Why Galileo was Wrong, Even Though He was Right

In the early seventeenth century a courageous and brilliant scientist, Galileo Galileo, confirmed heliocentrism, the idea first proposed a century earlier by Nicolaus Copernicus that the sun was at the center of the universe. Heliocentrism challenged geocentrism, the religiously motivated idea that a stationary earth was at the center of the universe. Galileo explained why heliocentrism was true and not surprisingly the church strongly opposed and persecuted the scientist. Ultimately, however, the truth could not be denied and church was forced to, once again, reluctantly give in to the objective truths of science.

That was the false history of the Galileo Affair according to later revisionists who promoted the view that science and religion were in conflict. In fact while Galileo indeed was brilliant, he also made it difficult for friendly voices to support him. Furthermore he did not confirm heliocentrism, and heliocentrism was not the only viable alternative to geocentrism. And geocentrism was hardly religiously motivated. The church had little objection to heliocentrism when Copernicus wrote of the model in the sixteenth century, and Copernicus was not the first to consider the idea.

The Galileo Affair is far more complex than the simple-minded warfare thesis supposes. Yes Pope John Paul II issued a declaration in 1992 acknowledging the church's errors. And the church was no doubt mistaken. But the church's action in the Galileo Affair was far more complex than simply opposing a scientific finding out of religious conviction (Galileo's trial focused on his insubordination, not heliocentrism). In fact, there were at least four reasons why the church opposed Galileo's heliocentrism which confound the naive warfare thesis.

First, in Galileo's day internal church politics had made it less receptive to new ideas such as heliocentrism. Second, Galileo's disobedience and style--such as satirizing his friend Pope Urban VIII who had been a supporter--fomented opposition. Third, it was understood that science could devise models that, on the one hand fit the data but on the other hand were not true or approximately so. In fact, geocentrism modeled the celestial motions quite accurately. And finally, in some cases where geocentrism did fail, another alternative--Tycho Brahe's hybrid model--succeeded.

An important failure of geocentrism were the phases of Venus which indicated it circled the sun, not Earth. Galileo expounded upon this point, but what he failed to mention was that the Tychonic system, in which the sun circles the earth and the inner planets in turn circle the sun, handled the phases of Venus just fine.

In fact new research reported on this week indicates another problem with Galileo's firmly held views. When observing stars through a telescope, as Galileo did, they do not appear as points of light, as they should, but as a small extended area, or disk, as did the planets. This disk appearance is due to the diffraction of light which was unknown at the time.

Of course the stars were assumed to be like the sun, and therefore much larger than the planets. Given their larger size the observed small disk meant they must have been much farther away than the planets. But the calculated distances to the stars were thousands of times less than what we now calculate. Yes the stars were far away, but those small disks were misleading. The diffracting light made the stars appear closer than they actually are.

Galileo was therefore assuming the stars were much closer than they actually are. Why is this important? Consider objects you observe out the window as you sit in a moving train. A nearby tree may be behind a closer tree, but as the train moves you will see the farther tree emerge as your angle changes. Two stars in the sky, on the other hand, do not move in relation to each other as you move along, because they are so far away.

Because Galileo calculated the stars to be much closer than they actually are, he would necessarily expect to see some change in their relative positions as the earth circled the sun in his heliocentric model. But no such relative change was observed. It was an important failure of Galileo's model which, again, he did not mention. And again, it was an observation that the Tychonic system handled just fine.

What the new research points out is that a contemporary of Galileo, the German astronomer Simon Marius, famous for naming the moons of Jupiter, was aware of these implications and followed them to their logical conclusion.

While Galileo was making high claims for heliocentrism, Marius had made clear in his 1614 book Mundus Iovialis (The Jovian World), that the observations confirmed the earth-centered Tychonic system.

The new paper, aptly entitled "How Marius Was Right and Galileo Was Wrong Even Though Galileo Was Right and Marius Was Wrong," is another example of how not just science, but the history of science, is more complicated than self-serving black-white renditions would have it.