Arizona State University (ASU) PHY101 Introduction to Physics Practice Exam

Copernicus explained retrograde motion as a result of the different orbital speeds of the planets. According to his heliocentric model, planets closer to the Sun, like Earth, move faster in their orbits compared to those that are further away, such as Mars. As Earth overtakes a planet in its orbit, that planet appears to move backwards, or in retrograde motion, against the background of the stars.

This explanation was revolutionary at the time, as it shifted the perspective from an Earth-centered (geocentric) view of the universe to one that recognized the Sun as the center of planetary motion. By understanding that the apparent backward motion of a planet is not an actual change in its orbit but rather a result of the relative speeds of the orbits of Earth and the other planet, Copernicus provided a clearer and more accurate model of celestial mechanics. This insight laid the groundwork for the later development of modern astronomy.