It was on this day in 1633 that Galileo Galilei (books by this author) stood trial before the Roman Inquisition, to defend the publication of his book Dialogue Concerning the Two Chief World Systems (1632).

It is commonly thought that Galileo was called to defend his scientific beliefs before the Church, who insisted on their own version of the universe. But the purpose of the trial was more complex than that, going back 17 years, to a technicality that occurred the first time that Galileo was officially chastised by the Church. The question was whether he was ordered to stop publishing or teaching anything about a Copernican view of the universe, or whether he was told that he could present it as a theory but not the absolute truth.

And really, it started 90 years earlier, when the Polish astronomer Nicolaus Copernicus published On the Revolutions of the Celestial Spheres (1543), laying out his revolutionary theory: that the earth and other celestial objects rotate around the sun, which is the fixed center of the universe. It was completely at odds with the accepted understanding — legitimized by the Church — that the Earth was at the center. There was a small stir, but Copernicus died at the same time he published his book, and his views were not particularly well publicized and sank into relative obscurity for decades.

But hearing those views from someone like Galileo was another story. Galileo was already famous, a respected scientist and lecturer, years before his involvement with any sort of controversial theory of the universe. After giving up the idea to become a monk, which his father opposed, he studied mathematics and medicine (his father's choice), but was more interested in math. He was one of the first scientists to show that math could be used to explain the laws of nature, and he used that understanding to conduct breakthrough experiments, including his work on accelerating objects. The story that he dropped cannon balls out of the Leaning Tower of Pisa is probably apocryphal, but he did show that falling objects accelerate at a uniform rate, and that this is true regardless of their weight.

At some point Galileo did become interested in the theory of the universe expressed by Copernicus, and then he discovered something that he thought would prove the theory beyond question: the telescope. A Dutch eyeglass maker is credited with inventing it in 1608, and as soon as he heard about it, Galileo set one up himself, and became the first person to use it to observe the sky. He deduced that the moon was illuminated by a reflection of the sun on the Earth, he saw that Jupiter was orbited by moons, and he studied Venus and realized that the only explanation for its changing phases was that it orbited the sun. He thought that, finally, no one could disagree that the planets orbited the sun, so he started talking openly about his ideas. He wrote and lectured for the educated public, figuring that they were a more receptive audience than scholars.

But of course people did disagree: The Church claimed it was at odds with the Bible, particularly a verse in the Book of Joshua that describes God stopping the sun in the sky, and one in Psalms that says Earth was put on its foundations and would not move. Galileo responded publicly by explaining that the truth of the Bible was not always literal, that it used metaphorical imagery. He wrote: "I do not feel obliged to believe that the same God who has endowed us with senses, reason and intellect has intended us to forego their use and by some other means to give us knowledge which we can attain by them. He would not require us to deny sense and reason in physical matters which are set before our eyes and minds by direct experience or necessary demonstrations."

And on this day of April in 1533, Galileo was called before the Inquisition to be questioned. They did not give him a chance to defend his view of the universe, nor in fact did they argue with his beliefs at all. Instead, their argument centered on the first time Galileo had been officially reprimanded, 17 years earlier. Galileo was interrogated by the full Inquisition again on April 30th, and he offered to write a sequel in which he argued against Copernicusm. This was not good enough. Weeks later, on June 16th, the pope decreed: "Galileo being interrogated on his intention, even with the threat of torture ... he is to abjure in a plenary session of the Congregation of the Holy Office, then is to be condemned to imprisonment at the pleasure of the Holy Congregation, and ordered not to treat further, in whatever manner, either in words or in writing, on the mobility of the Earth and the stability of the Sun; otherwise he will incur the penalties of relapse. The book entitled Dialogue of Galileo Galilei the Lincean is to be prohibited."

Eventually, he was allowed to return home under house arrest, where he became blind a few years later, and died in 1642. In 1718, the Church lifted its ban on Galileo's work, with the exception of the Dialogues, which was banned until 1822.