Class Notes on The Copernican Revolution 1543-1634

Background scientific Ideas inherited from late Medieval Philosophy

1. The established scientific theory of the universe of the 16th and early 17th centuries was based on the late medieval synthesis of the physics of Aristotle (c.325 B.C.) and the astronomy of Ptolemy (c.150 A.D.). In certain respects these were in tension since Aristotle provided a mechanism (crystalline spheres) to explain the motion of the planets, while Ptolemy was merely interested in devising mathematical calculating devices (epicycles and eccentrics) in order to predict where the heavenly spheres were to appear. These calculating devices postulated motions that could not be explained through Aristotle's crystalline spheres.

2. According to the physics of Aristotle there is a sharp distinction between motion below the orbit of the moon and that above the moon's orbit. a) Above the moon all motion is absolutely uniform and circular. The planets are essentially unchanging and move around the earth in a perfectly regular way. b) Below the moon, however, things are constantly being created and destroyed. Everything is in constant change.
      In the realm below the moon, all motion is either natural or violent (i.e. compulsory; see Matthews' Scientific Background, pp. 16-19). The natural motion of a body that is made of earth is downwards toward the center of the universe (i.e. the earth), while the natural motion of fire is upwards. Violent or compulsory motion occurs, for example, when we throw an object such as a ball against a wall. Aristotle argued that in order for the ball to keep moving toward the wall after it leaves your hand it requires a constant impulse: something must continue to push it. In his Physics he suggested two explanations, "mutual replacement" and displacement of air. According to the second explanation, the air which is pushed away in front of the ball gets displaced and moves around to the back of the ball to keep pushing it. 

3. According to the astronomy of Ptolemy, like that of Aristotle, the earth is at the center of the universe and does not move. The moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and the sphere of the fixed stars in their constellations all revolve around the earth—in that order. In thinking about the Ptolemaic system of the world, we need to remember that, to a great extent, it conforms with the appearances—with what we observe when we look up at the sky. For example, the sun appears to go around the earth; it rises in the east and sets in the west. One could argue that it is only common sense to believe that the sun goes around the earth. It certainly was common sense up until the seventeenth century.
     Ptolemy observed that there were motions of the planets that could not be accounted for by this simple geocentric scheme. For example, because (as we now know) its orbit is actually longer than that of the earth, Mars sometimes appears to have a retrogressive or backwards motion. (Think about how things appear when you are in a train that passes a slower train: the slower train appears to be going backwards while the train one is in appears to be standing still.) Moreover, Mars sometimes appears brighter and sometimes dimmer because (as we now know) the position of the earth in its orbit varies in relation to the position of Mars in its orbit. To understand Ptolemy, one needs to imagine how one would predict these observations on the common sense assumption that Mars moves around the earth. In order to "SAVE THE APPEARANCES", Ptolemy invented calculating devices to explain these anomalies while retaining the principle that all supra-lunar motion is uniform and circular. The most important of these were the epicycle and the eccentric:  http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html By using such calculating devices Ptolemy was able to predict the positions of the planets with a good deal of accuracy. However, his calculations were very complex.

4. The combined system of the world of Aristotle and Ptolemy was adopted by the Christian Church in the late Middle Ages. The earth is at the center of the universe, but outside the nine revolving spheres, where Aristotle placed what he called the Prime Mover, is heaven, the home of "God and all the Elect" (see the . diagram from Pierre Apian's Cosmographia below). Note that this is a spatially limited universe, bounded by God. Heaven is literally on the outside of this closed world. 

Copernicus to Kepler

5. In 1543 Nicolas Copernicus published his book On the Revolutions of the Heavenly Spheres, which postulated a sun-centered system. Copernicus challenged the accepted view from ancient times that the earth is stationary. The main advantage of his new system of the world from the point of view of astronomy was that it provided a simpler way of calculating the positions of the planets than the system of Ptolemy. (Note that, in his Commentarius, on page 37 of Matthews' Scientific Background, Copernicus writes that he could explain the appearances "with fewer and much simpler constructions than were formerly used" (my underlining). 
     While it is clear to us that Copernicus thought he was offering a true description of the universe, his book was published with a Preface by Ossiander, asserting that he was merely presenting a better calculating device than that of Ptolemy. This, and the fact that it was dedicated to the Pope, may explain why the book at first did not upset the authorities and was not considered heretical.
     It is also important to note that Copernicus did not confront the physical problems that would arise if his system of the world were really true. See the first new paragraph of page 35 of Matthews' Scientific Background for an indication of what these were. From this point of view, one can understand why an intelligent Aristotelian scientist of the day would have rejected Copernicus' world system.

6. In 1584 Giordano Bruno published his book On the Infinite Universe and its Worlds which proposed an infinite universe without any center. Bruno was denounced and arrested by the Inquisition in 1593 and burned at the stake in 1600.

7. In 1609, building his theory on the accurate observations of the movements of the planets by the Danish astronomer Tycho Brahe, Johannes Kepler published his book On the Motion of Mars. Kepler strongly supported the Copernican system. One of the greatest innovations was Kepler's discovery that the planets move in ellipses, and not circles.

8. In 1610 On the Revolutions of the Heavenly Spheres was put on the Index of banned books by Catholic Church.


Galileo's Defense of Copernicanism

9. Also in 1610, Galileo Galilei published a pamphlet called Starry Messenger in which described observations he had made with a telescope he had made using newly discovered Flemish lens grinding techniques, which he improved on and perfected. In this pamphlet he described the mountains he observed on the moon, as well as the moons that he discovered to revolve around Jupiter. The first observation challenged the established belief that creation and destruction only occur in the sub-lunary world, and the latter that all motion in the supra-lunary world is around the earth. Galileo later observed sun spots and the phases of Venus which further undermined the standard geocentric views.

10. In 1632 Galileo published his Dialogues Concerning the two Chief World Systems. In this book, Galileo adopted the form of philosophical writing used by Plato, a favorite form of writing of many important philosophers the early modern period. This genre was particularly important for Galileo since, for political reasons, he could not openly espouse the Copernican system as the truth about the physical world. In the Dialogues, the character Salviati challenges the views of Simplicio, an Aristotelian scientist. The third character, Sagredo, takes a more neutral position. 
       In the second Dialogue, Simplicio presents the main objections given by Aristotle to the view that the earth moves (see Matthews' Scientific Background , pp. 71-72). The most important of these (the fourth) is further explained by Salviati: he describes the "experiments" which Aristotelian scientists of his own day put forward to prove the falsity of the Copernican view that the earth turns on its axis (pp. 72-74). One of these experiments involves dropping a stone from a tower: the stone lands right at the foot of the tower. According to the Aristotelians, this experiment shows that the earth cannot be revolving on its axis at great speed from west to east, as the Copernicans claimed. For, if it were really doing that the tower would travel "many hundreds of yards to the east" in the time that the stone falls, and the stone would land that distance to the west of the tower (p. 72).
       There is a remarkable argument on pages 76-77 of our text in which Galileo shows the fallacy behind the Aristotelian use of this experiment to prove that the earth does not move. Salviati asks Simplicio how he proves that the stone falls straight down the side of the tower, perpendicular to the surface of the earth (Scientific Background, p. 76). Simplicio answers: "By means of the senses…." Salviati then goes on to show that the Aristotelians are guilty of the logical fallacy that we now call ‘begging the question' (petitio principii)—that is, assuming the conclusion of your argument in the premises. The argument of Simplicio may be formally represented in the following way:

1. If the earth moves [p], then the stone would not fall straight down the side of the tower [q]. 
2. If we see the stone fall straight down to the foot of the tower [r], the stone must really fall straight down the side of the tower [not-q].
3. We see the stone fall straight down to the foot of the tower [r]. (The "experiment" shows us this.)
4. The stone must really fall straight down the side of the tower [not-q]. (from 2 & 3 by modus ponens)
5. The earth does not move [not-p]. (from 1 & 4 by modus tollens)

Salviati then goes on to show that there is a fallacy in the argument because premise #2 begs the question. Premise #2 is only true if the earth does not move. If the earth moves there would actually be two separate motions of the stone, one by which it falls downwards "from top to bottom", and another motion by which it moves from west to east along with the tower. The combination of these motions would make it appear that the stone falls straight downwards, even though it actually falls with a "slanting" motion that combines these two separate motions.

Galileo develops his refutation of the Aristotelian argument in the Dialogues by showing that the Aristotelians assume that the stone can only have motion from west to east along with the tower if it were actually thrown from west to east. They thought that, even if the earth were in motion, the stone should fall straight downwards as soon as it is released. They thought this because they adopted Aristotle's theory about violent motion (see #1, above): according to that theory, the stone requires a constant impulse in order to keep moving eastward in the same direction of the earth and tower. But Galileo argued that in order to keep moving along with the tower the stone does not need any impulse from the air, or anything else. Galileo gave the first formulation of what later (after refinements by Descartes and Newton) came to be known as the principle of inertia: 

Keeping up with the earth is the primordial and eternal motion ineradicably and inseparably participated in by this [stone] as a terrestrial object, which it has forever and will possess forever.

To put the point another way, the question after Galileo was no longer why a moving object keeps moving, but rather why it stops. The ball lands at the foot of the tower, according to Galileo, because it is already moving from west to east along with the tower and earth, and it keeps moving along with them until it is stopped by landing on the ground. 

Thus Galileo saved the appearances by using his intellect or reason. Salviati puts it in the following way. He says that he can never

sufficiently admire the outstanding acumen of those who have taken hold of this opinion [Copernicanism] and accepted it is true; they have through sheer force of intellect done such violence to their own senses as to prefer what reason told them over that which sensible experience plainly showed them to the contrary.

Thus, Galileo is claiming that it is through intellect or reason that we must come to a correct conclusion about the true state of reality and the laws that underlie what nature is really like. We don't observe the principle of inertia through observation or through our senses, yet it is this principle was one of the main cornerstones of modern science.

11. Galileo's method of writing in dialogue form did not save him from being charged by the Church authorities. In 1633, after he had sent the Dialogues to the printer, he was ordered to come to Rome from Florence to appear before the Inquisition. He was forced to publicly withdraw his support for the Copernican system, and was put under house arrest by the Inquisition. Publication of his Dialogues was stopped and it was put on the Index of banned books by the Catholic Church.

Descartes' Reaction to Galileo's Arrest

12. Descartes was living in Holland in 1633. He had moved to Protestant Holland principally because it was the freest country in Europe. Yet he himself was a Catholic, in close touch with his Catholic friends in Paris. He had recently been working on a book called World in which, like Galileo, he supports the Copernican system. 

Descartes wrote the following in a letter to his friend Fr. Mersenne in Paris at the end of November 1633:

I had intended to send my book World as a New Year present, and only two weeks ago I was quite determined to send you at least a part of it, if the whole work could be copied in time. But I have to say that in the mean time I took the trouble to inquire in Leiden and Amsterdam whether Galileo's World System (i.e. Dialogues on the Two Chief Systems of the World) was available, for I thought I had heard that it was published in Italy last year. I was told that it had indeed been published but that all the copies had immediately been burnt, and that Galileo had been convicted and fined. I was so astonished at this that I almost decided to burn all my papers or at least to let no one see them. For I could not imagine that he... could have been made a criminal for any other reason than that he tried... to establish that the earth moves.... I must admit that if the view is false, so too are the entire foundations of my philosophy, for it can be demonstrated from them quite clearly.

In April of 1634 he writes again to Mersenne:

Though I thought they [his views on the solar system] were based on very certain and evident proofs, I would not wish, for anything in the world, to maintain them against the authority of the Church. I know that not everything which the Roman Inquisitors decide is automatically an article of faith.... But I am not so fond of my own opinions as to want to use such quibbles to be able to maintain them. I desire to live in peace and to continue the life I have begun under the motto ‘to live well you must live unseen'.... I do not altogether lose hope that... in time my World may yet see the light of day.

Descartes never did publish the book called World, though he published some of the main ideas in it (though not the claim that the earth moves!) in his Discourse on the Method of rightly conducting ones reason and seeking the truth in the sciences published in 1637, four years later. In the first part of the Discourse he gives an autobiographical account of his own life and education in which he summarizes some of the ideas in "a treatise which certain considerations prevent me from publishing" (Descartes: Selected Philosophical Writings, p. 41, top of the page and p. 46, the first pargraph of Part Six). The Discourse was published with Descartes' most important scientific treatises, including his Geometry, and his Optics.

The Copernican Revolution (1543-1687): Some important dates

•1543 Publication of Nicolas Copernicus’ On the Revolutions of the Heavenly Spheres
•1570-1600 Precise observations made by Tyche Brahe who held a modified Copernican system in which the earth still did not move, but the other planets moved around the sun
•1584 Giordano Bruno published his book On the Infinite Universe and its Worlds
•1600 Bruno is burned at the stake in Rome by the Inquisition
•1609 Johannes Kepler published On the Motion of Mars
•1610 Galileo Galilei published a pamphlet called Starry Messenger in which he carefully described the discoveries he made through a telescope about the surface of the moon and the existence of the satellites of Jupiter.
•1615 Galileo's writes various letters in which he argues that the Bible should not be read as opposing the Copernican System. (His arguments are summarized in his Letter to the Grand Duchess Chistina (not published until 1636). His views are opposed by Cardinal Bellarmine.
•1616 On the Revolutions of the Heavenly Spheres was put on the Index of banned books by the Congregation of the Index of the Church of Rome, and the teaching of Copernicus was declared contrary to the Bible. Catholic philosophers can neither hold nor teach the Copernican system.
•1632 Galileo sends his Dialogue Concerning the Two Chief World Systems to the printer after receiving official permission. In the book, a figure representing the views of Aristotle (Simplicio) argues with a figure representing the views of Galileo (Salviati).
•1633 Galileo is arrested by the Inquisition, forced to recant, and the book is put on the Index. Descartes learns of Galileo’s arrest decides not to publish The World, in which he too supported the Copernican system.
•1637 Descartes publishes the Discourse on Method including the Optics, Meteorology and Geometry.
•1638 Galileo's Two New Sciences, first great work in modern physics is published in Leyden (Holland).
•1644 Descartes’ Principles of Philosophy published.
•1687 Issac Newton publishes Mathematical Principles of Natural Philosophy in which the basic laws of motion are established, it is proved that these laws apply both on earth and in the movement of the planets, and the theory of universal gravitation is developed. Referring back to his predecessors Newton said that he "stood on the shoulders of giants."

Further Reading

Discoveries and Opinions of Galileo, translated with an introduction and notes by Stillman Drake (New York: Random House, 1957) QB36.G2 G313

Galileo Galilei, Dialogue concerning the two chief world systems, Ptolemaic & Copernican; translated by Stillman Drake (Berkeley: University of California Press, 1953) QB41 .G1356    

Richard Westfall, The Construction of Modern Science (Cambridge: C.U.P., 1977) QA.802.W47

Pietro Redondi, Galileo Heretic (Princeton: Princeton U.P., 1987) Q125.2.R4313 1987

Alexander Koyré, From the Closed World to the Infinite Universe (Baltimore: Johns Hopkins U.P., 1968) BD511.K67 1968

Peter Apian. Geocentric Diagram of the Universe, from the Cosmographia (1539). This schematic diagram illustrates the geocentric universe in the pre-Copernican era. The unmoving earth is  placed in the center and is surrounded by the nine moving spheres, containing, in sequential order, the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, the fixed stars and the empty sphere called the primum mobile. The ninth sphere, the primum mobile, was logically necessary in Aristotle's theory because it moved first and brought the other eight into motion. Beyond the ninth circle was the Empyyean, home of the Unmoved Mover in philosophy or God in theology.

A Perfit Description of the Celestiall Orbs

Thomas Digges  (1576)