by Dan Falk
SETTING THE EARTH IN MOTION
But these advantages came at a price: Copernicus’s system set the Earth in motion; it was now one of the planets, kin with Mercury, Venus, and the rest. “What appear to us as motions of the Sun arise not from its motion,” he wrote, “but from the motion of the Earth and our sphere, with which we revolve about the Sun like any other planet.” The model required one to think of the Earth as hurtling through space at enormous speeds—a shocking violation of common sense. No wonder Copernicus kept using the word “absurd” to describe his own theory.
Indeed, the notion of a speeding Earth had always been a key objection: Why don’t we feel the Earth’s motion? It is worth pausing to think about how our very conception of motion has evolved since Copernicus’s day, as this may help us see why the idea of a moving Earth seemed so preposterous four hundred years ago. Today we get on a jetliner and experience (barring turbulence) a perfectly smooth ride; many of us have no trouble falling asleep at thirty thousand feet, as the plane whips along at six hundred miles an hour. We have a sense from movies like Apollo 13, and the Star Trek franchise, that traveling though space (in the absence of laser blasts and asteroid impacts) is even smoother—as indeed it would be. But traveling in Renaissance Europe was a good deal rougher. As John Gribbin reminds us:
Remember that in the sixteenth century, motion meant riding on a galloping horse or in a carriage being pulled over rutted roads. The notion of smooth motion (even as smooth as a car on a motorway) must have been very difficult to grasp without any direct experience of it—as late as the nineteenth century there were serious concerns that travelling at the speed of a railway train, maybe as great as 15 miles per hour, might be damaging to human health.
A moving Earth seemed bizarre for many reasons. Think of a ball tossed into the air: If the Earth is rotating, shouldn’t the ball land some distance from the spot from which it was thrown? Why aren’t birds and clouds swept backward in the same manner? Indeed, Aristotle himself had ridiculed the idea of a moving Earth for just such reasons. (The same objections, incidentally, can be raised against the Earth’s daily rotation, as well as its yearly journey around the sun.) Copernicus, however, had an answer: The Earth’s atmosphere, and everything within it, must be carried along with it, so that no such motion is felt. (A complete explanation requires the concept of inertia, the tendency of moving objects to remain in motion and of stationary objects to remain at rest. Unfortunately, this idea was unknown at the time, making its appearance only with the work of Kepler and Galileo a half century later; it would eventually become a cornerstone of Newton’s mechanics.)
And there were more objections: If the Earth really moved around the sun, the stars should appear to shift in position over the course of a year; in astronomical jargon, they should display parallax.* However, no such shift was seen in the positions of the stars. And if the Earth moves in a vast orbit, then it must be nearer to certain stars at certain times of the year; therefore, the stars should vary in brightness over the course of the seasons. The answer, Copernicus reasoned, is that the stars must be very far away compared with the size of our solar system.
This view of a large, possibly infinite universe was a radical departure from the prevailing medieval view. Suddenly, the stars were seen to lie at distances that defy the imagination. Yet for Copernicus, this new, larger universe—with its more coherent cosmological picture—was easier to swallow than Ptolemy’s countless epicycles. “I think it is a lot easier to accept this [larger universe] than to drive ourselves to distraction multiplying spheres almost ad infinitum,” he wrote, “as has been the compulsion of those who would detain the Earth in the center of the universe.”
HYPOTHESIS OR HERESY?
As we have seen, science was not “at war” with religion at this time, and in fact many scientists, Copernicus among them, were members of the clergy. So what objection, if any, did religious thinkers have to this new arrangement of the heavens? To begin with, we must clear up one of the lingering misconceptions about Copernicus’s system: that it somehow “dethroned” the Earth from a privileged, central position in the cosmos.* In fact, the Earth’s position, according to the medieval view, was at the “lowest” position relative to the lofty spheres of the planets and the even loftier sphere of the fixed stars. At the center one would find hell—not a particularly privileged spot. Indeed, one could argue that Copernicus ennobled the Earth, lifting it up into the heavens with the other planets—closer to God, one might imagine. Having said that, the fact that Copernicus forced us to think of the Earth as a body in motion, and indeed one of many such bodies, must have been disconcerting. “Central” or not, the Earth had at least been unique, and static. As historian I. Bernard Cohen puts it, “The Aristotelian uniqueness of the earth, based on its supposedly fixed position, gave people a sense of pride that could hardly arise from being on a rather small planet (compared to Jupiter or Saturn) in a rather insignificant location (position 3 out of 7 successive planetary orbits).”
What Copernicanism threatened was Aristotelian physics rather than Christian dogma—but, as we’ve seen, these two ideologies had become intertwined by the sixteenth century. If the Roman Catholic Church had allied itself with some other way of thinking, history might have taken quite a different route. As Francis Johnson notes, “Had the Christian theologians not previously committed themselves to Aristotelian science, they would have found no great difficulty in reconciling passages in the Scriptures with the ideas of Copernicus.” At first, the Church showed little sign of any discomfort with the heliocentric model. It was seen primarily as a mathematical tool, one that held the promise, perhaps, of improved astronomical predictions. When a Vatican council convened in 1515 to consider reforming the Julian calendar currently in use, they wrote to Copernicus to ask for his opinion. Indeed, Pope Clement VII’s personal secretary lectured on the theory, with the pontiff and several cardinals in the audience.
And yet, the structure of the heavenly spheres—the physical interpretation of these complicated mathematical models—was another matter. This was something Church leaders were accustomed to dealing with, as they had spent centuries trying to reconcile Aristotelian physics with scripture, keeping only those views that were consistent with the Bible. A much-discussed biblical passage comes from the Book of Joshua, in which the Israelite leader commands the sun—not the Earth—to stand still in order to prolong the hours of daylight, allowing his army to prevail in battle:
Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon.
And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies.… So the sun stood in the midst of heaven, and hasted not to go down about a whole day.
(10:12–13)
It’s not clear that the heliocentric model received a much warmer reception from Protestant thinkers; like their Catholic counterparts, they seemed more than willing to embrace the Copernican model as a mathematical theory, and to simply ignore whatever its claims to “reality” might be. The reformer Martin Luther is said to have offered a snide dismissal of the theory, chastising astronomers who wished to sound “clever” by proposing “to turn the whole of astronomy upside down.” The truth, he declared, was set out clearly enough in the Bible: “I believe the Holy Scripture, for Joshua commanded the Sun to stand still and not the Earth.” On another occasion Luther is said to have called Copernicus a “fool.”*
When natural philosophy appeared to contradict scripture, there was little room for compromise. In 1270, the bishop of Paris issued a list of thirteen propositions, linked to the views of the more radical Aristotelians, which were deemed false and heretical. Seven years later the list was expanded to 219 items. More than twenty of them refer to cosmology. Among them was a prohibition against claiming “that the world is eternal as to all the species
contained in it; and that time is eternal, as are motion [and] matter.…”*
Perhaps, then, it is not so surprising that Copernicus, who had been quietly setting down a detailed exposition of his theory, was hesitant to publish. Only at Rheticus’s urging did he finally allow his great work to see the light of day. Under Rheticus’s supervision, the book was eventually printed in Nuremberg, as we’ve seen, with a copy of De revolutionibus reaching Copernicus as he lay on his deathbed.
“THE MARVELOUS SYMMETRY OF THE UNIVERSE”
Lengthy and highly mathematical, Copernicus’s book was structured in a fashion parallel to Ptolemy’s, with a series of sections (“books”) outlining the main arguments. (On the title page, in Greek, one finds the Platonic motto: “Let no one who is ignorant of geometry enter here.”) After that, however, came a surprise: Unknown to Copernicus, an anonymous preface had been added to the book. We now know it was penned by Andreas Osiander, a Lutheran clergyman who oversaw the final stages of the printing process after Rheticus was called away to an academic post at Leipzig. The preface served as a disclaimer, insisting that the heliocentric system was just a theoretical model rather than a true description of the cosmos; Osiander, it seems, was more nervous than Rheticus. The preface cautions the reader that “these hypotheses need not be true or even probable. On the contrary, if they provide a calculus consistent with the observations, that alone is enough.” Indeed, Osiander says that astronomy isn’t in the business of discovering “truth”:
So far as hypotheses are concerned, let no one expect anything certain from astronomy, which cannot furnish it, lest he accept as the truth ideas conceived for another purpose, and depart from this study a greater fool than when he entered it.
Whatever the reader may have thought of this preface, they would soon come to Copernicus’s own peace offering—a brief letter in which he dedicates the work to Pope Paul III. The letter begins:
Holy Father, I can guess already that some people, as soon as they find out about this book I have written on the revolutions of the universal spheres, in which I ascribe a kind of motion to the earthly globe, will clamor to have me and my opinions shouted down.
Copernicus recognizes that some of these attacks may be religiously motivated. There will be “babblers,” he fears, who “claim to be judges of astronomy although completely ignorant of the subject and, badly distorting some passage of Scripture to their purpose, will dare to find fault with my undertaking and censure it.” He confidently states that such attacks are “unfounded.”
Eventually he settles down to the business at hand. Having established the motion of the Earth, he summarizes this new heliocentric picture: “Truly indeed does the sun, as if seated upon a royal throne, govern his family of planets as they circle about him,” he writes. “Thus we discover in this orderly arrangement the marvelous symmetry of the universe, and a firm harmonious connection between the motion and the size of the spheres.…” As for the sun, what better place for it than at the center of this wonderful arrangement? “For who would place this lamp of a very beautiful temple in another better place than this wherefrom it can illuminate everything at the same time?” Osiander’s preface notwithstanding, Copernicus had been persuaded by the elegance and cohesive logic of the heliocentric model. The idea, if not the details, had convinced him that it was a true description of nature.
THE REVOLUTION THAT WASN’T?
In recent decades, scholars have taken a close look at the supposed “revolution” brought about by Copernicus’s work, and found it to be less than revolutionary. To begin with, his model did not necessarily lead to more accurate predictions for the positions of the planets than Ptolemy’s; indeed, from a purely mathematical perspective, they were virtually identical. (One may think of the Copernican system as roughly equivalent to the Ptolemaic system, but with the positions of the sun and Earth swapped.) In fact, the new model, in its simplest form, could not yield accurate planetary positions, because it contained a fatal error. Like Plato, Copernicus was committed to the idea that circles and spheres were the embodiment of perfection, and assumed that the planets moved with uniform speed along perfectly circular paths. It would be another seventy years before Johannes Kepler would correct the error, deducing their true shape (ellipses rather than circles).
Nor was the Copernican model “simpler” in any unambiguous way. In order to precisely match the observed motions of the sun, moon, and planets, the heliocentric system also needed eccentrics and epicycles, just as Ptolemy’s did; it was very much an intricate system of circles on top of circles, just as its predecessor had been. (Though it did manage to do away with equant points, along with the largest of Ptolemy’s epicycles, both of which are redundant in the Copernican model, and it managed to bring the total number of circles down from about eighty to thirty-four.) The complexity of the Copernican system may seem surprising, especially in light of the much-reproduced diagram that comes in book 1 of De revolutionibus—that famous drawing of neat and tidy concentric circles, seen in Fig. 0.1. Historians believe that this was intended as a schematic, meant to ease the reader into the book; later on, Copernicus lays out the intricacies of his theory in all its mathematical and geometrical glory. The structural similarities between De revolutionibus and the Almagest make it hard to argue that the newer book is simpler than its predecessor; one might even view Copernicus’s book more as a commentary on the earlier work (albeit a critical one) than as a “new” theory. “At the risk of verging upon the facetious,” notes S. K. Heninger, “we might say that Copernicus did no more than a bit of tinkering with the existing world-view.… Copernicus himself thought in terms of simplifying the old, not in terms of introducing the new.” And note that the title refers to the revolutions of “the heavenly spheres,” not “the planets.” As I. Bernard Cohen puts it, in De revolutionibus one finds “a kinship of geometrical methods and constructions” paralleling those of the Almagest, which “belies any simple claim that Copernicus’s book is in any obvious sense a more modern or simpler work.” Still, “simplicity” is not an easy word to define. One could argue that the Copernican model offers a more natural explanation of retrograde motion; while the epicycles are still there, they exist solely for the flexibility they provide.
Copernicus’s book is an almost paradoxical mix of conservatism and innovation. Though he warned the pope of his “absurd” new theory, he also stressed, at every turn, its continuity with ancient thinking. Again and again, he references one or another ancient Greek philosopher; as Peter Dear puts it, De revolutionibus “was presented explicitly as a renovation of the ancient Greek astronomical tradition.” And as Heninger notes, “only in retrospect does he assume the role of an intellectual radical.”
* * *
Was the Copernican remodeling of the heavens a seminal event that changed the course of history—or an academic exercise that changed nothing? We can certainly view it today as an earth-shattering—or more literally, “earth-moving”—event. Three cheers for the power of hindsight. But how was it seen in Copernicus’s time? How big a splash did his theory actually make? To I. Bernard Cohen, writing in the 1980s, De revolutionibus was so unrevolutionary as to be barely worthy of notice: “The idea that a Copernican revolution in science occurred goes counter to the evidence … and is an invention of later historians.” He adds that the revolution that we usually associate with the name Copernicus is more properly attributed to the work of Kepler, more than half a century later. But philosopher Richard DeWitt, writing in the 2000s, assures us that, from the time of Copernicus’s death to the end of the sixteenth century, “his theory was widely read, discussed, taught, and put to practical use.” Perhaps both are right: Copernicus’s book marked only the first phase in the revolution; but certain people definitely took notice. It was hardly, as Arthur Koestler once described it, “the book that nobody read.”
A WORLD MADE FOR WHOM?
At least one aspect of the Copernican system was, in fact, revolutionary. His cosmos was tru
ly vast: “So far as our senses can tell,” he wrote, “the earth is related to the heavens as a point is to a body and something finite is to something infinite.” That last word is tantalizing, though historians believe that Copernicus was unlikely to have imagined a literally infinite universe. Nonetheless, it was larger than anything the Western mind had had occasion to picture in the preceding centuries. One thirteenth-century astronomer had calculated that the most distant of the known planets, Saturn, lay some 73 million miles away—already “a staggering distance to the best-traveled medievals,” as one historian puts it, with the fixed stars, presumably, lying just beyond. Interestingly, because he accepted the ancient Greek value for the distance to the sun, Copernicus’s estimate of the distances to the planets actually shrank the solar system compared with the Ptolemaic model. The distances to the stars, however, increased tremendously. The absence of stellar parallax meant that the stars had to be much more distant than in the medieval view.* By one estimate, the Copernican universe was larger than its medieval predecessor by a factor of four hundred thousand. This was an age when it was taken for granted that the universe existed for our benefit. What, then, occupied all of this empty space between the planets and the stars? What could be its purpose?
Decades later, when Galileo was writing his treatise on the competing world systems, he was conscious of this (by now familiar) objection. His book was written in the form of a dialogue, and he has the character Simplicio, the defender of the traditional worldview, tackle the “empty space” problem:
Now when we see this beautiful order among the planets, they being arranged around the earth at distances commensurate with their producing upon it their effects for our benefit, to what end would there then be interposed between the highest of their orbits (namely, Saturn’s) and the stellar sphere, a vast space without anything in it, superfluous, and vain. For the use and convenience of whom?