De Revolutionibus Orbium Coelestium

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In 1592, when Galileo arrived at Padua and Bruno was arrested, Mikolaj Copernik - or Copernicus - had been dead for almost fifty years, but the force of his ideas was only just beginning to make itself seriously felt.

In his private life Copernicus was hardly a revolutionary, he was in fact a peaceful cleric. He lived a quiet life as a canon of the cathedral in the small town of Frauenburg in the semi-independent bishopric of Ermeland on the shores of the Baltic, now part of Poland. As a young man around the year 1500 he had spent some years studying in Italy, thanks to a rich uncle. He had even been at Il Bo in Padua, although without attracting the least attention.

Copernicus took his doctorate in canon law. But he had studied many disciplines and his greatest interest lay in astronomy. He knew - as did all other learned men - that the accepted Ptolemaic view of the world was hard to reconcile with precise astronomical observations. In order to make the system function after a fashion, Ptolemy had to introduce a number of "auxiliary orbits" or epicycles, small circular orbits that the planets described on their journey around the Earth. On his deathbed in 1543, Canon Copernicus published a book - De Revolutionibus Orbium Coelestium - which tried to demonstrate that the description of the universe would be a lot simpler and more correct if one altered one basic concept: instead of assuming that the Sun, stars, planets and Moon travelled in circles and epicycles around a fixed Earth, one could conjecture that the Earth and the other planets orbited the Sun, while at the same time revolving on their own axes.

This idea was not quite original. It had been proposed by Greek philosophers, but Copernicus was the first to try to develop it systematically.

One might have assumed that Copernicus' revolutionary idea with the Sun in the centre (the heliocentric system) would have proved instantly compelling for all professional astronomers and that, at a stroke, everything would have fallen into place. But this was decidedly not the case. An example of the scepticism this theory aroused is shown in this muted British reaction to a lecture given by Giordano Bruno at Oxford:

De Revolutionibus Orbium Coelestium 27

"Stripping up his sleeves like some jugler, and telling us much of chen-trum & chirculus & circumferenchia (after the pronunciation of his country language) he undertooke among very many other matters to set on foote the opinion of Copernicus, that the earth did goe round, and the heavens did stand still; whereas in truth it was his owne head which rather did run round, & his braines did not stand still."6

The Copernican system was almost impossible to square with plain common sense. Anyone could raise objections to it: why do we not notice that the Earth is turning round, let alone hurtling through space at enormous speed? Not even educated men well versed in physics and astronomy had good answers. At the University of Copenhagen in Protestant Denmark, the astronomer Tycho Brahe had been one of the very first to lecture on Copernican theory in the winter of 1574-75. But Brahe himself was not convinced, and instead put together his own cosmological model.

One thing was perfectly clear to everyone who touched upon Copernicus' notion of moving the Earth from its place at the centre of the world and reducing it to one of several planets orbiting the Sun: they would encounter solid resistance from a united front of conservative natural philosophers and theologians. For one thing Ptolemy's system was considered part of Aristotle's description of physical reality, but a far worse problem was the Bible's own words. One needed to look no further than Holy Scripture's first page, Genesis, chapter 1, verses 17-18, which said unequivocally of the Sun, Moon and stars that: God set them in the firmament of the heaven to give light upon the earth. And to rule over the day and over the night, and to divide the light from the darkness. Not a word about the Earth making day and night by revolving.

Even so, the idea captivated some, both because of its remarkable simplicity - away with all that complicated system of spheres and epicycles! - and also very much because of its revolutionary daring and intellectual challenge.

Professor Galilei loved intellectual challenges, and he despised the obstacle of ossified, conservative Aristotelian thought that hindered new ideas about natural phenomena. He could not but be attracted to the Copernican system. On his own initiative and without discussing the matter except with close friends, he studied the revolutionary ideas. In 1597 he announced that he was a "Copernican" in one of his rare letters to his German colleague, Johann Kepler.7

The new ideas were not represented in his teaching, however. Certainly Galileo must have felt himself superior to the friar-mystic Bruno and the simple miller with his unfortunate penchant for peasant philosophy. But the

Inquisition was just as much a part of his daily life too, a part he could not ignore.

The Inquisition's cumbersome bureaucracy and centralised structure meant that it was not particularly effective. The Holy Office was responsible for only a tiny fraction of the executions and downright homicide inflicted on members of minority faiths all over Europe in the 16th and 17th centuries.

But the institution was a reality no less omnipresent for all that. There is no indication that Galileo rehearsed anything but pure orthodox Ptolemaic theory from his lectern in Padua. During his lectures the Earth remained absolutely fixed as the definitive centre of the universe. A professor's responsibilities included not leading his students into heresy, not crossing that invisible - and pitifully ill-defined - boundary between science and religion.

In Florence the Grand Duke Ferdinando commissioned a huge planetarium, a model of the heavenly planets and bodies. The contrivance took five years to build. It was three metres high, made of wood completely covered with gold leaf and could be turned with the aid of a handle so that the Sun and planets moved.

But the Earth stood still in the centre of the model. The planetarium was an expression of Ferdinando's eccentric love of the spectacular, but it was also a demonstration of an absolute ruler's adherence to the prevailing astronomical and theological wisdom, and thus a discreet warning to those who thought otherwise.

But the situation for those who inclined towards the theory of the Sun in the centre, was not entirely hopeless. Copernicus' book had so far not been placed on the list of forbidden works. Of even greater importance was the clear tradition that had grown up within astronomical science for strictly distinguishing between astronomy and cosmology.

True astronomy was concerned with calculating planets' orbits, the position of stars, eclipses and that kind of thing. It could have a certain practical value, especially in navigation. As far as it went, this kind of astronomy could use many different models, provided they gave sensible results. Such "mathematical models" did not necessarily aim to represent the ultimate cosmological and physical truth about what the universe looked like. This gradually also came to apply to adjusted details of the Ptolemaic system, with its epicycles and other complications (for example, that the mathematical centre of the planets' orbits was not exactly the Earth). It was taken that this was an aid to calculation and not a real description of the world.

Viewed in this light, Copernicus' system was viable as a purely intellectual and mathematical model, without the Church needing to involve itself in the matter. And a number of experiments were made in this direction, without producing noticeably better results than the old model, as Copernicus had not been very precise in his specification.

The problem was that Copernicus himself did not regard his system as a useful aid for complicated computations. He saw it as a concrete representation of cosmological reality: the Sun stood still, the Earth moved around it. Of earlier astronomers' attempts at cobbling together a tenable geocentric model, he said contemptuously:

"They could not discover the main thing, namely the form of the heavens and the equilibrium of its parts. On the contrary, they are like one [a painter] who, from the best models selects hands, feet, head and other limbs, all of which are of the most excellent quality, but are not drawn as the picture of one single body, and therefore turn into a monster rather than a man when they are put together."8

Galileo also believed that the Sun actually stood still, forming a centre for the motions of the planets and the Earth. But he did not turn it into a bone of contention just then. Instead he returned to the pendulum and the falling spheres. For here, too, in miniature, there was much to learn about how the world really works.

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