What role did the Dialogo actually play in the ongoing cosmological/astronomical debate in the seventeenth century? The real answer is, given its reputation, surprisingly little. In reality Galileo was totally out of step with the actual debate that was taking place around 1630. Driven by his egotistical desire to be the man, who proved the truth of heliocentricity, he deliberately turned a blind eye to the most important developments and so side lined himself.



We saw earlier that around 1613 there were more that a half a dozen systems vying for a place in the debate, however by 1630 nearly all of the systems had been eliminated leaving just two in serious consideration. Galileo called his book Dialogue Concerning the Two Chief World Systems, but the two systems that he chose to discuss, the Ptolemaic/Aristotelian geocentric system and the Copernican heliocentric system, were ones that had already been rejected by almost all participants in the debate by 1630 . The choice of the pure geocentric system of Ptolemaeus was particularly disingenuous, as Galileo had helped to show that it was no longer viable twenty years earlier. The first system actually under discussion when Galileo published his book was a Tychonic geo-heliocentric system with diurnal rotation, Christen Longomontanus (1562–1647), Tycho’s chief assistant, had published an updated version based on Tycho’s data in his Astronomia Danica in 1622. This was the system that had been formally adopted by the Jesuits.



The second was the elliptical heliocentric system of Johannes Kepler, of which I dealt with the relevant publications in the last post.



Galileo completely ignores Tycho, whose system could explain all of the available evidence for heliocentricity, because he didn’t want to admit that this was the case, arguing instead that the evidence must imply a heliocentric system. He also, against all the available empirical evidence, maintained his belief that comets were sublunar meteorological phenomena, because the supporters of a Tychonic system used their perceived solar orbit as an argument for their system. He is even intensely disrespectful to Tycho in the Dialogo, for which Kepler severely castigated him. He also completely ignores Kepler, which is even more crass, as the best available arguments for heliocentricity were to be found clearly in Kepler published works. Galileo could not adopt Kepler’s system because it would mean that Kepler and not he would be the man, who proved the truth of the heliocentric system.



Although the first three days of the Dialogo provide a good polemic presentation for all of the evidence up till that point for a refutation of the Ptolemaic/Aristotelian system, with the very notable exception of the comets, Galileo’s book was out dated when it was written and had very little impact on the subsequent astronomical/cosmological debate in the seventeenth century. I will indulge in a little bit of hypothetical historical speculation here. If Galileo had actually written a balanced and neutral account of the positive and negative points of the Tychonic geo-heliocentric system with diurnal rotation and Kepler’s elliptical heliocentric system, it might have had the following consequences. Firstly, given his preeminent skills as a science communicator, his book would have been a valuable contribution to the ongoing debate and secondly he probably wouldn’t have been persecuted by the Catholic Church.

The Renaissance Mathematicus considers the importance that Galileo's famously controversial work, actually played in the development of modern astronomy:Like Giordano Bruno and the Library of Alexandria, Galileo's status as a secular saint and martyr rests almost entirely upon a false characterization of the historical events due to his utility in attacking the Roman Catholic Church and Christianity.