What a time to come on board the Catholic Astronomer blog with the purpose of writing about topics in the history of astronomy! Ten days ago I wrote about how it had been four hundred years since, on February 24, 1616, a committee of consultants for the Roman Inquisition assessed the heliocentric system as being “foolish and absurd in philosophy.” And now today it has been four hundred years since, on March 5, 1616, the Congregation of the Index (the office charged by the pope with licensing and censoring books) issued a public decree which stated that the doctrine of Earth’s motion was false, and was contrary to scripture, and that to prevent the Copernican opinion from making further progress against Truth, circulation of Copernicus’s De Revolutionibus was to be suspended until offending material could be edited out of it.
Why did the Inquisition consultants state that heliocentrism was scientifically untenable?* Why did the Congregation of the Index call heliocentrism false? What were those churchmen thinking? According to Galileo, they were thinking about the arguments of Francesco Ingoli.
Before we get to Ingoli, let us see how all this got started. It started in about 1608, with the invention of the telescope. After all, De Revolutionibus had been published in 1543; seven decades had passed with no committees or decrees. But the telescope and the discoveries Galileo made using it—like the phases of Venus, demonstrating that Venus must circle the sun—had made astronomy into something that was on the minds of people who probably would not have had astronomy on their minds otherwise.
Some such people were the powerful de’ Medici family of Tuscany. In December of 1613 Fr. Benedetto Castelli, a mathematics professor and one of Galileo’s friends and followers, had breakfast with the de’ Medicis. Grand Duke Cosimo II de’ Medici, the ruler of Tuscany, asked Castelli if he had a telescope. Castelli said that he did and that in fact the night before he had been observing Jupiter with its “Medicean planets” (the moons, so-named by Galileo). The mother of the Grand Duke, Her Most Serene Ladyship Christina of Lorraine, remarked, apparently to herself, that the “Medicean planets” had better be real and not an artefact of the telescope.
The Grand Duke asked another professor who was at the breakfast, Cosimo Boscaglia, about this. Boscaglia answered that they were real, as were all of Galileo’s astronomical discoveries. However, Boscaglia made the point to Her Ladyship that the Earth did not move, and that the Bible stood contrary to the idea of its motion. After the breakfast, the de’ Medicis summoned Castelli back, and Her Most Serene Ladyship Christina ended up arguing against Castelli, citing the Bible against any motion of the Earth. Castelli, who felt that Her Ladyship was challenging him largely to hear what he had to say, stood his ground despite being seriously intimidated by debating among the rich and the powerful. The Grand Duke and his wife sided with Castelli—the younger de’ Medici generation against the older.
On December 14 Castelli wrote a letter to Galileo about all this. Galileo responded on December 21, congratulating him. “What greater fortune can you wish,” wrote Galileo, “than to see their Highnesses themselves enjoying discussing with you, putting forth doubts, listening to your solutions, and finally remaining satisfied with your answers?” Then Galileo offered a rebuttal to Her Most Serene Ladyship’s Biblical objection to Earth’s motion. “Holy Scripture can never lie or err,” he wrote. Nevertheless, “its interpreters and expositors can sometimes err in various ways,” such as by limiting themselves to the literal meaning of its words. Then one would have to attribute to God things like regret, hate, and ignorance of future things (Galileo may have been thinking of passages such as Genesis 6:5-7, or Malachi 1:3). Scripture is written to accommodate the understanding of common people, he said, so it would be imprudent “to oblige scriptural passages to have to maintain the truth of any physical conclusions whose contrary could ever be proved to us.”
The letter became popular among Galileo’s followers and was copied and circulated. By February of 1615 a complaint had been filed with the Inquisition regarding the letter. However, a consultant for the Inquisition found that, except for some bad-sounding choices of words, there was nothing questionable in the letter. Galileo’s name was cleared, but the Inquisition decided to ask its consultants for an opinion on the Copernican system. And thus the events of February 24 and March 5, 1616.
Now we return to Francesco Ingoli. The Inquisition issued no formal condemnation of the Copernican system following the February 24 consultants’ assessment. The assessment was filed away. The March 5 decree of the Congregation of the Index was the public action. And in a 1624 letter to Ingoli, Galileo suggested that behind that decree stood an essay that Ingoli had written to Galileo two months prior to the decree. “The arguments you advanced [in the essay] were not lightly regarded by persons of authority who may have spurred the rejection of the Copernican opinion by the Congregation of the Index,” Galileo wrote, and noted that Ingoli’s essay had circulated widely. The historian Maurice Finocchiaro has written that Ingoli, a prominent Monsignor who would later be in charge of bowdlerizing De Revolutionibus for the Congregation of the Index, “had probably been commissioned by the Inquisition to write an expert opinion” on the heliocentrism controversy. Ingoli’s essay, says Finocchiaro, had likely “provided the chief direct basis for the recommendation by its committee of consultants that Copernicanism was philosophically untenable and theologically heretical.”
So what was in Ingoli’s essay?** Twenty-two anti-Copernican arguments. Of these, four were what Ingoli called “theological” arguments. The other eighteen he called “physical” or “mathematical” arguments.
Ingoli’s first argument is a “mathematical” argument regarding triangulating the distances to the sun and moon by observing them from different locations on Earth. Ingoli says the results of the triangulation are inconsistent with heliocentrism. Arguments #2 and #3 are mathematical arguments pertaining to how, were the Earth orbiting the sun, there would be observable effects visible in the stars, and there are not. #4 is another mathematical argument—that Copernicus was wrong regarding certain technical details of Mercury’s and Venus’s orbits.
Arguments #5 and #6 are “physical” arguments: #5 says that heavier materials move down, lighter materials up. #6 notes that in a circularly agitated sieve, the heavier bits of material collect at the center. These suggest that the Earth, which is heavy, would be at the lowest point—the center—in a universe of circling lighter celestial bodies.
Next come two “theological” arguments. #7 concerns the words of Scripture—the sun and moon being equally described in Genesis as lights in the firmament. #8 is on the location of hell.
Then comes a slew of mathematical arguments. A falling ball should deflect from the vertical (#9) and launched projectiles should behave differently at different latitudes (#10) on a rotating Earth. These things do not occur, so Earth does not rotate. #11 and #12 are more arguments about the Earth’s motion requiring observable effects (not observed) in the stars. #13 is that Earth’s motion should have an observable effect on daylight. #14 is that the observed motions of comets are inconsistent with a moving Earth. #15 through #17 deal with how Earth maintains its orientation relative to the stars (so that we have a “North Star”) even though it orbits the sun. Copernicus attributed this to a third motion of Earth—the first being rotation on its own axis, the second being revolution about the sun, and this third motion being to counter the second so as to maintain the orientation relative to the stars. Ingoli’s arguments were that this motion was unneeded (since Earth need not move at all—#15), as well as complicated and physically improbable (#16 and #17).
Next come three more physical arguments. Two relate to the idea of heaviness: Earth is heavy (#18) and hard to make move (celestial bodies were theorized to consist of a special lightweight material that moved naturally); heavy stuff moves naturally toward the center of Earth (#19) and so the Earth, being composed of heavy stuff, cannot move naturally around the sun. The third (#20) was that Copernicus lacked consistency in what he put into motion—both a dark body like Earth and bright bodies like the planets.
The final two are theological. #21 concerns the words of Scripture—Joshua commanding the sun to stand still. #22 cites the words of a vespers hymn (not to be taken lightly, Ingoli warns: Cardinal Bellarmine likes such arguments).
Ingoli borrows many of these arguments from the Danish astronomer Tycho Brahe, whose work he cites, complete with page numbers. Brahe, who had died fifteen years earlier, had been the most prominent astronomer of the age. He had been lord of his own island, where he ran a huge astronomical research program with all the best instruments and all the best observers, all funded by the King of Denmark.
Brahe had been an anti-Copernican. Recall Castelli’s breakfast with the de’ Medicis. Boscaglia had asserted that all Galileo’s astronomical discoveries were real. But he had also asserted that Earth did not move. Boscaglia could have this cake and eat it too, because Brahe, impressed with aspects of the Copernican system but rejecting the idea of Earth’s motion, had developed his own system.
Brahe had calculated that were the stars as distant as Copernicus had supposed—so that the Earth’s orbit would be nothing by comparison, producing none of the above-mentioned observable effects in the stars—then they would also have to be enormous in order to appear as large as they do in the sky (Brahe measured their apparent sizes). Every last star, even the smallest, would dwarf the sun. And that, Brahe said, was absurd.
By contrast, in Brahe’s system the stars were not so distant. In it, the sun, moon, and stars circled an immobile Earth while the planets circled the sun. The stars lay just beyond the planets, and, being not so distant, did not have to be so huge. In Brahe’s system, celestial bodies all fell into a consistent size range. The star size issue was a solid argument*' for Brahe’s geocentric system. Ingoli brought it up, right after his arguments #2 and #3. Of course he cited Brahe by page number.
Galileo’s discoveries were all fully compatible with Brahe’s system. Anti-Copernicans like Boscaglia could accept Galileo’s discoveries like those phases of Venus that proved it to circle the sun, because in Brahe’s system Venus circled the sun, which in turn circled an immobile Earth. And, like Ingoli, they could cite Brahe’s arguments and his scientific authority against Copernicus.
I will not analyze the scientific validity of the eighteen mathematical and physical arguments here. Some (like the star sizes) were solid, or would have seemed solid, given the knowledge of the time; some were weak, even given that knowledge. What is clear is that Ingoli’s challenge to Galileo, in the form of the essay that Galileo believed to have been influential in the action of the Congregation of the Index, was a challenge about matters of science. This is especially true since Ingoli suggested Galileo ignore the four arguments about religious matters. Respond, said Ingoli, to whatever arguments you wish, but at least to the better of the physical and mathematical arguments.
So what were those Inquisition and Congregation of the Index churchmen thinking when they rejected the system of Copernicus? According to Galileo they were thinking about the same thing Boscaglia was likely thinking about when he was with the de’ Medicis: the science of Tycho Brahe. How can you go wrong siding with the greatest astronomer of the age? And you don’t even have to mess with the literal words of the Bible. They probably thought the decree of March 5, 1616 was just a scientific “no brainer.”
*I Borrow this phrase from historian Maurice Finocchiaro.
**Ingoli’s essay was first published in English in 2015—a full translation with commentary and notes on Galileo’s 1624 response is included in Setting Aside All Authority. A partial translation with commentary and notes can be found at arXiv:1211.4244.
*'The apparent sizes of stars would eventually be shown to be spurious, but evidence for this would not begin to accumulate until more than five decades later.