In 1543, Nicolaus Copernicus (1473-1543) publicly defended the hypothesis that the earth is a planet and the sun a body resting near the center of a finite universe. But why did Copernicus make this bold proposal? And why did it matter? Copernicus's reordering of the universe mattered not least because it became the first in a string of new and daring scientific claims at odds with traditional representations of heavens. These well-known achievements included, among others, Tycho Brahe's great observation project to remap the stars' positions, Johannes Kepler's planetary rules (later generalized into laws), Giordano Bruno's conception of an infinite number of suns and planets in an infinite, homogeneous space, Galileo Galilei's mathematical analysis of falling bodies and telescopic discoveries, René Descartes's corpuscular philosophy of nature and Isaac Newton's stunning unification of natural philosophy that reduced all motion to a few physical laws.
"The Copernican Revolution," a term coined by the historian and philosopher of science Thomas S. Kuhn, is the prevailing historians' category under which these achievements usually fall. The Copernican Question reframes this pivotal moment in the history of science, centering the story on a conflict over the credibility of astrology that erupted in 1496 just as Copernicus arrived from Krakow to continue his studies in Bologna. This astrologers' war was ignited by the appearance of a massive attack on the conjoined subjects of astronomy and astrology, a compound discipline contemporaries called "the science of the stars." The author of this threatening critique was the Florentine humanist and polymath, Giovanni Pico della Mirandola (1463-1494). The astrologers defended themselves in various ways, including deployment of the spurious charge than an astrologer had accurately predicted Pico's death. Copernicus's strategy, on the other hand, took a sophisticated, high-end approach: to protect astrology by radically reconstituting its astronomical foundations.
De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres") changed the frame of reference (as we would now say it) from the Earth to the Sun; it claimed to be able to predict all the celestial phenomena already accounted for by the traditional planetary arrangement (e.g. apparent changes in brightness, speed and direction); and it further maintained that if the Earth is assumed to be a planet, then all the planets fall into a natural ordering with the periods of revolution increasing with distance from the Sun.
The new theory engendered enormous resistance in the universities from both theologians and traditional natural philosophers. Yet, the first generation of astronomer-astrologers to entertain Copernicus's proposals was, like him, also looking for ways to answer the astrological skeptics although, unlike him, they left intact the longstanding premise of the earth's centrality and stability. In practice, these first-generation interpreters of Copernicus's work selectively appropriated some of its calculational innovations while rejecting its counterintuitive physical claim that the earth moves. The central issue for Copernicus and his mathematically-skilled contemporaries was thus to provide a robust answer to Pico's skeptical attacks against astrology. Altogether, such efforts to bolster both astronomy and astrology converged to create a crucial, if hitherto unrecognized, theme that provides a consistent element of unity to the first century of the early modern scientific movement--a "long sixteenth century" from the 1490s to the 1610s. This long sixteenth century, in turn, set the stage for the great transformations in natural philosophy in the century that culminated with Newton.
To appreciate how astrology could have reached such a state of crisis and generated a spectrum of efforts to save it, consider some long-term background factors. Just over a century before Copernicus's birth in 1473, the Black Death decimated between a quarter and a third of Europe's population. Between 1347 and 1351, that devastating pandemic seriously depopulated the medieval cities and made people ask why God would let such a horrible event occur. Imagine being a physician at that time. It is no accident that all the Arabic astrological and medical writings that began to be translated in the West from the twelfth century onward would have held a special attraction for the poor, helpless doctors. So too did Roman treatises about the beneficial therapeutics of bathing. Then, in 1453, just as the cities were recovering and just as printing was introduced at Nuremberg (1451), Constantinople fell to the Ottomans and sent a great shiver through Europe, not to mention the Church in Rome. The fall of Constantinople also sent Greek Christian emigres fleeing into Europe, some of them bringing with them ancient mathematical manuscripts. Finally, in 1494, just as Copernicus decided to end his studies at Krakow, the French Valois king Charles VIII (1470-1498) crossed into Italy at the head of a massive, tightly unified army of 30,000 men. It was the largest military force to appear in centuries and it quickly upset the balance of power among the small Italian principalities and, ultimately, the entire European state-system. When the French entered Florence at the end of 1494, the ruling Medici family fled. By January 1495, the French were in Rome and, by late February, in Naples. Not only did the invasion disrupt the whole political geography of Italy, leading to unrelenting continent-wide wars for the next century and a half, but the armies also weakened peoples' immune systems and brought in a new disease that influenced the title of a long poem by the astronomer-physician Girolamo Fracastoro: Syphilis or the French Disease (1530). In the summer of 1496, as these events were unfolding, Copernicus arrived in Bologna to continue his studies, this time in canon law. He would remain in Italy for the next four years before returning to his native Poland and then, once again, returning in 1501 to study medicine at the University of Padua.
Massive, recurrent epidemics and chronic warfare fueled popular prophecy and end-time, apocalyptic expectation and, in such an environment, astrological prognostications dedicated to an entire region (rather than a horoscope for an individual) became one of the important, early products of the new print culture. What made the annual, regional forecasts different was that they dealt with social groupings rather than just individuals: merchants, students, clergy, professors, soldiers as well as specific rulers, like the king of France; and they dealt, of course, with great threats and uncertainties--war, disease, the Turkish menace and the weather.
Copernicus followed the practice of many students of that time: he found lodging in the home of one of the faculty--in this case, Domenico Maria Novara (1454-1504)--and we have quite specific evidence that the university required Novara to publish annual astrological prognostications every year for the city of Bologna. Unless Copernicus was living with Novara in their shared lodging with ears plugged and eyes blindfolded, there can be little doubt that he would have been aware of the master's activities; and there can be no doubt that Copernicus would have heard as well about Pico della Mirandola's detailed attack on the whole enterprise of astrological divination which issued from the same publisher who produced many of Novara's prognostications. Much of Pico's critique was based on disagreements among the astrologers and one of those difficulties concerned the uncertain ordering of Mercury and Venus with respect to the Sun. This was by no means a new concern, as one finds it already in Claudius Ptolemy's great work of theoretical astronomy, the Almagest (2 nd century, A.D.), and in Johannes Regiomontanus's Epitome of the Almagest (1496). Ptolemy had associated the combinations of physical effects the planets produced--moist/dry and hot/cold--directly to the ordering of the planets. The significant point is that Pico's objection occurred in the context of a criticism about this ordering of the astral-elemental qualities. And while Copernicus does not mention Pico by name in De revolutionibus, he makes an unmistakable reference to a passage in Pico's work where Pico discusses the ordering of Mercury and Venus; and furthermore, Copernicus's reference occurs in just that part of his book where he discusses the ordering of those planets.
This is not the whole story, but it constitutes a crucial element. Beyond Copernicus's specific engagement with Pico's critique, Piconian skepticism about astrology continued to be a major preoccupation, a recurrent point of controversy, throughout the sixteenth and seventeenth centuries. Thus, the issue is not whether Copernicus cast horoscopes--we still don't know whether or not he did--but to show that he was trying to find a way to defend the astronomical foundations of the theoretical part of astrology, the part that defined the planets' elemental qualities in relation to their order. The solution he offered was a new arrangement of the planets by their periods of revolution with respect to a central, resting Sun--from the speediest planets, Mercury (88 days), Venus (225 days) and Earth (365 days), to the slowest, Mars (2.1 years), Jupiter (12 years) and Saturn (30 years).
As for the sun-centered hypothesis itself, Copernicus already knew from his undergraduate education what every student then learned: that the Pythagoreans believed that the Earth is, as Aristotle says, "one of the stars, and creates night and day as it travels in a circle around the center." In fact, Aristotle used the Pythagoreans' claim to show that the consequences of their position are physically absurd and, hence, to affirm his own account that the center of the universe is not their "central fire" but Earth. In his earliest known formulation, dating to around 1510, Copernicus was also critical of the Pythagoreans--not because he rejected their position but because he rejected their justification for it. He realized that an improved version of the Pythagorean arrangement--taking the Earth's motion as a mathematical assumption in the style of Ptolemy--explained both the involvement of the Sun's apparent motion as a component of the other planets' motion as well as the ordering of Mercury and Venus. But not all lovely, unifying explanations are true and that is why Copernicus had to turn to probable/dialectical arguments based on likelihood rather than to the much stronger notion of a strict demonstration that rules out all possible alternatives.
Tycho Brahe (1546-1601) and Johannes Kepler (1571-1630) were also engaged in improving astrological prognostication. Brahe believed that astronomy and astrology's forecasts could be made better by taking on board Copernicus's principle of planetary modeling, which allowed only uniform motions around their proper centers; and, as is better known, he imposed a new and rigorous standard of precision on the quality of all celestial observations. But, for both physical and scriptural reasons Tycho rejected a moving Earth. He understood that a moving Earth would undermine the whole scheme whereby heavy bodies fall and planetary influences cascade down to the center of the universe. He thus designed an arrangement that kept the Earth in its traditional position, at the center and at rest. At the same time, Tycho did accept one part of Copernicus's arrangement as an answer to the Piconian problem by making all the planets revolve around the (still) revolving Sun--an arrangement that re-ordered Mercury and Venus.
Kepler was the only Copernican to attempt fully to revise the principles of astrology itself. In short, throughout the sixteenth century there were actually many different responses to Pico's objections, some of which accepted his criticisms completely (as was typical among theologians both Protestant and Catholic); some of which tried to answer Pico's objections in ways that involved reordering the planets around the centrally resting Sun (the Copernicans); others of which involved answering those objections by keeping the Earth at rest while centering all the planets on a revolving Sun (Tycho Brahe and other geoheliocentrists); and still others, like Kepler, who accepted Pico's argument that the zodiac was a human construction but nonetheless tried to provide an astrology that would permit the Earth's motion to be part of the scheme. These evolving engagements with Piconian astrological skepticism formed the crucial axis of controversy around which various motivations were organized in the sixteenth and early seventeenth centuries.
One might well wonder how this debate about the credibility of astrology would have proceeded had not European observers noticed and recorded the appearance of several unanticipated, one-time celestial events in 1572 (a new star), 1577 (a comet) and in 1604 (another new star). Protestant evangelicals of various shades interpreted the meanings of these unpredicted, short-term natural events in a religious framework, the master narrative of world history. They were read as signs of the imminent breakdown of nature as the divine Creation approached its predicted End--just as today beliefs analogous to such early modern ones are still at play in the politics of global warming. (Some of today's skeptics put climate scientists in the role of alarmist prophesiers foreseeing the end of a world of climactic regularities while the skeptics either deny any such change or regard technology and the free market as the source of worldly salvation).
In 1610, another unanticipated event occurred--the construction of a new kind of instrument that enabled unprecedented magnifications of distant objects, including entities like the ones that Galileo described as planets circulating around Jupiter and what he interpreted as craters on the surface of the moon. These new appearances revealed by the telescope differed from the earlier ones in that some of them displayed recurrent motions and--again unlike comets and novas--anyone with a good-enough instrument did not need to wait for divine intervention. A human being could make them reappear. These chance events, to which apocalyptic and astrological meanings were attributed, variously stimulated a handful of practitioners to connect novas, comets and Jupiter's "planets" with alternative planetary arrangements--and so fed the controversy ignited by Copernicus. This is how the debate about the foundations of astrology became interwoven with the eschatological narrative about the End of the World.
These historicized framings of the Copernican question within the controversy over the status of astrology and eschatology differ from the pattern of scientific change that Thomas Kuhn made famous fifty years ago in his account of radical rupture and revolutionary paradigm shifts. The Copernican Question is not a narrative of punctuated discontinuity but rather of the uncoordinated and unpredictable movement of questions, concepts, standards and logical resources between local communities. The main logical problem in the narrative is Copernicus's lack of a strict demonstration ruling out all alternatives. What must have convinced him to pursue the hypothesis of the Earth as a planet was that it entailed a unifying explanation of a good many phenomena. But that left him with the big problem of how to defend the truth of the main premise, the Earth's motion. And, in turn, that left him with the next best sort of proof--a weaker kind--that involved weighing and balancing probabilities on each side. What compels attention is that this weighing and balancing could go either way for two people considering the same evidence. And that pattern of indeterminacy would continue to play out across the sixteenth century at least until the appearance of three intellectually powerful followers of Copernicus: Giordano Bruno, Johannes Kepler and Galileo Galilei. More than any of their contemporaries, these three figures substantially increased the explanatory robustness of Copernicus's theory by variously putting forth all sorts of new entailments (such as Kepler's elliptical orbits, solar rotation, and a precise relationship between the size of an orbit and the time it takes a body to complete one revolution; or, Galileo's Jovian moons, Venusian phases and lunar imperfections). Yet, if logically, these discoveries enabled the Copernicans to shift the burden of proof to the traditionalists there were important differences among the early seventeenth-century Copernicans. They were deeply uncomfortable with one another's positions, socially fragmented, conceptually at odds over their physical premises and yet they shared a concern to eliminate the physical uncertainties that Copernicus had failed adequately to address. Bruno's claim that the universe is infinite and homogeneous was so radical that it eliminated the science of the stars as Kepler, Galileo and everyone else practiced it. By contrast, Kepler's project involved a massive rewriting and strengthening of all parts of the science of the stars--including the principles of theoretical astrology--so strange in its own way that Galileo would have nothing to do with it. Galileo would also reject Kepler's ellipses and everything to do with his metaphysics.
What did finally emerge in the 1620s and 30s was a consolidation of Kepler's and Galileo's arguments. Thus, by the 1640s the Jesuit Giambattista Riccioli (1598-1671) inherited a mass of new arguments and evidence that put him in a position to recast the probability arguments of the previous century. But without any precedent for managing such a volume of evidence, it was not clear how the weighing and balancing ought to be done--other than by counting up the arguments on both sides. Riccioli's situation distantly echoes a predicament of modern biological and medical science: how to evaluate a medical treatment by combining and measuring the results of many different kinds of individual studies.
The Copernican ordering achieved closure in the seventeenth century--an end to questioning and criticism from competing alternatives--in different ways among different audiences. Some believers in extraterrestrial life, for example, followed Bruno in postulating the existence of innumerable stationary suns, each with their own planetary systems. Similarly, Rene Descartes and his followers proposed an infinite number of whirlpools of tiny particles, each planet located in its own vortex that, in turn, circulated around the sun's vortex. But if high-end seventeenth-century natural philosophers like Newton and Descartes agreed about planetary order, they did not agree on the physical principles that constituted its foundation. Today, those who understand and accept Newtonian physics are easily able to justify their belief that the Earth is a planet revolving around the Sun. For many others, it is a proposition with no prehistory of controversy, a statement simply accepted on the authority of parents and trusted teachers.