How cooler heads could have avoided the ‘Galileo affair’

Conventional account ignores gaps in scientist’s evidence to Catholic Church inquisition

The 1633 trial of Galileo Galilei (1564-1642) before the Roman Inquisition is frequently cited to demonstrate that science and the Catholic Church are fundamentally at loggerheads. But, science and the church are not opposed. As I outlined in my last column, the church was largely responsible for the rise of modern science in medieval Europe.

The conventional account of the Galileo affair portrays a brilliant, uncomplicated scientist besieged by bigoted, ignorant Catholic clerics and forced to renounce his belief in the scientific fact that Earth orbits the sun because this contradicts certain passages in scripture. This story ignores gaps in Galileo’s evidence, the difficult personalities involved and the unsettled religious times. Today I offer a more nuanced account.

Galileo, rightly called the father of modern science, founded the modern scientific method of formulating theory based on results of careful experiments. He developed the astronomical telescope, discovered the phases of Venus and the moons of Jupiter and first saw mountains on the moon and spots on the sun. He discovered that all falling bodies, heavy or light, have the same constant acceleration, and more. Of course, Galileo was not infallible, for instance he formulated a wrong theory of tides. Temperamentally Galileo was vain, argumentative and his caustic tongue made enemies easily.

Galileo supported the heliocentric model, in which the Earth and the other planets in the solar system orbit the sun, proposed by Copernicus in 1453 to supplant the geocentric model where the sun and planets orbit the Earth. Galileo had telescopic evidence to support Copernicus. The phases of Venus demonstrate that Venus orbits the sun but do not prove the heliocentric model. Also, Jupiter’s moons showed that orbiting planets retain their satellites answering the objection that a moving Earth would lose the moon.


Although the church unjustly bullied Galileo, it did have some understandable reasons to oppose him. Firstly, Galileo’s evidence, all corroborated by Jesuit astronomers, fell well short of proving a sun-centred system. One compelling piece of evidence against an orbiting Earth was the absence of star parallax. If the Earth was moving, the observed position of stars in the sky would change with the seasons because the angle of observation changes – parallax. But no parallax could be detected and believers in a moving Earth could not explain this.

Secondly, astronomers were divided over the heliocentric model. Renowned astronomer Tycho Brahe (1546-1601) proposed an alternative model in 1583, accepted by many astronomers, in which the sun orbits Earth and the phases of Venus are explained.


Thirdly, the church was under intense pressure from the Reformation and sensitive to Protestant charges that Catholicism didn’t sufficiently respect the word of scripture. But here was world-famous Catholic Galileo declaring that scriptural passages describing a stationary earth were wrong because the Earth orbits the sun.

In 1624 Pope Urban VIII, Galileo’s friend, told Galileo he could publicly discuss the heliocentric model as hypothesis only, not as fact. Galileo agreed. This censorship was unfair but allowed Galileo to publicise his model while awaiting proof.

However, Galileo went back on his word in a very provocative manner. His 1632 book Dialogue Concerning the Two Chief World Systems is a conversation between two philosophers and a layman called Simplicio. Simplicio argues for an Earth-centred system, using, in part, some cosmological arguments favoured by Pope Urban VIII, and is answered by the philosophers. The word Simplicio had double meaning as the Italian for simple-minded is semplice. Simplicio was a straw man designed for demolition by the philosophers. Copernicanism wins the argument in the book.

An incensed pope, thinking Galileo was both disobeying and mocking him, referred Galileo to the inquisition in 1632 on suspicion of heresy. Galileo was convicted and the 69-year-old, after viewing the instruments of torture, formally retracted his position in June 1633. He was held under house arrest in his Florentine villa until his death in 1642.

Although Galileo couldn't prove his model, he had strong evidence. Church fathers such as Thomas Aquinas (1225-1274) had long taught that when scriptural descriptions of the natural world contradict valid scientific explanations, they should be reinterpreted. The church was grievously wrong to censure Galileo.

Galileo was vain and impetuous and the pope was a pompous know-it-all, accustomed to having his way – he had the Vatican garden birds killed because they disturbed him. Galileo should have been more cautious and the pope should never have activated the inquisition. Cooler heads would have avoided the "Galileo affair" – and its long, bitter legacy.

William Reville is an emeritus professor of biochemistry at UCC