By 1508, Copernicus had begun developing his own celestial model, a heliocentric planetary system. Heliocentric means that the sun is at the center of our solar system. During the second century A.D., Ptolemy had invented a geometric planetary model, which was inconsistent with Aristotle's idea that celestial bodies moved in a circular motion at different speeds around a fixed point, 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 of each planet's orbit depended on its distance from the sun.

Though his theory was viewed as revolutionary and met with some controversy, Copernicus was not the first astronomer to propose such a theory.

Centuries prior, in 270 B.C., ancient Greek astronomer Aristarchus of Samos had identified the sun as the solar system's central unit. But a heliocentric theory was quickly dismissed in Copernicus's era because Ptolemy's theories were far more eagerly accepted by the influential Roman Catholic Church, which adamantly supported the earth-based solar system theory. Still, Copernicus's heliocentric solar system proved to be more detailed and accurate than Aristarchus's, including a more efficient formula for calculating planetary positions throughout the year.