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In 1542, the European astronomer Nicolaus Copernicus sent his life’s work in astronomy, De Revolutionibus Orbium Coelestium, to the press. In the following year, the book was printed and published in Nuremberg – the first work of astronomy to seriously challenge the Ptolemaic system, which had dominated European astronomy for centuries. There are some who wish to tie the beginning of the Scientific Revolution to its publication. It was certainly a seminal work, and would give rise to a new cosmology that would eventually topple the medieval world view, but De Revolutionibus is a remarkably tradition-bound work when considered in light of its subsequent revolutionary effects. The structure of the work is obviously modeled on Ptolemy’s Almagest, and Copernicus made it clear in Book I of De Revolutionibus that he considered himself to be reviving ancient Greek doctrines. In the fifty or so years following Copernicus’ death, many astronomers spoke of him as a “second Ptolemy”. But while De Revolutionibus was largely traditional in its structure, Copernicus himself seems to have been aware of its revolutionary nature. He only assented to the publication of the work near the end of his life, and then only with reluctance, encouraged by a young disciple and admirer, the mathematician Georg Joachim Rheticus.

Missing in De Revolutionibus are many of the hallmarks of what later came to be known as the “astronomical revolution” – Copernicus did not reject the use of the epicycles and eccentrics used in Ptolemaic astronomy (although he did reject Ptolemy’s equant), or the Aristotelean conception of heavenly rotating crystalline spheres. He neither postulates an infinite universe nor does he think the sun is the same sort of body as the stars, and his work does not treat the Earth as “just another planet” -- i.e. the Earth’s movement is not accounted for in the same fashion as that of the other planets, via epicycles and eccentrics. These aforementioned breaks with the ancient astronomical and cosmological tradition would only surface later in the works of his successors. Yet De Revolutionibus contained the seed of a revolution which would eventually result in the abandonment of the dominant Aristotelian-Ptolemaic view of the universe. In that sense, Copernicus’ work was simultaneously radical and traditional – it has, in other words, a “dual nature” (Kuhn 1957). But what exactly were the radical innovations that Copernicus introduced in De Revolutionibus?

The first and undoubtedly most famous of these was the movement from a geocentric model to a heliocentric one. In the Ptolemaic world-view, the Earth rests at the centre of the universe, with all the heavenly bodies revolving around it. In the Copernican astronomical model, however, the planets, including the Earth, revolve around an immobile Sun near the centre of the universe. The “lantern of the world” is thus positioned because it is the best place from which to “simultaneously illuminate all things” (Copernicus, 1976). This meant that the heavenly bodies no longer all revolved around a single point – the Earth and planets revolve around the central Sun while the Moon revolves around the Earth. The second innovation was the introduction of the Earth’s motion, in stark contrast to the immobile central Earth of Ptolemy – instead of rotating, the outermost crystalline sphere of stars is immobile like the Sun, and the apparent movement of the stars and Sun is acounted for by making the Earth spin on its axis whilst simultaneously revolving around the Sun. A third motion keeps the Earth correctly tilted.

It is important to note that neither of these breaks with the Ptolemaic tradition were entirely new ideas, as Copernicus is at pains to point out in the preface to De Revolutionibus. Inspired by the spirit of Renaissance humanism, he refers to past authorities to lend weight to his case. The idea of a moving Earth (and axial rotation) had been suggested by a number of scholars from antiquity to the Middle Ages, amongst them Heraclides of Pontus and Nicolas Oresme. Likewise, the notion of heliocentrism can be traced to the philosophers of the ancient world, back to Philolaus and his disciple Aristarchus. These ideas, then, were hardly innovative in the proper sense of the word, and Copernicus’ historical signficance cannot be attributed solely to the fact that he expounded them. Had he only proposed the heliocentric cosmological theory without a sophisticated astronomical model, he might very well have remained a relatively marginal historical figure, like his Greek heliocentrist predecessors. But Copernicus distinguished himself from his predecessors in at least one important way: he developed and published a detailed professional mathematical account of a heliocentric system, thus providing subsequent astronomers with a fully mathematically worked out and viable alternative to the system of Ptolemy.

In the decades immediately following the publication of De Revolutionibus, the Copernican system did not attract many adherents. It was a sophisticated and complex mathematical work, for the most part incomprehensible to anyone without the benefit of professional astronomical and mathematical training – and its cosmological implications were at odds with the received view of his day. Only 50 or so years later did it start to gain significant credibility. Why was this the case?

It is tempting to attribute the slow adoption of Copernicus’ system to a general reluctance to abandon an anthropocentric universe – and hence, a geocentric one. Man, it might be argued, is a vain creature and would inevitably be loath to give up a world-view which placed himself and his immediate environment at the centre of the universe. But in the Aristotelean cosmology, the Earth is a place of constant generation and decay whereas the heavens are constant and unchanging, in circular motion, the most mathematically perfect of shapes. The medieval synthesis of Aristoteleanism and Christianity found in the writings of Thomas Aquinas extends this even further, with the unchanging heavens a perfect realm reflecting the divinity of the creator, in sharp contrast to the fickle and tumultuous habitat of men, below which burned the fires of Hell. The central position of the Earth may therefore not have been regarded as one of particular eminence, and the initial scorn with which Copernicanism met probably had other sources.

A more plausible cause of Copernicanism’s poor reception is the fact that it required an entirely new physical theory – if the Earth was no longer the centre of the Universe, then the dominant Aristotelean physical theory would have to be either radically reformulated or abandoned entirely. Aristotle maintained that the elements in the terrestrial sphere naturally sought either to get closer to or move away from the centre of the universe – but this account obviously only accords with observations of objects near the Earth under the assumption of geocentricity. According to Copernicanism, then, the centre of the world could no longer serve as the centre of all gravity, and the theory thus posed a challenge not only to the accepted Ptolemaic astronomy, but also to the physical theory of the day. A synthesis of Copernicanism and traditional Aristotelean physics was well nigh impossible, while Copernicus offered no fully developed physical theory as a replacement for that of Aristotle. He also left unanswered the unpleasant problem of accounting for why the Moon revolved around the Earth and not the Sun, unlike the other planets.

Another major problem was the notion of a moving Earth – which may very well have been the most dominant and persuasive counter-argument against Copernicanism in the decades following the publication of De Revolutionibus. The idea that the Earth moved was counter-intuitive and contrary to the “common sense” of the day. If the Earth actually moved in the way described by Copernicus’ theory, it would have to be moving at enormous speed – about 1000 miles an hour via axial rotation and about 70,000 miles an hour in orbit around the Sun – but this speed could not be felt at all by observers on the Earth’s surface. Lacking a more advanced physical theory, a moving Earth must have seemed preposterous to most of Copernicus’ contemporaries. Understandably, therefore, the idea was met largely with scorn or bemused contempt. To exacerbate things, Copernicanism required that the universe be much larger than men had previously believed. If the Earth did indeed move, then the positions of the stars ought to change substantially throughout the year. No such change could be observed. To solve this problem, Copernicus placed the the sphere of the stars a vast distance away from the centre of the universe. This suggested an enormous vacuum, which constituted another major attack on Aristotelean physics.

Viewed from a practical or instrumental standpoint, Copernicus’ model was only a marginal improvement on that of Ptolemy in terms of predictive accuracy. Erasmus Reinhold, who worked out new astronomical tables based on the Copernican theory, found that the model expounded in De Revolutionibus was more accurate than Ptolemy’s Almagest only two times out of three. This poor predictive accuracy may at least partly be attributed to the flawed observational data inherited from his astronomical predecessors in the ancient world, to which he tried to adapt his system. Due to the need to accommodate faulty data, his system became more complex than it need have been. But however marginal the improvements in predictive accuracy, Copernicus insisted on the greater simplicity and harmony of his system, as did his disciple Rheticus. Indeed, the advanced mathematical model contained within De Revolutionibus impressed professional astronomers, even if they did not accept the cosmological premises on which it was based. While Ptolemy’s account of planetary motions was in a sense arbitary, Copernicus’ account allowed the position of any one planet to be worked out from another and can in that sense be said to be more harmonious.

The Christian church authorities, whether Lutheran or Catholic, made no attempts to ban De Revolutionibus upon its publication, and it would only be placed on the Vatican’s index of prohibited books 73 years later. By the time church authorities took action, knowledge of the Copernican theory had long disseminated to the universities and centres of learning in Europe. This slow reaction may at least partly be attributed to the inaccessibility of the work – its contents were only fully comprehensible to erudite technical astronomers, not the general public – and partly to the fact that the Copernican theory, whatever its mathematical and astronomical merits, may not have been taken entirely seriously as a cosmological theory. Many astronomers seem to have been content with making use of Copernicus’ mathematical system while rejecting the cosmological implications of his work. The Earth’s motion could be treated as a convenient fiction for getting correct calculations of planetary positions. This was the stance suggested by Osiander, the Lutheran theologian who oversaw the initial publication of De Revolutionibus and wrote a preface to the book. An instrumentalism of this sort was in many ways a convenient solution – astronomers could derive benefits from Copernicus’ work while avoiding the physical and theological difficulties associated with a moving Earth and heliocentrism. And this approach to astronomical texts will not have been new – Ptolemy’s Almagest was also commonly regarded as presenting a useful model for calculation without committing astronomers to believing that planets actually moved in the way described. Additionally, Copernicanism, with all its new problems, may also have seemed an implausible rival to the firmly entrenched Aristotelean-Ptolemaic tradition; for the case for heliocentrism, as presented by Copernicus, was not particularly strong. His model, was only slightly better at prediction, but still about as complicated as that of Ptolemy, while it clashed with Aristotelean physics and the “common sense” of the day. It is not difficult to imagine it arousing disdain, rather than intellectual angst, amongst established scholars in the dominant tradition. Ptolemaic astronomy was not undergoing a crisis at this time – Why abandon a familiar and well-tested theory for a problematic and seemingly preposterous rival? Such attitudes may have prevented the churchmen from discerning the ultimately revolutionary and theoretically subversive nature of De Revolutionibus.