significantly deviating from Aristotle's idea that celestial bodies moved in a fixed circular motion around the earth. In an attempt to reconcile such inconsistencies, Copernicus's heliocentric solar system named the sun, rather than the earth, as the center of the solar system. Subsequently, Copernicus believed that the size and speed of each planet's orbit depended on its distance from the sun.
Though his theory was viewed as revolutionary and met with controversy, Copernicus was not the first astronomer to propose a heliocentric system. Centuries prior, in the third century B.C., ancient Greek astronomer Aristarchus of Samos had identified the sun as a central unit orbited by a revolving earth. But a heliocentric theory was dismissed in Copernicus's era because Ptolemy's ideas were far more accepted by the influential Roman Catholic Church, which adamantly supported the earth-based solar system theory. Still, Copernicus's heliocentric system proved to be more detailed and accurate than Aristarchus's, including a more efficient formula for calculating planetary positions.
In 1513, Copernicus's dedication prompted him to build his own modest observatory. Nonetheless, his observations did, at times, lead him to form inaccurate conclusions, including his assumption that planetary orbits occurred in perfect circles. As German astronomer Johannes Kepler would later prove, planetary orbits are actually elliptical in shape.