The trial of Galileo Galilei in 1633 is the standard example of religion suppressing science, and the standard version is not very accurate. The proceedings turned on a specific legal question about a document from 1616, a book that broke the terms under which it had been licensed, and a personal offence given to a pope who had been Galileo’s patron.
None of that makes the outcome defensible. It does make it comprehensible, which the simplified version does not.
The Warning of 1616
By 1610 Galileo had turned his telescope on the sky and published Sidereus Nuncius. The moons of Jupiter, the phases of Venus and the mountains of the Moon did not prove that the Earth moves, but they demolished several Aristotelian assumptions and made the Copernican system considerably more plausible.
Galileo campaigned for it energetically, including in a letter to the Grand Duchess Christina arguing that Scripture is written in the language of ordinary people and should not be used to settle questions of natural philosophy. Arguing about how the Bible should be interpreted was, for a layman in 1615, a considerably riskier move than arguing about astronomy.
In February 1616 a panel of theologians assessed two propositions. That the Sun is the centre of the world and immobile was judged formally heretical. That the Earth moves was judged erroneous in faith. Copernicus’s De revolutionibus was suspended pending correction, and returned to circulation in 1620 with about ten passages altered so that the theory read as a mathematical hypothesis rather than a physical claim.
Cardinal Robert Bellarmine, the leading theologian of the period, summoned Galileo and delivered a warning. Galileo agreed to comply and was given a certificate stating that he had not abjured anything and had merely been informed of the ruling: he was not to hold or defend the opinion.
There is also an unsigned minute in the Vatican files, dated the same week, recording a stricter injunction delivered by the Commissary General: that he must not hold, defend or teach the doctrine in any way whatsoever. Whether that meeting occurred as described, and whether the document is a genuine record or a later insertion, has been argued about ever since. It matters because the entire 1633 prosecution rests on it.
The Friendly Pope
In 1623 Cardinal Maffeo Barberini was elected as Urban VIII. He was an admirer of Galileo, had written a poem in his honour, and had opposed the 1616 decision.
Galileo travelled to Rome and was granted six audiences. He came away believing he had permission to write about the two systems, provided he treated the Copernican account as a hypothesis rather than a demonstrated truth, and provided he included an argument that Urban was fond of: that God, being omnipotent, could produce the observed appearances by means beyond human comprehension, so no astronomical model can be proven necessarily true.
The Dialogue
Dialogo sopra i due massimi sistemi del mondo appeared in Florence in 1632, with the approval of the censors in both Rome and Florence.
It is written as a conversation over four days between three characters. Salviati argues the Copernican case, Sagredo is an intelligent neutral who is gradually persuaded, and Simplicio defends the Aristotelian and Ptolemaic position. The book is a masterpiece of scientific prose, written in Italian rather than Latin so that it could be read outside the universities, and it is very funny at Simplicio’s expense.
Two problems were immediately apparent to anyone reading it.
The first is that it is not balanced. Salviati wins comprehensively, and the promised hypothetical framing is barely maintained. The book that was licensed as an impartial comparison is an argument for one side.
The second was worse. Urban’s favourite argument, the one Galileo had been required to include, appears at the very end, and it is placed in the mouth of Simplicio, whose name suggests simpleton and who has spent four hundred pages being wrong about everything. Whether or not Galileo intended the insult, the Pope took it as one, and enemies at court were happy to encourage the reading.
The Political Weather
The timing was disastrous for reasons that had nothing to do with astronomy.
The Thirty Years War was going badly for the Catholic side, and Urban was under attack for having supported France against the Habsburgs and for being insufficiently firm on doctrine. In a consistory in March 1632, Cardinal Gaspare Borgia publicly accused him of being soft on heresy, and the confrontation nearly turned violent.
A pope in that position, presented with a book by a personal protégé that appeared to flout a standing ruling of the Holy Office and to mock him personally, had limited room to be generous.
What Galileo Actually Proved
There is a scientific point here that gets lost, and Galileo’s opponents were not wrong about it.
The Dialogue’s culminating argument, presented on the fourth day as the physical proof that the Earth moves, is a theory of the tides. Galileo held that the combination of the Earth’s daily rotation and its annual orbit causes the water in the ocean basins to slosh, producing tides in the way water sloshes in a moving barge.
This is wrong. It predicts one tide a day rather than two, and it cannot account for the connection between the tides and the position of the Moon, which had been known for centuries. Kepler had proposed lunar attraction as the cause, and Galileo dismissed the suggestion as occult nonsense.
The genuine proofs of the Earth’s motion arrived much later: stellar aberration in 1729, the annual shift of the Foucault pendulum’s plane and, definitively, stellar parallax measured in 1838. In 1633, no conclusive physical demonstration existed, which is precisely what the Church required before allowing the reinterpretation of Scripture.
The Trial
Galileo was summoned to Rome in late 1632, pleading illness and age, and arrived in February 1633. He was sixty eight.
The proceedings were narrower than the popular account suggests. The charge was not that heliocentrism is false but that he had violated the 1616 injunction and had obtained his publishing licence without disclosing it. The unsigned minute was produced. Galileo produced Bellarmine’s certificate, which said only that he must not hold or defend, and testified that he had no memory of the stricter version.
He argued that the Dialogue actually refutes Copernicanism, a defence nobody could take seriously. Under a plea arrangement he admitted that vanity had led him to make the Copernican arguments stronger than he intended.
On 22 June 1633 he was found vehemently suspect of heresy, required to abjure, and sentenced to imprisonment, immediately commuted to house arrest. The Dialogue was banned.
He did not mutter eppur si muove, and yet it moves, as he rose from his knees. The phrase first appears in print in 1757, more than a century later.
The Best Work Came After
Galileo spent his remaining years under house arrest, first in Siena and then at his villa at Arcetri, going blind, forbidden to publish.
During those years he wrote the Discorsi e dimostrazioni matematiche intorno a due nuove scienze, which is a better book than the Dialogue and far more important to physics. It contains the law of falling bodies, the parabolic trajectory of projectiles, the beginnings of the concept of inertia, and the first serious treatment of the strength of materials. There is no astronomy in it and nothing for the Inquisition to object to.
The manuscript was smuggled to the Netherlands and printed in Leiden in 1638. It is the foundation on which Newton built, and it exists because a man under house arrest went back to the problems he had set aside forty years earlier. Our editions of the Dialogo and the Discorsi present both books, the one that caused the trial and the one written after it.
He died at Arcetri in January 1642.
Afterwards
The prohibition on books teaching heliocentrism as fact was lifted in 1758, with the works of Copernicus, Kepler and Galileo remaining on the Index until 1835.
In 1979 John Paul II asked for the case to be reexamined, and the commission reported in 1992. Its conclusion was that Galileo’s theological judges had failed to distinguish between a matter of faith and a question of physical fact, and had wrongly transposed a scientific question into the realm of doctrine.
What the Case Shows
The useful lesson is not that authority opposes discovery, which is too general to be informative. It is more specific.
Galileo was right about the fundamental question and could not prove it. His opponents were correct that the demonstration was missing and used that gap to enforce a position they held for other reasons. The Church’s requirement, that Scripture should not be reinterpreted until a physical proof exists, sounds reasonable and was applied in a way that made producing the proof harder.
The astronomical argument was settled in the following century by other people, and the physics that made it inevitable was written by Galileo himself while under sentence, in a book about falling weights that nobody thought to ban.
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