Galileo's own health was not good. Fever and rheumaticky bouts kept him in bed for weeks on end. His work was set back a good deal. The worst thing was that his illness prevented him from observing the most interesting astronomical phenomenon of the period.
In fact it was a triple phenomenon, three comets of varying brightness that appeared in quick succession in the autumn of 1618.
These comets were the first to be visible in Europe after the invention of the telescope, and therefore of enormous astronomical interest. That they were also harbingers of disaster, as comets usually were, there could be little doubt: earlier that year rebellious Protestants in Prague had thrown three of the Emperor's henchmen out of the windows of the Hradcany Palace and had thus precipitated the pan-European catastrophe known subsequently as the Thirty Years War.
At all events these comets presaged a heated debate amongst astronomers and natural philosophers. There were two main trends ofthought: those who still sided with Aristotle, who had of necessity to maintain that the comets were nearer than the Moon, they had to be part of the earthly and mutable. Aristotle assumed that comets were composed of vapours from the Earth which ignited when they ascended high enough, and were then driven by the Moon's sphere to move.
But Tycho Brahe had observed a comet in 1577 and, incomparably accurate observer that he was, had managed with the naked eye to take sufficiently good measurements to calculate the comet's parallax. From these measurements he concluded that comets had to be much further away than the Moon, probably somewhere near the orbit of Venus, and that they revolved around the Sun - not in circular, but in oval orbits.
But Galileo had not witnessed the phenomena himself. As a result he was at first little inclined to wade into the debate. But in 1619 a pamphlet entitled An Astronomical Discussion of the Three Comets of 1618 appeared. The work was officially anonymous, but it was well known that the author was the man who now held the chair at the Collegio Romano that had once belonged to Clavius and Grienberger, the Jesuit priest, Orazio Grassi.
Father Grassi was a gifted, melancholic man. In an intelligent and well-meaning attempt to understand the comets he accepted a good deal of Tycho Brahe's argumentation. Grassi had access to Jesuit observations from all over Europe and could calculate the parallax. He admitted that the comets were further away than the Moon, and in so doing distanced himself from Aristotle. It was a step the Jesuits had already taken, as they had supported Galileo's telescopic observations. On the other hand, he could not follow Brahe in his assertion that comets moved round the Sun. As a material description of a heavenly phenomenon it was dangerously close to the prohibited Coper-nican ideas.
But in the discussion that raged in Rome's scientific and ecclesiastical circles, Grassi's assumptions were portrayed as a weighty argument against Copernicus. This irritated Galileo, but perhaps not so much as something else: Grassi's slender pamphlet does not mention Galileo once. In truth, the Grand Duke's mathematician had not distinguished himself in comet research. He had never personally observed a single comet, but it appears that Galileo continued to regard the astronomical use of the telescope as his own private and sacrosanct territory. Furthermore, he felt his position as Europe's most fashionable astronomer under threat. He was getting enquiries about the comets from several quarters including the French court. And he had nothing to say about them - whilst Grassi was bringing out new observations and theories.
He therefore made up his mind to reply. It is true that the pamphlet Discourse on the Comets was published in the name of one of his students, Mario Guiducci, but the surviving manuscript is almost entirely in Galileo's handwriting, and no one was in doubt as to its true author.
The pamphlet had a double objective. It was to repudiate both Aristotle's and Tycho Brahe's comet theories. In so doing it would indirectly show that a new, third theory was required, one that, for very good reasons could not be postulated, because it had to be founded on Copernican thinking. But Galileowas toothoroughinhis repudiation. In ordertobestthe Jesuit Grassi, he totally rejected Brahe's completely correct observations, and settled on the theory that comets actually were vapours from the Earth and they were closer than the Moon.
There was a "Copernican" basis for this faulty conclusion. If the comets and the Earth really did orbit the Sun, the comets should display retrograde movement in certain phases, in other words, move "backwards" through the sky during the periods when the Earth was "catching up" with them, just as the planets do. The fact that such movement cannot be observed, was used as an anti-Copernican argument. (The real reason is that comets are only visible from the Earth for a short period as they move towards the Sun.)
But the main motivation for Galileo's contention was probably psychological rather than astronomical. Grassi had promulgated an argument based on telescopic observations, namely that comets were not much enlarged by the instrument and therefore had to be correspondingly further away, an argument he believed not everyone had understood and accepted.
With his highly developed sense of pride Galileo managed to construe this as an attack upon himself - on the maestro of the telescope, the first, most skilful and practised telescope-user of them all! He felt he just had to counter such an argument. The result was that Discourse on the Comets, which was published in July 1619, not only built on a completely false concept of what comets were; but worse, the pamphlet was a stinging personal attack on Grassi and the entire Jesuit milieu around the Collegio Romano.
Honour was equally highly developed in that quarter. Orazio Grassi hit back the same autumn. Using the easily detectable pseudonym of Lotario Sarsi, he published The Astronomical and Philosophical Balance, in which proofs and assertions about comets were to be carefully weighed. The book is a strange mixture of stringent and up-dated thinking in the fields of astronomy and optics, mixed with uncritical quotations from the ancients -and some well-chosen, sarcastic attacks on Galileo:
"I fancy I hear a small voice [in Galileo's text] whispering discreetly in my ear: the motion of the Earth. Get thee behind me thou evil word, offensive to truth and to pious ears! (...) But then certainly Galileo had no such idea, for I have never known him otherwise than pious and religious."49
At first Galileo did not realise how deeply he had offended Grassi, and refused to believe it was he who had written The Balance. But soon he had to recognise that it was indeed the case.
The truth was that in the course of the discussions about sunspots and comets, Galileo had managed thoroughly to irritate his most important scientific allies - the Jesuits in Rome.
But Galileo had been even more incautious. Discourse on the Comets had aimed some hefty swipes at Christopher Scheiner too, the German Jesuit astronomer that Galileo had fallen out with over the discovery of sunspots. Father Scheiner now lost all hope of any objective exchange of views with Galileo, which he had tried to encourage by sending him his Mathematical Discourses and a courteous accompanying letter some years before.
Grassi had to be answered. It was imperative for Galileo's honour and position, something to which his friends at the Accademia dei Lincei especially drew his attention. The affair also involved the academy's honour. A number of its members were highly critical of the Jesuits and were keen to see Grassi thoroughly taken down.
Nor was Galileo bereft of support in Rome - Maffeo Barberini continued to keep in contact. The Cardinal had great notions of his own worth as a poet, and he unleashed his talents in a private poetic eulogy to the mathematician -Adulatio perniciosa, in which he praised Galileo's discoveries of planets and sunspots. In the covering letter he wrote: "The respect I have always entertained for your person and for the virtues within you, have informed this composition which I enclose. I greet you with all my heart, in the hopes that Our Lord gives you contentment."
And, as he always did, Cardinal Barberini signed it - come fratello. The shaping of his answer to Grassi took Galileo some time, however. Partly on account of caution, and partly illness. Unfortunately the pilgrimage to Loreto and the Santa casa had brought no relief.
In the meantime two more highly important people had died, Cardinal Bellarmine and the Pope himself, Paul V. The latter's successor was a typical compromise candidate, Gregory XV Ludovisi, an old cardinal from Bologna, who was remarkable for little except his incipient senility.
This papal election took place in February 1621, and it was clear that the new Pope would not increase the chances of freer expression on cos-
mological models, even though Bellarmine was gone. But almost at once a third important person died: Grand Duke Cosimo II. His son and heir, Ferdinando II, was only ten years old, so it was clear that his mother the Grand Duchess and his grandmother the Dowager Grand Duchess Christina would have an even tighter grip on the reins than before. What this would mean for the support Galileo might expect, was very unclear.
Galileo's response to Grassi was not ready until well into the autumn of 1622. By then it had, however, turned into an entire book which he sent to his Lyncean friends in Rome so that they could comment on the manuscript, get the censor's permission to print it (imprimatur), and carry out the printing process. All this took time, and Prince Cesi only had an edited manuscript ready for the press by the summer of 1623. Just then there was another in the long series of deaths that affected Galileo's destiny during these years. Pope Gregory XV passed away after two years on St Peter's throne.
The old Pope had managed to accomplish a couple of things. He had appointed Richelieu, the widowed French Queen Maria de' Medici's young counsellor, as a cardinal. And he had altered the rules governing the elections of future popes.
The new procedure meant that the election of Gregory's successor took a long time, for there was bitter conflict within the College of Cardinals. In the intense heat of the Roman summer the cardinals sat shut away in the Sistine Chapel for almost a month before they reached some kind of agreement. But on 6 August the white smoke drifted up from the chapel, and the College's spokesman emerged and cried out the longed-for words across Rome: Habemus papam - "We have a pope".
When Prince Cesi heard the news of who had finally won the papal nomination, he immediately halted the printing of Galileo's book. A new dedication was absolutely imperative. The book must be seen as a tribute to the new man in the Vatican, who had now taken the name Urban VIII.
This was Galileo's great opportunity. The new, absolute ruler of the Catholic Church's spiritual realm and the Papal States' temporal lands was the author of the eulogy Adulatio perniciosa, his admirer, countryman and friend "like a brother", the Florentine Maffeo Barberini.
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