—Ecclesiastes
Amazed, and as if astonished and stupefied, I stood still, gazing for a certain length of time with my eyes fixed intently upon it…. When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes.
—Tycho, on the supernova of 1572
Mikolai Kopernik, though rightly esteemed as a great astronomer, was never much of a stargazer. He did some observing in his student days, assisting his astronomy professor at Bologna, Domenico Maria de Novara, in watching an occultation of the star Aldebaran by the moon, and he later took numerous sightings of the sun, using an instrument of his own devising that reflected the solar disk onto a series of graph lines etched into a wall outside his study. But these excursions served mainly to confirm what Kopernik and everybody else already knew, that the Ptolemaic system was inaccurate, making predictions that often proved to be wrong by hours or even days.
Kopernik drew inspiration less from stars than from books. In this he was very much a man of his time. The printing press—invented just thirty years before he was born—had touched off a communications revolution comparable in its impact to the changes wrought in the latter half of the twentieth century by the electronic computer. To be sure, Greek and Roman classics had been making their way from the Islamic world to Europe for centuries, and with enlightening effect—the first universities had been founded principally to house the books and study their contents—but the books themselves, each laboriously copied out by hand, were rare and expensive, and frequently were marred by transcription errors. All this changed with the advent of cheap, high-quality paper (a gift of Chinese technology) and the press. Now a single competent edition of Plato or Aristotle or Archimedes or Ptolemy could be reproduced in considerable quantities; every library could have one, and so could many individual scholars and more than a few farmers and housewives and tradespeople. As books spread so did literacy, and as the number of literate people increased, so did the market for books. By the time Kopernik was thirty years old (and printing itself but sixty years old), some six to nine million printed copies of more than thirty-five thousand titles had been published, and the print shops were working overtime trying to satisfy the demand for more.
Kopernik was as voracious a reader as any, at home in law, literature, and medicine as well as natural philosophy. Born in 1473 in northern Poland, he had come under the sponsorship of his powerful and calculating uncle Lucas Waczenrode, later bishop of Warmia, who gave him books and sent him to the best schools. He attended the University of Cracow, then ventured south into the Renaissance heartland to study at the universities of Bologna and Padua. He read Aristotle, Plato, Plutarch, Ovid, Virgil, Euclid, Archimedes, and Cicero, the restorer of Archimedes’ grave. Steeped in the literature and science of the ancients, he returned home with a Latinized name, as Nicolaus Copernicus.
Like Aristotle, Copernicus collected books; unlike Aristotle, he did not have to be wealthy to do so. Thanks to the printing press, a scholar who was only moderately well off could afford to read widely, at home, without having to beg admission to distant institutions of learning where the books were kept chained to the reading desks. Copernicus was one of the first scholars to study printed books in his own library, and he studied none more closely than Ptolemy’s Almagest. Great was his admiration for Ptolemy, whom he admired as a thoroughly professional astronomer, mathematically sophisticated and dedicated to fitting his cosmological model to the observed phenomena. Indeed, Copernicus’s De Revolutionibus (On the Revolutions), the book that would set the earth into motion around the sun and bring about Ptolemy’s downfall, otherwise reads like nothing so much as a sustained imitation of Ptolemy’s Almagest.
It is widely assumed that Copernicus proposed his heliocentric theory in order to repair the inaccuracies of the Ptolemaic model. Certainly it must have become evident to him, in his adulthood if not in his student days, that the Ptolemaic system did not work very well: “The mathematicians are so unsure of the movements of the sun and moon,” notes the preface to De Revolutionibus, “that they cannot even explain or observe the constant length of the seasonal year.”1 Prior to the advent of the printing press, the failings of Ptolemy’s Almagest could be attributed to errors in transcription or translation, but once reasonably accurate printed editions of the book had been published, this excuse began to evaporate. Copernicus owned at least two editions of Almagest, and had read others in libraries, and the more clearly he came to understand Ptolemy’s model, the more readily he could see that its deficiencies were inherent, not incidental, to the theory. So considerations of accuracy may indeed have helped convince him that a new approach was required.
But by “new,” Copernicus the Renaissance man most often meant the rediscovery of something old. Renaissance, after all, means “re-birth,” and Renaissance art and science in general sprang more from classical tradition than from innovation. The young Michelangelo’s first accomplished piece of sculpture—executed in the classical style—was made marketable by rubbing dirt into it and palming it off, in Paris, as a Greek relic. Petrarch, called the founder of the Renaissance, dreamed not of the future but of the day when “our grandsons will be able to walk back into the pure radiance of the past”2 (emphasis added); when Petrarch was found dead, at the age of seventy, slumped at his desk after an all-night study session, his head was resting not on a contemporary volume but on a Latin edition of his favorite poet, Virgil, who had lived fourteen centuries earlier. Copernicus similarly worked in awe of the ancients, and his efforts, like so much of natural philosophy then and since, can be read as a continuation of the academic dialogues of Plato and Aristotle.
Aristotle, the first of the Greeks to have been rediscovered in the West, was so widely revered that he was routinely referred to as “the philosopher,” much as lovers of Shakespeare were to call him “the poet.” Much of his philosophy had been incorporated into the world view of the Roman Catholic Church. (Most notably by Thomas Aquinas—at least until the morning of December 6, 1273, when, while saying mass in Naples, Thomas became enlightened and declared that “I can do no more; such things have been revealed to me that all I have written seems as straw, and I now await the end of my life.”) From Aristotle, Copernicus acquired an enthusiasm for the universe of crystalline spheres—although, like Aristotle, Copernicus never could decide whether the spheres actually existed or were but a useful abstraction.
Copernicus also read Plato, as well as many of the Neoplatonic philosophers whose work ornaments and obfuscates medieval thought, and from them absorbed the Platonic conviction that there must be a simple underlying structure to the universe. It was just this unitary beauty that the Ptolemaic cosmology lacked. “A system of this sort seemed neither sufficiently absolute nor sufficiently pleasing to the mind,” Copernicus wrote.3 He was after a grasp of the more central truth. He called it “the principal thing—namely the shape of the universe and the unchangeable symmetry of its parts.”4
Rather early on, perhaps during his student days in sunny Italy, Copernicus decided that the “principal thing” was to place the sun at the center of the universe. He may have drawn encouragement from reading, in Plutarch’s Morals, that Aristarchus of Samos “supposed that the heavens remained immobile and that the earth moved through an oblique circle, at the same time turning about its own axis.”5 (He mentions Aristarchus in De Revolutionibus, though not in this context.) Possibly he encountered more recent speculations about the motion of the earth, as in Nicole Oresme, the fourteenth-century Parisian scholar who pointed out that
if a man in the heavens, moved and carried along by their daily motion, could see the earth distinctly and its mountains, valleys, rivers, cities, and castles, it would appear to him that the earth was moving in daily motion, just as to us on earth it seems as though the heavens are moving…. One could then believe that the earth moves and not the heavens.6
&nb
sp; Copernicus was influenced by Neoplatonic sun worship as well. This was a popular view at the time—even Christ was being modeled by Renaissance painters on busts of Apollo the sun god—and decades later, back in the rainy north, Copernicus remained effusive on the subject of the sun.* In De Revolutionibus he invokes the authority of none other than Hermes Trismegistus, “the thrice-great Hermes,” a fantastical figure in astrology and alchemy who had become the patron saint of the new sun-worshipers: “Trismegistus calls [the sun] a ‘visible god,’ Sophocles’ Electra, ‘that which gazes upon all things.’”7 He quotes the Neoplatonist mystic Marsilio Ficino’s declaration that “the sun can signify God himself to you, and who shall dare to say the sun is false?”8 Finally, Copernicus tries his hand at a solar paean of his own:
In this most beautiful temple, who would place this lamp in another or better position than that from which it can light up everything at the same time? For the sun is not inappropriately called by some people the lantern of the universe, its mind by others, and its ruler by still others.9
Trouble arose not in the incentive for the Copernican cosmology, but in its execution. (The devil, like God, is in the details.) When Copernicus, after considerable toil, managed to complete a fully realized model of the universe based upon the heliocentric hypothesis—the model set forth, eventually, in De Revolutionibus—he found that it worked little better than the Ptolemaic model. One difficulty was that Copernicus, like Aristotle and Eudoxus before him, was enthralled by the Platonic beauty of the sphere—“The sphere,” he wrote, echoing Plato, “is the most perfect… the most capacious of figures … wherein neither beginning nor end can be found”10—and he assumed, accordingly, that the planets move in circular orbits at constant velocities. Actually, as Kepler would establish, the orbits of the planets are elliptical, and planets move more rapidly when close to the sun than when distant from it. Nor was the Copernican universe less intricate than Ptolemy’s: Copernicus found it necessary to introduce Ptolemaic epicycles into his model and to move the center of the universe to a point a little away from the sun. Nor did it make consistently more accurate predictions, even in its wretchedly compromised form; for many applications it was less useful.
Copernicus’s model of the solar system is generally portrayed in simplified form, as in this illustration based upon one in his De Revolutionibus. In its details, however, it was as complex as Ptolemy’s geocentric model.
This, in retrospect, was the tragedy of Copernicus’s career— that while the beauty of the heliocentric hypothesis convinced him that the planets ought to move in perfect circles around the sun, the sky was to declare it false. Settled within the stone walls of Frauenburg Cathedral, in a three-story tower that afforded him a view of Frisches Haff and the Gulf of Danzig below and the wide (though frequently cloudy) sky above—“the most remote corner of the earth,”11 he called it—Copernicus carried out his sporadic astronomical observations, and tried, in vain, to perfect the heliocentric theory he had outlined while still a young man. For decades he turned it over in his thoughts, a flawed jewel, luminous and obdurate. It would not yield.
As Darwin would do three centuries later, Copernicus wrote and privately circulated a longhand sketch of his theory. He called it the “ballet of the planets.” It aroused interest among scholars, but Copernicus published none of it. He was an old man before he finally released the manuscript of De Revolutionibus to the printer, and was on his death bed by the time the final page proofs arrived.
One reason for his reluctance to publish was that Copernicus, like Darwin, had reason to fear censure by the religious authorities. The threat of papal disapproval was real enough that the Lutheran theologian Andreas Osiander thought it prudent to oil the waters by writing an unsigned preface to Copernicus’s book, as if composed by the dying Copernicus himself, reassuring its readers that divine revelation was the sole source of truth and that astronomical treatises like this one were intended merely to “save the phenomena.” Nor were the Protestants any more apt to kiss the heliocentric hem. “Who will venture to place the authority of Copernicus above that of the Holy Spirit?” thundered Calvin,12 and Martin Luther complained, in his voluble way, that “this fool wishes to reverse the entire science of astronomy; but sacred Scripture tells us that Joshua commanded the sun to stand still, and not the earth.”13*
The book survived, however, and changed the world, for much the same reason that Darwin’s Origin of Species did—because it was too technically competent for the professionals to ignore it. In addition to presenting astronomers with a comprehensive, original, and quantitatively defensible alternative to Ptolemy, De Revolutionibus was full of observational data, much of it fresh and some of it reliable. Consequently it was consulted regularly by astronomers—even by non-Copernicans like Erasmus Reinhold, who employed it in compiling the widely consulted Prutenic Tables—and thus remained in circulation for generations.
To those who gave it the benefit of the doubt, Copernicanism offered both a taste of the immensity of space and a way to begin measuring it. The minimum radius of the Copernican sphere of stars (given the unchanging brightnesses of the zodiacal stars) was estimated in the sixteenth century to be more than 1.5 million times the radius of the earth. This represented an increase in the volume of the universe of at least 400,000 times over al-Farghani’s Ptolemaic cosmos. The maximum possible size of the Copernican universe was indefinite, and might, Copernicus allowed, be infinite: The stars, he wrote, “are at an immense height away,” and he expressed wonderment at “how exceedingly vast is the godlike work of the Best and Greatest Artist!”14
Interplanetary distances in Ptolemy were arbitrary; scholars who ventured to quantify them did so by assuming that the various orbits and epicycles fit snugly together, like nested Chinese boxes. The Copernican theory, however, precisely stipulated the relative dimensions of the planetary orbits: The maximum apparent separation of the inferior planets Mercury and Venus from the sun yields the relative diameters of their orbits, once we accept that both orbit the sun and not the earth. Since the relative sizes of all the orbits were known, if the actual distance of any one planet could be measured, the distances of all the others would follow. As we will see, this advantage, though purely theoretical in Copernicus’s day, was to be put to splendid use in the eighteenth century, when astronomical technology reached the degree of sophistication required to measure directly the distances of nearby planets.
The immediate survival of Copernicanism was due less to any compelling evidence in its favor than to the waning fortunes of the Ptolemaic, Aristotelian model. And that, as it happened, was prompted in large measure by changes in the sky—by the apparition of comets, and, most of all, by the fortuitous appearance of two brilliant novae, or “new stars,” during the lifetimes of Tycho, Kepler, and Galileo.
Integral to Aristotle’s physics was the hypothesis that the stars never change. Aristotle saw the earth as composed of four elements—earth, water, fire, and air—each of which naturally moves in a vertical direction: The tendency of earth and water is to fall, while that of fire and air is to rise. The stars and planets, however, move neither up nor down, but instead wheel across the sky. Aristotle concluded that since objects in the sky do not partake of the vertical motion characteristic of the four terrestrial elements, they must be made of another element altogether. He called this fifth element “aether,” from the Greek word for “eternal,” and invested it with all his considerable reverence for the heavens. Aether, he argued, never ages or changes: “In the whole range of time past,” he writes, in his treatise On the Heavens, “so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts.”15
Aristotle’s segregation of the universe into two realms—a mutable world below the moon and an eternal, unchanging world above—found a warm welcome among Christian theologians predisposed by Scriptures to think of heaven as incorruptible and the earth as decaying an
d doomed. The stars, however, having heard neither of Aristotle nor of the Church, persisted in changing, and the more they changed, the worse the cosmology of Aristotle and Ptolemy looked.
Comets were an old problem for the Aristotelians, since no one could anticipate when they would appear or where they would go once they showed up.* (It was owing to their unpredictability that comets acquired a reputation as heralds of disaster—from the Latin dis-astra, “against the stars.”)* Aristotle swept comets under the rug—or under the moon—by dismissing them as atmospheric phenomena. (He did the same with meteors, which is why the study of the weather is known as “meteorology.”)
But when Tycho Brahe, the greatest observational astronomer of the sixteenth century, studied the bright comet of 1577, he found evidence that Aristotle’s explanation was wrong. He triangulated the comet, by charting its position from night to night and comparing his data with those recorded by astronomers elsewhere in Europe on the same dates. The shift in perspective produced by the differing locations of the observers would have been more than sufficient to show up as a difference in the comet’s position against the background stars, were the comet nearby. Tycho found no such difference. This meant that the comet was well beyond the moon. Yet Aristotle had held that nothing superlunar could change.
The other great empirical challenge to Aristotle’s cosmological hegemony came with the opportune appearance, in the late sixteenth and early seventeenth centuries, of two violently exploding stars—what we today call Supernovae. A star that undergoes such a catastrophic detonation can increase a hundred million times in brightness in a matter of days. Since only a tiny fraction of the stars in the sky are visible without a telescope, Supernovae almost always seem to have appeared out of nowhere, in a region of the sky where no star had previously been charted; hence the name nova, for “new.” Supernovae bright enough to be seen without a telescope are rare; the next one after the seventeenth century did not come until 1987, when a blue giant star exploded in the Large Magellanic Cloud, a neighboring galaxy to the Milky Way, to the delight of astronomers in Australia and the Chilean Andes. The two Supernovae that graced the Renaissance caused quite a stir, inciting not only new sights but new ideas.
Coming of Age in the Milky Way Page 6