Chicago Coalition of Library Friends Meetup

On December 20, 2025, six of us continued our conversation on Mario Livio’s Galileo and the Science Deniers. We looked at a photograph of a falling tennis ball taken with a strobe light that flashed every 1/20 of a second. It showed that the distance the ball traveled was longer than the previous interval of time, and showed that the distance the ball traveled at each interval is proportional to the square of the elapsed time. Without the advantage of today’s photographic technology, Galileo found this relationship by slowing the speed of a falling body—letting a ball roll downward on a grooved inclined plane—and measuring the time with a water clock. He demonstrated that objects fall at the same rate regardless of their mass, correcting Aristotle’s conclusion that heavy objects fall faster than light ones when he had incorrectly generalized from his observations of objects falling through water without accounting for the negligible viscosity of air. Without air, when Astronaut David Scott dropped a hammer and a feather, both hit the Moon’s surface at the same time. Perhaps, Galileo dropping a large and a small ball from the Tower of Pisa was only a story. Was Aristotle an Aristotelian? Did Galileo embody the spirit of “trust but verify,” while the Aristotelians embodied “just trust me?”

The Church trusted Aristotle enough to put all of their theological eggs into his philosophical basket. His astronomical ideas were based on what the naked eye saw day after day, night after night, where the sun, moon, planets, and stars moved eternally, and where the earth seemed to be the center of the universe. This celestial regularity was mirrored in the ancient Greek Antikythera Mechanism, a complex astronomical analog computer where dozens of gears meshed and moved symbols for the sun, moon, and planets. It was not a time machine as portrayed in the Indiana Jones movie: The Dial of Destiny, but it was a calculating machine that predicted lunar phases, solar eclipses, and the positions of the planets with reasonable accuracy. Centuries later, the Church trusted Aristotle’s geocentric logic as the basis of its theology, but it used epicycles where a planet would move in a small circle while its epicycle moved around the circumference of a larger one, accounting for the retrograde motion of the outer planets, Mars, Jupiter, and Saturn. Copernicus questioned the geocentric model because epicycles added complexity to explain planetary retrograde motion for the outer planets, and found a simpler and elegant solution by placing the Sun at the center, which explained the retrograde motions by having the Earth speed ahead in its orbit, overtaking the slower-moving outer planets in their larger orbits.

Galileo supported Copernicus's theory by 1597, and further unraveled Aristotle’s assertions by improving the optics of a Flemish invention, the telescope, and he first pointed it at the Moon. He found it was pocked with craters and had tall mountains, and it was not a smooth, perfect sphere as Aristotle thought. He then pointed the telescope at Jupiter and found four new “stars” that change positions regularly. This was unexpected since the Sun, Moon, planets, and stars were supposedly fixed to a perfect, transparent, incorruptible crystalline substance of the celestial spheres. The periodic changing of positions of Jupiter’s moons suggested that they moved around Jupiter and were not cemented on a celestial sphere along with Jupiter. With his telescope, he also saw Saturn, and it appeared to have two moons orbiting closely to it, but his telescope was not powerful enough to see Saturn’s rings.

Cinching the heliocentric model was Galileo’s observation of the phases of Venus. In the geocentric system, Venus stayed on an epicycle between the Earth and the Sun, meaning it would only show crescent phases. Galileo found Venus had the same phases as the Moon, changing from a crescent at its closest approach to the Earth to a fully illuminated disk when it was far from us. A "full" Venus proved it had to pass behind the Sun, which is impossible in the Ptolemaic model. By projecting the sun’s image on a sheet of paper with a telescope, Galileo also saw sunspots, showing more blemishes on another celestial object.

To paraphrase Shakespeare’s Hamlet, "Galileo found more things in heaven and earth than are dreamt of in Aristotle’s philosophy." By publishing his observations in Italian, not using Latin, the reclusive and scholarly language, John Brockman notes that Galileo created a third culture, an addition to C. P. Snow’s two cultures of humanities and sciences. It was a culture where Galileo intended that scientific discoveries be shared with everyone. However, the Renaissance coincided with the launch of Martin Luther’s Reformation, with his strong belief that ordinary people should also read the Bible, contrary to the Catholic doctrine that reserved scriptural interpretation for the clergy. While promoting science for the people, Galileo fell perilously into the Inquisition of the Church’s Counter-Reformation.

We invite you to find out more about the denial of Galileo’s science in our discussion of Mario Livio’s Galileo and the Science Deniers, QB36.G2L658 2020, on January 10, 2026, from 2 PM to 4 PM.