The Copernican Revolution
Alessia Tami (University of Zurich)
On the Revolutions of the Heavenly Spheres
The Book That Placed the Sun at the Center
Published in 1543, De revolutionibus orbium caelestium (On the revolutions of the heavenly spheres) by the Polish astronomer and Catholic canon, Nicolaus Copernicus, introduced a new model of the cosmos that placed the Sun, not the Earth, at the center of the universe. Copernicus had already outlined his Sun-centred model of the cosmos in the Commentariolus (Brief sketch) in 1514, but he kept it secret for several years, only circulating the manuscript to a select few.
Copernicus devised an imaginary point which he called the ‘mean Sun’, or the average position of the Sun, and placed it at the center of the universe. He argued that the Earth and other planets rotated around the Sun, adding that the Earth made a complete turn on its axis once a day.
De revolutionibus orbium caelestium was Copernicus’s magnus opus. He spent the final thirty years of his life refining his model with observations and mathematical calculations. He died the same year the book was published.
In the years to come, Copernicus’s controversial but well-substantiated assertions set in motion a process of scientific observation and discoveries whereby humanity was to gradually change how it understood its place in the universe. We now term this period of great astronomical change the Copernican Revolution.
Learn more about the Commentariolus.
Why Did Copernicus Rethink the Universe?
Ancient and medieval astronomers were aware that the motion of the planets followed a rhythm that depended on their position relative to the Sun. The Moon, which was also considered a planet, was the lone exception since it orbited the Earth.
Predicting the positions of the planets was extremely complicated since they moved faster or slower at different times during the year. In his book, the Almagest, the Alexandrine astronomer Ptolemy employed three strategies to account for these variations. First, he shifted each of the planetary spheres so that the Earth was no longer located exactly at the center of the cosmos. Furthermore, he suggested that as the planets traveled along these eccentric orbits, they also moved in a smaller circular path called an epicycle. Finally, to reconcile the planets’ changing velocities with Plato’s belief that the planets exhibited uniform circular motion, he introduced the idea of the equant: a point within a planet’s orbit from which it appeared to move at a constant angular speed.
What prompted Copernicus to place the Sun at the center of the universe were a pair of concerns with Ptolemy’s geocentric cosmos. He strongly objected to his predecessor’s reliance on the equant, which he considered a violation of the Platonic ideal of planets moving in perfect circles. In addition, medieval commentators had become increasingly aware of errors in astronomical calculations based on the Ptolemaic model, which complicated efforts to determine the timing of religious holidays like Easter.
Copernicus proposed an elegant solution to both of these problems. He argued that it was easier to determine each planet’s position if the Sun replaced the Earth at the center (or slightly off-center) of the cosmos. In this new arrangement, the Sun only seemed to move due to the Earth’s year-long revolution around it, with day and night resulting from a rotation every 24 hours.
While Copernicus continued to use eccentric orbits and epicycles to model planetary motion, his heliocentric system eliminated the need for the equant. It also provided a simplified explanation of retrograde motion, when Mars, Jupiter, and Saturn briefly reversed direction as they moved along their orbits. Ptolemy had relied on additional epicycles to account for this behavior, but Copernicus showed how it could also result from the Earth’s tendency to outpace the outer planets during their respective trips around the Sun.