Theological objections, the missing stellar parallax, the physics problem of what keeps the Earth moving, and the conflict with common sense that made Copernicanism controversial.

Why Was Copernicus’s Theory Controversial?

The heliocentric theory is so familiar to us that it is easy to forget how strange it sounded in 1543, when Nicolaus Copernicus published De Revolutionibus. To most of his contemporaries, the idea that the Earth moved — that the solid ground beneath their feet was whirling through space at thousands of miles an hour, that the Sun stood still and the heavens rotated around them once a day — was not just false. It was absurd. It contradicted common sense, scripture, and the entire Aristotelian physics on which the late-medieval university was built. It took more than a century, and the work of Kepler, Galileo, Descartes, and Newton, to make the heliocentric theory the consensus view of European natural philosophy. This page looks at the reasons the heliocentric theory was controversial: the theological objections, the missing stellar parallax, the physics problem of a moving Earth, and the broader conflict with common sense. For the contents of Copernicus’s book, see What Did Copernicus Write in De Revolutionibus?. For the longer-term consequences of his theory, see How Did Copernicus Change Astronomy?. For the life of Copernicus and the publication of the De Revolutionibus, see the Copernicus page. The trial of Galileo is the most famous episode in the controversy, and is treated in Why Was Galileo Put on Trial?. The wider religious reception is treated in the religion and the Church.

The Theological Objections

The first wave of objections to the heliocentric theory was theological. The Bible, read literally, describes a stationary Earth and a moving Sun. In Joshua 10:13, Joshua commands the Sun to “stand still” over Gibeon, not the Earth. In the Psalms, the Earth is “founded” upon the seas and “established” so that it cannot be moved. In Ecclesiastes, “the Earth abideth forever; the Sun also ariseth, and the Sun goeth down, and hasteth to the place where he arose.” To many Christian readers, these passages seemed to settle the question. Luther, in a famous table-talk of 1539, dismissed Copernicus as “the fool who wishes to reverse the entire science of astronomy; but sacred scripture tells us that Joshua commanded the sun, not the Earth, to stand still.” Calvin, in a commentary on the Psalms, asked who would venture to place the authority of Copernicus above that of the Holy Spirit. Catholic writers were for a long time more cautious; the De Revolutionibus was, after all, dedicated to the pope, and Copernicus was a canon. But the underlying objection was the same: the heliocentric theory seemed to contradict the plain meaning of scripture. The Catholic Church did not formally act against Copernicanism until 1616, when it placed De Revolutionibus on the Index of Forbidden Books pending correction. The theologians of the Sacred Congregation could not, in fact, agree on what the right corrections were, and the corrections were never actually made. The book remained on the Index in a kind of suspended status until 1758, when the Index was revised. The wider context of the Church’s engagement with the new science is surveyed in the religion and the Church. the theological objections were not uniform. Some Catholic theologians, including the great Spanish Jesuit commentator on Aquinas, Francisco de Toledo, had argued in the late sixteenth century that the Bible should be read “according to the manner in which it is customary for it to speak,” accommodating itself to the ordinary mode of human perception. This position, won out. But in 1543 it was, at best, a minority view.

The Missing Stellar Parallax

The second, and most decisive, observational objection to the heliocentric theory was the absence of observed stellar parallax. If the Earth revolves around the Sun, then the nearer stars should appear to shift back and forth against the background of the more distant stars, in the same way that a foreground tree appears to shift against the background of a forest when we move our head. The effect should be most pronounced for the nearest stars and least for the most distant. In fact, no such shift had ever been observed, with any naked-eye instrument, in 1543 or for the next two and a half centuries. There were two ways to account for this. One was that the heliocentric theory was wrong, and the Earth really was at rest. The other was that the stars were so far away that the parallax was too small to detect. Copernicus himself had proposed the second explanation, in the fourth postulate of De Revolutionibus. The Earth–Sun distance, he said, was less than the radius of the celestial sphere as the Earth’s radius is less than the Earth–Sun distance. The stars were unimaginably far away. This explanation made the heliocentric theory difficult to test, but not impossible. The first successful measurement of stellar parallax was made in 1838 by Friedrich Bessel, who detected the parallax of the star 61 Cygni, and at about the same time by Thomas Henderson, who detected that of Alpha Centauri. The parallax of 61 Cygni is 0.314 arcseconds, an angle about 180,000 times smaller than the diameter of the full moon. To detect it required the best telescopes of the early nineteenth century, and the patience to measure the position of a single star to a few hundredths of an arcsecond over the course of a year. That Copernicus’s theory should have predicted this immeasurably small effect, and that the effect should have been detected three centuries later, is one of the great confirmations in this history.

The Physics Problem

A third set of objections was physical. The Aristotelian physics, which dominated the European universities well into the seventeenth century, held that the natural motion of the Earth — the heavy element — was toward the center of the universe. If the Earth were not at the center, why should bodies fall downward? Why should a stone thrown upward come back to the thrower’s hand? Why should the atmosphere not be left behind by a rapidly rotating Earth? Why should a tower built perpendicular to the surface remain perpendicular, if the surface were moving? Copernicus, in Book I of De Revolutionibus, had addressed several of these objections. He argued that falling bodies share the Earth’s motion, so that their apparent vertical fall is consistent with a moving Earth. He argued that the atmosphere rotates with the Earth. He argued that the Earth’s motion is “natural” rather than “violent.” These arguments were intelligent but, by modern standards, incomplete. The principle of inertia — that bodies in motion continue in motion unless acted upon — had not yet been formulated, and without it, the physics of a moving Earth was hard to construct. The physics problem was eventually solved, but only by Newton, in the Principia of 1687. Newton’s first law of motion — that every body continues in a state of rest or of uniform motion in a straight line, unless compelled to change that state by impressed forces — provided exactly the framework that Copernicus had lacked. Combined with the law of universal gravitation, Newton’s mechanics explained why the Earth could rotate and revolve without flinging off its atmosphere or its inhabitants. The story is told in How Did Newton Discover Gravity?.

The Conflict with Common Sense

A fourth set of objections, less respectable but in some ways more powerful, was the conflict with common sense. The heliocentric theory asked people to believe that the ground beneath their feet was in constant motion, that the Sun did not move, that the stars were so far away that their distance was unimaginable, and that the Earth was just one of several planets. Each of these claims ran against the immediate deliverances of the senses. This conflict is one of the most durable themes in the history of popular reaction to the new science. From Luther’s table-talk to Galileo’s contemporaries, the most common objection to heliocentrism was not that it contradicted scripture or even that it failed the observational tests. It was that it was absurd. People can see that the Sun moves across the sky, that the stars wheel overhead at night, that a stone falls in a straight line. To say that the senses are deceiving us, that what we see is the consequence of a much more elaborate underlying reality, is a strong claim, and it took generations of patient teaching to make it acceptable. Galileo made a famous attempt to defuse this objection in the Dialogue Concerning the Two Chief World Systems (1632), and his argument deserves to be remembered. The senses, he pointed out, do not deceive us about the immediate appearance of things. The Sun does appear to move, and the Earth does appear to be at rest. The question is what the underlying physical reality is that produces these appearances. The senses cannot answer this question. Only reason and observation, properly combined, can. This is, in essence, the modern view. It was not, in 1632, the common-sense view.

The Aesthetic and Methodological Objections

A fifth set of objections was aesthetic and methodological. Many of Copernicus’s contemporaries did not object to the heliocentric theory on religious or physical grounds; they objected to it on the grounds that it was mathematically uglier than the geocentric system it was supposed to replace. The Ptolemaic system, with its many centuries of refinement, produced tables that were at least as good as Copernicus’s, and better. The Copernican system retained epicycles, eccentrics, and most of the apparatus of the Ptolemaic system. It did not, observationally, justify the radical step of moving the Earth. Tycho Brahe, the great Danish observer whose data Kepler would use, objected on just these grounds. He proposed the “geo-heliocentric” or Tychonic system, in which the Sun and Moon revolve around a stationary Earth while the other planets revolve around the Sun. This system preserved the physics of a stationary Earth, the scriptural reading of the cosmos, and the practical accuracy of the Copernican ordering of the planets. It was a serious competitor, and it remained popular into the early seventeenth century, especially among Catholic astronomers who wanted the best of both worlds. The Tychonic system was eventually routed by Kepler’s discovery that the orbits of the planets were ellipses, not circles, and that the heliocentric system, in its Keplerian form, predicted the positions of the planets more accurately than any rival. The way Kepler’s laws clinched the heliocentric case is told in How Did Kepler’s Laws Support Heliocentrism?.

The Long Resolution

The heliocentric theory was controversial for more than a century after the publication of De Revolutionibus. The controversy was not resolved by a single decisive event, but by a slow accumulation of arguments, observations, and theoretical innovations. The major steps were the following. First, Tycho Brahe’s observations (1576–1601) raised the empirical standard of astronomy. They showed that the available planetary tables, both Ptolemaic and Copernican, were not very accurate, and they provided the data Kepler needed. Second, Kepler’s three laws (1609, 1619) gave the heliocentric theory the precise mathematical form that made it empirically superior to any rival. The orbit of Mars, in particular, was incompatible with the circular orbits of the Ptolemaic and Tychonic systems. Kepler’s work is described in detail in Johannes Kepler: The Celestial Harmonist and in What Are Kepler’s Three Laws of Planetary Motion?. Third, Galileo’s telescopic discoveries (1610–1613) gave the heliocentric theory direct observational support. The phases of Venus, the moons of Jupiter, the mountains on the Moon, the sunspots — each was a direct blow against the Aristotelian-Ptolemaic synthesis. The discoveries are described in What Did Galileo Discover With His Telescope?. Fourth, Descartes’ mechanical philosophy (1644) and Newton’s Principia (1687) provided the theoretical framework. The heliocentric system, in the Principia, was a consequence of the law of universal gravitation and the three laws of motion. It was, by 1687, no longer a controversial hypothesis. It was a theorem.

Why It Mattered

The controversy over the heliocentric theory mattered not just because the Earth moved, but because of what the heliocentric theory implied about the status of human knowledge. If the obvious deliverances of the senses could be wrong about something as fundamental as whether the ground was moving, then the whole question of how to reason about the natural world was reopened. The philosophers of the seventeenth century — Bacon, Descartes, Hobbes, Locke, and others — wrestled with this question. The methodological answers they developed, including Bacon’s inductive method and Descartes’ deductive method, are described in the Bacon and Descartes page. The controversy also mattered because of what it implied about the relationship between science and religion. The Bible, read literally, seemed to say one thing; the astronomers, working with mathematical models and telescopic observations, said another. The way this conflict was eventually resolved — with the broad acceptance of the view that scripture speaks of the natural world in a manner accommodated to ordinary perception — was one of the most important intellectual achievements of the early modern period. The story is told in the religion and the Church.

Why Was Copernicus's Theory Controversial?