Galileo:
"A New Theory of the Nature of Theory"

By Gabriel Blanchard

Galileo's notorious condemnation as "vehemently suspect of heresy" conceals a far more interesting story.

In 1609, Galileo discovered that the sun and not the earth was the center of the Solar system, which nobody had discovered since Copernicus in 1514. Whether a textbook credits Galileo or his predecessor with first forming heliocentric theory seems to be a matter of taste.

The late Renaissance was a period of intellectual foment throughout Europe. The most famous example is, of course, the Protestant Reformation, but this fit into a larger and stranger pattern. Scholars at the time (Protestant and Roman Catholic) accepted the model of the universe set forth by ancient astronomers such as Aristotle, Hipparchus, and above all Ptolemy, whose Almagest remained the standard astronomy textbook for well over a thousand years. This was a geocentric system—the earth was the center of the universe, and everything above the moon was made of a different kind of matter called aether, not subject to the changefulness we see on earth. Modern writers often dismiss this cosmology with some contempt, but there were good reasons for it at the time.

The constancy of the heavens was a matter of direct observation. The earth shows all sorts of fluctuation, both regular (like the seasons) and irreversible (like decay). The stars show no such fickleness: even planets and comets describe the same path decade after decade. It seemed ludicrous to suppose that they were made of the same stuff when they behaved so differently, which led Aristotle to propose a fifth element in addition to the classical four (fire, air, water, and earth). This was the aether, which existed only beyond the moon’s orbit, where change obviously ceased.

As for heliocentrism, ancient scholars had considered the possibility—and rejected it on mathematical grounds. If the earth orbited the sun, they argued, then we ought to see slight changes in the positions of the stars, a phenomenon called stellar parallax. But no hint of parallax could be found; ergo, the earth must be stationary. This was actually due to the immense distance of the stars, which made parallax incredibly small—far too minute for the naked eye, or even for the first telescopes. (No astronomer successfully documented stellar parallax until 1838.) But this was of course unknown in the third century, and for that matter in the seventeenth.

This synthesis began to break down in the sixteenth century, however. And it had less to do with Copernicus than with Tycho Brahe, an astronomer from Denmark, who in 1572 observed a dramatic change in the supposedly changeless heavens: a supernova. There was no aether. In a way, the Reformation may have seemed ordinary enough; priests, peasants, and kings alike had rebelled against Rome’s authority before. But the supernova must have made people feel as though the sky were falling.

Philosophy is written in that most great book which lies continually open to our eyes (I speak of the universe), but we cannot understand it, if we do not first learn to understand its language and to grasp the kind of characters in which it is written.

Galileo Galilei

Galileo received his first scholarly appointment in Pisa in 1589. Twenty years later he perfected the refracting telescope, which became popular not only with navigators but with astronomers like himself. With it he studied a second supernova, Kepler’s, and discovered the rings of Saturn and Jupiter’s four largest moons (Io, Callisto, Ganymede, and Europa). He even spotted Neptune, though he did not realize it was a planet.

But it was Galileo’s discovery of the phases of Venus that put him on a collision course with the Catholic Church. In Ptolemy’s model, no more than two phases of Venus could ever be visible from the earth; Galileo observed all four. This exploded the Ptolemaic model. However, it left room for a new model that Tycho Brahe proposed: the planets orbited the sun, and the sun orbited the earth. The Tychonic model was rather fashionable in Galileo’s time, but he rejected it. He insisted that the fully heliocentric Copernican model was not only plausible, but fact, even though no evidence of parallax then existed. And it was this, not heliocentrism, that offended the Inquisition.

C. S. Lewis is best known today as an author of children’s books, but he was also a professor of Medievalism and the Renaissance and knew a good deal about their approach to science. In The Discarded Image, he writes:

A scientific theory must “save” or “preserve” the appearances, the phenomena, it deals with, in the sense of getting them all in, doing justice to them. … The astronomical or chemical theory can never be more than provisional. It will have to be abandoned if a more ingenious person thinks of a supposal [that is simpler]. This would, I believe, be recognized by all thoughtful scientists today. It was certainly recognized in the Middle Ages. The real reason why Copernicus raised no ripple and Galileo raised a storm, may well be that whereas the one offered a new supposal about celestial motions, the other insisted on treating this supposal as fact. If so, the real revolution consisted not in a new theory of the heavens but in “a new theory of the nature of theory.”

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Every week, we publish a profile of one of the figures from the CLT author bank. For an introduction to classic authors, see our guest post from Keith Nix, founder of the Veritas School in Richmond, VA.

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