Hence I feel no shame in asserting that this whole region engirdled by the moon, and the center of the earth, traverse this grand circle amid the rest of the planets in an annual revolution around the sun. Near the sun is the center of the universe. Moreover, since the sun remains stationary, whatever appears as a motion of the sun is really due rather to the motion of the earth.
What major new ideas did Copernicus discuss in this excerpt? What was the source of these ideas? Why might one say that European astronomers finally destroyed the Middle Ages?
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Kepler published his first two laws of planetary motion in 1609. Although at Tbingen he had accepted Copernicus’s heliocentric ideas, in his first law he rejected Copernicus by showing that the orbits of the planets around the sun were not circular but elliptical, with the sun at one focus of the ellipse rather than at the center. In his second law, he demonstrated that the speed of a planet is greater when it is closer to the sun and decreases as its distance from the sun increases. This proposition destroyed a fundamental Aristotelian tenet that Copernicus had shared—that the motion of the planets was steady and unchanging. Published ten years later, Kepler’s third law established that the square of a planet’s period of revolution is proportional to the cube of its average distance from the sun. In other words, planets with larger orbits revolve at a slower average velocity than those with smaller orbits.
Kepler’s three laws effectively eliminated the idea of uniform circular motion as well as the idea of crystalline spheres revolving in circular orbits. The basic structure of the traditional Ptolemaic system had been disproved, and people had been freed to think in new ways about the actual paths of planets revolving around the sun in elliptical orbits. By the end of Kepler’s life, the Ptolemaics ystem was rapidly losing ground to the new ideas. Important questions remained unanswered, however: What were the planets made of? And how could motion in the universe be explained? It was an Italian scientist who achieved the next important breakthrough to a new cosmology by answering the first question and making important strides toward answering the second.
Johannes Kepler. Abandoning theology in favor of mathematics and astrology, Kepler became a key figure in the rise of the new astronomy. Building on Tycho Brahe’s vast store of astronomical data, Kepler discovered the three laws of planetary motion that both confirmed and modified the Copernican theory. They also eliminated the Ptolemaic-Aristotelian ideas of uniform circular motion and crystalline spheres moving in circular orbits. This portrait was done by an unknown painter three years before Kepler’s death.
Musée de l’Oeuvre Notre Dame, Strasbourg//© Imagno/Austrian Archives/Getty Images
Galileo
Galileo Galilei (1564–1642) taught mathematics, first at Pisa and later at Padua, one of the most prestigious universities in Europe. Galileo was the first European to make systematic observations of the heavens by means of a telescope, thereby inaugurating a new age in astronomy. He had heard of a Flemish lens grinder who had created a “spyglass” that magnified objects seen at a distance and soon constructed his own after reading about it. Instead of peering at terrestrial objects, Galileo turned his telescope to the skies and made a remarkable series of discoveries: mountains and craters on the moon, four moons revolving around Jupiter, the phases of Venus, and sunspots. Galileo’s observations demolished yet another aspect of the traditional cosmology in that the universe seemed to be composed of material substance similar to that of the earth rather than ethereal or perfect and unchanging substance.
The Telescope. The invention of the telescope enabled Europeans to inaugurate a new age in astronomy. Shown here is Johannes Hevelius, an eminent German-Polish astrologer (1611– 1697), making an observation with his telescope. Hevelius’s observations were highly regarded. He located his telescope on the roof of his own house, and by the 1660s, his celestial observatory was considered one of the best in Europe.
© Bibliothèque Nationale, Paris/SuperStock
Galileo’s revelations, published in The Starry Messenger in 1610, stunned his contemporaries and probably did more to make Europeans aware of the new picture of the universe than the mathematical theories of Copernicus and Kepler did. The English ambassador in Venice wrote to the chief minister of King James I in 1610:
I send herewith unto His Majesty the strangest piece of news … that he has ever yet received from any part of the world; which is the annexed book of the Mathematical Professor at Padua [Galileo], who by the help of an optical instrument … has discovered four new planets rolling about the sphere of Jupiter… . So upon the whole subject he has first overthrown all former astronomy… . By the next ship your Lordship shall receive from me one of the above instruments [a telescope], as it is bettered by this man.7
During a trip to Rome, Galileo was received by scholars as a conquering hero. Grand Duke Cosimo II of Florence offered him a new position as his court mathematician, which Galileo readily accepted. But even in the midst of his newfound acclaim, Galileo found himself increasingly suspect by the authorities of the Catholic Church.
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Kepler and the Emerging Scientific Community
The exchange of letters between intellectuals was an important avenue for scientific communication. After receiving a copy of Johannes Kepler’s first major work, the Italian Galileo Galilei wrote to Kepler, inaugurating a correspondence between them. This selection contains samples of their letters to each other.
Galileo to Kepler, Padua, August 4, 1597
Your book, highly learned gentleman, which you sent me through Paulus Amberger, reached me not days ago but only a few hours ago, and as this Paulus just informed me of his return to Germany, I should think myself indeed ungrateful if I should not express to you my thanks by this letter. I thank you especially for having deemed me worthy of such a proof of your friendship… . So far I have read only the introduction, but have learned from it in some measure your intentions and congratulate myself on the good fortune of having found such a man as a companion in the exploration of truth. For it is deplorable that there are so few who seek the truth and do not pursue a wrong method of philosophizing. But this is not the place to mourn about the misery of our century but to rejoice with you about such beautiful ideas proving the truth… . I would certainly dare to approach the public with my ways of thinking if there were more people of your mind. As this is not the case, I shall refrain from doing so… . I shall always be at your service. Farewell, and do not neglect to give me further good news of yourself.
Yours in sincere friendship,
Galilaeus Galilaeus
Mathematician at the Academy of Padua
Kepler to Galileo, Graz, October 13, 1597
I received your letter of August 4 on September 1. It was a double pleasure to me. First because I became friends with you, the Italian, and second because of the agreement in which we find ourselves concerning Copernican cosmography. As you invite me kindly at the end of your letter to enter into correspondence with you, and I myself feel greatly tempted to do so, I will not let pass the occasion of sending you a letter with the present young nobleman. For I am sure, if your time has allowed it, you have meanwhile obtained a closer knowledge of my book. And so a great desire has taken hold of me, to learn your judgment. For this is my way, to urge all those to whom I have written to express their candid opinion. Believe me, the sharpest criticism of one single understanding man means much more to me than the thoughtless applause of the great masses.
I would, however, have wished that you who have such a keen insight into everything would choose another way to reach your practical aims. By the strength of your personal example you advise us, in a cleverly veiled manner, to go out of the way of general ignorance and warn us against exposing ourselves to the furious attacks of the scholarly crowd… . But after the beginning of a tremendous enterprise has been made in our time, and furthered by so many learned mathematicians, and after the statement that the earth moves can no longer be regarded as
something new, would it not be better to pull the rolling wagon to its destination with united effort? … For it is not only you Italians who do not believe that they move unless they feel it, but we in Germany, too, in no way make ourselves popular with this idea. Yet there are ways in which we protect ourselves against these difficulties… . Be of good cheer, Galileo, and appear in public. If I am not mistaken there are only a few among the distinguished mathematicians of Europe who would dissociate themselves from us. So great is the power of truth. If Italy seems less suitable for your publication and if you have to expect difficulties there, perhaps Germany will offer us more freedom. But enough of this. Please let me know, at least privately if you do not want to do so publicly, what you have discovered in favor of Copernicus.
What does the correspondence between Galileo and Kepler reveal about an emerging spirit of scientific inquiry? What other notable achievements must European society have reached even to make this exchange of letters possible? What aspects of European material culture made the work of these scientists easier?
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GALILEO AND THE INQUISITION In The Starry Messenger, Galileo had revealed himself as a firm proponent of Copernicus’s heliocentric system. The Roman Inquisition (or Holy Office) of the Catholic Church condemned Copernicanism and ordered Galileo to reject the Copernican thesis. As one cardinal commented, “The intention of the Holy Spirit is to teach us not how the heavens go, but how to go to heaven.” The report of the Inquisition ran:
That the doctrine that the sun was the center of the world and immovable was false and absurd, formally heretical and contrary to Scripture, whereas the doctrine that the earth was not the center of the world but moved, and has further a daily motion, was philosophically false and absurd and theologically at least erroneous.8
Galileo was told, however, that he could continue to discuss Copernicanism as long as he maintained that it was not a fact but a mathematical supposition. It is apparent from the Inquisition’s response that the church attacked the Copernican system because it threatened not only Scripture but also an entire conception of the universe. The heavens were no longer a spiritual world but a world of matter. Humans were no longer at the center, and God was no longer in a specific place. The new system raised such uncertainties that it seemed prudent simply to condemn it.
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The Starry Messenger
The Italian Galileo Galilei was the first European to use a telescope to make systematic observations of the heavens. His observations, as reported in The Starry Messenger in 1610, stunned European intellectuals by revealing that the celestial bodies were not perfect and immutable but composed of material substance similar to that of the earth. In this selection, Galileo describes how he devised a telescope and what he saw with it.
Galileo Galilei, The Starry Messenger
About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons gave credence while others denied them. A few days later the report was confirmed to me in a letter from a noble Frenchman at Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to inquire into the means by which I might arrive at the invention of a similar instrument. This I did shortly afterwards, my basis being the theory of refraction. First I prepared a tube of lead, at the ends of which I fitted two glass lenses, both plane on one side while on the other side one was spherically convex and the other concave. Then placing my eye near the concave lens I perceived objects satisfactorily large and near, for they appeared three times closer and nine times larger than when seen with the naked eye alone. Next I constructed another one, more accurate, which represented objects as enlarged more than sixty times. Finally, sparing neither labor nor expense, I succeeded in constructing for myself so excellent an instrument that objects seen by means of it appeared nearly one thousand times larger and over thirty times closer than when regarded without natural vision.
It would be superfluous to enumerate the number and importance of the advantages of such an instrument at sea as well as on land. But forsaking terrestrial observations, I turned to celestial ones, and first I saw the moon from as near at hand as if it were scarcely two terrestrial radii. After that I observed often with wondering delight both the planets and the fixed stars, and since I saw these latter to be very crowded, I began to seek (and eventually found) a method by which I might measure their distances apart… .
Now let us review the observations made during the past two months, once more inviting the attention of all who are eager for true philosophy to the first steps of such important contemplations. Let us speak first of that surface of the moon which faces us. For greater clarity I distinguish two parts of this surface, a lighter and a darker; the lighter part seems to surround and to pervade the whole hemisphere, while the darker part discolors the moon’s surface like a kind of cloud, and makes it appear covered with spots… . From observation of these spots repeated many times I have been led to the opinion and conviction that the surface of the moon is not smooth, uniform, and precisely spherical as a great number of philosophers believe it (and the other heavenly bodies) to be, but is uneven, rough, and full of cavities and prominences, being not unlike the face of the earth, relieved by chains of mountains and deep valleys.
What was the significance of Galileo’s invention? What impressions did he receive of the moon? Why were his visual discoveries so stunning, and how did he go about publicizing them?
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Galileo, however, never really accepted his condemnation. In 1632, he published his most famous work, Dialogue on the Two Chief World Systems: Ptolemaic and Copernican. Unlike most scholarly treatises, it was written in Italian rather than Latin, making it more widely available to the public, which no doubt alarmed the church authorities. The work took the form of a dialogue among Simplicio, a congenial but somewhat stupid supporter of Aristotle and Ptolemy; Sagredo, an open-minded layman; and Salviati, a proponent of Copernicus’s ideas. There is no question who wins the argument, and the Dialogue was quickly perceived as a defense of the Copernican system. Galileo was dragged once more before the Inquisition in 1633, found guilty of teaching the condemned Copernican system, and forced to recant his errors. Placed under house arrest on his estate near Florence, he spent the remaining eight years of his life studying mechanics, a field in which he made significant contributions.
GALILEO AND THE PROBLEM OF MOTION One of the problems that fell under the heading of mechanics was the principle of motion. The Aristotelian conception, which dominated the late medieval world, held that an object remained at rest unless a force was applied against it. If a force was constantly exerted, then the object moved at a constant rate, but if it was removed, then the object stopped. This conception encountered some difficulties, especially with a projectile thrown out of a cannon. Late medieval theorists had solved this problem by arguing that the rush of air behind the projectile kept it in motion. The Aristotelian principle of motion also raised problems in the new Copernican system. In the Ptolemaic system, the concentric spheres surrounding the earth were weightless, but in the Copernican system, if a constant force had to be applied to objects to cause movement, then what power or force kept the heavy earth and other planets in motion?
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A New Heaven? Faith Versus Reason
In 1614, Galileo wrote a letter to the Grand Duchess Christina of Tuscany in which he explained why his theory that the earth rotated around the sun was not necessarily contrary to Scripture. To Galileo, it made little sense for the church to determine the nature of physical reality on the basis of biblical texts that were subject to different interpretations. One year later, Cardinal Robert Bellar-mine, a Jesuit and now a member of the church’s Inquisition, wrote a letter to one of Galileo’s followers that laid out the Catholic Church’s approach to the i
ssue of Galileo’s theory.
Galileo, Letter to the Grand Duchess Christina, 1614
Some years ago, as Your Serene Highness well knows, I discovered in the heavens many things that had not been seen before our own age. The novelty of these things, as well as some consequences which followed from them in contradiction to the physical notions commonly held among academic philosophers, stirred up against me no small number of professors—as if I had placed these things in the sky with my own hands in order to upset nature and overturn the sciences… .
Contrary to the sense of the Bible and the intention of the holy Fathers, if I am not mistaken, they would extend such authorities until even in purely physical matters—where faith is not involved—they would have us altogether abandon reason and the evidence of our senses in favor of some biblical passage, though under the surface meaning of its words this passage may contain a different sense… .
The reason produced for condemning the opinion that the earth moves and the sun stands still is that in many places in the Bible one may read that the sun moves and the earth stands still. Since the Bible cannot err, it follows as a necessary consequence that anyone takes an erroneous and heretical position who maintains that the sun is inherently motionless and the earth movable.
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