The great irony of the Galileo affair is that until Galileo forced the issue into the realm of theology, the Church had been a willing ombudsman for the new astronomy that emerged in the 16th century. In 1543, Nicolai Copernicus, a Polish canon and devout Catholic, published his epochal book supporting the heliocentric (earth around the sun) model at the urging of two Catholic prelates, dedicating it to Pope Paul III, who received it cordially.
If the issue had remained purely scientific, Church authorities would have shrugged it off. Galileo's mistake was to push the debate onto theological grounds. Galileo told the Church: Either support the heliocentric model as a fact (even though not proven) or condemn it. He refused the reasonable middle ground offered by Cardinal Bellarmine: You are welcome to hold the Copernican model as a hypothesis; you may even assert that it is superior to the old Ptolemaic model; but don't tell us to reinterpret Scripture until you have proof.
Galileo's response was his theory of the tides, which purported to show that the tides are caused by the earth's rotation. Even some of Galileo's supporters could see that this was nonsense. Also, ignoring the work of Kepler, he insisted that the planets go around the earth in perfect circles, which the Jesuit astronomers could plainly see was untenable. In fact, the Copernican system was not strictly "proved" until 1838, when Friedrich Bessel succeeded in determining the parallax of star 61 Cygni.
The real issue in the Galileo affair was the literal interpretation of scripture. In 1616, the year of Galileo's first trial, there was precious little elasticity in Catholic biblical theology. But this was also the case with the Protestants: Luther and Melanchthon had vehemently opposed the heliocentric model on scriptural grounds. Another irony of the affair, pointed out by John Paul II, is that Galileo's argument that scripture makes use of figurative language and is meant to teach "how to go to heaven, not how the heavens go" was eventually taught by two great papal encyclicals, Leo XIII's Providentissumus Deus (1893) and Pius XII's Divino Afflante Spiritu (1943).
There are fundamentalists out there, Protestant and Catholic, who do not understand this simple point: Scripture does not teach science. The Book of Genesis was written in the archaic, prescientific idiom of the ancient Palestinians. The author of Genesis could not have told us that the universe is twelve billion years old, because the ancient Hebrews did not have a word for one billion, and even if they had, the fact is hardly necessary for our salvation.
If the universe were roughly 6,000 years old, as a literal reading of Genesis would suggest, then we would not be able to see the Milky Way. The light would not have reached the earth yet.
As Catholics, we must believe that every word of scripture is inspired by the Holy Spirit, a claim the Church won't make even for ex cathedra pronouncements. But we must not think of the sacred writers as going into a trance and taking automatic dictation in a pure language untouched by historical contingency. Rather, God made full use of the writers' habits of mind and expression. It's the old mystery of grace and human freedom.
Once we understand how to read scripture, the vexed subject of evolution should not present a problem. That evolution per se is not an issue for Catholics was made clear by John Paul II during a brilliant series of catechetical talks on creation at his Wednesday audiences in 1986:
The theory of natural evolution, understood in a sense that does not exclude divine causality, is not in principle opposed to the truth about the creation of the visible world as presented in the Book of Genesis. . . . It must, however, be added that this hypothesis proposes only a probability, not a scientific certainty. . . . it is possible that the human body has evolved from antecedent living beings.
The pope got it exactly right. Not only is Darwinism not proved, almost every aspect of it is currently subject to a heated debate among geneticists and paleontologists. Darwin's model of gradual evolution does not square with the fossils, which show species appearing fully formed, staying around for a million years or whatever, and then suddenly disappearing (99 out of 100 known species are extinct). There are no transitional forms between any of the major animal groups, and even in "thought experiments," smooth intermediates between, say, reptiles and birds are almost impossible to construct.
Darwinism also does not square with breeding experiments; dogs remain dogs, fruit flies remain fruit flies. While DNA allows a certain elasticity in a species for ecological adjustment, it programs living things to remain stubbornly what they are. The essence of Darwinism is the unwarranted extrapolation of the small changes that happen all the time within species into the really big jumps (reptile to bird); as any statistician will tell you, extrapolation is a dangerous business, and in the case of Darwin it goes flat against the evidence.