Dialogue Concerning the Two Chief Systems of the World, Ptolemaic and Copernican - Aftermath - A More Perfect Heaven: How Copernicus Revolutionized the Cosmos - Dava Sobel

A More Perfect Heaven: How Copernicus Revolutionized the Cosmos - Dava Sobel (2011)

Part III. Aftermath

Chapter 11. Dialogue Concerning the Two Chief Systems of the World, Ptolemaic and Copernican

The constitution of the universe, I believe, may be set in first place among all natural things that can be known, for coming before all others in grandeur by reason of its universal content, it must also stand above them all in nobility as their rule and standard. Therefore if any men might claim extreme distinction in intellect above all mankind, Ptolemy and Copernicus were such men, whose gaze was thus raised on high and who philosophized about the constitution of the world.

—GALILEO GALILEI, Dialogue Concerning the
Two Chief Systems of the World, 1632

In an exchange of letters in Latin between Galileo and Kepler in 1597, prompted by the publication of Kepler’s Mysterium, the Italian Catholic professor admitted to having long been “a secret Copernican” who could not openly espouse his belief in a moving Earth for fear of ridicule from colleagues. In reply, the German Lutheran exhorted him to join the pro-Copernican movement: “Would it not be better to pull the rolling wagon to its destination with united effort?”

Galileo answered Kepler with silence. Not until 1610, after refining the optical instrument he called a spyglass, and discovering through its lenses heavenly marvels such as the moons of Jupiter, did he outwardly avow his support for Copernicus.

Before Galileo’s innovations refined the rudimentary spyglass, instruments had aided astronomers in defining only the positions of bodies. Galileo’s telescopes enabled observers to glimpse composition as well. The lunar landscape, for example, erupted in rocky mountains and fell into deep valleys, mirroring the surface of the Earth. The Sun exhaled dark spots that gathered and glided across its face like windblown clouds. The telescope further upset the balance of the heavens by exposing unknown bodies—not “new” entities such as Tycho’s nova of 1572 (or Kepler’s of 1604) that suddenly caught the naked eye, but never-before-seen objects beyond the reach of human vision, including ear-like appendages flanking Saturn and hundreds of faint stars filling in the outlines of the constellations. Also the planet Venus revealed a cycle of phases, from crescent to full, demonstrating beyond doubt that it revolved around the Sun. Venus’s phases fit equally well into the Tychonic system or the Copernican, but Ptolemy’s universe could not embrace such a phenomenon. Galileo published his findings. The Starry Messenger, a slim volume expounding his “message from the stars,” sold out within one week of its printing in Padua in March of 1610. After that, he could not build telescopes fast enough to keep pace with the demand.


Galileo’s telescopic discovery of the four largest moons of Jupiter in January 1610, described and diagrammed here in his own hand, convinced him that the Earth was not the only center of motion in the universe, and he became an outspoken “Copernican.”

News of the new discoveries spread quickly to tremendous acclaim, but Galileo also became a lightning rod for all the criticism, ridicule, and outrage that Copernicus had dreaded. Thanks in part to Galileo’s loud praise of it, On the Revolutions came to the suspicious attention of the Sacred Congregation of the Index, a watchdog arm of the Church created late in the sixteenth century to proscribe books that threatened faith or morals.

Copernicus had predicted trouble from “babblers who claim to be judges of astronomy although completely ignorant of the subject,” who would distort passages of Scripture to censure him. Rheticus had also anticipated such calumny, and tried to ward it off by rectifying tenets of the Copernican system with chapter and verse, with Bishop Giese’s wholehearted approval. Even Osiander, whose anonymous note “To the Reader” had so offended Giese and Kepler, probably intended to protect the book by dismissing Copernicus’s bold assertions as clever calculation devices. And, as expected, On the Revolutions provoked the ire of religious authorities almost from the moment it appeared.

Pope Paul III, the dedicatee, had established the Roman Holy Office of the Inquisition in 1542, a year before the book’s publication, as part of his campaign to quash the Lutheran heresy. Whether through the efforts of Rheticus or Giese, His Holiness duly received a gift copy of On the Revolutions. He turned it over to his personal theologian, Bartolomeo Spina of Pisa, Master of the Sacred and Apostolic Palace. Spina took sick, however, and died before he could review the book, leaving that task to his friend and fellow Dominican friar Giovanni Maria Tolosani. In an appendix to the treatise On the Truth of Holy Scripture, published in 1544, Tolosani denounced the deceased Copernicus as a braggart and a fool who risked straying from the faith.

“Summon men educated in all the sciences, and let them read Copernicus, Book I, on the moving Earth and the motionless starry heaven,” Tolosani challenged. “Surely they will find that his arguments have no solidity and can be very easily refuted. For it is stupid to contradict a belief accepted by everyone over a very long time for extremely strong reasons, unless the naysayer uses more powerful and incontrovertible proofs, and completely rebuts the opposed reasoning. Copernicus does not do this at all.”

Thus panned, On the Revolutions eluded official denunciation for a time. All works by Rheticus, however, along with those of Martin Luther, Johann Schöner, and numerous other Protestant authors, took their places on the Roman Index of Prohibited Books in 1559. Petreius’s name turned up that same year on the appended list of forbidden printers, prompting some unknown number of zealots to destroy their copies of On the Revolutions because of its association with a prohibited press. Fortunately, the Index of 1564 reversed the situation by removing Petreius’s name. Two years later, when his relative Petri issued the Basel edition, a few Catholic readers obediently excised its bonus text of the First Account with scissors or knives. Some also deleted Rheticus’s name from that edition’s title page, by crossing it out or pasting a slip of paper over it.

In Protestant regions, where the Index carried no weight, On the Revolutions nevertheless stood open to attack on religious grounds. Kepler therefore defended the Copernican idea in the introduction to his 1609 New Astronomy. He argued that the Holy Scriptures spoke by turns colloquially and poetically about common things such as the Sun’s apparent motion through the sky—“concerning which it is not their purpose to instruct humanity.” Given the Bible’s emphasis on salvation, Kepler advised readers to “regard the Holy Spirit as a divine messenger, and refrain from wantonly dragging Him into physics class.”

Galileo seconded Kepler’s stand on biblical interpretation. “I believe that the intention of Holy Writ was to persuade men of the truths necessary for salvation,” he wrote in a position paper in 1613, “such as neither science nor any other means could render credible, but only the voice of the Holy Spirit. But I do not think it necessary to believe that the same God who gave us our senses, our speech, our intellect, would have put aside the use of these, to teach us instead such things as with their help we could find out for ourselves, particularly in the case of these sciences of which there is not the smallest mention in the Scriptures; and, above all, in astronomy, of which so little notice is taken that the names of none of the planets are mentioned. Surely if the intention of the sacred scribes had been to teach the people astronomy, they would not have passed over the subject so completely.”

Galileo greatly expanded his comments two years later, in 1615, in reaction to rumors that the Holy Office planned to list On the Revolutions on the Index. Addressing himself to the Medici matriarch, the Grand Duchess Cristina, he pointed out the folly of such an action:

“To ban Copernicus now that his doctrine is daily reinforced by many new observations and by the learned applying themselves to the reading of his book, after this opinion has been allowed and tolerated for those many years during which it was less followed and less confirmed, would seem in my judgment to be a contravention of truth, and an attempt to hide and suppress her the more as she revealed herself the more clearly and plainly. Not to abolish and censure his whole book, but only to condemn as erroneous this particular proposition, would (if I am not mistaken) be a still greater detriment to the minds of men, since it would afford them occasion to see a proposition proved that it was heresy to believe. And to prohibit the whole science would be but to censure a hundred passages of Holy Scripture which teach us that the glory and greatness of Almighty God are marvelously discerned in all His works and divinely read in the open book of Heaven.”


Galileo Galilei, philosopher and mathematician to the Grand Duke of Tuscany, in a drawing by Ottavio Leoni.

Galileo confronted Joshua head-on. He considered the miracle first from the Ptolemaic—Earth-centric, Earth-static—point of view, and then claimed the Copernican universe far better equipped to answer Joshua’s prayers.

“Now let us consider the extent to which it is true that the famous passage in Joshua may be accepted without altering the literal meaning of its words, and under what conditions the day might be greatly lengthened by obedience of the Sun to Joshua’s command that it stand still.

“In the Ptolemaic system, this could never happen at all. For the movement of the Sun through the ecliptic is from west to east, and hence it is opposite to the movement of the primum mobile, which in that system causes day and night. Therefore it is obvious that if the Sun should cease its own proper motion, the day would become shorter, and not longer. The way to lengthen the day would be to speed up the Sun’s proper motion; and to cause the Sun to remain above the horizon for some time in one place without declining towards the west, it would be necessary to hasten this motion until it was equal to that of the primum mobile. This would amount to accelerating the customary speed of the Sun about 360 times. Therefore if Joshua had intended his words to be taken in their pure and proper sense, he would have ordered the Sun to accelerate its own motion in such a way that the impulse from the primum mobile would not carry it westward. But since his words were to be heard by people who very likely knew nothing of any celestial motions beyond the great general movement from east to west, he stooped to their capacity and spoke according to their understanding, as he had no intention of teaching them the arrangement of the spheres, but merely of having them perceive the greatness of the miracle.”

Galileo next considered the possibility that Joshua meant to halt the primum mobile, and all heavenly movement along with it. “And indeed Joshua did intend the whole system of celestial spheres to stand still, as may be deduced from his simultaneous command to the Moon, which had nothing to do with lengthening the day. And under his command to the Moon we are to understand the other planets as well, though they are passed over in silence here as elsewhere in the Bible, which was not written to teach us astronomy.”

Turning to Copernicus, Galileo reminded Madama Cristina of his own discovery of the Sun’s monthly rotation, which he propounded in his 1613 Letters on Sunspots.

“If we consider the nobility of the Sun, and the fact that it is the font of light which (as I shall conclusively prove) illuminates not only the Moon and the Earth but all the other planets, which are inherently dark, then I believe that it will not be entirely unphilosophical to say that the Sun, as the chief minister of Nature and in a certain sense the heart and soul of the universe, infuses by its own rotation not only light but also motion into other bodies which surround it. And just as if the motion of the heart should cease in an animal, all other motions of its members would also cease, so if the rotation of the Sun were to stop, the rotations of all the planets would stop too.”

Thus, stopping the Sun sufficed to halt “the whole system of the world.” All the heavenly revolutions ceased as a result, and “day was miraculously prolonged.” For emphasis, Galileo pointed out how “exquisitely” his scenario agreed with “the literal sense of the sacred text.”

Continuing with brio, Galileo brought up the matter of the Sun’s having stood still “in the midst of the heavens,” according to Joshua 10:13, and he elaborated on that point.

“Grave theologians raise a question about this passage, for it seems very likely that when Joshua requested the lengthening of the day, the Sun was near setting and not at the meridian. … For if it had been near the meridian, either it would have been needless to request a miracle, or it would have been sufficient merely to have prayed for some retardation.” This conundrum had forced several biblical scholars, named here by Galileo, to equivocate in their interpretation of the phrase “in the midst of the heavens.” But all inconsistencies vanished “if, in agreement with the Copernican system, we place the Sun in the ‘midst’—that is, in the center—of the celestial orbs and planetary rotations, as it is most necessary to do. Then take any hour of the day, either noon, or any hour as close to evening as you please, and the day would be lengthened and all the celestial revolutions stopped by the Sun’s standing still in the midst of the heavens; that is, in the center, where it resides.”

In his enthusiasm for the Copernican system, Galileo apparently forgot that Catholic law forbade such exegesis by a layman. Only the Holy Fathers of the Church were empowered to probe the deep meaning of the Bible. The Protestant Kepler could emulate Luther and reach a personal understanding of Holy Writ in his own country with impunity. But Galileo, according to the decrees of the Council of Trent, issued in 1564, dared not interpret Scripture “in any way other than in accordance with the unanimous agreement of the Fathers.”

The Fathers counted saints and martyrs of old among their number, and also the cardinal inquisitors of Galileo’s day, including the pope’s Jesuit theological adviser, Roberto Bellarmino, who put down Galileo’s arguments with his own authoritative declaration:

“The words ‘the Sun also riseth and the Sun goeth down, and hasteth to his place where he arose, etc.’ were those of Solomon, who not only spoke by divine inspiration but was a man wise above all others and most learned in human sciences and in the knowledge of all created things, and his wisdom was from God. Thus it is not likely that he would affirm something which was contrary to a truth either already demonstrated, or likely to be demonstrated. And if you tell me that Solomon spoke only according to the appearances, and that it seems to us that the Sun goes around when actually it is the Earth which moves, as it seems to one on a ship that the shore moves away from the ship, I shall answer that though it may appear to a voyager as if the shore were receding from the vessel on which he stands, rather than the vessel from the shore, yet he knows this to be an illusion and is able to correct it because he sees clearly that it is the ship and not the shore that is in movement. But as to the Sun and the Earth, a wise man has no need to correct his judgment, for his experience tells him plainly that the Earth is standing still and that his eyes are not deceived when they report that the Sun, Moon, and stars are in motion.”

On February 23, 1616, a panel of eleven theologians put the Copernican idea to a vote. They deemed “the quiescence of the Sun in the center of the world” to be “formally heretical” because it contradicted Scripture. They further found the heliocentric universe philosophically “foolish and absurd.” Although the Earth’s motion seemed to them an equally ridiculous concept, they declared it merely “erroneous in faith,” since it did not explicitly deny the truth of Holy Writ. These judgments formed the crux of an official edict issued on March 5, denouncing Copernicus’s teachings as “false and contrary to Holy Scripture.”On the Revolutions would be named in a decree appended to the Index of Prohibited Books. But instead of being banned and destroyed—the fate of other forbidden titles—On the Revolutions was to be suspended until corrected. In the decades since its publication, the book had proved so useful that the Church could not justify condemning it outright. Indeed, the much-desired calendar reform that engaged Copernicus during his lifetime had since been implemented with the help of his text. On the Revolutions and the Prutenic Tables provided the mean length of the tropical year and synodic month that enabled the Jesuit Father Christoph Clavius of the Roman College to create the so-called Gregorian calendar, which replaced the Julian in 1582, during the pontificate of Gregory XIII.

In 1619 another decree regarding the Index banned Kepler’s Epitome of Copernican Astronomy, along with “all other works by this author.” The following year a further decree enumerated ten specific corrections to be made to On the Revolutions. These few changes—only ten points in more than four hundred pages—conformed Copernicus’s text to Osiander’s note. They rephrased every testimony to the Earth’s motion so as to sound hypothetical. The censors deleted the part of the preface claiming “astronomy is written for astronomers,” for they had appropriated the subject to themselves. The line appeared in the paragraph that voiced Copernicus’s nightmare vision of “babblers who claim to be judges of astronomy although completely ignorant of the subject,” who might bend “some passage of Scripture to their purpose” and bludgeon him with it.

It was left to each individual owner of On the Revolutions to insert the specified changes where indicated. Galileo made all of them, perhaps in the belief that his copy might be examined by clerical authorities. He had himself been advised personally by Cardinal Bellarmino, in Rome in 1616, to end his teaching and writing about Copernicus, and he had acquiesced. Several years later, however, in 1624, a seemingly broad-minded new pope, Urban VIII, encouraged Galileo to write a definitive comparison of the Ptolemaic system with the Copernican. This book, Galileo’s Dialogue Concerning the Two Chief Systems of the World, published in Florence in 1632, led to immediate accusations of heresy. The formal trial of Galileo by the Inquisition took place the following year, and ended with his abjuration. The Dialogue then joined On the Revolutions on the next Index of Prohibited Books. They both remained listed there—the one banned, the other suspended until corrected, both the subjects of continuing controversy and commentary—through the ensuing two centuries.


In his best-known work, Dialogue Concerning the Two Chief Systems of the World, Galileo staged a four-day conversation among three intellectuals. The frontispiece to the first edition pictured those men as Aristotle, Ptolemy, and Copernicus (at right, holding a representation of his Sun-centered cosmos).