Heliocentric harmony - Introduction
On April 12, 1615, Cardinal Roberto Bellarmine, the leading Catholic theologian, wrote an often-quoted letter to Paolo Antonio Foscarini, a Carmelite monk from Naples who had published a tract defending the Copernican system. Bellarmines letter, which was obviously intended as much for Galileo as for Foscarini, opened on a conciliatory note: "For to say that assuming the earth moves and the sun stands still saves all the appearances better than eccentrics and epicycles is to speak well. . . . But to affirm that the sun is really fixed in the center of the heavens and that the earth revolves very swiftly around the sun is a dangerous thing, not only irritating the theologians and philosophers, but by injuring our holy faith and making the sacred scripture false".
Bellarmine made very clear that he was unwilling to concede the motion of the earth in the absence of an apodictic proof when he added, "If there were a true demonstration, then it would be necessary to be very careful in explaining Scriptures that seemed contrary, but I do not think there is any such demonstration, since none has been shown to me. To demonstrate that the appearances are saved by assuming that the sun is at the center is not the same thing as to demonstrate that in fact the sun is in the center and the earth in the heavens."
Bellarmines letter sets the stage for a challenging inquiry: what kind of evidence convinced Galileo and Kepler that the Copernican system was the correct, physically real description of our universe, and yet failed to convince Bellarmine? What would it have taken to convince Bellarmine? For example, most astronomy textbooks today list the Foucault pendulum as the proof of the earths rotation, and the annual stellar parallax as the proof of the earths yearly revolution around the sun. Would these evidences have converted Bellarmine to the Copernican doctrine, and if not (as I shall argue), why not? Framing the question in these terms will enable us to distinguish between proof and persuasion, and to gain some insight into the matter of truth in science.
Copernicus himself does not state directly what induced him to work out the heliocentric arrangement, apart from some rather vague dissatisfaction with his perceived inelegance of the traditional geocentric pattern. But Copernicus was nothing if not a unifier. In the Ptolemaic astronomy each planet was more or less its own independent entity. True, they could be stacked one after another, producing a system of sorts, but their motions were each independent. The result, Copernicus wrote in the preface to his book, was like a monster composed of spare parts, a head from here, the feet from there, the arms from yet another creature. Each planet had a main circle and a subsidiary circle, the so-called epicycle. Copernicus discovered that he could eliminate one circle from each set by combining them all into a unified system, and when he did this, something almost magical happened. Mercury, the swiftest planet, circled closer to the sun than any other planet. Lethargic Saturn automatically circled farthest from the sun, and the other planets fell into place in between, arranged in distance by their periods of revolution.
His monumental treatise, De revolutionibus, was published in the year he died, 1543, and in chapter 10 of Book I Copernicus summed up his aesthetic vision: "In no other way do we find a wonderful commensurability and a sure harmonious connection between the size of the orbit and the planets period" . It is the most soaring cosmological passage in his entire book. The key word is commensurability, the translation of Copernicus symmetria (literally syn = common and metria = measure). The common measure was the earth-sun distance, which provided the measuring rod for the entire system.
Once this heliocentric unification was accomplished, the system showed other advantages. There was, for example, the curious fact that whenever Mars or Jupiter or Saturn went into its so-called retrograde motion, the planet was always directly opposite the sun in the sky. As Gemma Frisius was to describe it soon after the publication of De revolutionibus, from antiquity this had been merely a "fact in itself," but in the Copernican system it became a reasoned fact . In the cosmological chapter I,10 of book, Copernicus noted that the heliocentric arrangement finally provided a natural explanation of this otherwise unexplained coincidence. He mentioned as well that it explained why the retrograde motion of Jupiter was smaller than that of Mars, and why that of Saturn was still smaller. As Copernicuss only student and disciple, Georg Joachim Rheticus put it, "All these phenomena appear to be linked most nobly together, as by a golden chain; and each of the planets, by its position and order and every inequality of its motion, bears witness that the earth moves and that we who dwell upon the globe of the earth, instead of accepting its changes of position, believe that the planets wander in all sorts of motions of their own" .
1. Bellarmine to Foscarini, 12 April 1615, Opere, 12, 171-72; abridged from the translation in Stillman Drake, Discoveries and Opinions of Galileo (Garden City: Doubleday, 1957), pp. 162-64. (The Galileo Opere cited here is the so-called National Edition edited by Antonio Favaro, Florence, 1890-1909, reprint 1968.)
2. My translation from Nicolaus Copernicus, De revolutionibus orbium coelestium (Nuremberg, 1543), Book I, chapter 10.
3 . Reiner Gemma Frisius in Johannes Stadius, 5 (Cologne, 1560), signatures b3-b3v.
4 . Georg Joachim Rheticus, Narratio prima (1540), translated in Edward Rosen, Three Copernican Treatises (New York: Octagon Books, 1971), p. 165.