The title “Dialogue Concerning the Two Chief World Systems” certainly sounds like a heavy topic. It was heavy in 1632 when Galileo published it. The two systems were the Ptolemaic and the Copernican theories of cosmology. It is less controversial and easier to understand today.
Ptolemy, following the tradition of Aristotle, believed that the Earth was the center of the universe, and everything — Sun, Moon, planets, and stars — revolved around it.
Copernicus, on the other hand, posited that the Sun is the center of the universe, and though we seem to be standing still, we are in fact hurtling through space as we circle the star.
I used to have a quotation in my middle school classroom for my students that said “You are not the center of the universe” – Copernicus. Nicholas didn’t say exactly that quote, and he wasn’t specifically referencing my young teen students, but it was a good point-of-departure quote for discussion.
Galileo had spoken with Pope Urban VIII earlier and discussed his tide theory as proof that the Earth moved through space – not that the Sun was the center of the universe. The Pope granted him permission to write “Dialogue on the Tides” but that the Copernican theory should be treated as hypothetical in the book. Wisely, Galileo wrote the book as a series of discussions between two philosophers. One believed in Copernicus, one believed in Ptolemy, and a neutral but well-educated layman served as a moderator. That got it past the Catholic censors.
But Galileo was Copernican all the way and the popular book did not please Pope Urban VIII who had Galileo tried by the Inquisition. They ruled that he was “vehemently suspect of heresy” and too close to endorsing Copernican theory and the book was placed on the Catholic Church’s Index of Forbidden Books.
Galileo was ordered to recant and recite weekly psalms of penitence. He spent the rest of his life under house arrest, and none of his later books were permitted to be published in his lifetime.
The Dialogue on Opposing World Systems remained on the Index of Forbidden Books until 1835. Change is slow in religion – but not in science.
Further Reading
The Essential Galileo
Weekends in Paradelle 