Science and Religion series -- "The Galileo Affair, Part 1: Copernicus"

In this fourth essay in our Science and Religion series, Fr Jonathan Jong tells the story of the Galileo affair, beginning several generations earlier with Copernicus, whose heliocentric model has since replaced Ptolemy’s geocentric model.

Obviously, the Earth revolves around the Sun. This is obvious now in part because we—maybe not me or you, but someone—can go to space, and see it happening. The trouble is that is was not at all obvious in 1543.

Nicolaus Copernicus died in 1543: he lived just long enough to see the publication of his magnum opus, On the Revolutions of the Heavenly Spheres. It is said that he held a copy of it on his death bed. Revolutions is infamous for proposing that the Earth revolves around the Sun: some say that it marks the beginning of modern science, and others add that it is the first major battle in the warfare between science and religion. The Church is suppose to have suppressed Revolutions and its heliocentric thesis, not only in Copernicus’s lifetime but more dramatically in Galileo’s nearly a century later. But let’s not get ahead of ourselves.

Lucas Watzenrode the Younger was the Prince-Bishop of Warmia from 1489 to 1512, and the uncle of Nicolaus Copernicus. Watzenrode arranged for Copernicus to study, first in Krakow and then in Bologna and then in Padua. Copernicus did not complete a degree in Krakow, but it seems that his interest in astronomy and mathematics was sparked there. In Bologna, he began a doctorate in canon law and in Padua he studied medicine. It is not clear whether he completed his medical degree (though he was licensed to practice), and when he was eventually awarded a doctorate in canon law, it was from the University of Ferrera instead of Bologna.

The word nepotism comes form the sorts of shenanigans medieval ecclesiastics like Watzenrode pulled as when he made Copernicus a canon of Frombork Cathedral, a senior administrative position. Not that he went to Frombork: until his uncle’s death in 1512, Copernicus served as his secretary and physician. When he eventually did more to Frombork, he continued in his administrative duties, as well as serving as a consulting physician. Throughout this early part of his career, Copernicus did make astronomical observations, and by 1514 had already committed his idea of a heliocentric universe to paper, in six leaves titled the Commentariolus, never published but circulated among friends.

People argue about whether Copernicus was ordained to the priesthood. This seems unlikely; there is, in any case, no positive evidence that he was. He was, however, considered for his uncle’s episcopal seat in 1537: had he been appointed, he would have had to be ordained if he was not already. In any case, he spent his entire career working for the church, rather than for a university. He conducted his research from this job, and when he died, he was buried in Frombork Cathedral, where his body remains to this day.

It is a long time from 1514 to 1543, and Copernicus spent much of that time making astronomical observations and calculations to support his theory. But let’s start in 1514, or indeed a few years earlier when Copernicus first explores heliocentrism. The obvious question is: why? The short answer is that he was dissatisfied with Ptolemy’s old geocentric model. He was not alone in this. By the 16th century, Claudius Ptolemy’s model would have been about 1,400 years old. Over time, its inadequacies had been noticed and patched up in various ways. Even Ptolemy’s own model is a version of a more general kind of Aristotelian theory that had (a) the Earth unmoving at the centre of the universe, and (b) other celestial bodies, including the Sun and planets, moving in (c) circular orbits around the Earth (d) at uniform speed.

The simplest version of the idea is just a series of concentric circles, all with the Earth at the perfect centre (Figure 1; apologies for the terrible diagrams). The problem with this simple model is that it does not fit astronomical observations at all well. Very early on, long before Ptolemy, ancient astronomers fiddled with the simple model by moving the Earth from the exact centre. Despite being called a geocentric model, Ptolemy’s—following his predecessors—instead revolved around a hypothetical point called an eccentric (Figure 2).

Ancient astronomers also noticed that the planets did not seem to move in uniform circles. Indeed, they noticed that planets might mostly move along an orbit, but then they would seem to reverse direction for a few nights before resuming to move forward again! This is called retrograde motion. To solve this problem, astronomers—including Ptolemy—added epicycles and deferents (Figure 3). A deferent is just a good old fashioned circular orbit around the Earth, except that it has an epicycle attached to it. An epicycle is a secondary circular orbit that moves long the deferent.

To make matters worse, there were also major and minor epicycles introduced, so that we have circular orbits that moved along circular orbits that moved along circular orbits around the eccentric (Figure 4).

Ptolemy’s innovation was to introduce the equant (Figure 5). This was to account for another lack of uniformity in the circular motion. Not only did the celestial bodies fail to appear to always move in one direction on their orbits: they also failed to move at uniform speed, at least from the standpoint of the orbital centre (i.e., the eccentric). So, Ptolemy posited that the celestial bodies did move at uniform speed, when observed from this new hypothetical point, located at the same distance but on the other side of the eccentric from the Earth. Now, Ptolemy had a system in which (a) the Earth unmoving a bit off centre of the universe, and (b) other celestial bodies, including the Sun and planets, moving in (c) circular orbits around other circular orbits around the eccentric (d) at uniform speed from the standpoint of the equant.

Copernicus was dissatisfied with this picture of how celestial bodies moved. He was not dissatisfied with the fact that it was geocentric, not that it was really geocentric (see the bit about eccentrics above). Nor was he dissatisfied with the accuracy of the model in matching astronomical observations: the Ptolemaic model really wasn’t bad in terms of predicting the locations and movements of planets. Ptolemy’s introduction of the equant was a substantial improvement compared to previous geocentric models, but in the 1,400 years between Ptolemy’s death and Copernicus’s work, the further tweaks made to the model were minimal.

Copernicus didn’t like equants. We know this because he says so in the Commentariolus, but also because his model is really substantially similar to Ptolemy’s except in two ways. The first is the obvious one that Copernicus’s model is heliocentric. The second is that it doesn’t contain equants. It contained eccentrics, deferences, and epicycles, but no equants. (Incidentally, he really had no problem with epicycles. Without getting bogged down in the details: Copernicus’s system begins with fewer epicycles than Ptolemy’s, but the eradication of the equants brought more back.) So, what can we say comparing Ptolemy’s and Copernicus’s models. First, they are equally accurate at accounting for the observational data. Second, Ptolemy’s is more conservatively Aristotelian with respect to the location of the Earth. Thirdly, Copernicus’s is more conservatively Aristotelian with respect to circular motion of uniform speed, having dispensed with Ptolemy’s fudge. Fourth, both theories are quite complicated, with their systems of circles within circles revolving around eccentrics; arguably, containing one fewer type of astronomical construct, the equant, Copernicus’s is the more elegant of the two.

From a purely astronomical standpoint, it’s not obvious that Copernicus’s model was—at the time—really better than Ptolemy’s. From a physical standpoint—again at the time—it was very much worse. During Copernicus’s day, practically everyone believed that the Earth sat stationary at the centre of the universe. This was not because they hadn’t thought of the possibility that it instead revolved around the Sun. Certainly Ptolemy considered this possibility, but found that the evidence—as he understood it at the time—counted decisively against it. First, there was the common sense argument that the Earth sure doesn’t seem to be moving. When we normally sit atop a moving object—say a horse or a double-decker bus—we can usually feel the motion: for example, we might feel the wind in our faces. Now we know that this isn’t always true, not just because we know that the Earth does in fact revolve around the Sun, but also because we have the experience of sitting in cars and aeroplanes without noticing very much that we are moving very quickly, except when there are sudden changes in acceleration or gravity. Of course, there were no cars or planes back in Ptolemy’s or Copernicus’s day, so neither of them had the benefit of these experiences.

There was also the argument from falling objects. Say we are riding on the aforementioned horse. Now say we throw a ball high up in the air, as the horse is moving quickly forward. As the ball is in the air, we—on the moving horse—would move ahead of it, and the ball would eventually fall behind us. In the same way, if we were standing still on a moving Earth and threw a ball up high in the air, wouldn’t the fact that the Earth were moving mean that we would’ve moved along with it while the ball was in midair, so that by the time it comes back down it’d land behind us? Remember that Newton’s laws of motion and gravity were still more than 150 years away from Copernicus sitting at his desk in Frombork. (Galileo’s work on motion would come to Copernicus’s rescue first, about 70 years after the latter’s death.)

Then there was the argument from stellar parallax. The best way to explain this is to have you experience it for yourself. Pick up an object near to you, say a pen, and hold is up it front of you. Now, close your left eye, so that you are only looking through your right eye. Notice the relationships in space between the pen and the objects around it, say a lamp or bookcase or whatever it is in the room now. Now, close your right eye, so that you are only looking through your left eye, and notice the relationships in space between the pen and those same objects. What you’ll see is that those relationships have changed. The pen might now look closer to the lamp and further from the bookcase, or whatever. This is parallax, the apparent shift in the positions of objects due to our motion rather than the motion of the objects in question. I had you use one eye rather than the other, but the same effect can be achieved if you bob your head from left to right. You can try that too.

Let’s now expand our field of vision, from the pen and furniture in your room to a star and the other celestial objects in the night sky. The motion of our eyes or heads affects how we perceive the pen’s location relative to other objects in our visual field. By the same line of reasoning, the motion of the Earth should affect how we perceive the relative locations of celestial bodies like stars. But—according to observations at the time, from Ptolemy to Copernicus—the angular distance of stars remained constant from night to night, as if the Earth were stationary. Now, it turns out that there is stellar parallax: but it was undetectable even in Copernicus’s time, and so he and his supporters did not have access to this evidence that we now have. Stellar parallax was first observed only in the 19th century.

All of which is to say that if you were around in 1514—and even in 1543—it would not have been at all unreasonable of you to have rejected Copernicus’s heliocentric model. You would now be very unreasonable to do so, but things were different in the 16th century. And though people certainly did reject Copernicus’s theory for many decades, he had some supporters too. The Commentariolus received generally positive feedback, though some people did point out that it disagreed with passages of the Bible about the fixity of the Earth (e.g., Psalm 93.1) and the Sun miraculously standing still, which implies that it usually moves (Joshua 10). Martin Luther—a contemporary of Copernicus’s—was quite snooty about heliocentrism on these grounds too. Other churchmen were much more supportive. In the 1530s, Copernicus had been encouraged to publish his theory by Bishop Tiedemann Giese and Cardinal Nicholas Schönberg, but it was only in the 1539 or so that things went seriously underway. This was in part due to the instigation of an earnest and enthusiastic young matemathician, Georg Joachim Rheticus, who had as his patron the great Reformation theologian Philipp Melanchthon.

After 1540, Copernicus began preparing Revolutions for publication. In it, he thanked Giese and Schönberg—Catholic churchmen—and added a dedication to Pope Paul III, a shrewd political move. He did not mention Rheticus, who was a Protestant: as we shall revisit in the second part of this miniseries on the Galileo Affair, these were difficult religious times. He did however hand the manuscript over to Rheticus to have it printed in Nuremberg: Protestants were better at printing than Catholics were. Unfortunately, Rheticus could not see the publication to its completion, as he was called away Leipzig. He handed the task over to Andrew Osiander, a Lutheran minister. This is where things get a bit pear-shaped. Osiander—consulting nobody—added an anonymous preface to the book, saying that it merely expresses hypotheses that “need not be true nor even probable”. This was certainly not Copernicus’s position: Giese and Rheticus both protested to this intervention. It was only later, thanks largely to Johannes Kepler—we will get to him later in the series too—that the true origin of the preface would be widely known.

There is no simple way to characterise how Revolutions was received. In 1543, 400-500 copies were printed; in 1566, another 500 copies were printed [1]. It was a technical book, quite unlike Galileo’s later work and Darwin’s centuries later. Most of the readers were astronomers, who seem to have received it warmly, though they were aware that a moving Earth was physically implausible. Broadly speaking, astronomers liked the banishment of the equant, but few of them were thorough-going Copernicans. The historian Robert Westman counts ten in the three generations after the publication of Revolutions. He even names them in his book, The Copernican Question. Others were with Osiander: they did not believe that the Earth revolved around the Sun, but they liked Copernicus’s model more than Ptolemy’s as an algorithm. Others still—most notably Tycho Brahe—came up with a hybrid model, in which the Sun, Moon, and stars revolved around the Earth, while the planets revolved around the Earth. The reasons for the rejection of Copernicus are mixed and difficult to disentangle. There were physical reasons and theological reasons, both of which I have already mentioned. It is true that some of the true Copernicans were theologically suspect—Giordano Bruno, for example—but others were perfectly orthodox by the standards of their day, and on either side of the Catholic-Protestant divide: consider Rheticus and Kepler, for example. Motivations for accepting physical heliocentrism were also complicated. Some—like Rheticus—emphasised the harmony in heliocentrism: he felt that the motions Copernicus posited were more unified or inter-connected than those in Ptolemy’s. Others argued that Copernicanism explained the relationship between retrograde motion and the Sun’s location: the Prolemaic system describes this relationship too, but does not account for it.

The problems would come later, and in 1616 the Church placed Revolutions on the Index of Forbidden Books, saying that it was “suspended until corrected”. Corrections were proposed to nine sentences in the book, and the corrected versions were permitted from 1620 onward. Examinations of extant copies of pre-1620 editions from the time suggest that if there were attempts made to correct books already in circulation, then they were not very effective. Instead, some people—Rheticus most notably—had scribbled over Osiander’s preface rather than over Copernicus’s text. In 1758, Pope Benedict XIV removed the original from the Index. 1616 was also the year of Galileo’s first trial. This was not a coincidence.

Further Reading

DeWitt, Richard. (2010). Worldviews: an introduction to the history and philosophy of science. Chichester: Wiley-Blackwell.

Gingerich, O. (2004). The book nobody read: chasing the Revolutions of Nicolaus Copernicus. London: Random House.