The Theory of Heliocentrism

After Galileo Galilei returned to Tuscany from Rome in 1616, he moved into a lovely farm villa perched on a high hill above the southern bank of the Arno River. As commercially minded as ever, he grew wheat and beans, which he sold alongside his telescopes in the bustling markets of Florence. Bright students from the University of Pisa and elsewhere frequently came to stay with him. Galileo treated them generously, even as he was growing more irascible with age in his relationships with natural philosophers closer to his own rank. Yet he seemed to have learned his lesson when it came to the forbidden theory of heliocentrism. Whatever he may have said to friends and students in the privacy of his home, he did not argue for its physical truth in public. Indeed, he dutifully followed the Church’s instructions to cross out offending passages in his personal copy of Copernicus’s On the Revolutions of the Heavenly Spheres.

Whilst thus steering clear of the minefield of heliocentrism, Galileo turned his astronomical focus to the subject of comets, of which a couple of smaller examples passed over his head there in Tuscany during this period. In yet another proof, should we be in need of one, that he was far from infallible, Galileo insisted that comets were not physical objects at all, but rather strictly optical phenomena. He based this conclusion on their stubborn refusal to reveal themselves properly to the observer standing behind a telescope; no matter how much they were magnified, they remained blurry in outline. We know now that this is because of the diffuse cloud of gas and dust that comes to surround them when they are heated by the Sun, but Galileo had no way of deducing this. When he argued that they were not physical objects — very much a minority view in astronomy since at least the time of Tycho Brahe — he was simply applying empirical reasoning to the best of his ability. In a small book called The Assayer, he made perhaps the most eloquent argument yet for the emerging scientific method, for making the theory suit the data instead of trying to make the data suit the theory. That he used his inductive empirical reasoning to come to the false conclusion that comets do not physically exist should serve as a reminder to every scientist that theories must always be conditional, pending new data that might disprove them.

The Assayer showed that Galileo’s facility as an intellectual jouster hadn’t been dampened one whit by his advancing age. Much of it was directed toward an author of the old-fashioned Aristotelian school who wrote under the name of Lothario Sarsi. This Sarsi became the hapless stand-in for thousands of others just like him who preferred airy philosophical abstractions about the way things surely ought to be to meeting the world where it lived.

Perhaps Sarsi thinks that philosophy is a book of fiction created by one man, like the Iliad or the Orlando Furioso, books in which the least important thing is whether what is written in them is true. Sarsi, this is not the way matters stand. Philosophy is written in that great book which ever lies open before our eyes — I mean the universe…

I wrote above that Galileo seemed to have learned his lesson about heliocentrism during this period. I used that word advisedly: in truth, he was merely biding his time, waiting for a change in the intellectual climate. He suspected that the change he wished for might be nigh when Pope Paul V died on January 28, 1621. He suspected this even more so a month later, when he learned that Pope Paul’s successor was Gregory XV, the first Holy Father to be recruited from the ranks of the Jesuit order. Founded a century earlier in the heat of the schismatic crisis caused by Martin Luther, this Society of Jesus was reform-minded and friendlier to science than many other orders within the Church. Then, on September 17, 1521, Robert Bellarmine, the chief attack dog of the Church’s conservative wing, passed away as well. It looked like the poles were shifting, such that Galileo would might soon no longer have to hide his belief in heliocentrism as a physical truth. A sympathetic papal secretary wrote to him to tell him that he hoped to persuade the pope soon to do away with the restrictions on “those admirable ideas with which you have enlightened our age.”

As it happened, Pope Gregory died on July 8, 1623, before he could get around to that task. That said, his successor, Pope Urban VIII, appeared at first to bode equally well for Galileo’s cause, for all that he was not a Jesuit. Cardinal Maffeo Barberini, the name by which Urban had been known prior to his elevation, had actually been present at Galileo’s dressing-down by Cardinal Bellarmine in early 1616. But he had not been among those at that meeting who had been most eager to condemn the astronomer for promoting heliocentrism. On the contrary, he had been the closest thing Galileo had to an ally in that room; Barberini had displayed distinct discomfort with much of what was said that day. Now, reports reached Galileo from Rome that one of Urban’s servants had been been directed to read The Assayer to him while he took his meals. Thus encouraged, Galileo decided he ought to meet Urban personally for the second time, hopefully under more genial circumstances than their last encounter.

So, in April of 1624, Galileo made the fourth trip of his life to Rome. He was accorded as cordial a reception by the new pope and his court as he could have asked for. He had recently been experimenting with compound microscopes, which were essentially telescopes in reverse, with the concave lens placed farther from the eye, the convex lens closer. He delighted Rome’s chattering class by showing them insects through these different but no less magical tubes. Gentlemen goggled and ladies clutched their pearls as the microscope revealed flies and ants to be grotesque monsters out of a book of mythology.

But Galileo found that the pope and his court kept dodging the real agenda that had brought him to Rome. Much to his disappointment, and contrary to the earlier statements of his network of sympathizers in the capital of Catholicism, there was no great appetite for removing Copernicus from the Index of Forbidden Books and repealing Pope Paul V’s edict that heliocentrism as a physical truth was “utterly heretical.” With the Thirty Years War spreading across the lands north of Italy and the threat posed by Protestantism still very much at top of mind, the Church had enough on its plate. What benefit was there in promoting a theory that might sow further doubt among the faithful, even if in only a small way? Pope Urban, exclaimed the frustrated Galileo, “rests content with [his] own convenient way of having the heavenly bodies moved, without the slightest difficulty, by angels.” Galileo went home again after just six weeks.

Still, he did feel that the changing of the papal guard had given him some space to speak his mind publicly about heliocentrism once again. He chose to do so in his typical jousting fashion, by writing a pamphlet to serve as a riposte to an anti-Copernican book written by a Roman priest named Francesco Ingoli. He went at his opponent hard, as was his wont.

Signor Ingoli, if your philosophical sincerity and my old regard for you will allow me to say so, you should in all honesty have known that Nicolaus Copernicus spent more years on these very difficult problems than you spent days on them. You should have been more careful, and not allowed yourself to be so lightly persuaded that you could knock down such a man, especially with the sort of weapons you use, which are among the most common and trite objections advanced in this subject…

If anyplace in the world is to be called its center, this is the center of celestial revolutions, and anyone who is competent in this subject knows that it is the Sun rather than the Earth that is found there.

Not understanding the full ramifications of gravity, Galileo had developed a theory that the tides on Earth were caused by the oceans sloshing around in their basins as a result of the planet’s movements. “If the Earth is at rest,” he wrote now, “the tides cannot occur.” This was going far out on a limb indeed, in that Galileo was pointing to the tides as palpable, physical evidence in favor of a peripatetic Earth rather than being content to say that heliocentrism merely made the math work out better. Even if his version of the meeting he held in 1616 with Cardinal Bellarmine was the correct one, and he had simply agreed on that occasion not to advocate for Copernicus’s theories as physical rather than instrumental truths, he was now quite blatantly violating the agreement.

And yet no condemnation came down from Rome in response to his pamphlet, which reached the Eternal City before 1624 was over. Taking this silence as a tacit acknowledgment that the rules of the game had changed and he need no longer be bound by the promise he had made to Cardinal Bellarmine, Galileo embarked upon what he fully expected to be the last and most important book of his life, his final, eloquent argument for heliocentrism. Its creation would consume him intermittently for the next five years, a period during which he was often ill. Now entering his seventh decade on Earth, he felt he was in a race against time.

Galileo’s Dialogue Concerning the Two Chief World Systems was an odd amalgamation that managed to illustrate both how much progress Europe’s intellectual vanguard had made in recent decades and how much they were still in debt to their ancient forebears. It was modeled on Plato, who had presented much of his philosophy in the form of transcribed conversations between his own mentor Socrates and the citizens of Athens. Because Socrates himself never put any of his ideas down on the page, we cannot know how accurately Plato represented these conversations, or for that matter whether they ever really took place at all. But the Socratic dialogue as a means of hashing out the case for and against an idea has lived on right to the present day; plenty of modern-day science-fiction novels, for example, include discussions between characters that function in just this way, whether the subject under debate be terraforming Mars or traveling through time. It is an extremely useful way of coming to grips with a subject when it is used in good faith, but, at the same time, it can be all too easily twisted by a partisan to belittle or trivialize the opposition rather than enlighten them; it is, after all, practically the definition of a straw-man argument. As anyone who knew his appetite for verbal combat might have guessed, Galileo landed closer to the extreme of the unscrupulous hatchet man than that of the studiously fair-minded neutral arbiter.

His “Two World Systems” are of course heliocentrism and geocentrism. In place of the wise Socrates, who would naturally be an advocate for heliocentrism if he was still with us, Galileo places one Filippo Salviati, an old friend and patron from the Florentine upper crust. In the role of Salviati’s geocentrist antagonist, he places a fictional Aristotelian named Simplicio; the similarity of the name to “simpleton” was not accidental. And then there is Giovanfrancesco Sagredo, a real Venetian nobleman who hosts the discussion in his palace and serves as a stand-in for Galileo’s ideal reader, a bright, interested, and open-minded layman. Put another way, Salviati and Simplicio will each argue their case for the benefit of Sagredo and the reader, who will then be entrusted to pass judgment. Galileo uses Simplicio to set up the standard arguments against heliocentrism one by one, then deploys Salviati to knock them down.

Simplicio posits that the heavens are fundamentally different from the terrestrial world, in that they were created by God to be perfect and unchanging. The “unchanging” part at least cannot possibly be true, says Salviati in reply. Have astronomers not been watching comets and supernovae come and go since ancient times? More recently, have astronomers not observed spots moving about the surface of the Sun, expanding and contracting and then disappearing altogether, to be replaced by new patterns? If we accept these things as proofs that the heavens are not unchanging, they cannot be perfect either, because there can be only one state of perfection; here Galileo uses the philosophy of the Aristotelians against them. The evidence indicates, then, that the heavenly realm visible above us is an extension of the earthly one, subject to the same set of physical laws and capable of being made comprehensible using the same set of intellectual tools. The importance of this idea alone to the future of astronomy — think of Isaac Newton and his theory of gravity — can scarcely be overstated.

Undaunted in his dull intransigence, Simplicio next asks why, if the Earth is simultaneously spinning on its axis and moving through space, everything and everyone on it aren’t flung off of it. Salviati answers that the Earth imparts its own motion to everything upon it, a statement which is in line with the physics that human beings like him and Simplicio experience every day. To demonstrate this, Salviati proposes a thought experiment to his benighted interlocutor. Imagine you are in the hold of a ship. A fly will buzz around the hold, a fish will swim around in a bowl, and water will flow from a tap into a bucket in just the same way whether the ship happens to be sailing on the ocean or resting at anchor in a harbor. It is in fact impossible to determine from the motion of the fly, the fish, or the water whether the ship is in motion or not. Why should the objects on Earth, that enormous spaceship, be any different?

Thus stymied, Simplicio falls back on the philosophical tenet that is often referred to as Occam’s Razor, which states that the simplest explanation for any phenomenon which isn’t contradicted by the evidence of our senses is probably the correct one. We can see the Sun rising and setting, he notes, see the Moon and planets and stars wheeling around above us. Why make these things more complicated than they need to be? The problem with this argument, Salviati patiently explains, is that geocentrism does contradict the evidence of our senses as soon as we begin to look more closely — for example, when we notice that some of the planets appear to stop in the midst of their wheeling and suddenly start moving in the opposite direction at arbitrary times. When that happens, Occam’s Razor becomes heliocentrism’s friend instead of its foe. Which explanation is simpler and therefore more likely: that Mars likes to shift its direction of travel from time to time for no apparent reason, or that both Mars and the Earth are orbiting the Sun?

None of these arguments and counterarguments were new; certainly we have seen them all before. The most novel and exciting parts of the Dialogue are the ones where Galileo almost seems to be channeling Giordano Bruno. He has Salviati state that the Moon and the planets, being physical bodies like the Earth, might very well be the home of alien lifeforms utterly unlike those that live on Earth. And then he has Sagredo, his thoughtful layman, go a step further. Sagredo says that absence of evidence for other planets beyond the familiar five that we can see is not evidence for the absence of such worlds, of which there may be an infinite number, orbiting stars which are not our Sun. Even as Galileo has convinced himself of the nonexistence of comets as physical bodies, he has made the leap to imagining the stars as something other than mere pinpricks of light on the inner surface of a cosmic sphere. No: they are bodies like our own Sun, which is itself just another star, special only because it is so close to us. Despite having yet to even prove that the Sun rather than the Earth might be the center of the universe to the satisfaction of many of his peers, Galileo is now on the verge of leaving Copernicus’s theory of heliocentrism behind for something even more discomfiting: an infinite universe, or at least an unbelievably big one, with neither the Earth nor the Sun at its center, nor unique in any way whatsoever.

What does it mean to say that the space between Saturn and the fixed stars is empty of world bodies? That we do not see them perhaps? Then did the four satellites of Jupiter and the companions of Saturn come into the heavens when we began seeing them and not before? Were there not innumerable fixed stars before men began to see them? The nebulae were only little white patches. Have we with our telescopes made them become clusters of many bright and beautiful stars? Oh, the presumptuous, rash ignorance of mankind!

The real Giovanfrancesco Sagredo might well have wondered whether his friend Galileo was really doing him a favor by putting such words in his alter ego’s mouth. For this position was fraught with grave and perilous theological implications, as eloquently described by Galileo’s biographer David Wootoon.

In place of a universe with humanity at its centre, a universe made to serve humanity’s purposes, a universe designed to make possible humanity’s salvation, Galileo offers a mysterious universe whose purposes are unknown, whose size is unfathomable and which may contain other beings quite different from ourselves. We are now simply “noi Terreni,” us terrestrials. You do not have to force the text to find in it Bruno’s heresy: around other stars there may be other planets, other worlds which make the mistake of thinking that they are at the centre of everything, and that their experiences are the true measure of reality.

When push came to shove, Galileo’s theories could be true or Christianity could be literally true. But it was hard to see how humanity could be God’s special, chosen creations in a universe potentially teeming with intelligent life. Why should God give so much attention to us alone? “Presumptuous, rash ignorance” indeed!

Bold and bracing though it often is, Dialogue Concerning the Two Chief World Systems is by no means an airtight argument for either Bruno’s infinite universe or Copernicus’s Sun-centered one. For example, it details at rather exhausting length Galileo’s misguided theory that the tides are caused by the motion of the Earth, an argument that directly contradicts his example of the fishbowl in the ship’s hold. This contradiction notwithstanding, Galileo trots the tides out at the very end of the book as Salviati’s coup de grâce against Simplicio.

Then, too, the Dialogue is almost as notable for what it leaves out as what it includes. Galileo strives to discredit only the geocentrism of Claudius Ptolemy, not the hybrid cosmos of Tycho Brahe, to which there were probably more adherents among educated astronomers by this point than there were obstinate pure geocentrists. And Galileo doesn’t deign to go into the myriad ways that Copernicus’s system just doesn’t hang together as well as one might wish, how it has to be kludged up with epicycles upon epicycles to make the math work out. If Galileo had read one of Johannes Kepler’s recent books, Occam’s Razor could have told him that the latter’s elliptical universe was the real solution to this conundrum. Lacking that advantage, he had to choose whether to make his argument using gnarly mathematics that he himself struggled to truly comprehend or just to hand-wave away the problems. He chose the hand-wave. In an ironic way, Galileo’s heliocentrism was as much a matter of faith as empirical science when you got right down to it. He loved to rage against the Aristotelians, but in his later years especially he showed himself to be willing to bend the empirical facts to suit his agenda time and time again. “The truth is that after his sensational discoveries in 1610,” writes Arthur Koestler, “Galileo neglected both observational research and astronomic theory in favour of his propaganda crusade.”

But if it had its share of flaws as a scientific text, the Dialogue was positively scintillating to read. This was the Galileo who had been delighting audiences with his intellectual élan for decades, still at the height of his rhetorical powers in his early sixties. He was no longer writing for his fellow astronomers; he was writing to win over popes and kings, bishops and nobles. He was no longer a practicing scientist; he was now a partisan popularizer — or, as Koestler will have it, a propagandist.

Galileo had a close friend in Rome, a Prince Fredrico Cesis, who kept him up to date on the goings-on there. The reports Cesis sent back to Tuscany were for the most encouraging for Galileo’s cause. Pope Urban was proving something of a tyrant, but he was a tyrant of a different stripe than Pope Paul. For Urban was a throwback to the popes of the High Renaissance, being more concerned with his and his family’s wealth and status than the niceties of religious doctrine. He had ordered all of the songbirds around his palace killed because they disturbed his sleep. He had made his brother and two nephews cardinals, one of them at the ripe old age of nineteen. Convinced that he was a gifted poet, he wrote his own psalms and hymns and demanded that the clergy recite them during Mass. He plundered bronze from the ancient Pantheon to erect a monument to himself inside Saint Peter’s Basilica. “What the barbarians did not do, the Barbarins did,” muttered the citizens of Rome to one another.

Galileo believed that Urban’s worldly predilections would redound to his benefit as he made his case for heliocentrism. Why should the pope spend time and energy parsing astronomical texts when there were family members to be enriched and monuments to himself to be built? Galileo had no way of knowing that Urban’s vanity would in time prove as dangerous to him as Paul’s zealotry — more so, in fact. For now, he thought that all of the signs looked positive. He heard from the well-connected Prince Cesis that Urban was rumored to have expressed regret about the inclusion of Copernicus’s book on the Index and the blanket condemnation of heliocentrism that Pope Paul had issued at the same time: “It was never our intention, and if it had been up to us that decree would never have been issued.” Galileo and Cesis decided to have the Dialogue printed in Rome itself; it seemed the perfect way to advertise that the conventional wisdom about the heavens had well and truly shifted. The aging astronomer arrived in Rome for the fifth time in his life, this time with a handwritten manuscript in hand, on May 3, 1630.

It turned into another disappointing visit; it seemed that Prince Cesis may have been overoptimistic in his rumor-mongering. Galileo had his second audience with Pope Urban, but still couldn’t pin him down on the one subject that he really wanted to discuss with him. The same went for just about everyone else he met in Rome. Finally, he and Cesis decided to take a lack of open opposition as permission. Cesis paid a scribe to copy Galileo’s manuscript out. He would see to its printing there in Rome, he promised, while Galileo, who was becoming ever more of a homebody as he aged, returned to his beloved Tuscan villa.

But hardly had Galileo reached his home than he received a piece of shocking news from the city he had just left: Prince Cesis had suddenly died at the age of 45, leaving behind no will and an estate that, it was now revealed, owed money to just about every creditor in Rome. Galileo would have to come up with a new plan for getting his Dialogue published.

This event was followed by an even worse one, not only for Galileo but for everyone around him. The chaos, filth, and privation unleashed by the Thirty Years War was a fine breeding ground for disease. That summer of 1630, the bubonic plague made its way down into Italy from the burnt-out farmers’ fields and corpse-strewn battlefields of Germany. The death toll was horrifying, adding up to more than one person in two in many parts of Tuscany and points north. Even more at risk than most thanks to his age and generally fragile health, Galileo huddled inside his country villa, turning away all comers. A specific understanding of the germ theory of disease transmission may have been over 200 years in the future, but it was nevertheless well known that contact with the smitten dramatically increased one’s own chances of coming down with the plague. “My work has been put aside in a corner [and] my life wastes away,” complained Galileo. (He couldn’t know that this was literally true of Johannes Kepler, who died shivering and unshriven in a humble inn in Regensburg that autumn.)

While he waited out the plague, Galileo tried to obtain a license to print his book from Rome, something which the Church had now come to insist upon in these increasingly censorious times; no legitimate printer in Italy would take on a book that didn’t have the Church’s stamp of approval. Galileo made his application to Niccolò Riccardi, Pope Urban’s Master of the Sacred Palace, whose responsibilities included acting as the Church’s official censor. Riccardi stipulated that Galileo, in keeping with longstanding Church policy, pay lip service to heliocentrism as merely an instrumental truth in a preface, either not realizing or not caring that the rest of the book would so forthrightly and even scathingly argue for the theory’s physical truth. Galileo grudgingly complied: “I have taken the Copernican side in the discourse and, proceeding as with a pure mathematical hypothesis, I have tried to show, by even means, that it is more satisfactory than supposing the Earth motionless.” He sent the preface to Riccardi, who deemed it sufficient to approve the book to which it was to be attached. Importantly for our story, Riccardi did not know about Galileo’s 1616 run-in with Cardinal Bellarmine, nor of his pledge to either, depending on whom you asked, not discuss heliocentrism at all in the future or not discuss it as a physical truth.

Not until the second half of 1631 did Galileo dare to venture into the tragically empty streets of Florence in search of a printer. He settled on one named Giovan Battista Landini, who found him to be an exacting and demanding customer, but was happy enough for his business in the depopulated city. On February 21, 1632, Landini heaved a sigh of relief. “Praise be to God, today I finished Galileo’s book,” he wrote to a friend. As he was doing so, he had 1000 copies of Dialogue Concerning the Two Chief World Systems sitting in his storeroom, ready to make their mark. Within weeks, some of them would be in Rome.

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(A full listing of print and online sources used will follow the final article in this series.)