In part one I outlined the clash, which took place between Galileo and Foscarini on the one side and the Catholic Church on the other in the second decade of the seventeenth-century. I ended by saying that this initial confrontation had very few consequences for Galileo at the time, who continued to be the highly feted darling of the North Italian in-crowd, including the higher echelons of the Catholic Church. Of course the events of 1615/16 would come back to haunt Galileo when he was tried for writing and publishing his Dialogo in the 1630s but that is a very complex topic that require a post of its own sometime in the future. I also wrote that the books of Foscarini and of the Protestant Copernicans, Michael Maestlin and Johannes Kepler were placed on the Index of Forbidden Books. Interestingly De revolutionibus was only placed on the Index until corrected. It is here that we will pick up the thread and examine the consequences of the Church’s actions on the development of astronomy in the seventeenth-century.

What did it mean when I say that De revolutionibus was only placed on the Index until corrected? This means that De revolutionibus was not forbidden but that only those statements within the book, which claimed that heliocentricity was a proven fact, were to be removed. This mild censorship, only a handful of passages in the whole book were affected, was carried out comparatively quickly and the thus censored version was given free to be used by astronomers already in 1621. The whole of this episode demonstrates that the powers that be within the Church were well aware that De revolutionibus was an important astronomical text and should, despite the judgement of the eleven members of the commission set up to adjudicate on the affair that the idea that the Sun is stationary is “foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture…”; while the Earth’s movement “receives the same judgement in philosophy and … in regard to theological truth it is at least erroneous in faith”, remain available to Catholic astronomers for their studies.

There is a widespread popular perception that the Church’s theological rejection of the theory of heliocentricity led to a breakdown of astronomical research in Catholic countries in the seventeenth-century. Nothing could be further from the truth. As mentioned in the first part of this post, some historians think that Cardinal Bellarmino’s admission in his letter to Foscarini that … if there were a real proof that the Sun is in the centre of the universe, that the Earth is in the third sphere, and that the Sun does not go round the Earth but the Earth round the Sun, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary …, was interpreted by many Jesuit and Jesuit educated astronomers as a challenge to find an empirical proof for heliocentricity. As we shall see there is quite a lot of circumstantial evidence to support this claim.

An important historical fact to be born in mind when considering the development of astronomy in the seventeenth-century was that there existed no empirical proof for the heliocentric hypothesis, whether it be in the original form proposed by Copernicus or the much more sophisticated form developed by Kepler. The astronomers would have to wait until 1725 before James Bradley delivered the first proof of the earth’s annual orbit around the sun with his discovery of stellar aberration and slightly longer before the geodesists demonstrated that the earth is an oblate spheroid thus confirming a prediction made by both Newton and Huygens that diurnal rotation would result in the earth having this form thus proving indirectly the existence of diurnal rotation. This tends to be forgotten or simply ignored by those claiming that the Church should have accepted heliocentricity as a fact in 1615. In reality the heliocentricity became accepted by almost all astronomers whether Catholic or non-Catholic by around 1660, long before any empirical proof existed, on the basis of accumulated circumstantial evidence and the lack of a convincing alternative. A lot of that circumstantial evidence was delivered by Catholic astronomers, who despites the Catholic theological position, continued to work avidly on the development of the modern astronomy.

It is also important to realise that although the Church banned claiming that heliocentricity was a fact, the heliocentric theory, it was still perfectly possible to speculate about heliocentricity, the heliocentric hypothesis. Throughout the seventeenth-century Catholic astronomers in Italy adopted an interesting strategy to deal with the Church’s ban of the heliocentric theory. They would preface their works with a statement of the fact that in its wisdom the Church had shown the heliocentric theory to be contrary to Holy Scripture and thus factually false and then proceed to discuss this interesting mathematical hypothesis without claiming it to be true. This strategy sufficed for the Inquisition’s guardians of the truth and thus the astronomers continued to discuss and disseminate heliocentricity with impunity.

Scientific theories are not only disseminated by their supporters but often also by their opponents. Long before Galileo muddy the waters with his heated challenge to the Church’s exclusive right to interpret the Bible it is certain that more people learnt of the existence of the heliocentric hypothesis and its basic details from the works of Christoph Clavius, a convinced defender of geocentricity, than from De revolutionibus. In his commentary on the Sphere of Sacrobosco, an introductory astronomy textbook, Clavius discussed Copernicus’ heliocentric hypothesis sympathetically, respecting its mathematical sophistication, whilst firmly rejecting it. This book went through numerous editions and was the most widely disseminated and read, by both Catholic and Protestant students, astronomy textbook throughout most of the seventeenth-century and was for many their first introduction to the ideas of Copernicus. It was also Clavius’ postgraduate students, in his institute for mathematical research at the Collegio Romano, who provided the very necessary empirical confirmation of Galileo’s telescopic discoveries in 1611, shortly before Clavius’ death. This activity by Jesuit astronomers pushing the boundaries of astronomical knowledge did not cease following the decisions of 1616.

There was a slowdown in the development of modern astronomy in the second and third decades of the seventeenth-century that has nothing to do with the Church’s ban but was the result of a lack of technological advance. In the four years between 1609 and 1613 European astronomers had discovered everything that it was possible to discover using a Dutch or Galilean telescope with a convex objective and a concave eyepiece. The only new discoveries were the observations of a transit of Mercury by Gassendi in 1631 and a transit of Venus by Horrocks in 1639 neither of which had an immediate impact because they didn’t become widely known until much later. For various reasons, not least Galileo’s very public rejection of it as inferior, the superior Keplerian or astronomical telescope, with two convex lenses, didn’t start to become established until the 1640s. However once established the new discoveries began to flow again: the moons of Saturn, the rings of Saturn, diurnal rotation of the planets. Many of these new discoveries, which added new circumstantial evidence for heliocentricity, were made by Giovanni Domenico (Jean-Dominique) Cassini (1625–1712) a Jesuit educated Catholic astronomer. It was also Cassini, with the support of his teachers the Jesuits Giovanni Battista Riccioli and Francesco Maria Grimaldi, who proved, using the heliometer constructed for this purpose in the San Petronio church in Bologna, that either the sun’s orbit around the earth or the earth’s orbit around the sun must be an ellipse, as required by Kepler. Although this proved that the orbit is an ellipse it didn’t show which system was correct.

Cassini, who would go on to become the leading observational astronomer in Europe, always avoided committing himself to any systems simply delivering empirical results and leaving the cosmological interpretation to others. Although confirming Cassini’s heliometer results Riccioli stayed committed to semi-Tychonic system, in which the inner planets orbited the Sun, which in turn together with Saturn and Jupiter orbited the Earth. Riccioli presented this rather bizarre mongrel in his Almagestum Novum published in 1651. Riccioli’s Almagestum contained descriptions of all the various possible systems, including the Copernican, and became a very widely disseminated and read technical textbook for astronomers, both Catholic and Protestant. Like Clavius before him, Riccioli introduced many to heliocentricity for the first time. The Almagestum contained 126 arguments concerning the Earth’s motion 49 pro and 77 contra the most extensive discussion of the problem ever. You can read Chris Graney’s English translation of the arguments here. Although Riccioli came out against heliocentricity his analysis was sympathetic enough to the concept that he was actually investigated by the Inquisition.

Having been made available by the Index copies of De revolutionibus appear only to have been actually censored within Italy nearly all the surviving censored copies, including Galileo’s, coming from there. Outside of Italy, with the notable exception of Descartes, nobody seems to have taken very much notice of the Inquisition’s ban. Descartes appears to have withheld publication of his The World, in the 1630s, containing his defence of heliocentricity, out of respect for his Jesuit teachers. Publishing his views, in modified form, first in his Principles of Philosophy in 1644.

Another Frenchman, Pierre Gassendi like Descartes educated by the Jesuits, who became professor of mathematics at the Collège Royal in Paris in 1645 published his views on astronomy in his Institutio astronomica, although formally a supporter of the Tychonic system, Gassendi’s presentation of the Copernican system is so sympathetic that many historians have interpreted him as a secret supporter of heliocentricity. Gassendi also published biographies of Tycho, Peuerbach, Regiomontanus and Copernicus. Like Riccioli, Gassendi’s astronomical writings were very popular and very widely read, again leading to a widespread dissemination of the principles of heliocentricity.

Another leading French Catholic astronomer, Ismael Boulliau was an open and avid supporter of the Keplerian elliptical astronomy and was indeed the first to hypothesise that gravity ought to be an inverse quadrate force, a significant step in the road to acceptance of heliocentricity. It was Boulliau’s dispute with the English astronomer Seth Ward about Kepler’s second law, which nobody liked, both parties offering alternatives, that first made Newton aware of Kepler’s system.

By about 1660 enough circumstantial evidence had been accumulated that most astronomers in Europe both Catholic and Protestant, with the necessary education to do so, had accepted heliocentricity as a fact with a small minority still holding out for a Tychonic system. In the end the Tychonic system had fallen victim of Ockham’s razor being viewed as overly complex in comparison with the Keplerian elliptical system for which more and more evidence had accumulated throughout the preceding fifty years. A significant advance in the development of modern physics in which Galileo’s Discorsi had played an important role also contributed crucially to this acceptance, dealing as it did with the physical problems of terrestrial motion. A detailed analysis of these developments would make this already over long post even longer and must be dealt with separately.

Although by no means an exhaustive presentation of the development of astronomy in the seventeenth-century, I think the above contains enough to demonstrate that the Church’s ban of the heliocentric theory had very little negative influence on that development and that Catholic astronomers played a leading role within it. Returning to my earlier speculation, I feel justified in saying that had Galileo and Foscarini not forced the Church’s theologians into a corner in 1615, then the Catholic astronomers, and in particular the Jesuits and their pupils, would have led the Church to an acceptance of heliocentricity within the seventeenth-century.