Four centuries ago, the Italian scientist Galileo Galilei put his liberty and life on the line to convince the religious establishment that the Copernican model of the solar system—in which the Earth and the other planets revolved around the sun—represented physical reality.

Following his own observations and the findings by other astronomers, no one could really argue anymore that what one saw through the telescope was an optical illusion, and not a faithful reproduction of the world. The only defense remaining to those refusing to accept the conclusions first proposed by Nicolaus Copernicus, a Renaissance-era mathematician and astronomer, and bolstered by accumulating facts and scientific reasoning, was to reject the interpretation of the results.

Theologians concluded that a moving Earth and a stationary sun were in conflict with literal interpretations of scripture, and with the Ptolemaic geocentric model, which had been adopted as the Catholic Church’s orthodoxy. The deniers cited, for example, the book of Joshua, in which, at Joshua’s request, God commanded the sun, and not the Earth, to stand still over the ancient Canaanite city of Gibeon.

WATCH: How the Earth Was Made on HISTORY Vault

Inquisition of Galileo Is Launched Under Pope

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Galileo Galilei before members of the Holy Office in the Vatican in 1633.

Galileo, however, went on to publish his book, Dialogue Concerning the Two Chief World Systems, in which he derided those who refused to accept the Copernican system. On April 12, 1633, chief inquisitor Father Vincenzo Maculano, appointed by Pope Urban VIII, launched an inquisition of Galileo and ordered the astronomer to appear in the Holy Office to begin trial. 

The trial of Galileo, a man described by Albert Einstein as “the father of modern science,” took place in three sessions, on April 12, April 30 and May 10 in 1633. The sentence was delivered on June 22.

In the first session, prosecutor Maculano introduced a warning issued against Galileo 17 years earlier, in which Galileo was ordered by the Church’s Commissary General to abandon his Copernican ideas and not to defend or teach them in any way. This document was significant, since in his book (published in 1632), Galileo presented arguments favoring the Copernicus model, even though he added a preface and a coda which appeared to imply that one couldn’t conclude which of the two models was correct.

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

When asked what instructions he had received in 1616, Galileo said, “Lord Cardinal Bellarmino [who had been Chief Theologian of the Holy Office] told me that since Copernicus’s opinion, taken absolutely was contrary to Holy Scripture, it could neither be held or defended, but it could be taken and used suppositionally.” Galileo even produced a copy of the letter given to him by Bellarmino, which stated as much.

From a purely legal perspective, this brought the evidence incriminating and vindicating Galileo practically to a draw, since, while the injunction document spoke of “not to hold, teach, or defend in any way, either verbally or in writing,” Bellarmino’s letter used the much weaker language of “not to hold or defend Copernicanism.”

But a special commission appointed to examine Galileo’s Dialogue and to determine whether he violated the prohibition to hold, teach or defend Copernicanism in any way, issued a report concluding that in writing the book, Galileo had disobeyed the injunction. One member, the Jesuit Melchior Inchofer, stated that Galileo was “vehemently suspected of firmly adhering” to the Copernican opinion, and “indeed that he holds it.”

Undoubtedly feeling intimidated and fearing for his life, Galileo then admitted that in certain parts of his book the arguments in favor of Copernicanism appeared stronger than they should have been, due to, he said, “vain ambition, pure ignorance, and inadvertence.” He offered to make any amends to the book ordered by the court, finishing with a plea for leniency, based on his age and infirmity.

A summary of the trial proceedings turned out to be extremely damaging to Galileo. It even contained false allegations raised against him some 18 years earlier, such as that he had been heard to state that God was an “accident.”

Galileo Is Convicted and Forced to Recant His Work

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English poet John Milton visiting Galileo when a prisoner of the Inquisition.

On June 22, 1633, Galileo was ordered to kneel as he was found “vehemently suspected of heresy.” He was forced to “abandon completely the false opinion” of Copernicanism, and to read a statement, in which he recanted much of his life’s work.

From its extremely narrow perspective, the Church did act within its legal authority: Galileo was convicted because of two indisputable facts. By writing the Dialogue he violated the injunction issued by the Commissary General in 1616, not to defend or teach the Copernican model. Also, he obtained the Church’s permission to print the book without revealing that such an injunction existed.

Galileo was an elderly, blind man still under house arrest when a then little-known poet, John Milton, visited him 1638. Milton later referred to his visit with the scientist as he argued against licensing and censorship in a speech to English Parliament in 1644. 

The poet warned his countrymen, “this was it which had damped the glory of Italian wits; that nothing had been there written now these many years but flattery and fustian. There it was that I found and visited the famous Galileo grown old, a prisoner to the Inquisition, for thinking in Astronomy otherwise than the Franciscan and Dominican licencers thought.”