Reprinted with permission of the International Planetarium Society
Savannah Science Museum
4405 Paulsen Street
Savannah, Georgia 31405
One of the little fictions that planetarium lecturers like to tell is that of Galileo confronting the Inquisition. Accused of holding the heretical belief that the Earth moves around the sun, Galileo stands defiantly—the enlightened man of science—facing the entrenched dogma of the Church. It is a story told so often that we have come to believe it ourselves.
Unfortunately, history does not support such a picture. Galileo may not have been guilty of heresy, but he was guilty of several other things: (l) some of his scientific "facts" were wrong; (2) he claimed to have proof when no proof existed; (3) he was unaware of Kepler's exposition of planetary motion, though Kepler's book was in his own bookcase; and (4) he had made enemies—bitter enemies—quite needlessly.
Nor was the Roman Church the main villain in the piece. Galileo's real enemies were the university professors. They, quite understandably, resented Galileo's outright rejection of their revered Aristotle, whose concepts constituted almost everything they taught. And they fumed at their inability to answer Galileo's arguments rationally. For twenty years they debated him and lost. Eventually, when Galileo embraced Copernicism, they were able to enmesh the clerics in their battle.
In order to understand his trial, we must first understand Galileo himself. He was born in 1564 at the height of the Italian Renaissance, and he was the total Renaissance man, in all that the word implies: brilliant, witty, articulate, incisive, charming. Not only a scientist and mathematician, he was an accomplished musician on both lute and organ. His skills in communication were particularly outstanding: he could write clearly in Latin (he had committed all of Virgil to memory), and he could write even more clearly in Italian. Some say he wrote the best Italian prose of his time.
Galileo was a constant experimenter and a keen observer of nature. He was convinced that nature's laws could be reduced to mathematical equations. Early in his career he set forth the concept of center of gravity, and he invented the hydrostatic balance, an instrument for determining density. He deduced the law of the pendulum, and he clarified misconceptions on the subject of buoyancy. More than any one individual, Galileo may be considered the founder of the modern "scientific method".
Aristotle had a different approach to science. He attempted to describe all of nature not only in form but also in purpose. Everything had to be placed on its proper level within the natural order of things. Thus, his "scientific method" was not so much measurement as it was classification. His universe was an elaborate structure tying together all of nature. A flaw in any part of the structure would weaken the whole. Although his descriptions of plants and animals was astonishingly accurate, his physics erred badly. For example, he believed that a heavy body falls faster than a light one, though a simple experiment using a rock and a pebble would have shown him otherwise. Nevertheless, Aristotle's teachings were unquestioned. After eighteen centuries, universities accepted them as if they had been written in stone. Galileo would not accept them at all. He became convinced that experimentation, not introspection, was the key to understanding.
So far, so good. But Galileo had another side to his nature. He was pugnacious, argumentative, and vain. He refused to believe that any of his contemporaries could be as enlightened as he. Thus, he always showed bitter disdain for Tycho Brahe, though Tycho was by far the better astronomer: Tycho's measurements of the motions of the planets were superb, and he was able to determine the parallax of the moon even without a telescope.
Galileo's opinion of Kepler was not much better. (After all, Kepler had suggested the "puerility" that the moon might cause the tides). In 1597, when Kepler wrote Galileo that he had been converted to Copernicus' theory of a sun-centered universe, Galileo could not accept that Kepler was ahead of him. He wrote back that he too was a Copernican, and had been so for several years. However, history shows no evidence for such a claim: Galileo continued to teach the Aristotelian system at Padua University for another ten years.
As for his argumentative nature, it was evident even in his undergraduate days. He frequently engaged in debates with his professors, so much so that he earned the nickname "the Wrangler". And, not content with winning an argument, he would skewer his opponents with their own errors, and then expose them to the ridicule of the public.
As a young professor at Pisa, he taunted his colleagues. He wrote a satirical poem concerning a rule that required professors to wear the academic gown at all times. "Even when engaged in amorous activities?" he asked, and added little digs against the clergy with their equally long cassocks. He made his enemies early.
Most of all he lampooned the professors for their Aristotelian beliefs. Overthrowing Aristotle became an obsession of his. He once wrote Kepler, "Your new book is going to discomfit these stuffed robes of learning. How I love to see them stuck, silent with popping eyes, the figure of ignorance". The professors longed to see him stuck as well. They packed his classes and hissed at his attacks against Aristotle. Soon they were plotting actively, and formed a league to try to discredit him.
In 1609, Galileo heard of the discovery of the telescope, and he soon was able to make a much better one. When he trained this on the heavens, his life was transformed. Aristotle was wrong, over and over.
Aristotle had taught that the heavens were immutable and that the heavenly bodies were ethereal, not composed of fire, air, earth, and water as were earthly substances. The earth was the center of the universe, while the sun, moon, planets and stars orbited it in perfect circles.
This arrangement did not agree well with experimental measurements, so in 150 AD, Ptolemy embellished it with a system of epicycles—circles within circles—several dozen being required. Ptolemy's model became the standard.
A generation before Galileo, Copernicus had expressed the view that "such a complicated system to explain the motion of only seven heavenly bodies is unworthy of the hand of the Creator". In 1543, he tried to simplify things by proposing instead that the sun was at the center, and that the planets were bodies much like the earth, all orbiting the sun. But Copernicus, like Aristotle and Ptolemy, could not bring himself to believe that the planets moved in anything but perfect circles. And, to embody this belief, he had to retain 34 (some say 48) of the epicycles. His system was still unwieldy, and it did not predict planetary motion any more accurately than Ptolemy's.
But Galileo was able to bring new evidence that made Copernicus' model seem more reasonable. With his telescope he saw that the moon had mountains and seas. It was not composed of an ethereal substance, but was similar to the earth. Clearly Aristotle was wrong.
Next, Galileo discovered that Jupiter had four moons, again contradicting Aristotle, who had taught that there were only seven heavenly bodies. Not only that, but the moons were not left behind as Jupiter moved, again contradicting Aristotle, who had claimed that everything had "its natural place", and if the earth moved, the moon would be left behind.
The Milky Way was also changed: it was not a band of light, but a river of stars. More than any other observation, it gave the universe a dimension outwards.
He invited the university professors to see the heavenly wonders for themselves, but was met with hostility. Some refused to look through the telescope at all, others looked but professed to see nothing, others claimed that what they saw was a flaw in the optics.
Galileo had better luck with the Jesuit astronomers in Rome. They had obtained a telescope and had confirmed what Galileo had found. Galileo went to Rome for a triumphal visit. Even old Father Clavius, the author of the Gregorian calendar, who had scoffed earlier, gave in gracefully. Cardinal Bellarmine, the chief theologian of the Church, asked the Jesuits for an official opinion of Galileo’s views, and got "the most favorable letter you could think of".
Having successfully made friends and converts, Galileo proceeded to make them enemies. In 1612, a Jesuit astronomer, Christopher Scheiner in Bavaria, had written a letter reporting that he had seen spots on the sun, using a telescope of Kepler's design. Galileo could not accept that someone was ahead of him: he claimed that he had been observing sun-spots for over a year, and so had priority of discovery. History raises its eyebrows: Galileo usually reported in writing anything new and startling that he saw, even if only in letters to his close friends. There is no evidence of this. Understandably, Scheiner was incensed.
Ten years later, Galileo was even more insulting: He implied that Scheiner had heard of Galileo's work, had repeated it, and had published without giving Galileo any credit. Scheiner was outraged: It was bad enough to be plagiarized, but to be accused of plagiarism by the one who had been doing the plagiarizing …. The Jesuits closed ranks and turned against Galileo.
In Florence a Dominican, Tommaso Caccini, entered the controversy. He delivered a sermon starting with a masterful pun from Acts I, 11: "Men of Galilee (Viri Galilaei), why stand ye gazing up into heaven?" He denounced Galileo's moving earth as heresy. Though his superior apologized, his attack brought Galileo's views to the attention of the Inquisition, which began to make inquiries.
Cardinal Bellarmine looked into the matter. He stated with some accuracy: "If there is real proof that the sun is the center of the universe, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary. But as for myself, I do not believe that there is such a proof, since none has been shown me. It is not a proof that if the sun be put at the center of the universe everything works out the same as if it were the other way around".
Galileo had no proof. His best evidence was his observations of Venus: Venus had phases like the moon, and got larger in his telescope as the crescent got smaller. Clearly Venus was orbiting the sun, not the earth. But Tycho had been able to reconcile this with Ptolemy, by proposing that Venus orbits the sun, and that the sun orbits the earth, carrying Venus with it.
The Church demanded that Galileo recant. He was admonished by Cardinal Bellarmine, and admitted that he had erred. He was granted a friendly audience with Pope Paul V, not the most open-minded of Popes, who assured him that the plotting of his enemies was known, and that he had nothing to fear.
For the next three years, Galileo continued his experiments with physics. Then in 1618 three comets appeared, and he again entered into controversy. Comets were always hard to explain—they arrive unexpectedly, and might appear in any part of the sky, not just along the path followed by the moon and the planets. Years earlier, Tycho had proved that comets were farther away than the moon. Yet the paths of these comets were clearly not circles, and Galileo, like Copernicus and Aristotle before him, believed that heavenly bodies could only move in a circular orbit. They could not move first nearer to the sun and then farther away, unless there was some force acting on them. Kepler had suggested some kind of magnetism, but Galileo rejected this as mysticism.
He decided that comets must be of earthly origin: that they were vapors that had escaped from the earth and had mounted up beyond the moon, where they reflected sunlight back to the earth. This explanation was unworthy of Galileo: total nonsense; as bad as any of Aristotle's conjectures.
Another opinion about comets was expressed by Father Orazio Grassi of the Jesuit College in Rome. He used Tycho's model of the universe, and held that comets orbit the sun but the sun orbits the Earth. This was more correct than Galileo's hogwash, but it did not deter Galileo. He wrote a 200-page book, The Assayer, that flayed Grassi. It was a masterpiece of polemics. He dedicated it to the new Pope, Urban VIII, who is said to have enjoyed it. But it turned the Jesuits even more against Galileo.
Galileo now decided that he should make another try at promulgating the Copernican theory. He would write a book A Dialogue About the Two Chief World Systems in which he would present the views of both Copernicus and Ptolemy. Three characters would be involved: Salviati, the Copernican; Sagredo, the undecided; and Simplicio, the Ptolemian. It was no accident that the name Simplicio could also imply "simple-minded".
Galileo reasoned that inasmuch as Urban was an old friend and sympathetic to science, perhaps he could be persuaded to lift the ban on Copernicism. Urban granted Galileo six audiences—more than given to the ambassadors of most states. They discussed the pros and cons of Copernicus. Galileo presented his arguments, as he had done to his students and disciples many times before.
But Urban was no ordinary intellect, swayed by clever explanations and half-truths. He was a competent mathematician in his own right, and a patron of the sciences. He remained unconvinced by Galileo's arguments.
Galileo then presented his "proof": The tides in the sea could only be explained if the earth both rotated on its axis and revolved around the sun. The reasoning was long and complex, and Galileo devoted the entire final day of his Dialogue to it.
Urban's responded to this "proof" with these words (which he ordered be inserted in the Dialogue): "I maintain that your explanation of the tides is neither true nor conclusive, and that if you were asked whether God by his infinite power and wisdom might confer the reciprocal motion of the oceans in some other way than by making the contained vessel to move, you would say that he could, and in many ways, some beyond the reach of our intellect".
In this, Urban was no less than prophetic! The orbiting of the earth about the sun—Galileo's main thesis—has nothing to do with the tides. Galileo's "proof" was wrong. The true nature of the tides was indeed "beyond the reach of Galileo's intellect".
Now Galileo committed an incredible blunder. Whether deliberately or unthinking, he put Urban's words into the mouth of Simplicio: Simplicio, the simple-minded! And he had Salviati reply sarcastically, "A truly angelic doctrine"!
The Pope was furious. He withdrew all support for Galileo. The machinery of the Church moved inexorably toward the trial, which took place in 1633. The issue: Had he obeyed the early admonition not to defend the Copernican position as truth? Galileo recognized the danger of his position and tried to equivocate. "No!", he protested, "I have neither maintained nor defended the opinion that the earth moves and that the sun is stationary, but have rather demonstrated the opposite of the Copernican opinion, and shown that the arguments of Copernicus are weak and inconclusive".
This, of course, was nonsense. The Dialogue is an impassioned argument for the sun-centered universe. The judges concluded that Galileo was playing them for fools. They found him guilty and banned his book.
Galileo realized that the issue was no longer one of science, but of obedience. And he had no wish to be considered anything but a loyal son of the Church. On his knees he recanted. Legend has Galileo, still defiant, murmuring "Yet it does move!", but this is most unlikely. One might ask instead whether Galileo, in an uncharacteristic flood of humility, might have wondered whether he had let Copernicus down.
If so, he was correct. On his own bookshelf was Kepler's The New Astronomy, that Kepler had sent him in 1609. Kepler showed that the motions of the planets about the sun could be explained in excruciating detail if they were assumed to move in ellipses rather than circles. With one stroke all the circles and epicycles were removed.
Galileo would have been even more intrigued with Kepler's Second Law: that a line drawn from the planet to the sun will sweep out equal areas in equal times. This hints at a mathematical basis for the solar system. And Kepler's Third Law propounded in 1619, would have been compelling: The cube of a planet's distance to the sun is proportional to the square of its period. The mathematics here could leave no doubt that it must be right. Even the Pope might have been convinced. But Galileo, believing that Kepler could not have anything new or worthwhile to say, never read the book.
Galileo spent the last decade of this life under house arrest in Tuscany, cared for by his daughter. He continued his experiments and his writings, and published his best scientific work Discourses on Two New Sciences (statics and dynamics) in 1638. He died in 1642 at the age of 77.
Of course, in the end he triumphed. Once Newton explained Kepler's ellipses with his Law of Gravitation, there was no further doubt about the Earth's motion about the sun. Galileo had suffered a defeat, but the Church had suffered a catastrophe. Science moved north, leaving the Church behind and in a untenable position. Eventually, the Church relented. In 1822, Galileo's books were removed from the prohibited list. And in this past decade, the Church admitted it had erred in condemning Galileo.
John Appeldoorn, guest lecturer at the Savannah Science Museum, has had one career as a research chemist with Exxon. He finds that in retirement he has the opportunity to devote to astronomy the time that it deserves.
This article originally appeared in the Planetarian, Vol 19 #4, pages 15-17 & 19, December 1990. The author has likely moved since. The Planetarian is the quarterly journal of the International Planetarium Society, and this article is copyrighted by the International Planetarium Society