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Long-Lost Letter Reveals How Galileo Tried to Trick the Inquisition

Galileo had evidence suggesting that Earth orbits the sun (not the other way around), but he also knew it was a dangerous claim.
Galileo Galilei (1564-1642) explaining his theories on the solar system.

Galileo Galilei (1564-1642) explaining his theories on the solar system.

A long-lost letter written by Galileo Galilei reveals an effort by the 17th-century astronomer to soften his public stance against the Catholic Church’s doctrine that the sun orbits the Earth. The letter, uncovered at the Royal Society in London, appears to solve a four-century-old mystery over Galileo’s original language on the celestial matter.

In the letter, written in 1613, the famed astronomer-philosopher-physicist-mathematician argued for the first time against the concept that the sun orbited the Earth (and not the other way around). When a copy of the letter was later forwarded to the Inquisition in Rome, Galileo claimed the language had been altered to make it more heretical, and produced a toned-down version he claimed was the original. In fact, as this new discovery shows, it was Galileo who had done some altering.

The newly rediscovered document, which had been misdated in the Royal Society library’s catalog, shows that Galileo himself had made changes to his original text, in an effort to protect himself from the Inquisition’s wrath.

The Polish astronomer Nicolaus Copernicus had argued in his 1543 book On the Revolutions of the Heavenly Spheres that the sun lay at the center of the universe, while Earth was a planet orbiting it. Though Copernicus himself did not live to see the impact of his revolutionary heliocentric theory, the mathematician Giordano Bruno was convicted of heresy in 1600 for his support of Copernicus’ theory, and burned at the stake.

Through his telescopic experiments, Galileo found evidence that supported the Copernican model. On December 21, 1613, he wrote to his friend Benedetto Castelli, a mathematician at the University of Pisa in Italy, about his findings. He argued that passages in the Bible mentioning astronomical events could not be taken literally, and that the Copernican theory was not necessarily incompatible with the Bible.

Due to the controversial nature of the letter, copies were circulated, and one was sent to the Inquisition in Rome in 1615. Shortly after that, Galileo wrote to a cleric friend claiming that the letter forwarded to the Inquisition had been altered to amplify the heresy of Galileo’s claims. He enclosed what he said was the original, and asked his friend to pass it on to the Vatican.

The two versions of Galileo’s letter that survived confused later scholars, who didn’t know which represented the astronomer’s original text. But in August 2018, Salvatore Ricciardo, a postdoctoral science historian at Italy’s University of Bergamo, visited the Royal Society library and was browsing the online catalog when he came across an entry for the letter Galileo had written to Castelli. Though the date in the catalog said October 1613, Ricciardo examined the document and soon realized its importance.

“I thought, ‘I can’t believe that I have discovered the letter that virtually all Galileo scholars thought to be hopelessly lost,’” Ricciardo told Nature. “It seemed even more incredible because the letter was not in an obscure library, but in the Royal Society library.”

Ricciardo co-authored an article about his discovery, to be published in the Royal Society journal Notes and Records.

Signed “G.G.,” the letter includes scratched-out text and additions, and handwriting experts have confirmed it is Galileo’s writing. Underneath the corrections, the text is the same as in the version sent to the Inquisition, suggesting that Galileo sought to edit the text himself and create a blander, less controversial version. In one example, Galileo had originally written that certain biblical passages were “false if one goes by the literal meaning of the words,” but he crossed out “false” and wrote “look different from the truth.”

Despite these efforts, the Inquisition warned Galileo in 1616 to stop voicing his support for Copernicus’ model. It also withdrew On the Revolutions of the Heavenly Spheres from circulation and banned other books on the heliocentric model.

Sixteen years later, Galileo published his book Dialogue Concerning the Two Chief World Systems, which laid out his theories of the universe, including arguments for the Copernican model. Though he claimed to be writing hypothetically, the Inquisition didn’t buy it, and called Galileo to stand trial in Rome. In 1633, it convicted him on “vehement suspicion of heresy” and banned his book. His prison sentence was commuted, but he would spent the last nine years of his life under house arrest. 

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