400-year-old Galileo letter reveals his damage control strategy

Imagine you’re at a Royal Society library in the UK, browsing through the online catalog for some light reading. You’re looking at old manuscripts and documents, when suddenly, you come across an age-old letter marked-up with various amendments and cuts. It looks like any other letter from the 1600s, complete with spots of dark, aged parchment and nearly illegible script; but upon closer inspection, you realize that it’s a copy of Galileo’s infamous letter to the Catholic Church – edited by none other than Galileo Galilei himself.

Postdoctoral science historian Salvatore Ricciardo found himself in this situation in early August as he researched documents related to Benedetto Castelli, a friend of Galileo. While searching for any copies of Galileo’s letters with handwritten marks, Ricciardo hit the jackpot with this new discovery.

As an observational astronomer in a time period where the Roman Catholic Church had immense power, Galileo faced constant pushback for his ideas. His discovery of the phases of Venus and the four largest moons of Jupiter led him to believe that Copernicus’s heliocentrism theory was correct – that the earth and the planets revolve around the sun. These discoveries were met with opposition from the Catholic church, whose official position at the time was that the sun revolved around the Earth. The church wasn’t going to re-interpret well-established Biblical verses based off of a loosely-knit theory that was actually rejected by most astronomers at the time.

Because Castelli was such a close friend, Galileo felt safe expressing his opinions surrounding the Church – and express his opinions he did. In his letter, Galileo argued against church doctrine that “the scant references in the Bible to astronomical events should not be taken so literally.” Copies of the letter were circulated, causing huge controversy, until eventually Dominican friar Niccolò Lorini forwarded it to the Inquisition in Rome. The Inquisition, whose job was to conduct trials for crimes of religious natures, warned Galileo to abandon the heliocentric theory before they were forced to bring him to trial. Ignoring this warning, Galileo published Dialogue Concerning the Two Chief World Systems, where he argued for the heliocentric theory and against the Church’s earth-centered theory. Because of this, Galileo was summoned to stand trial and was convicted of “vehement suspicion of heresy.”

All this controversy stemming from one source – his letter to Castelli; and up until Ricciardo’s discovery, the letter continued to be a source of frustration. This is because there are two known surviving versions of this letter: the Castelli letter, and a much milder version sent to friend and cleric Pietro Dini. In the Dini letter, Galileo had suggested that the “version of the letter Lorini had given to the Inquisition may have been doctored,” made to be worse than it actually was. He had forwarded a milder version of the Castelli letter to Dini, insisting that it was the correct version, and asked him to send it to the Vatican. The question of whether or not the inflammatory Castelli letter was the original has stumped historians for years.

But here is where Ricciardo’s discovery comes into play: the letter he had found was a copy of the Castelli letter, amended to be less inflammatory. Galileo himself had doctored his words, trying to mitigate the fallout from the Church and do some damage control. As Ricciardo points out, the differences in the original wording are significant: “In one case, Galileo referred to certain propositions in the Bible as 'false if one goes by the literal meaning of the words.' He crossed through the word ‘false,’ and replaced it with ‘look different from the truth.'” This is proof that the Castelli letter was actually the original, and shows historians that Galileo actively attempted to moderate his language to avoid harsher backlash. “I can’t believe that I have discovered the letter that virtually all Galileo scholars thought to be hopelessly lost. It seemed even more incredible because the letter was not in an obscure library, but in the Royal Society library,” says Ricciardo.

The circumstances surrounding the discovery add another element of strangeness: the letter had gone unnoticed for centuries, being in the possession of the library since at least the mid-18th century. It was repeatedly misdated and miscatalogued – it’s most likely because of this mismanagement that no one had ever connected the letter to the timeline of the Galileo affair. Researchers are now trying to track how long it was at the Royal Society Library, and to piece together how it arrived there in the first place.

While ultimately unsuccessful, Galileo had attempted to trick the Catholic Church with a carefully edited version of his incendiary Castelli letter. “Galileo’s letter to Castelli is one of the first secular manifestos about the freedom of science – it’s the first time in my life I have been involved in such a thrilling discovery” says Ricciardo’s supervisor, Franco Giudice. And now, with this discovery, historians can finally settle a years-long debate.