Empirical proof and/or persuasion
Neither Kepler nor Galileo tells us precisely why he became a Copernican. Kepler always justified his choice in terms of the Holy Trinity, but this could hardly have been the starting point. Surely it was the aesthetic appeal that arrested their attention, the sheer geometrical beauty of an arrangement that included the distant promise of a new physics. And it was Kepler who first glimpsed this new physics when he discovered not only that Mars moved in an orbit with the sun at one focus of the ellipsethat focal point is far more important than the elliptical shape itselfand also that the earth in its orbit had the property of speeding up when it was closer to the sun. I hasten to point out that this momentous physical discovery was not present in De revolutionibus and had to be teased out through Keplers insight into the nature of the problem. These discoveries were made by 1605, though publication of Keplers Astronomia nova was delayed until 1609.
And it was then that Galileo turned his optical tube, not yet named the telescope, to the heavens. In the following January he found the four bright satellites of Jupiter, and by April of 1610 his Sidereus nuncius was in print. And there he allowed himself a Copernican remark :
I would suggest that this realization that the earth could likewise keep the moon in tow was absolutely central to Galileos conversion to a strong, enthusiastic heliocentrism. Later, when he had determined the periods of the circumjovials, he realized that the innermost satellite rounded Jupiter the most quickly, the outer satellite was the slowest, and so on. Behold! a miniature Copernican system! This could not but help authenticate the Copernican arrangement, and Galileo presented it as such in his Dialogo of 1632, the book that got him into trouble with the Inquisition.
But meanwhile, toward the end of 1610, Galileo made another discovery that bore directly on the viability of the Ptolemaic system. In the Ptolemaic arrangement, the epicycle of Venus always lay between the earth and then sun, so if the planet shone by reflected sunlight, it could never show a full phase. By late December Galileo had confirmed that "the mother of loves" (as he encoded her) displayed the entire gamut of phases from full to crescent, and therefore it had to go around the sun as in the Copernican arrangement.
Was this the brilliant confirmation of a Copernican prediction? A.D. White, in his infamous A History of the Warfare of Science with Theology in Christendom (published in 1896) had it so. The so-called Galileo affair played a central role in his account, introduced by the following wholly fictitious episode :
Copernicus had, in fact, mentioned the possible phases of Venus in the opening of this cosmological chapter 10. The context was that those who held that Venus was a dark body, shining by reflected light, argued that its interposition between us and the sun would diminish the suns light, and since this was never observed, Venus must lie farther than the sun. That was it, nothing more. Copernicus passing remark may have provided the basis for a few comments made by the English astronomer John Keill in a Latin textbook he published in 1718 . Thus the seeds for the myth were planted. With each retelling the story was more richly embroidered, reaching its apotheosis with Whites well-embellished vignette.
Galileo indirectly informed Kepler of the phases of Venus, and Kepler promptly published the news. Galileo himself publicized his discovery at the end of his book on sunspots, printed in 1613. The Ptolemaic system was thus destined for the scrapheap, and this was situation in 1615 when Bellarmine wrote his letter to Foscarini. Recall what Bellarmine said:
In other words, the Copernican system very nicely explained the appearances, the phases of Venus, but this did not guarantee that the sun was fixed in the center. Why not? Because Tychos geo-heliocentric arrangement also had Venus going around the sun, albeit a mobile sun, and therefore the Tychonic system explained the Cytherian phases equally well.
Earlier I asked the question, what would have it taken to persuade Bellarmine that the earth moved? Suppose that the Foucault pendulum had been set in motion with its shifting orientation of the swing. What would Bellarmine have made of that? Well, why not suppose that the influences of the whirling stars caused the plane of oscillation of the pendulum to rotate? This is not a frivolous way out, for it is the general relativistic explanation. And what if the annual stellar parallax had been found? Well, why not let each star have its own tiny epicycle, cycling around each year? I think such an explanation would have naturally occurred to Bellarmine. You may immediately think of Ockhams razor, that the simpler explanation would surely prevail. But remember that Ockhams razor is not a law of physics. It is an element of rhetoric, in the tool kit of persuasion. In the absence of new physics, a myriad epicycles might not have been an obstacle to keeping the earth safely fixed.
And the absence of an observed stellar parallax worked seriously against the acceptance of the Copernican system throughout the seventeenth century. Copernicus himself recognized the problem, and he addressed it in the final sentences of his cosmological chapter 10. The parallax was not seen because the stars were so far away. "So vast, without any question, is the Divine Handiwork of the Almighty Creator" . When in 1616 Copernicus book was placed on the Index of Prohibited Books "until corrected," one of the corrections ultimately made was to excise that sentence. It was not that the censors thought the argument was faulty. Rather, they feared that Copernicus made it read as if that was the way God had actually created the cosmos.
In 1674 Robert Hooke summarized the state of play of the arguments. The problem of the earths mobility, he wrote, "hath much exercised the Wits of our best modern Astronomers and Philosophers, amongst which notwithstanding there hath not been any one who hath found out a certain manifestation either of the one or the other Doctrine" . Thus, he suggested, people let their prejudices reign. Some, "have been instructed in the Ptolemaik or Tichonick System, and by the Authority of their Tutors, over-awed into a belief, if not a veneration thereof: Whence for the most part such persons will not indure to hear Arguments against it, and if they do, tis only to find Answers to confute them."
Hooke then confirms what I have been arguing, namely that the best and most persuasive reason for adopting the Copernican system up through his time was the proportion and harmony of the world: "On the other side, some out of a contradicting nature to their Tutors; others, by as great a prejudice of institution; and some few others upon better reasoned grounds, from the proportion and harmony of the World, cannot but embrace the Copernican Arguments."
But, Hooke allows, "what way of demonstration have we that the frame and constitution of the World is so harmonious according to our notion of its harmony, as we suppose? Is there not a possibility that things may be otherwise? nay, is there not something of a probability ? may not the Sun move as Ticho supposes, and that the Planets make their Revolutions about it whilst the Earth stands still, and by its magnetism attracts the Sun and so keeps him moving about it?" There is needed, Hooke declares, an experimentum crucis to decide between the Copernican and Tychonic systems, and this he proposed to do with a careful measurement of the annual stellar parallax. I will not describe Hookes attempt, which used what might well be described as the first major instrumentation set up for a single purpose, but let me merely state that Hooke thought he had confirmed the effect and therefore the Copernican arrangement.
While it soon became apparent that Hookes handful of observations had not established a convincing annual parallax, further attempts led James Bradley to the discovery of stellar aberration, published in 1728 . This phenomenon, easily explained in terms of a moving earth, did not have the historical cachet that the quest for parallax had. Hence, ironically, what persuaded the Catholic Church to take Copernicus book off the Index was an ultimately false claim for the discovery of an annual stellar parallax. The new edition of the Index appearing in 1835 finally omitted De revolutionibus, three years before a convincing stellar parallax observation was at last published .
10. Galileo Galilei, The Starry Messenger, translated in Stillman Drake, Discoveries and Opinions of Galileo (Garden City: Doubleday, 1957), p. 57.
11. Andrew Dickson White, A History of the Warfare of Science with Theology in Christendom (New York: D. Appleton, 1896), p. 130.
12. See Edward Rosen, "Copernicus on the Phases and the Light of the Planets," pp. 8198 in his Copernicus and His Successors (London: The Hambledon Press, 1995), esp. p. 84.
13. Copernicus, op. cit., at the very end of the chapter, (note 2).
14. Robert Hooke, An Attempt to Prove the Motion of the Earth from Observations, (London, 1674), pp. 1, 3.
15. James Bradley, "An account of a new-discovered motion of the fixed stars," Philosophical Transactions, 35 (172728), 63761.
16. See Pierre-Noël Mayaud, S.J., La Condamnation des Livres Coperniciens et sa Révocation: á la lumière de documents inédits des Congregation de lIndex et de lInquisition, (Rome: Editrice Pontificia Universita Gregoriana, 1997).