by Mario Livio
Galileo’s observations of the phases of Venus dealt a fatal blow to the Ptolemaic geocentric systems, but they could not definitively dispose of Brahe’s geocentric-heliocentric compromise, in which Venus and all the other planets revolved around the Sun, while the Sun itself orbited the Earth. This left a potential escape route for those Jesuit astronomers who were still determined to avoid Copernicanism.
I LIKEN THE SUNSPOTS TO CLOUDS OR SMOKES
Galileo did not neglect the object that, in the Copernican view, was central and most important in the solar system—the Sun itself—and his observations had led to the detection and first coherent explanation of the relatively dark areas of the Sun’s surface known as sunspots. Galileo was definitely not the first person to discover these mysterious spots. Chinese and Korean astronomers may have seen them centuries earlier; for example, there are records from China that date back to the Han dynasty, 206 BCE to 220 CE. Sunspots had certainly been talked about at the time of Charlemagne, who ruled much of western Europe in the late-eighth to early-ninth centuries, and the Italian poet and painter Raffael Gualterotti described a sunspot he had seen on December 25, 1604, in a book published the following year. The English mathematician and astronomer Thomas Harriot observed sunspots with a telescope in December 1610 but didn’t publish his results. His observations became known only in 1784 and were not published until 1833. Finally, the Frisian (northwest Germany) astronomer Johannes Fabricius observed sunspots with a telescope on February 27, 1611, and described them in a pamphlet (of which Galileo was unaware) published the same year in the city of Wittenberg.
Observations of the Sun with the telescope were tricky, since without covering the lens with some protective material, such as soot, one could easily be blinded. Fortunately for Galileo, his talented former student Benedetto Castelli came up with the clever idea of simply projecting the image of the Sun formed by the telescope onto a screen or a sheet of paper. Originally, Galileo described his observations of sunspots in the preface to his book Bodies in Water. At that stage, he still entertained two possibilities for the nature of the spots. He thought that they could be either directly on the Sun’s surface (in which case their motion indicated that the Sun rotated about its axis), or they could be planets revolving around the Sun very close to its surface. By the time of the book’s second printing in the fall of 1612, however, Galileo was convinced that the spots had to be on the Sun’s surface and “carried around by rotation of the Sun itself,” and he added a paragraph to that effect.
The findings of another astronomer forced Galileo to turn his full attention to sunspots. In March 1612 he received from his German correspondent Markus Welser three letters describing sunspots, written under the pseudonym “Apelles latens post tabulam” (“Apelles hiding behind the painting”). The letters, later published as a pamphlet, had been written by the Jesuit priest and astronomer Christoph Scheiner, a professor at the University of Ingolstadt. Scheiner was forbidden from publishing under his real name for fear that if he happened to be wrong, the publication could discredit the Jesuits. Consequently, he used a pseudonym coined after the fourth-century Greek artist who used to hide behind his paintings to listen to viewers’ criticism, implying that Scheiner was waiting for comments before revealing his identity. The astronomer maintained that the spots were projected shadows of many small planets orbiting the Sun in very tight orbits.
While there is no doubt that his ideas were inspired mainly by an attempt to rescue the Sun from imperfection, Scheiner based his model on three main arguments: First, the spots did not return to the same points, which to him implied that they were not contiguous to the surface of a rotating Sun. Second, Scheiner believed that the spots were darker than the unilluminated portions of the lunar surface, which he thought to be impossible if they were really on the Sun’s surface. Third, the spots appeared thinner near the Sun’s edge than when they were near the center of the solar disk, which he interpreted as an example of phases, like the ones seen in the case of Venus.
In addition to his comments about sunspots, Scheiner called attention to what he regarded as more convincing evidence than the phases, that Venus was really orbiting the Sun. Scheiner’s proof relied on the fact that tables based on the Ptolemaic model, known as ephemerides, predicted that Venus would transit (that is, pass in front of, as seen from Earth) the Sun on December 11, 1611. Yet no such long-duration transit had been observed.
Welser sent the letters to Galileo to ask for the famous scientist’s opinion on Scheiner’s ideas, apparently assuming that Galileo would appreciate the scientific approach exhibited in the letters. However, the answer to “Apelles” that he received from Galileo was quite different from what he had expected. On one hand, Galileo’s reply was witty, quite courteous, and certainly scientifically brilliant, but on the other, it also used highly critical and rather patronizing language. For example, referring to what he regarded as Scheiner’s obstinate adherence to some Aristotelian concepts (such as the hardness and immutability of the Sun), Galileo wrote that Apelles “cannot yet totally free himself from those fancies previously impressed on him.”
Galileo’s response was delivered in several installments. First, he sent two letters written in Italian (“because I must have everyone able to read it”) in May and October. Then, after Scheiner replied to the first letter with a letter of his own, and Welser published the entire series of Scheiner’s letters under the title A More Accurate Disquisition of Sunspots and the Stars Wandering Around Jupiter, Galileo dispatched a third letter in December. These three letters were also published in Rome by the Lincean Academy in March 1613, entitled History and Demonstrations Concerning Sunspots and Their Phenomena (Figure 4.5).
Galileo, never known for taking criticism well, was particularly annoyed by Scheiner’s claim that the failure to detect Venus’s transit constituted superior evidence that Venus orbited the Sun. He pointed out that Scheiner erred in estimating the planet’s size and, in addition, that it would have sufficed for Venus to possess just a tiny bit of intrinsic brightness to render the absence of the transit useless in terms of proof for Venus’s orbit.
Figure 4.5, Title page of Galileo’s book on sunspots.
Following this captiousness, Galileo moved on to dismantling Scheiner’s explanation for sunspots. He made it clear that the spots were not really dark; they only appeared dark relative to the Sun’s bright disk but were, in fact, brighter than the surface of a full Moon. He then argued correctly that the fact that the spots moved at varying speeds and changed positions with respect to one another showed unambiguously that they couldn’t be satellites, since “anyone who wished to maintain that the spots were congeries of minute stars would have to introduce into the sky innumerable movements, tumultuous, uneven, and without any regularity.” Instead, Galileo placed the spots squarely on the Sun’s surface or no farther from the surface of the Sun than clouds would be (relatively) from Earth. Like clouds, he commented, the spots appeared suddenly, changed shape, and disappeared without warning. Using an intuition gained through his artistic education in drawing, Galileo also demonstrated that the spots’ apparent narrowing as they approached the edge of the solar disk was due simply to the foreshortening that is observed when something moves on the surface of a sphere (Figure 4.6). Finally, and perhaps most important, from the motion of the spots, Galileo estimated that the Sun takes approximately a month to rotate about its axis. Indeed, we know now that the solar rotation period at the equator is 24.47 days.
Figure 4.6. The visual phenomenon of foreshortening circles drawn on the surface of a sphere.
Materially for Galileo’s problems with the Catholic Church in later years, he also asked Cardinal Carlo Conti for his opinion on sunspots. The cardinal answered in July 1612 that there was nothing in Scripture to support the Aristotelian notion of an incorruptible Sun. Concerning Copernicanism in general, however, Conti advised that this theory was inconsistent with Scripture, and that a different interpretation of the biblical lan
guage “should not be admitted unless it is really necessary.”
Galileo’s observations and interpretation of sunspots were of critical significance for two main reasons. First, they demonstrated that a celestial object could be spinning around its axis without either slowing down or leaving behind cloudlike features. This could instantly remove two serious objections raised against the idea of a spinning Earth in the Copernican model. The deniers asked: How can the Earth keep spinning? And: Why aren’t clouds (or birds, for that matter) falling behind? Second, by publishing his results on the rotating Sun in Bodies in Water—a book ostensibly about floating bodies—Galileo signaled the first appearance of a unified theory of the physics of the Earth and of the heavens. This type of unification would later help produce Newton’s universal theory of gravitation (which brought together phenomena as diverse as apples falling on Earth and planets orbiting the Sun), and would inspire all the attempts today to formulate a “theory of everything”—a framework merging all the fundamental interactions (electromagnetic, strong and weak nuclear, and gravitational).
As he had often done, Galileo used the opportunity of the correspondence about sunspots to offer a glimpse into his philosophy with respect to disseminating knowledge. In a letter to his friend Paolo Gualdo, the archpriest at the Padua Cathedral, he made a few remarkable comments about the fact that science should not be exclusively the province of scientists. He explained that he hoped that from his letters to Welser, even those who “became convinced that in those ‘big books there are great new things of logic and philosophy and still more that is over their heads’ ” would see that “just as nature has given them, as well as the philosophers, eyes with which to see her works, so she has also given them brains capable of penetrating and understanding them.” Here Galileo establishes himself firmly as a member of what author John Brockman dubbed the “third culture”: a direct conduit between the scientific world and laypeople. The key point that Galileo made was that scientific knowledge, when presented adequately, is not beyond the grasp of nonscientists, and since he regarded it as an essential part of human culture, literally everybody should strive to acquire it.
Fascinatingly, Galileo expressed here even less surprise at the human capacity to fathom the cosmos than Einstein did in 1936: “The eternal mystery of the world is its comprehensibility.… The fact that it is comprehensible is a miracle.” Galileo’s comments on the human ability to decipher nature’s secrets were also echoed in his famous Letter to Benedetto Castelli, when he said that he did not believe “that the same God who has given us our senses, reason, and intelligence wished us to abandon their use.”
Today we know that sunspots are indeed regions on the surface of the Sun that are somewhat cooler (temperature of about 4,000 Kelvin) than the surrounding area (about 6,000 Kelvin), and therefore appear darker. The lower temperature results from concentrations of magnetic field flux that suppress heat transport by convection (fluid motion). Sunspots typically last anywhere from a few days to a few months, and their sizes vary widely, from a few tens of miles across to a hundred thousand miles. Sunspot activity cycles last about eleven years; over the course of a cycle, the number of spots initially increases rapidly and then declines more slowly.
Galileo’s Letters on Sunspots not only gave him a scientific victory over Christoph Scheiner at the time, but also brought Copernicanism to the attention of a larger readership. In 1615 Scheiner sent Galileo another work, entitled Sol Ellipticus (The Elliptical Sun), and asked for Galileo’s opinion on it, but he never received a response. Scheiner himself eventually published in 1630 an impressive and authoritative book on sunspots, which, in honor of his protector, Prince Paolo Orsini, he entitled Orsini’s Rose, or the Sun’s Variations in Accordance with the Observed Appearance of Its Flares and Sunspots. In this book, Scheiner conceded that the spots were on the solar surface, but he claimed that Galileo’s conclusions on this topic had not been based on scientific reasons. Unfortunately, there is no doubt that Galileo’s rather disparaging letters, his disregard for Scheiner’s work in 1615, and some further comments he made later in his book The Assayer, which the Jesuit astronomer took to be directed at him personally, did turn Scheiner into an unappeasable enemy. This marked just the beginning of a conflict with the Jesuits, which would culminate in the punitive actions against Galileo in 1633.
CHAPTER 5 Every Action Has a Reaction
Given the magnitude of Galileo’s celestial discoveries with the telescope and the fact that Sidereus Nuncius quickly turned him into an international celebrity, it was only to be expected that the reactions would be intense, passionate—and mixed. Indeed, controversy erupted almost before the ink dried on the book’s pages. There were several reasons for the initial skepticism, and those could be traced to the long-lasting dominance of Aristotle’s ideas and the almost religious acceptance of his general approach to science.
First, Galileo’s methodology introduced a radically new element into what he asserted could be considered as constituting evidence. Fundamentally, Galileo claimed that his new device—the telescope—was revealing unimaginable truths that couldn’t be perceived by the unaided senses. This flew in the face of the established Aristotelian tradition. How could anyone be sure that what Galileo was seeing was a genuine heavenly phenomenon and not a spurious artifact produced by the telescope itself? The telescope was, after all, the very first gadget presented as a means to boost and expand the power of sensory faculty.
A second problem that Galileo’s discoveries in both mechanics and astronomy encountered had to do with his proclamation that the universe was “written in the language of mathematics.” That is, he introduced the mathematization of the physical world. This notion ran completely contrary to Aristotelian reasoning, according to which mathematics had little if anything to do with reality or with the makeup of the cosmos. Until Galileo’s time, astronomers were expected to use mathematics only to calculate planetary orbits and the apparent motion of the Sun, and thereby to create maps of the sky at particular times. These, in turn, were supposed to aid in estimating time, in establishing a calendar, in navigation, and in the production of astrological charts. Astronomers were not meant to construct physical models of the universe or of any phenomena within it. Here is how the Aristotelian Giorgio Coresio—the person who claimed that balls dropped from the Tower of Pisa confirmed Aristotle’s assertions about free-falling bodies—had put it: “Let us conclude, therefore, that he who does not want to work in darkness must consult Aristotle, the excellent interpreter of nature.” Compare this submissiveness to authority to Galileo’s almost poetic later pronouncement in The Assayer: “It [the universe] is written in the language of mathematics, and the characters are triangles, circles, and other geometrical figures, without which it is humanly impossible to comprehend a single word of it, and without which one wanders in vain through a dark labyrinth.”
Vincenzo di Grazia, a professor at Pisa, expressed his views on what he regarded as the contradistinction between mathematics and the natural sciences in even stronger terms:
Before we consider Galileo’s demonstrations, it seems necessary to prove how far from the truth are those who wish to prove natural facts by means of mathematical reasoning, among whom, if I am not mistaken, is Galileo. All the sciences and all the arts have their own principles and their own causes by means of which they demonstrate the special properties of their own object. It follows that we are not allowed to use the principles of one science to prove the properties of another [emphasis added]. Therefore, anyone who thinks that he can prove natural properties with mathematical argument is simply demented, for the two sciences are very different.
Galileo could not have disagreed more with this attempt at hermetic compartmentalization of the different branches of science. “As if geometry in our day was an obstacle to the acquisition of true philosophy; as if it were impossible to be a geometer as well as a philosopher, so that we must infer as a necessary consequence that anyone who knows geo
metry cannot know physics, and cannot reason about and deal with physical matters physically!… as if knowledge of surgery was opposed to medicine and destroyed it,” he mocked di Grazia. Einstein would fully agree with Galileo more than three centuries later, writing: “We may in fact regard [geometry] as the most ancient branch of physics.… Without it, I would have been unable to formulate the theory of relativity.”
These two problems—the legitimacy of the telescope as an instrument enhancing the senses on one hand, and the role of mathematics in revealing truths about nature on the other—combined in the minds of the Aristotelians to form what they considered to be a powerful argument against Galileo’s findings. Not only wasn’t there a convincing theory of optics that could demonstrate that the telescope doesn’t deceive, they contended, but also the validity of such a theory in itself, being based on mathematics, was questionable. On top of these philosophical matters, weighed, of course, the fact that all of Galileo’s celestial discoveries defied Aristotelian ideas that the conservative establishment had revered for almost two millennia.
No wonder, then, that the immediate reaction in many circles was one of confusion. People from all ranks and spheres, ranging from state rulers and high church officials, to the lay public, turned to prominent scientists for opinion and advice. Even the German scholar Markus Welser, who later was instrumental in helping to spread Galileo’s ideas, wrote to Christopher Clavius at the Collegio Romano, asking for his judgment:
With this occasion, I cannot neglect to tell you that it has been written to me from Padua as a certain and secure thing that with a new instrument called by many visorio, of which he makes himself the creator, Mr. Galileo Galilei of that university has discovered four planets, new to us, having never been seen, as far as we know, by a mortal, and also many fixed stars, not known or seen before, and marvelous things about the Milky Way. I know very well that “to believe slowly is the strength of wisdom,” and I have not made up my mind about anything. I ask Your Reverence, however, candidly to tell me your opinion about this fact in confidence.