How Did Copernicus Change Astronomy?
From a teaching aid to a true cosmology: how Copernicus's heliocentric theory unified celestial and terrestrial matter and prepared the way for Kepler and Newton.
How Did Copernicus Change Astronomy?
The standard story of the Scientific Revolution begins with Copernicus, and the standard story of how Copernicus changed astronomy is summarized in two sentences: he moved the Earth from the center of the universe, and he put the Sun in its place. There is something true about this, but it is also misleading, because what Copernicus actually changed was not, in the first instance, observation, and not, in the first instance, prediction. What he changed was the structure of the question. This page looks at the ways in which the heliocentric theory altered the practice of European astronomy — the slow, fitful, century-long transition from a cosmology in which the Earth sat at the center of a finite, hierarchically ordered cosmos to one in which the Earth was a planet among planets, circling an unremarkable star in an indefinitely large universe. For Copernicus’s life and the contents of his book, see the Copernicus page and What Did Copernicus Write in De Revolutionibus?. The reasons the new theory met with so much resistance are surveyed in Why Was Copernicus’s Theory Controversial?. The longer scientific context is the heliocentrism and astronomy section.
The Cosmology That Copernicus Replaced
To see what Copernicus changed, it helps to see the system he was changing. The cosmology of the late-medieval European university was a synthesis of Aristotelian physics with Ptolemaic astronomy, formalized by Thomas Aquinas in the thirteenth century and taught in the universities until well into the seventeenth. The structure was roughly this: the universe is finite, spherical, and has a single center; the Earth sits at this center and consists of the four elements — earth, water, air, and fire — arranged in concentric spheres; beyond the sphere of fire is the lunar sphere, marking the boundary between the changeable, terrestrial world and the unchanging, celestial world; beyond the lunar sphere are the spheres of the planets and the sphere of the fixed stars, all composed of a fifth element, the “quintessence” or “ether,” and all moving in perfectly circular, uniform motion; beyond the sphere of the fixed stars is the “empyrean,” the dwelling place of God and the elect. The mathematical structure of this cosmology was Ptolemaic. The planets moved on circles whose centers were sometimes on other circles, and the whole system was, as Copernicus’s teacher Cracow had said, “an astronomical monster” of circles upon circles, epicycles and eccentrics, deferents and equants. The system worked, in the sense that it produced tables that predicted the positions of the planets reasonably well. The arithmetic was intricate, but the result was usedful. The astronomers of the late fifteenth and early sixteenth centuries — and Copernicus was trained as one of them — were competent practitioners of this tradition.
The First Change: From Model to Cosmology
The most important change Copernicus made was conceptual. Before Copernicus, the system of epicycles and deferents was, for most astronomers, a mathematical model. It was a way of computing positions; it was not, necessarily, a description of the actual physical arrangement of the heavens. The Greeks had always distinguished the “appearances” — what we observe — from the “real” arrangement of the cosmos, and most medieval astronomers were nominalists in this respect. They used the Ptolemaic apparatus because it worked, and they did not feel obliged to defend the literal truth of every detail. Copernicus, by contrast, insisted that the heliocentric arrangement was the actual physical arrangement. The Earth really did rotate on its axis. The Earth really did revolve around the Sun. The Moon, the planets, and the fixed stars really were arranged in the order he had specified. This is the famous “Copernican” move, and it is more important than the change in mathematical predictions, because it changed the question. From Copernicus onward, astronomy was no longer a matter of saving the appearances with clever mathematics. It was a matter of describing reality, and the description had to be defended physically. In this respect, Copernicus stood at the head of a long line of natural philosophers who would insist that mathematical models are true of the world, not just useful for computing. This insistence is one of the defining features of modern science, and it is one of Copernicus’s principal legacies. The implications of this move — the unification of celestial and terrestrial matter, the demolition of the sphere of the fixed stars, the openness to an infinite universe — were not fully developed by Copernicus himself, but they were the necessary consequences of taking the heliocentric system literally.
The Second Change: Unification of Celestial and Terrestrial Matter
A second change that Copernicus made, and that took time to develop, was the unification of celestial and terrestrial matter. The Aristotelian cosmos had distinguished sharply between the four terrestrial elements and the celestial fifth element. The heavens were made of a different kind of substance from the Earth; they were perfect, unchanging, eternal, and moved in perfect circles. The Earth was the realm of change, decay, generation, and corruption. The heliocentric system began to dissolve this distinction. If the Earth was a planet, then the planets were earths — made of the same kind of material, subject to the same kind of change, perhaps inhabited by the same kind of creatures. The same reasoning worked in reverse: if the planets were like the Earth, they too had surfaces, mountains, atmospheres, weather, and the rest. Giordano Bruno made this argument explicitly in the 1580s, and the telescopic discoveries of Galileo — mountains on the Moon, clouds on the planets, the resolution of the Milky Way into a swarm of faint stars — made it empirically obvious. The unification of celestial and terrestrial matter was one of the great conceptual achievements of the Scientific Revolution, and it prepared the way for Newton’s universal gravitation. For Newton to argue that the force holding the Moon in its orbit around the Earth was the same force that made a stone fall to the ground, the conceptual distinction between the celestial and terrestrial realms had to be removed. Copernicus, by relocating the Earth, did not complete this work, but he made it possible.
The Third Change: A New Ordering of the Planets
A third change, more concrete, was the new ordering of the planets. In the Ptolemaic system, the order of the planets was determined by the periods of their apparent motions, with the Moon closest to the Earth and Saturn the farthest of the known planets. The Copernican system reordered the planets by their actual distance from the Sun, with Mercury closest, then Venus, then the Earth, then Mars, Jupiter, and Saturn. This new ordering was not, at first, observationally justified; it was justified on aesthetic and quasi-physical grounds. But it was enormously fruitful. It made sense, for the first time, of the fact that Mercury and Venus were always seen near the Sun, while Mars, Jupiter, and Saturn could be seen at any elongation; it explained the variation in the apparent brightness of the planets as a function of their distance from the Earth; and it gave a natural way to predict the existence of a planet between Mars and Jupiter, where the Titius-Bode law be invoked. By the time Newton wrote the Principia in 1687, the ordering of the planets was an unproblematic fact.
The Fourth Change: Setting Up Kepler’s Problem
A fourth change, the most technically important, was that Copernicus’s system set up the problem that Kepler would solve. Copernicus, like all astronomers before Kepler, had insisted on uniform circular motion. But the Earth, if it were moving around the Sun, did not appear to move uniformly. Its speed, as measured by the rate at which the Sun appeared to move against the fixed stars, varied through the year. The Ptolemaic system had handled this variation by means of an eccentric circle, with the Earth offset from the center. Copernicus handled it by the same means, in a heliocentric translation. This was, in fact, an oddity. Why should the Earth move at a non-uniform speed around the Sun, if the celestial motions were all uniform and circular? The Ptolemaic system had also had this problem, of course. But in the Copernican system, the problem was sharper, because the heliocentric arrangement was supposed to be more economical, more elegant, more harmonious. Kepler attacked this problem directly in the Astronomia Nova (1609), using Tycho Brahe’s observations of Mars, and discovered that the Earth’s orbit was actually an ellipse with the Sun at one focus. This discovery broke the circle-orthodoxy of two thousand years and laid the groundwork for Newton’s gravitational theory. The detailed story of the three laws is told in What Are Kepler’s Three Laws of Planetary Motion?.
The Fifth Change: The Immeasurable Firmament
A fifth change, often overlooked, was Copernicus’s claim that the celestial sphere was immeasurably large. In the fourth of the seven postulates of De Revolutionibus, Copernicus asserted that the ratio of the Earth’s distance from the Sun to the height of the firmament is less than the ratio of the Earth’s radius to its distance from the Sun, so that the firmament’s distance is immeasurably great. This was a striking claim, with several implications. It made the absence of observed stellar parallax intelligible. (Stellar parallax, the apparent back-and-forth motion of a nearby star against the background of more distant stars as the Earth moves around the Sun, would be a direct confirmation of heliocentrism; but it was too small to detect with the naked eye. If the stars are vastly farther away than the Sun, the parallax would be too small to see.) It also opened the way for the idea of an infinite universe. If the celestial sphere is immeasurably large, why not imagine it as actually infinite, and the stars as other suns? Bruno made this argument. So, in different ways, did Descartes. By the time Newton wrote the Principia, the question of whether the universe was infinite was an open one, and Newton’s argument that an infinite, uniform universe would collapse to its center was one of the most influential arguments in the natural theology of the eighteenth century.
The Long Delay
For all these reasons, the heliocentric theory was, revolutionary. But it is important to note that the revolution was slow. The heliocentric system was published in 1543 and did not become the consensus view of European astronomers until the late seventeenth century. For most of the sixteenth century, the De Revolutionibus was used as a source of tables and a mathematical curiosity, but its cosmology was treateded as an open question. The reasons for the slow spread of Copernicanism are many. They include the absence of direct observational proof, the persistence of Aristotelian physics, the difficulty of constructing a physics of a moving Earth, and the strong theological objections to the idea that the Earth was not at the center of God’s creation. These are surveyed in Why Was Copernicus’s Theory Controversial?. The slow spread of Copernicanism is also the story of Kepler, Galileo, and Newton. Kepler gave the heliocentric theory the precise mathematical confirmation that Ptolemaic and Tychonic systems could not match; the detailed story is told in How Did Kepler’s Laws Support Heliocentrism?. Galileo gave it direct observational support, with the telescope. Newton, in the Principia, gave it the unified mechanical foundation that the heliocentric theory had lacked. The full these changes is the central narrative of the key figures page.
The Cumulative Effect
The effect of Copernicus’s work was that, by the early eighteenth century, European natural philosophers were no longer debating whether the Earth moved. They were debating how the motion was caused, what kept the planets in their orbits, and what the structure of the universe as a whole might be. These were questions that could not even be formulated in the Ptolemaic system, because in that system the Earth was at rest. By relocating the Earth, Copernicus changed the question. That is the deepest sense in which he “changed astronomy.”