Why was Copernicus a Copernican?

Nicholas Copernicus (1473 – 1543) was the first man since antiquity to propose that the sun was at the center of the cosmos and that the earth was in motion around it. The question is: Why did Copernicus break with centuries of accepted astronomical wisdom?

By the late fifteenth century, when Nicholas Copernicus was studying astronomy, first at Cracow and later at various Italian universities, there was broad consensus about the general structure and substance of the cosmos. However, the study of the heavens was hardly static, and there were several questions and controversies that are directly relevant to understanding Copernicus’ momentous decision to shift the sun to the center of the cosmos and to set the earth in motion around it. One major problem was that the mathematical models derived from Ptolemy and from later Arabic astronomers were not accurate enough. Predictions of phenomena like eclipses were frequently off by several days. This lack of accuracy was a problem for making astrological predictions. It was also a serious problem for the Spanish and Portuguese, who by the fourteenth century had begun to embark upon voyages of exploration and conquest. Precise astronomical information was crucial to accurate navigation. Most astronomers in the fifteenth and sixteenth centuries believed that Ptolemy’s models needed correction, but they believed this could be accomplished by making newer and more accurate observations of celestial events and using them to adjust the existing models.

In addition to problems with the accuracy of the system, there were two important debates among astronomers about the architecture of the cosmos. The first of these debates was over the structure of the orbs carrying the planets, sun, moon and stars; and the second was about the ordering of the planets. The majority of astronomers were prepared to accept that the celestial orbs included devices like epicycles. (Remember, planets did not ORBIT around the earth. Rather, they were carried by ORBS that rotated around the earth.) However, in the twelfth century, the Spanish Muslim philosopher Ibn Rushd (1126 – 1198), known in the Latin West as Averroes, argued that these models violated the tenets of Aristotelian physics. All motion in the heavens, argued Averroes, should be uniform, circular and centered on the earth. A device like the epicycle-on-deferent had two centers of motion, only one of which was the earth. Averroes proposed instead a system where each celestial body’s motion was the combined motion of several nested spheres, each rotating at different speeds in different directions around different axes. This system was very similar to that of Eudoxus. The key advantage to this system, for Averroes and his followers, was that the motion of every single celestial orb was centered on the earth. The key disadvantage of this system, as far as the opponents of Averroes were concerned, was the lack of calculational accuracy. That is, no Averroist ever devised a system that actually allowed accurate prediction of the positions of celestial bodies, rendering the Averroist systems useless for astrology or navigation. While few astronomers were willing to trade the Aristotelian purity of the Averroists for the predictive accuracy of Ptolemy’s followers, this remained a live area of controversy on into the sixteenth century, and the problem of how to reconcile astronomical models with the Aristotelian demand for perfect circular motion was one that influenced Copernicus, as we shall see.

The second debate was over the ordering of the planets. The ordering of the outer planets, Mars, Jupiter and Saturn, was generally agreed upon. Saturn was farthest away from the earth because it had the longest period of rotation. Jupiter had the second longest and Mars the third. And there was no question that the moon was the celestial body closest to the earth. However, the ordering of the remaining three heavenly objects, the sun, Mercury and Venus, was a subject of debate. While the moon, Mars, Jupiter and Saturn fell into a neat order based on their periods, in a geocentric cosmos, the sun, Mercury and Venus all have periods of about a year. Thus, the rationale for putting Mercury after the moon, followed by Venus, followed by the sun, as Ptolemy did, was a bit shaky, and not everyone agreed with this ordering. In fact, in the fifth century, the Roman writer Martianus Capella, author of a teaching text on the liberal arts (including astronomy) that was highly influential in the early Middle Ages, posited that Mercury and Venus rotate around the sun, while the sun rotates around the earth. The fourteenth-century theologian Henry of Langenstein (d. 1397) suggested that it was possible the sun was between the moon and Mercury. This lack of clarity on the ordering of the inner planets had serious ramifications for astrology, where the distance of each planet from the earth affected its influence.

The foregoing account of the state of astronomy in the late fifteenth and early sixteenth century is intended to provide context for my discussion of Copernicus’ On the revolutions (1543), the first fully developed heliocentric model of the cosmos. This context does not, of course, fully explain why Copernicus rejected the centuries old geocentric models of the cosmos. His education and influences were not at all unusual and yet he alone of all of his contemporaries proposed a heliocentric model of the cosmos. However, it is important to see his system as an answer to questions astronomers of his time were asking, even if his answer was one nobody expected and few accepted at first.

Copernicus described his system in preliminary form in around 1514 in a manuscript known as the Commentariolus, or “little commentary” on Ptolemy’s Almagest. In this manuscript, he asserted that the sun is stationary at the center of the cosmos and that the earth, and five planets rotated around the sun. (The moon still rotated around the earth.) Mercury was the closest planet to the sun, followed by Venus, the earth/moon system, Mars, Jupiter, Saturn and the “fixed” stars on the starry vault. Thus the apparent motion of the sun around the earth, which had defined the solar year, was actually caused by the motion of the earth around the sun, a motion that took one year. Not only did the earth rotate around the sun, but it rotated once every twenty four hours around an axis passing through the north and south poles. This diurnal rotation accounted for the daily rising and setting of the celestial bodies. All the planets, like the earth, moved around the sun in the same direction though at various rates. The apparent retrograde motion of the planets was a result of the changes in the earth’s position. (Review the animations on Craig McConnell’s site.) Copernicus eventually worked out mathematical models for each planet, including the earth, that would allow their positions at any point in time to be calculated. He finally published his complete system in 1543 as On the revolutions of the heavenly spheres. Copernicus died in 1543, and thus was not around to witness reactions to his work or to respond to criticisms.

Copernicus’ work was not based on new observations of the heavens. His accomplishment was to reinterpret existing data in a radical new way. This certainly raises the question, why then did he propose his heliocentric model? Despite the problems and debates in astronomy that I discussed above, there was little reason to believe that the entire system was in need of total overhaul. Copernicus himself never states, at least not in any extant writing, what factor or factors were decisive in his intellectual path to heliocentrism. This has led to considerable and ongoing debate about Copernicus’ motivations. To quote Peter Barker, “Here is an outstanding intellectual scandal in the history of science: there is no generally agreed and historically respectable answer to the question of why Nicholas Copernicus adopted heliocentrism.”

Two plausible motivations have been suggested, each related to the two debates in astronomy that I sketched out earlier. One explanation for Copernicus’ rejection of the geocentric model is that he, like the Averroists, was bothered by the fact that Ptolemy’s mathematical models seemed to violate the Aristotelian principle that motion in the heavens was uniform and circular. Copernicus was not, like the Averroists, bothered by the epicycles. Indeed, his own models of planetary motion contain epicycles. However, he was bothered by another mathematical device of Ptolemy’s called the equant. In the equant model, a planet is embedded in a sphere that rotates around the earth and the planet sweeps out equal angles in equal times as measured from what is called the “equant point,” which is not the center of the circle. To be clear, if the planet was moving with uniform circular motion, it would sweep out equal angles in equal times as measured from the center of the circle.  (Again, review Craig McConnell’s site for an animated equant model.)  The equant point is some distance from the center, which means that the planet actually moves with varying speed. This, to Copernicus, and indeed to many of his predecessors and contemporaries, did not seem to fit the requirement that planetary motions be combinations of uniform circular motion. By putting the sun at the center, and the earth in motion, and by employing mathematical models devised by Arabic astronomers and mathematicians (I will return to the subject of Copernicus’ debt to Arabic science), Copernicus was able to eliminate the equants from his models of celestial motion.

The second motivation has to do with the ordering of the planets. As I noted above, the ordering of Mercury and Venus was the subject of some debate in Copernicus’ day. In a 2002 article, Bernard Goldstein proposed that Copernicus adopted the heliocentric system because it allowed him to order the planets according to their periods of rotation. This was the basic principle used to order the planets in the geocentric system, except that Mercury, Venus and the sun had nearly identical periods of rotation around the earth. In a heliocentric system, all the planets have different periods of rotation around the sun and thus can be neatly arranged. Mercury has a period of eighty days and so is closest to the sun. Venus comes next with a period of nine months, then the earth (one year), Mars (two years), Jupiter (twelve years), and finally Saturn (thirty years). In 2011, Robert Westman proposed an even bolder thesis. He argues that Copernicus was stimulated to solve the problem of how to order the planets by debates about astrological theory and practice that he was exposed to while studying in Italy. Specifically, in date Giovanni Pico della Mirandola (1463 – 1494) published a scathing attack on astrology in which he cited the lack of agreement among astronomers about the correct ordering of Mercury, Venus and the sun as a strong reason for doubting the claims of astrologers. Westman sees Copernicus’ adoption of heliocentrism as a move to defend the legitimacy of astrology. This is the strongest and most provocative argument to date for the importance of astrology in the Scientific Revolution.

The case of Copernicus provides the most telling and well-documented historical example of the ongoing influence of Arabic science and mathematics in early modern Europe. In 1957, Otto Neugebauer discovered that Copernicus’ model for the moon was identical to that of the Damascene astronomer Ibn al-Shatir (1304 – 1375). Neugebauer’s work was subsequently expanded by Noel Swerdlow, who demonstrated even more similarities between Copernicus’ planetary models and those of Ibn al-Shatir. In 1973, Willy Hartner showed that one of the mathematical models Copernicus used was identical to a model devised about three hundred years earlier by Nasir al-Din al-Tusi (d. 1274). This device, known as a Tusi couple, generates linear motion from multiple circular motions. Surprisingly, despite the quantity and quality of the evidence, there is still considerable reluctance among some historians of science to accepting that Copernicus was in any way influenced by Islamic intellectuals. For example, André Goddu, author of the most recent intellectual biography of Copernicus, flatly states that, “Experts have exaggerated the supposed identity between Copernicus’s and al-Shatir’s models and the Tusi couple.”  It is true that Copernicus did not acknowledge the work of al-Shatir or Tusi in On the revolutions, that he did not read Arabic, and that there are no extant manuscripts of the work of Tusi or al-Shatir in languages that Copernicus is known to have read. However, as Saliba points out, “there were enough Arabists in various European cities who were . . . competent enough to read the technical contents of scientific manuscripts and to understand their import and thus pass them on either orally or even by request in a tutorial fashion.” It seems that the unwillingness to accept that Copernicus borrowed from Arabic astronomers owes more to a deeply entrenched commitment to Western exceptionalism than to a lack of historical evidence. In fact, Copernicus may be just the tip of the iceberg. Research into the scientific connections between Europe and the Islamic world in the early modern period is still very much in its infancy.

Assignment: Read Nicholas Copernicus, De revolutionibus (On the revolutions) (1543) through chapter 10.

References:

Robert S. Westman, The Copernican Question: Prognostication, Skepticism, and Celestial Order (Berkeley and Los Angeles: University of California Press, 2011).

George Saliba, Islamic Science and the Making of the European Renaissance (Cambridge, MA; London: MIT Press, 2007), esp. pp. 196-221.

André Goddu, Copernicus and the Aristotelian Tradition: Education, Reading, and Philosophy in Copernicus’s Path to Heliocentrism (Leiden; Boston: Brill, 2010).

Peter Barker and Matjaž Vesel, “Goddu’s Copernicus: An Essay Review of André Goddu’s Copernicus and the Aristotelian Tradition” Aestimatio 9 (2012): 304–336.