Discuss Mario Livio’s Galileo and the Science Deniers

On January 10, 2026, our group of six continued our discussion of Mario Livio’s Galileo and the Science Deniers. We began by noting how, to the naked eye, the stars appear attached to a domed ceiling—a celestial sphere rotating East to West around an imaginary axis. In Chicago, we can trace the sun’s path, the ecliptic, noting its northernmost position on June 21 and its southernmost on December 21. These celestial motions are the basis of our clock and calendar, and they are so regular, we can predict solar eclipses by calculating when a new Moon’s path will intersect with the Sun on the ecliptic.

Ptolemy organized these objects into nested spheres centered around a stationary Earth. He placed the Moon in the closest sphere, followed by Mercury, Venus, the Sun, and finally the outer planets and stars. To account for "retrograde motion"—where planets appear to move backward—Ptolemy utilized "epicycles," or circles within circles. While this allowed for accurate predictions, it was immensely complex. Copernicus later simplified this by placing a spinning Earth and the other planets in orbit around the Sun. Wary of challenging Church doctrine and Aristotelian philosophy, Copernicus waited until he was on his deathbed to publish On the Revolutions of the Heavenly Spheres.

In contrast, Galileo’s The Starry Messenger (1610) was an immediate sensation. A runaway bestseller, its 550 copies sold out almost instantly. Unlike the dense, math-heavy tomes of his contemporaries, Galileo’s book was accessible and filled with striking illustrations of the Moon’s jagged craters and the four moons of Jupiter. By showing that celestial bodies were not "perfectly smooth ethereal spheres," Galileo effectively brought the heavens down to Earth, becoming an international celebrity in the process.

However, this fame made Galileo a target. Critics like Cesare Cremonini refused to even look through the telescope, while others argued that the stars visible only through the lens were mere optical illusions or "artifacts" of the instrument. To defend his findings, Galileo sent his telescopes to the Jesuits at the Collegio Romano and the French Queen, Marie de Medici, who eventually verified his observations.

The tension escalated when the Grand Duchess Christina of Lorraine expressed concern over contradictions between Copernicanism and the Bible. In response to these concerns, Galileo wrote to his student, Benedetto Castelli, praising his defense of the new astronomy but cautioning him not to use science to justify scripture. This correspondence drew the scrutiny of Cardinal Roberto Bellarmino, the chief defender of Church doctrine. Bellarmino had been a judge in the trial of Giordano Bruno, who was burned at the stake in 1600 for various heresies, including the claim that the Earth moved and that stars were suns with their own planets.

During his subsequent exchanges with Bellarmino, Galileo famously quoted Cardinal Cesare Baronio: “The intention of the Holy Spirit is to teach us how one goes to heaven and not how the heavens go.” This distinction went unheeded. By separating religious faith from physical observation, Galileo did more than just defend a map of the solar system; he helped establish the scientific method by developing procedures of verifying claims before trusting them, the very foundation of the scientific revolution. Are there parallels between the 17th-century reaction to Galileo and modern-day science denial?

We invite you to find out more about the denial of science in our discussion of Mario Livio’s Galileo and the Science Deniers, QB36.G2L658 2020, on January 24, 2026, from 2 PM to 4 PM.