In the Catholic world prior to Galileo's conflict with the Church, the majority of educated people subscribed to the Aristotelian (or Ptolemaic) geocentric view (Aristotle's theory of homocentric spheres, and Ptolemy's mechanism of eccentrics and epicycles) that the earth was the centre of the universe and that all heavenly bodies revolved around the Earth, despite the use of Copernican theories to reform the calendar in 1582.

After 1610, when he began publicly supporting the heliocentric view, which placed the Sun at the centre of the universe, he was opposed by astronomers, philosophers and clerics. One of the latter, Niccolò Lorini, eventually lodged an informal complaint against Galileo with the prefect of the Congregation of the Index, and another, Tommaso Caccini, formally denounced him to the Roman Inquisition, early in 1615. The subsequent investigation led to the Catholic Church's condemning heliocentrism as "false" and "altogether contrary to the Holy Scripture" in a decree by the Congregation of the Index in February 1616. Although Galileo was not then judged to have committed any offence, he was nevertheless warned to abandon his support for heliocentrism - which he promised to do. When he later defended his views in his most famous work, "Dialogue Concerning the Two Chief World Systems", published in 1632, he was tried by the Inquisition, found "vehemently suspect of heresy", forced to abjure, and spent the remaining nine years of his life under house arrest.