Galileo's Daughter (13 page)

The three comets of 1618

In September, just when Galileo’s student assistants had finished this preliminary work, another bout of illness prevented him from building on it as planned. The delay might have been merely temporary, except that while Galileo languished, the heavens sent him a new mystery to ponder, and this apparition initiated a cascade of events that postponed the publication of his motion studies for another two decades.

A small comet glowed in the skies over Florence that September of 1618. Though unspectacular, as comets go, it was nevertheless the first comet to appear since the birth of the telescope. Other astronomers took to their rooftops with instruments of Galileo’s design, but Galileo himself remained indoors an invalid. Then another comet arrived in mid-November, while Galileo unfortunately fared no better than before. And even by the end of November, when a truly brilliant third comet burst on the scene to garner the attention of observers all over Europe, Galileo still could not stand among them.

“During the entire time the comet was visible,” he reported later, “I was confined by illness to my bed. There I was often visited by friends. Discussions of the comets frequently occurred, during which I had occasion to voice some thoughts of mine which cast doubt upon the doctrines that have been previously held on this matter.” In fact, Galileo saw only one important comet his whole life—the big bright one of 1577, in his youth—and never did figure out what these objects really were.

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 sphere’s 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 vapors—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 skeptical 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.

Tycho Brahe’s system of the world

Just at this juncture, Galileo’s student Mario Guiducci got elected consul of the prestigious Florentine Academy, which honor 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 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 analyze 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 specter 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
Der 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 favor 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 toward 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 favored, and the prevailing dependence on received wisdom or majority opinion. “I cannot but be astonished,” Galileo wrote,

that Sarsi should persist in trying to prove by means of witnesses something that I may see for myself at any time by means of experiment. Witnesses are examined in doubtful matters which are past and transient, not in those which are actual and present. A judge must seek by means of witnesses to determine whether Pietro injured Giovanni last night, but not whether Giovanni was injured, since the judge can see that for himself. But even in conclusions which can be known only by reasoning, I say that the testimony of many has little more value than that of few, since the number of people who reason well in complicated matters is much smaller than that of those who reason badly. If reasoning were like hauling I should agree that several reasoners would be worth more than one, just as several horses can haul more sacks of grain than one can. But reasoning is like racing and not like hauling, and a single Barbary steed can outrun a hundred dray horses.

It took Galileo two years to complete
The Assayer,
beset as he was throughout by many family and official matters. Marina Gamba died in February 1619, leaving Galileo’s children officially motherless. Having helped both his daughters take the veil, Galileo now atoned for the messy circumstances of his son’s birth by getting Grand Duke Cosimo II to legitimize Vincenzio on June 25, two months before the boy’s thirteenth birthday. Cosimo handled this matter-of-factly enough, knowing his own Medici forebears to have fathered at least eight illustrious illegitimate sons, two of whom had become cardinals—and one of those had traded the cardinal’s biretta for the pope’s tiara as His Holiness Clement VII.

Meanwhile Galileo’s mother, Madonna Giulia, grew older and ever crankier at the house in Florence where she had stayed when her son moved to Bellosguardo. “I hear with no great surprise that our mother is being so dreadful,” Galileo’s brother, Michelangelo, commiserated in October 1619 from the safe distance of Munich. “But she is much aged, and soon there will be an end to all this quarreling.”

Madonna Giulia died in September 1620, at eighty-two. Her death was soon followed by the publicly mourned passing of the grand duke, only thirty years of age, in February 1621. Cosimo II, who had come to power at nineteen, now bequeathed the Grand Duchy of Tuscany to the eldest of his eight children, ten-year-old Ferdinando II. The boy also inherited Cosimo’s chief mathematician and court philosopher, for Galileo’s appointment carried a lifetime tenure. Until Ferdinando reached majority, however, he deferred perforce in all matters to the judgment of his regents: his mother, Archduchess Maria Maddalena of Austria, and his grandmother, the dowager Grand Duchess Cristina of Lorraine.

The necrology of the year 1621 also listed two major figures behind the anti-Copernican edict: Roberto Cardinal Bellarmino, later to be canonized Saint Robert Bellarmine, and Pope Paul V, the founder of the Vatican Secret Archives, where certain papers pertaining to Galileo’s 1616 trip to Rome already resided among centuries’ worth of private papal documents. Paul V, who had promised to protect Galileo for the rest of his life, died of a stroke January 28. A little over a week later, on February 9, the Sacred College of Cardinals suddenly and unanimously acclaimed his successor, Alessandro Ludovisi of Bologna, as Pope Gregory XV. But the new pontiff’s frail health, of which the cardinals had been well aware at the time of his selection, would end his papacy in less than two years.