Most of Galileo’s contemporaries feared comets as evil omens. (Indeed the three 1618 examples were presently seen, with hindsight, as heralds of the Thirty Years’ War, which broke out in Bohemia the same year.) Aristotelian philosophers figured comets for atmospheric disturbances. The fact that comets came and went, changing their fuzzy-glow appearance all the while, automatically relegated them to the sublunar sphere between the Earth and the Moon, where they were thought to be ignited by friction of the spheres’ turning against the upper reaches of the air.
It may seem incredible that Galileo resisted the temptation to go outdoors in the autumn of 1618 long enough to view any one of the three comets, especially since he felt well enough to enter into intellectual discussions with visitors. But in fact the November night air held terrible danger for him, a man well past fifty now, who had spent most of the current year battling one malady after another. Moreover, as Galileo no doubt knew from his friends’ accounts, he would not have seen much even if he had risked his own study of these objects. A comet, or ‘hairy star’, retained its blurred contours despite the aid of the most powerful telescope.* Unlike the fixed stars that resolved into points of light when the telescope stripped them of their rays, or planets that turned to tiny globes, a comet could not be brought into sharp focus. And Galileo held back because he believed – in agreement for once with his Aristotelian contemporaries, though not for the same reasons – that comets belonged to the Earth’s atmosphere.
Galileo thus rejected the findings of his Danish predecessor, Tycho Brahe, who had observed the great comet of 1577 and another in 1585. Tycho, probably the most able naked-eye stargazer who ever lived, followed that comet every night with his oversized measuring instruments to determine its position. It lay beyond the Moon, he discovered through position studies, perhaps as far as Venus, and that meant one of two things to his sixteenth-century way of thinking: either the comet had come crashing through Aristotle’s crystalline celestial spheres, or the celestial spheres did not exist. Tycho chose the latter scenario, emboldened by having been the first European, in 1572, to identify a nova, which convinced him that changes could occur in the ‘immutable’ heavens.
Galileo, when he witnessed the next nova in 1604, backed the deceased Tycho’s interpretation of the new star’s nature and significance. But he despised Tycho’s planetary system for its poor compromise between Ptolemy and Copernicus. And as for the comet Tycho had tracked so carefully, Galileo dismissed it as a will-o’-the-wisp. He took comets to be anomalous illuminations in the air – most likely reflections of sunlight bounced off high-altitude vapours – not heavenly bodies per se. You could no more gauge the distance to a comet, Galileo believed, than you could catch a rainbow or contain the aurora borealis.
None of the news, notes or queries on the 1618 comets that reached Galileo shook him from his sceptical stance. Nor was he impressed by the pamphlet sent him from Rome containing a comet lecture delivered at the Collegio Romano and published in early 1619. Its author, Jesuit astronomer Father Orazio Grassi, argued on the basis of his studies that the path of the late-November comet carried it between the Sun and the Moon. This was a remarkable conclusion for any Jesuit to reach, because the Collegio Romano did not dispute Aristotle lightly. Nevertheless, Galileo doubted Father Grassi’s distance estimates, just as he had questioned Tycho’s, on the grounds that comets had no substance. Father Grassi furthermore committed several mathematical mistakes in his calculations that led him to estimate the volume of the comet, body and ‘beard’ together, at billions of times the size of the Moon – a ridiculous exaggeration in Galileo’s view. Worse, in describing his telescope observations of the comet, Father Grassi exposed his ignorance of the instrument’s fundamental principles, inviting Galileo’s scorn.
Just at this juncture, Galileo’s student Mario Guiducci was elected consul of the prestigious Florentine Academy, which honour obliged him to present a pair of lectures in the spring of 1619. He chose comets as his topic. Galileo wrote much of the content for him, expressing his own bewilderment while negating the work of Tycho and Father Grassi: ‘Hence we must be content with what little we may conjecture here among shadows, until there shall be given to us the true constitution of the parts of the universe, inasmuch as that which Tycho promised us still remains imperfect.’
Father Grassi took umbrage at the published version of these talks, which appeared in June 1619 under the title Discourse on the Comets. Galileo – for everyone rightly assumed him to be its author – seemed to have singled out the Jesuits as targets of attack: first Father Scheiner (the ‘Apelles’ of the Sunspot Letters) and now Father Grassi – even though the Jesuit Collegio Romano had always upheld Galileo’s discoveries and treated him with great respect.
Father Grassi’s angry, offended published rebuttal followed swiftly in the book Libra Astronomica, or Astronomical and Philosophical Balance, which he wrote in Latin under the pen name Lothario Sarsi, a purported student of his. As its title promised, the Libra of 1619 hung Galileo’s ideas about comets on a steelyard balance scale and found them weightless.
Compelled to respond and silence the noisy barking of his opponents, Galileo began retorting right on the title page of his riposte. He called it II Saggiatore, or The Assayer – thus replacing the crude scale of the Libra with the more delicate balance assayers used to analyse the quantity of pure gold in gold ore. Father Grassi, retaliating again later in his turn, accidentally on purpose referred to this book as Assaggiatore, or Winetaster – to imply that Galileo, a notorious lover of good wine, had been drinking when he wrote The Assayer.
In 1620, as the tenor of the comet debate turned nastier, the Holy Congregation of the Index raised the spectre of the Edict of 1616 by announcing at last the necessary corrections that must be made to Copernicus’s text, De revolutionibus, in order to have it removed from the Index of Prohibited Books. The congregation insisted on watering down some dozen statements by Copernicus affirming the Earth’s motion, in order to make them sound more like hypothetical suggestions. Galileo dutifully penned the required changes into his own copy of De revolutionibus, though he took care to cross out the offending passages with very light strokes.
Galileo ventured no mention of the Copernican theory in The Assayer. Such discussion would have been imprudent, given the edict, but also irrelevant: Copernicus had not discussed comets in his book, and Galileo’s view of comets as optical illusions automatically divorced them from the order of the Sun and planets as far as he was concerned. He even derided ‘Sarsi’ and ‘his teacher’ for granting comets the status of quasi-planets. ‘If their opinions and their voices have the power to call into existence the things they have considered and named,’ quipped Galileo, ‘why then I beg them to do me the favour of considering and naming “gold” a lot of old hardware that I have about my house.’
Indeed, Galileo persisted, the play of the Sun’s light could set the most mundane objects aglitter, to fool the unsuspecting: ‘Sarsi has but to spit upon the ground and undoubtedly he will see the appearance of a natural star when he looks at his spittle from the point towards which the Sun’s rays are reflected.’
Galileo took the occasion of The Assayer to mock the philosophical terms that masqueraded as scientific explanations in his day. He noted that sympathy, antipathy, occult properties, influences and their like were all too often ‘employed by some philosophers as a cloak for the correct reply, which would be: “I do not know.”’
‘That reply’, he reiterated, ‘is as much more tolerable than the others as candid honesty is more beautiful than deceitful duplicity.’
Avoiding the forbidden topic of the world system, The Assayer thus considered the current comet controversy in the larger context of the philosophy of science. Galileo drew an unforgettable distinction between the experimental method, which he favoured, and the prevailing dependence on received wisdom or majority opinion. ‘I cannot but be astonished that Sarsi should persist in trying to