by Mario Livio
Why did the Jesuit mathematicians remain silent? We shall probably never know for sure, but their passive attitude may have reflected a misguided form of scientific caution. There is no doubt that the Jesuit astronomers realized, as Clavius himself had admitted, that the Aristotelian doctrine was no longer tenable. In the absence of direct, convincing proof for the Earth’s motion, however, the Jesuits may have opted to sit on the fence regarding the scientific question, relying on the fact that a compromise theory (Tycho Brahe’s geocentric-heliocentric model) had not yet been ruled out definitively, and it was not in conflict with Scripture. In theological matters per se, the Jesuits could not compete with or claim superiority over the Dominicans. Be that as it may, the outcome was deplorable, and the situation was going to become even gloomier and more tragic with Galileo’s trial in 1633. The fact remains that even at lectures starting the academic year at the Collegio Romano in 1623, Jesuit professors still spoke against “finders of novelties in the Sciences.”
Over the past four centuries, there have been several attempts, especially in Catholic apologetic writings, to argue that some of the responsibility for the prohibition of Copernicanism lies with Galileo himself, because he wouldn’t keep his mouth shut. Such claims are outrageous. As shown clearly in his Letter to Benedetto Castelli, his letters to Cardinal Dini, and his Letter to the Grand Duchess Christina, Galileo intended for the Church authorities to recognize Copernicanism—for which he saw compelling scientific evidence—as a potentially viable theory, and to defer judgment on it rather than to authoritatively and definitively condemn it. In his Letter to the Grand Duchess Christina, Galileo reaffirmed his belief in the truth of Scripture but emphasized the importance of interpretation: “Holy Scripture can never propose an untruth, always on condition that one penetrates to its true meaning, which—I think nobody can deny—is often hidden and very different from that which the simple signification of the words seems to indicate.” Even if his declared religiousness was partly for tactical purposes, simply to defend himself, the logic of Galileo’s argument must be admitted. Moreover, independently of Galileo, Foscarini had a similar goal, yet Ciampoli predicted correctly that Foscarini’s book would be condemned.
The key point remains that, unlike in the history of art or even in the history of religious ideas, in the history of science, we can eventually know who was right. Galileo was right, and the Church in this case abused its disciplinary power. As Pope John Paul II admitted in 1992: “This led them [the theologians who condemned Galileo] unduly to transpose into the realm of the doctrine of the faith, a question which in fact pertained to scientific investigation.” Such acknowledgments, however, didn’t come for almost four centuries. In 1619 Galileo’s already complex relationship with the Jesuit astronomers was about to seriously deteriorate.
CHAPTER 8 A Battle of Pseudonyms
Comets had fascinated people since antiquity. When three comets appeared in succession toward the end of 1618, they generated quite a sensation. The third one, in particular, was first detected on November 27, and around mid-December it became exceptionally impressive, with a long, spectacular tail. Historically, many considered comets to be bad omens, supposedly forewarning of deaths of kings or of bitter wars. As fate would have it, the appearance of the comets did coincide roughly with the beginning of the devastating Thirty Years’ War in Central Europe, which resulted in no fewer than 8 million fatalities.
While Galileo may have intended to keep a low profile after the disturbing events against Copernicanism in 1616, it was clear that the advent of the comets would not allow him to remain silent for much longer. At first, Galileo couldn’t comment directly on the comets, since he had been bedridden with pain for the entire period during which they were visible and hence unable to observe them with his own eyes. The situation became thornier when a Jesuit mathematician of the Collegio Romano, Orazio Grassi, published in 1619 the contents of his public lecture on the topic, entitled An Astronomical Discussion on the Three Comets of 1618.
Grassi, who was a highly educated scientist, an opera stage designer, and an architect, replaced Grienberger as the chair of mathematics in 1617. Like Scheiner before him, Grassi published his treatise anonymously—again, for fear of any potential embarrassment to the Jesuits, in case his ideas turned out to be wrong. Grassi’s theory of comets deviated courageously from the Aristotelian view, which placed the comets at about the distance of the Moon. Instead, following Tycho Brahe, Grassi proposed that the comets were considerably farther out, somewhere between the Moon and the Sun. He based this conclusion on “an established law according to which the more slowly they move, the higher they are, and since the motion of our comet was midway between that of the sun and of the moon, it will have to be placed between them.” Grassi still adhered to a scheme in which the comets, the Moon, and the Sun orbited the Earth. Brahe’s original idea about the distance of comets, by the way, was based on the nondetection of any appreciable parallax (shift with respect to background stars) in the observations of the comet of 1577.
As to the actual nature of comets, many astronomers at the time were still adopting Aristotle’s theory, which stated that these represented exhalations of the Earth that became visible above a certain height due to combustion, disappearing from view as soon as that inflammable material was exhausted. Grassi, however, again followed Brahe in suggesting that comets were some sort of “imitation planets.” In this, as it turned out, Grassi was more in the right than Galileo, who later defended a view in which comets represented optical effects rather than real objects.
Galileo was alerted to Grassi’s publication in the first part of 1619. Even though Galileo’s name was never mentioned in the treatise, nor was there anything even remotely offensive to him in it, Galileo was also informed that both the Jesuits of the Collegio Romano and an influential group of Roman intellectuals that included Francesco Ingoli—the person who drew up the Church’s modifications to Copernicus—were using the treatise to argue against Copernicanism. Ingoli was employing Brahe’s old argument that if the Earth was really moving around the Sun, observations taken six months apart should have revealed a parallax in the position of any celestial object, resulting from the Earth’s motion. In the absence of such a detected parallax, Ingoli concluded: “From the motion of the comet, it seems possible not only to refute the Copernican theory, but also to draw forth arguments, whose efficacy is not to be disdained, in favor of the stability of the Earth.”
Faced with this open attack on Copernicanism on several fronts at a time when he was still extremely bitter toward the Jesuit mathematicians for deserting him, but also encouraged to intervene by a few of his correspondents in Rome, Galileo decided to respond. Still, understanding the risks involved, he presented his comments not under his own name; instead, he had a former student and recently appointed consul of the Florentine Academy, Mario Guiducci, speak on his behalf. Accordingly, Guiducci gave a series of three lectures about the comets in Florence, and the lectures were published as an essay entitled Discourse on the Comets of Guiducci, at the end of June 1619.
Even a cursory examination of this manuscript in the late nineteenth century by Antonio Favaro, the editor of the National Edition of Galileo’s Works, revealed that it had largely been written (and the rest revised) by Galileo’s own hand. While he did not use his most offensive language in the “Discourse” itself, Galileo’s annotated copy of Grassi’s lectures contains quite a few ill-tempered insults such as “pezzo d’asinaccio” (“piece of utter stupidity”), “bufolaccio” (“buffoon”), “elefantissimo” (“most elephantine”), and “baldordone” (“bumbling idiot”). Specifically, Galileo, posing as Guiducci, addresses in the “Discourse” several points. First, he questions whether one could really use parallaxes for comets at all, since it was not obvious at the time that comets indeed represented solid bodies rather than optical phenomena caused by the reflection of light in vapors (similar to rainbows, aurorae, or haloes). Galileo notes:
“There are two kinds of visible objects, the first are true, real, individual and immutable, while the others are mere appearances, reflections of light, images and wandering simulacra. These are so dependent for their existence upon the vision of the observer that not only do they change position when he does, but I believe that they would vanish entirely if his vision were removed.”
Another argument in Grassi’s publication attracted opprobrium from Galileo. Grassi wrote: “It has been discovered by long experience and proved by optical reasons that all things observed with this instrument [the telescope] seem larger than they appear to the naked eye, yet according to the law that the enlargement appears less and less the farther away they are removed from the eye, it results that fixed stars, the most remote of all from us, receive no perceptible magnification from the telescope. Therefore, since the comet appeared to be enlarged very little, it will have to be said that it is more remote from us than the moon.” Grassi appeared to cite here a “law,” according to which the magnification of the telescope depended on the distance of the object.
Unfortunately, no such law exists. Galileo, who took all things personally, seems to have thought that this remark was questioning his own understanding of the telescope. Naturally, he wouldn’t let it pass without a response. The astronomer who had the best understanding of the optics of the telescope at the time was Kepler. The magnification of a telescope is determined only by the focal lengths—the distance behind the lens over which rays of light that strike it parallel to its central axis are focused—of the objective lens and the eyepiece. The fact that Grassi, who later wrote extensively about optics and who had read Kepler’s book, would make such a statement is somewhat puzzling, and it indicates that perhaps he just misspoke.
Even though Galileo’s understanding of optics was not of the highest caliber—for example, he confused an increase in the size of the image with the formation of an out-of-focus image—he correctly attacked Grassi’s law. Galileo pointed out that if the law were true, one could determine the distance to objects on Earth by merely checking how much the objects were magnified when viewed through the telescope, which was obviously wrong. For example, two telescopes of different power would have shown different magnifications for the same object.
Galileo also took issue with Tycho’s original suggestion that comets moved in circular orbits, proposing instead that a motion along a straight line away from the Earth fitted the observations better for the third comet of 1618. We know now that comets indeed move along highly elongated elliptical orbits, locally more resembling motion along a straight line than in a circle.
Galileo never advanced a genuine theory on the nature of comets. In his survey of past ideas, Guiducci/Galileo mentioned favorably the suggestion that comets could represent mere reflections of sunlight by vapors rather than real objects, but he added, “I do not say positively that a comet is formed in this way, but I do say that just as there are doubts about this, so there are doubts about the other schemes employed by other authors.” Since Guiducci/Galileo did, however, question the idea that comets represented solid objects, Grassi was justified in concluding that Galileo believed comets to be reflections of sunlight from vapor, with those vapors being exhalations from Earth, rising into the skies. Whereas this hypothetical model was very close to Aristotle’s ideas, we should note that Galileo did differ from Aristotle in two important aspects: First, the source of the comet’s light was reflected sunlight rather than Aristotle’s suggestion of a fiery combustion. Second, Galileo claimed specifically that the comets were far beyond the Moon and therefore well into Aristotle’s “celestial” region, which was supposed to be inaccessible to “terrestrial” vapors.
“Discourse on the Comets” was not one of Galileo’s best scientific works. Not only was he unable to produce even a provisional, viable theory of comets, but also it contained an inexplicable inconsistency, or internal contradiction. The discrepancy involved Galileo’s treatment of the question of the parallax. On one hand, Galileo wanted to refute Grassi’s claim that the nondetection of a semiannual parallax in the comets proved that the Earth was not moving around the Sun. To this end, he argued that one could not really apply parallaxes to comets, since they did not appear to be solid bodies, as evidenced by the fact that one could observe stars through the comets’ tails. On the other hand, Galileo did not hesitate to use “the smallness of the parallax observed with utmost care by so many excellent astronomers” to infer the comets’ supralunar distances. The fact that these incompatible arguments escaped Galileo’s notice is extremely surprising, and Grassi pounced on it in his response to Guiducci’s treatise.
There was another serious problem with Galileo’s ideas about comets, one which he did realize and remarked upon (via Guiducci):
I shall not pretend to ignore that if the material in which the comet takes form had only a straight motion perpendicular to the surface of the earth, the comet should have seemed to be directed precisely toward the zenith, whereas, in fact it did not appear so, but declined toward the north. This compels us either to alter what was stated, even though it corresponds to the appearances in so many cases, or else to retain what has been said, adding some other cause for the apparent deviation.
The last sentence alluded to the fact that due to the prohibition on discussing Copernicanism, Galileo didn’t feel free to speak his mind. Galileo/Guiducci added that “we should be content with that little bit that we can conjecture amidst the shadows, until we are told the true constitution of the parts of the world, because what Tycho had promised remained imperfect.” In other words, Galileo recognized that even his hypothetical scenario did not quite agree with observations, but he thought that Brahe’s theory was undermined by its own set of challenges. For instance, Brahe suggested that comets move in the opposite direction to that of the other planets. At the same time, Galileo felt formally forbidden to discuss any potential remedies to the model he had examined that could perhaps be provided by the Copernican scenario. In fact, in two works published in 1604 and 1619, Kepler suggested that comets move along straight lines, with the apparent deviation in their path being caused by the Earth’s motion. While Galileo was almost certainly inspired by Kepler’s ideas, he remained utterly silent about them.
Today we know that comets are small solar system bodies that orbit the Sun along orbits that are either very elongated (highly eccentric) ellipses or hyperbolic. They consist of nuclei that range in size from a few hundred yards to tens of miles across, and are composed primarily of ice, rock, and dust (they are “dirty snowballs”), as well as frozen carbon dioxide, methane, and ammonia. When the comets pass closer to the Sun, solar radiation vaporizes volatile material, which starts streaming out of the nucleus, forming an extended atmosphere, or coma, and two tails, one of dust and one of gas. The dust tail reflects sunlight directly, while the gas tail glows as it is being ionized. The ion tail can be as long as the distance between the Earth and the Sun. In the solar system, there are two reservoirs of comets. One is the Kuiper belt, a disk of comets just beyond Pluto that supplies most of the comets that orbit the Sun with orbital periods of less than a century. The second source, the Oort cloud surrounds the outer solar system, and its outer edge reaches almost a quarter of the way to the nearest star. The Oort cloud may contain as many as a trillion comets, and it supplies the long-period comets. Halley’s Comet, arguably the most famous comet, returns to Earth’s vicinity about every seventy-five years. It was last seen in 1986.
Galileo was correct in associating comets with the process of releasing gas, and their light with effects triggered by the proximity of the comets to the Sun, but he was wrong in even hypothesizing that the gases were released by the Earth. We should remember, though, that Galileo’s goal was not to formulate a definitive theory of comets but mainly to discredit and raise doubts about the model of Tycho Brahe, whose scenario for the solar system he had always regarded as a silly and irritating compromise.
Galileo’s ultimate objecti
ve was, of course, to refute the Jesuit claim that comets proved Copernicus wrong. In attempting to achieve that, however, he brought upon himself (nobody doubted that Guiducci’s publication was Galileo’s handiwork) the fury of Orazio Grassi, who complained about Galileo “vilifying the good name of the Collegio Romano,” of the Jesuits in general (“The Jesuits are much offended,” Giovanni Ciampoli informed him), and even of Scheiner personally, whose work on sunspots was unfavorably and unnecessarily mentioned in the treatise. The arena was thus set for the second round.
GRASSI’S COUNTERATTACK
Guiducci’s treatise appeared at the beginning of the summer of 1619, and Grassi took only about six weeks to respond. His stinging, harsh essay, entitled “The Astronomical and Philosophical Balance,” hit the press in the fall of the same year. However, this continued to be a battle between two masked men. Just as Grassi’s original “Astronomical Discussion” was published with the declaration that it had been authored by “one of the Fathers of the Collegio Romano,” and Galileo’s response appeared as if it was the work of Guiducci, the Balance was published under the somewhat defective (and rather transparent) anagrammatic pseudonym Lothario Sarsio Sigensano, instead of Oratio Grassi Salonensi, with “Sarsi” pretending to be a student of Grassi’s. The title contained the word Balance because it purported to carefully weigh Galileo’s opinions.