Galileo Galilei

Despite the numerous controversies of interpretation which swirl around the Galileo Galilei affair, the basic facts of the case are now quite well known. Furthermore a delineation of these facts reveals the key parameters which have characterized the relationship between science and religion ever since. This at least allows us to identify the factors which define the science-religion dispute, which is our main concern, without becoming bogged down in evaluating the many controversies of the partisans on both sides.

With this in mind we will now first give an historical sketch of what happened in the Galileo Galilei affair, and then list five lessons to be learned from it. One of the basic facts about the Galileo affair, which has considerable consequences for our interests in this lecture, is that it consisted of two trials, not just one. The first trial took place in February of 1616; the second trial seventeen years later in the spring of 1633. The first trial made a judgment about a question of truth; the second about a question of legal culpability. The first trial dealt with matters of fact; the second with matters of authority. The first trial occurred at an abstract level; the second at a personal level. Let us see whether we can explain this more concretely.

We know from his personal correspondence that at least by 1597 Galileo Galilei had become personally convinced that the sun is located in the center of the world, and that the earth and the other planets revolve around the sun in orbits having regular periods. This was an old theory, first entertained by some of the ancient Greek astronomers, and then reintroduced by Copernicus in 1543. In the period between Copernicus and Galileo Galilei it remained just that, a theory, which the professional astronomers debated warmly among themselves, but which did not have much of a wider influence.

The turning point came in 1610 by which time Galileo Galilei had constructed his telescope, which he then turned for the first time to the observation of the heavens. He made a host of new discoveries of great importance; for example, the mountains and valleys on the moon, four of the moons of Jupiter, the rings of Saturn (although he did not realize what they were), sunspots, and of special importance, the phases of Venus. He began to publish these observations, along with his interpretations of them, and in the process the heliocentric theory was transformed from a curious speculation among astronomers to a view of the world which seemed, even to the generally informed reader, to be more and more probably the true description of the solar system.

In these early writings Galileo's previous convictions about heliocentrism gradually became more explicit. But it is very important to emphasize that he did not have a strict proof. The closest he came at that time was the evidence that Venus goes through a full set of phases, as does our own moon. This proves that Venus (and probably Mercury also) revolves around the sun, but it does not determine whether the earth also revolves around the sun, rather than vice versa. Galileo Galilei was quite well aware that he did not have a conclusive proof of this point, as is evident from the fact that for the rest of his life he kept up his search for the conclusive proof. But all of his efforts in this direction failed him, including his famous claim that the ocean tides are caused by the double motion of the earth required by Copernicus' theory. Galileo's work on these questions were methodologically routine scientific in quiries in which theories are proposed and tested against the authority of empirical facts and their rational interpretation.

But another type of authority soon entered into the picture. There are many apparently conflicting passages in the Bible which state either directly or by implication that the earth is at rest in the center of the universe, and that the sun revolves around the earth from east to west. Galileo's opponents were quick to point out this apparent inconsistency between the new astronomy and the Scriptures.