Rheticus and the First Report
Copernicus declined, but agreed to let Rheticus publish a simple analysis of the first four books of his work. So in 1540 the young scholar printed at Danzig the first report on On the Revolutions of the Heavenly Spheres, and sent a copy with high hopes to Melanchthon. The kindly theologian was not convinced. When Rheticus returned to Wittenberg early in 1540 and taught the Copernican hypothesis in his class, he was, as he himself writes, "ordered" to teach Johannes Sacrobosco's Sphere instead. On 16 October 1541 Melanchthon wrote to a friend that some think it a great feat to defend an absurdity, like that Prussian astronomer who moves the earth and holds the sun still; wise rulers, he said, ought to restrain the unruliness of human minds.
In the summer of 1540 Rheticus returned to Frauenburg and stayed until September 1541. He repeatedly urged his master to give the full text of his work to the world. When two high-ranking churchmen joined him, Copernicus, perhaps because he felt one foot was surely in the grave, gave way. He added last supplements to his manuscript and allowed Rheticus to send it to a Nuremberg publisher who took all costs and risks (1542). Later, when Rheticus left Wittenberg to teach at Leipzig, he entrusted supervision of the printing to his friend Andreas Osiander, a Lutheran pastor of Nuremberg.
Osiander's Preface
Osiander had written to Copernicus on 20 October 1541 that it would be better to offer the new view as a hypothesis, not as a proven truth, and in another letter of the same date he told Rheticus that in this way "the Aristotelians and theologians will keep their peace." Copernicus himself, both in the Commentariolus and in his main book, had repeatedly called his theories hypotheses, yet in the dedication he claimed that he had based his views on "the clearest evidence." Nothing is known of his reply to Osiander. At any rate, Osiander, without signing it, added to the book a preface addressed "To the reader concerning the hypotheses of this work."
In substance it says: no doubt many scholars, because of the reputation these novel hypotheses have gained, will be deeply offended by the book's scientific theories. But the hypotheses need not be true, or even probable; it is quite enough if the calculation based on them agrees with astronomical observation. The astronomer will gladly follow the hypotheses easiest to grasp; the philosopher may seek greater probability; but neither will reach certain reality unless truth is revealed to them by divine revelation. So let us accept that these new hypotheses take their place beside the old, which are no more likely to be real; besides, they are admirable and simple, and in studying them we shall gain a great treasure of learned observations. Beyond this, no one should expect certainty from astronomical hypotheses, which the astronomer cannot give; whoever accepts as truth what was devised for other ends will probably leave this science more ignorant than he entered it. (The source gives this preface with omissions.)
The preface has often been condemned as a presumptuous distortion. Copernicus was probably deeply hurt by it, for the old man, who had lived thirty years with his theory, had come to feel it as part of his life and blood and to regard it as a description of the world's realities. Yet Osiander's preface was both prudent and fair, since it noticeably reduced the natural resistance of many minds to a new and revolutionary idea; and it is still a good reminder that our descriptions of the universe are no more than the fallible statements of drops of seawater about the sea, which may in their turn be rejected or corrected.
The Death of Copernicus and the Dedication
At last, in the spring of 1543, the book appeared under the title The First Book of the Revolutions of the Heavenly Spheres, later called On the Revolutions of the Heavenly Spheres. One of the first copies reached Copernicus on 24 May 1543. He was on his deathbed. He opened the title page, read it, smiled, and within the hour closed his eyes on the world. The dedication to Pope Paul III was itself an attempt to disarm religious resistance to a theory that plainly contradicted the express text of Scripture, as Copernicus knew very well.
He began his preface with reassuring words of faith: he still believed we must avoid accepting scientific theories that are wholly contrary to religious principles. He had long hesitated to publish, asking himself whether it would not be better to follow the Pythagoreans, who disclosed the secrets of their philosophy not in books but by word of mouth, and only to relatives and friends. Learned churchmen such as Nicholas Schonberg, Cardinal of Capua, and Tiedemann Giese, bishop of Kulm, had urged him to publish his discoveries. (Copernicus thought it wise not to mention the Lutheran Rheticus in this connection.) He acknowledged his debt to the Greek astronomers, though inadvertently he left out the name of Aristarchus. He believed astronomers needed a better theory than Ptolemy's, since on the earth-centered basis they ran into many scientific difficulties and could not even compute the length of the year exactly. Finally Copernicus turned to the pope, as a man "of eminent rank ... and a lover and protector of every science, even mathematics," and asked him to take him under his protection against the "bites of slanderers," who without sufficient knowledge of mathematics "allow themselves to pass judgment on such matters," or who "attack my theory because it conflicts with certain passages of Scripture."
The Argument of the Book
The exposition begins with certain assumptions. First, the universe is spherical. Second, the earth is spherical, that is, left to itself it tends toward one center of gravity and therefore naturally takes the form of a sphere. Third, the motions of heavenly bodies are uniform and circular, or compounded of circular motions, since the circle is the "most perfect form" and since "the mind shrinks in dread" at the thought that heavenly motions might not be uniform.
Copernicus points to the relativity of motion: any apparent change of place comes either from the motion of the observer or of the thing observed, or from change in both, provided they differ; for if things move equally with respect to each other, no motion is perceived between the object and the observer. The apparent daily motion of the heavens around the earth may therefore come from the earth's real daily rotation on its axis, and the apparent yearly motion of the sun around the earth can be explained by supposing that the earth circles the sun once a year.
He anticipates the objections. Ptolemy had argued that if the earth turned, clouds and objects on it would be flung away or left behind. Copernicus replies that the same objection applies with greater force to the claim that the great heavenly bodies revolve around the earth, since their immense distances require them to travel enormous orbits at inconceivable speed. Ptolemy had also said that an object thrown straight up from a rotating earth should not fall back to its starting point; Copernicus answers that such objects, like clouds, are "part of the earth" and share its motion. To the objection that the earth's yearly circuit should shift the apparent positions of the "fixed" stars as seen from two opposite points of its orbit, he replies that the earth does move in orbit, but the sphere of the stars is so far away that the change in their relative positions is imperceptible to us. (With modern instruments this shift has become observable to some degree.)
He sums up his arrangement of the heavens in a few compact sentences. First and highest is the sphere of the fixed stars, which contains itself and all else and is therefore motionless. Among the moving bodies, first is Saturn, which completes its orbit in thirty years; then Jupiter, with a twelve-year circuit; then Mars, which completes its orbit in two years. In fourth place is the yearly revolution around the sun, in whose orbit lies the earth, with the moon's orbit as an epicycle. Fifth is Venus, which circles the sun in nine months. Sixth is Mercury, which completes its circuit in eighty days. And in the midst of all stands the sun. Some have called it the lamp of the world, others its mind, others its ruler; and rightly, for the sun, sitting as on a royal throne, governs the family of stars that circle around it. In this ordered arrangement, he says, we find a marvelous symmetry in the universe and a fixed harmony between the motion and the size of the orbits, such as could be found in no other way.
What the System Kept
Every development in a scientific theory usually carries along remnants of the one it replaces. Copernicus built his proofs on observations inherited from Ptolemy and kept much of the Ptolemaic machinery of epicycles and eccentrics, which Kepler later had to discard. Most curious was his calculation that the sun is not exactly at the center of the earth's orbit; he said the center of the universe is a point "three times the sun's diameter" from the sun (the source's figure), and that the centers of the other planetary orbits lie outside the sun's globe and are not alike. Copernicus transferred to the earth and sun two ideas of the day now wholly discarded: that the sun is the approximate center of the universe, and that the sun is at rest. He also thought that besides its rotation and revolution the earth had a third motion, which he believed necessary to explain the tilt of its axis and the precession of the equinoxes.