A Brief History of Faithful Science

The Copernican Model

portrait of Nicolaus Copernicus
Nicolaus CopernicusThis is a 16th-century portrait of Copernicus, by an unknown artist. Source: Wikimedia Commons (public domain)

The story of the scientific revolution begins with Nicolaus Copernicus, a Polish astronomer and clergyman of the Roman Catholic Church. He is famous for challenging geocentrism, the ancient idea (made popular by Greek philosopher Aristotle) that the earth is at rest in the center of the cosmos. However, Copernicus was not the first to propose that the earth orbits the sun rather than vice versa. To understand the context and significance of his work, let’s rewind history and look again at ancient Greece.

As early as the fourth century BC, Greek philosopher and astronomer Heraclides Ponticus—a contemporary of Aristotle—argued that the apparent motion of the sun and stars is due to the daily rotation of the earth on its axis; and in the third century BC, Greek astronomer Aristarchus of Samos proposed that the earth also orbits the sun. However, this heliocentric (sun in the middle) theory was quickly forgotten as Aristotelian philosophy gained popularity. Aristotle had argued that the earth is at rest in the center of the universe, while celestial bodies (the sun, moon, stars, and planets) are attached to transparent, rotating spheres that surround the earth.

In the second century AD, the Greco-Egyptian astronomer Claudius Ptolemy developed a mathematical model of the universe, based largely on Aristotle’s ideas. (In science, the word model usually refers to a mathematical representation of something, not a physical object like a model airplane.) Ptolemy used complicated geometric and trigonometric formulas to describe the motions of celestial bodies in precise mathematical terms. The perfectly circular orbits suggested by Aristotle didn’t quite line up with observations; so to achieve greater accuracy in his model, Ptolemy added epicycles (little circles that moved along bigger circles) to the supposed orbits of planets around the earth. He also placed the rotating spheres slightly off center, so that the middle of each heavenly sphere didn’t correspond exactly to the center of the earth. When finished, his intricate mathematical model of the cosmos was remarkably successful in describing the apparent motions (as viewed from Earth) of stars and planets. It also successfully predicted a variety of celestial phenomena, including eclipses of the sun and moon. This Ptolemaic model of the universe was perhaps the longest-lived scientific theory ever devised, remaining in use for roughly 1500 years.

diagram of Ptolemy's geocentric model
Ptolemy's geocentric model of the universe
(not to scale)

Christian thinkers in the medieval period incorporated much of Aristotle’s philosophy into their theological views, and by the time of Copernicus, most educated Christians accepted Ptolemy’s geocentric model of the cosmos as well. For this reason, Copernicus’s heliocentric model was controversial among both Catholic and Protestant Christians. Nonetheless, some Christians were attracted to the theory, and encouraged Copernicus to publish his work when he was hesitant to do so. Among these was Nikolaus von Schönberg, a retired Cardinal of the Roman Catholic Church, who wrote the following letter to Copernicus in 1536:

Nicholas Schönberg, Cardinal of Capua,
to Nicholas Copernicus, Greetings.
Some years ago word reached me concerning your proficiency, of which everybody constantly spoke. At that time I began to have a very high regard for you, and also to congratulate our contemporaries among whom you enjoyed such great prestige. For I had learned that you had not merely mastered the discoveries of the ancient astronomers uncommonly well but had also formulated a new cosmology. In it you maintain that the earth moves; that the sun occupies the lowest, and thus the central, place in the universe … I have also learned that you have written an exposition of this whole system of astronomy, and have computed the planetary motions and set them down in tables, to the greatest admiration of all. Therefore with the utmost earnestness I entreat you, most learned sir, unless I inconvenience you, to communicate this discovery of yours to scholars, and at the earliest possible moment to send me your writings on the sphere of the universe together with the tables and whatever else you have that is relevant to this subject. Moreover, I have instructed Theodoric of Reden to have everything copied in your quarters at my expense and dispatched to me. If you gratify my desire in this matter, you will see that you are dealing with a man who is zealous for your reputation and eager to do justice to so fine a talent. Farewell.
Rome, 1 November 1536This letter, along with Copernicus's book and numerous other correspondences, are available here.

Contrary to popular belief, Copernicus was never persecuted for his ideas. The church did require modest emendations to his book, clarifying that the heliocentric model was a hypothesis rather than a proven fact or doctrine. However, that happened more than 70 years after Copernicus’s death, in response to rising controversy surrounding Galileo’s work. We’ll get to that story in a moment.

diagram of the Copernican model
Copernicus's heliocentric model
(not to scale)

Although the Copernican model placed the sun at the center of the cosmos, in other respects it was very similar to Ptolemy’s model. Copernicus retained the old Aristotelian idea that celestial bodies are attached to hollow spheres; and like Ptolemy, he hypothesized that planets move in perfectly circular epicycles, which in turn move along perfectly circular orbits. The predictions of his model were at best only slightly more accurate than the predictions of Ptolemy’s geocentric model. Since there was little evidence to support the new heliocentric model over the well-established geocentric model (which also had a robust history of Aristotelian philosophy in its favor), the Ptolemaic model remained the dominant view until two other Christians—a Lutheran Protestant and a Roman Catholic—promoted heliocentrism nearly three quarters of a century after Copernicus published his work. Their names were Johannes Kepler and Galileo Galilei.