Yet these explanations were not enough to win the day. Astronomers of the sixteenth century belonged to a long tradition that had distinguished astronomy from physics. At the universities astronomy was taught as part of the quadrivium, the four advanced topics of the seven liberal arts. The astronomer instructed his students in the celestial circles, the geometry of planetary mechanisms, and the calculation of positions required for making up horoscopes. But the physical nature of the heavens was described not in Aristotle’s De coelo, but in his Metaphysica, and that text belonged to the philosophy professor. The distinction was rather clearly stated in the anonymous "Introduction to the Reader," added to De revolutionibus by the Lutheran clergyman Andreas Osiander, who had served as proofreader for the publication. "You may be worried that all of liberal arts will be thrown into confusion by the hypotheses in this book," Osiander wrote (and I paraphrase). "But not to worry. It is the astronomer’s task to make careful observations, and then form hypotheses so that the positions of the planets can be calculated for any time. But these hypotheses need not be true not even probable. A philosopher will seek after truth, but an astronomer will just take what is simplest. And neither will find truth unless it has been divinely revealed to him" [5].

Bellarmine certainly understood Copernicus in this light. In the opening lines of his letter to Foscarini he stated, "First, I say that it appears to me that your Reverence and Signor Galilei did prudently to content yourselves with speaking hypothetically, as I have always supposed Copernicus did" [6]. Osiander has been much castigated for having had the presumption to preface Copernicus’ treatise in this manner, but he was preaching to the choir in what he added. The Protestants in Wittenberg endorsed the interpretation, and surely would have invented it if Osiander hadn’t already clearly stated it. And the Catholics likewise fell in line, as Bellarmine’s opinion reveals. When Galileo was negotiating with Cosimo de Medici for his new position in the Florentine court, he was comparatively indifferent about his salary, but he was insistent on the title: Mathematician and Philosopher to the Grand Duke. In other words, he wanted to be credentialed not just to make mathematical astronomical models or hypotheses, but he intended to speak authoritatively about how the universe was really constructed.

Along these same lines Tycho Brahe, the second most distinguished astronomer of the sixteenth century, remarked, "This innovation expertly and completely circumvents all that is superfluous or discordant in the system of Ptolemy. On no point does it offend the principles of mathematics. Yet is ascribes to the earth, that hulking, lazy body, unfit for motion, a motion as fast as the aethereal torches, and a triple motion at that" [7]. Thus Tycho had no problem with the Copernican system as a mathematical construction, but he believed that Copernicus fell short with respect to physics. Copernicus had attempted to describe the earth’s motion as "natural" in a sort of Aristotelian manner, but he was not persuasive. It is interesting to notice that Tycho always put physics first when he criticized the Copernican doctrine, saying that it went against both physics and holy Scripture. Surely if the earth was spinning at a dizzying speed, stones thrown straight up would land far away. And if the earth was wheeling around the sun, how could it keep the moon in tow? These consequences would require new physics, which wasn’t anywhere in sight. But it was not just a problem with the physics. Philosophers and churchmen surely felt threatened by a potential challenge to traditional sacred geography. Where would heaven and hell be found in the new picture? And did not Psalm 104 say that the Lord God laid the foundation of the earth, that it would not be moved forever? Surely the task of reading the evidence was confused, scientifically as well as culturally.

Nevertheless Tycho, being a perceptive and highly motivated scientist, set out to distinguish observationally between the Ptolemaic and Copernican arrangements. He knew that in the Ptolemaic system, the epicycle of Mars always lay beyond the sun, whereas in the Copernican arrangement, Mars at its closest was only half that distance away. Because Tycho, like Copernicus and Ptolemy before him, accepted an erroneously small earth-sun distance (in fact, too small by a factor of 20), he believed that he had a chance to triangulate the distance to Mars using as his baseline the difference in viewpoint between an evening and a morning observation, the so-called diurnal parallax. We know today that this parallax is actually too tiny for naked-eye visibility, though if the solar distance had been as small as he believed, he could just have managed to detect it.

Tycho’s quest for the parallax of Mars was a driving factor during the golden years at his Uraniborg observatory, in the 1580s. At first, when he found no parallax, he believed that the Copernican arrangement had to be rejected since Mars seemed even at it closest approach to be farther than the sun. But he continued his assault on the problem and two years later discovered that he had to correct for differential refraction of the earth’s atmosphere. As it subsequently worked out, his refraction table had an error exactly equal to the effect he was seeking, which led to a spurious result for the distance to Mars. Believing that he had proved that Mars came closer than the sun, he then declared against the Ptolemaic arrangement. Interestingly, however, he did not endorse the Copernican system, but rather, he adopted his own geo-heliocentric scheme. In the Tychonic system the earth remained fixed in the center of the cosmos, with the two great luminaries cycling around it. In turn, the sun carried a retinue of planets around it. These were spaced with intervals exactly as in the Copernican system, except that the fixed earth broke the pattern, as may be seen in the detail from the frontispiece of Riccioli’s Almagestum novum.

Tycho’s geo-heliocentric system, with a fixed, central earth, hangs more weightily in Urania’s balance than Copernicus’ heliocentric system in this detail from the frontispiece of Giovanni Battista Riccioli’s Almagestum novum (Bologna, 1651). [Click image to enlarge.]

Consequently, by the 1590s there was no unambiguous evidence in favor of a moving earth. Why, then, did Kepler and Galileo both opt for the Copernican arrangement at that time, when the choices were so confused? The sole observational distinction between the Ptolemaic and Copernican blueprints resided in Tycho’s claim about the parallax of Mars, which remained unpublished until he printed an unsubstantiated remark in his 1596 volume of letters. As Galileo would say, he could not sufficiently admire those who had embraced the heliocentric arrangement despite the violence to their own senses [8]. As for the advantages pointed out by Copernicus, most of these inhered equally in the Tychonic arrangement.

Nevertheless, what was perhaps the most attractive aesthetic feature of the Copernican arrangement was shattered by Tycho’s alternative. This was the sheer beauty of all the planets arrayed around the bright central sun, with the planets naturally ranked according to their periods of revolution. "In the center of all rests the sun," Copernicus wrote. "For in this most beautiful temple, could we place this luminary in any better position from which it can light up the whole at the same time? For the sun is rightly called by some the lantern of the universe, by others the Mind, and by still others its Ruler. . . . So the sun, sitting as upon a royal throne, governs the family of planets that wheel around it" [9]. In placing his paean to the sun at this central juncture in his soaring cosmological chapter, Copernicus must have understood that this would necessarily be the crux of his argument and the key to the new physics. Traditionally the driving power for the planets had come from outside, from the prime mover that spun the entire system in its swift daily motion, with each successively further inward sphere lagging more and more behind, so that the moon circled the earth in about 24 hours. Hence, compared to the starry background the moon appeared to move the fastest, though in reality it was the tardiest. It was all tied into a very neat package with Aristotle’s remark that it was the love of god that kept the prime mover spinning, so from the beginning the arrangement of the heavens had theological overtones.

Now to anyone who thought in deeply physical terms, as both Kepler and Galileo did, an alternative source of motion would be required for the Copernican system, because in it the stars, in the outermost sphere, were fixed. Somehow the sun had to offer this motive power, and Copernicus had hinted at it with his statement that "the sun, sitting as upon a royal throne, governs the family of planets circling round it." In this regard the Tychonic arrangement was a very mixed bag. For Tycho the stars still wheeled around a centrally fixed earth each day, but how would the sun in turn control the planets? As a unified physical system, it didn’t quite make it. In other words, it was simply not persuasive.