These days, you’d expect to find a long-lost letter by Galileo in some dingy archive scarcely visited by scholars, rather than in the collections and catalogs of a distinguished library that has been scoured time and time again. Not so, it seems.

The letter written and subsequently edited by Galileo Galilei was found in the Royal Society library, of all places, by postdoctoral science historian Salvatore Ricciardo of the University of Bergamo. The serendipitous discovery is covered in an exclusive feature by Nature, and the details of the letter are currently in press at the Royal Society journal Notes and Records.

The seven-page letter dodged the eyes of researchers for centuries due to a fairly simple, human-centered mistake: it was placed in a misdated catalog. Oops! 

The letter, dated on December 21, 1613, reveals Galileo softened his language to try to reel in the commotion he created with his arguments in support of a Sun-centered universe, first proposed by Polish astronomer Nicolaus Copernicus decades earlier. 

At the time, the Catholic Church staunchly claimed the Earth was the center of the universe and the Sun revolved around it. Anyone who claimed otherwise stood in denial of Church doctrine and was stamped a heretic. This means Galileo’s impassioned claims of a universe in which the Earth orbited the Sun placed him in the perilous spotlight of the pope and Catholic Inquisition.

Now, the long-lost letter clarifies a conflicting account of what happened when Galileo sent a written argument to mathematician Benedetto Castelli on why astronomical research should not be tied down by theological doctrine. That letter made its way into the hands of the Catholic Inquisition in Rome, and now rests in the Vatican Secret Archives.

Multiple copies of the letter exist, however the language in them differs subtly. Galileo claimed that the one in the clergymen’s possession had been doctored to lend further weight to the assertion he's a heretic. However, this newly discovered letter suggests the astronomer lied. Perhaps upon reflecting on the severity of the situation, Galileo edited the original version and sent it to his friend Piero Dini, a cleric, with the request he send the “true” copy to the Vatican.

This “true” copy was the same as the Inquisition’s but with edits to tone down his language, according to Nature. For example, he first penned that certain statements in the Bible are “false if one goes by the literal meaning of the words,” but scratched that to say they “look different from the truth.” In another edit, he wrote that the Scriptures were “concealing” its basic dogmas, but on second thought swapped the word out for “veiling”.

Scholars are waiting to comment further until the letters are published and detailed in the Royal Society journal. For now, they are surprised by its unexpected appearance.