by Dava Sobel
Fortunately, Galileo’s endeavors led him to a new source of supplemental income. In the course of teaching military architecture and fortification to private students, he had invented his first commercial scientific instrument in 1597, called the geometric and military compass. It looked like a pair of metal rulers joined by a pivot, covered all over by numbers and scales, with screws and an attachable arch to hold the compass arms open at almost any angle. By 1599, after various modifications, the device functioned as an early pocket calculator that could compute compound interest or monetary exchange rates, extract square roots for arranging armies on the battlefield, and determine the proper charge for any size cannon. Shipwrights at the nearby Venetian Arsenale also adopted Galileo’s revolutionary compass, to help them execute and test new hull designs in scale models before building them full-size.
Galileo crafted the first few compasses himself but soon required the services of a full-time, live-in instrument maker to meet the popular demand. The hired craftsman moved into Galileo’s house with wife and children in tow, to work in exchange for salary, room and board for his whole family, all production materials, and a two-thirds share of the price of finished brass instruments, which sold for five scudi each. Galileo would not have made much money under these conditions, except that he charged every visiting student nearly twenty scudi to learn how to use the compass, and all of this was his to keep. At first he gave out a personally handwritten instruction manual as a learning aid; then in 1603 he hired an amanuensis to help generate enough copies— until three years later, when he hit on the idea of publishing the booklet for sale with the instrument.
He called his treatise Operations of the Geometric and Military Compass of Galileo Galilei, Florentine Patrician and Teacher of Mathematics in the University of Padua. Its 1606 title page notes that the book was printed “in the Author’s House” and cannily dedicated to the future grand duke of Tuscany, Don Cosimo de’ Medici.
Galileo’s geometric and military compass
“If, Most Serene Prince,” Galileo addressed his young patron in the dedication, “I wished to set forth in this place all the praises due to your Highness’s own merits and those of your distinguished family, I should be committed to such a lengthy discourse that this preface would far outrun the rest of the text, whence I shall refrain from even attempting that task, uncertain that I could finish half of it, let alone all.”
Cosimo, a lad of sixteen, had become Galileo’s most elite private pupil the previous summer. The heir apparent to the House of Medici, he bore the name of his resolute grandfather, Cosimo I, who had expelled all rival and foreign influences from Florence, annexed the city of Siena to the Duchy of Tuscany, and then pressured Pope Pius V to create for him the title of grand duke in 1569. Thus the self-made Medici family, who had been successful bankers holding high government positions in the old Republic of Florence throughout the fourteenth and fifteenth centuries, assumed the aura and authority of royalty in Galileo’s time.
Medici coat or arms
Galileo, who typically returned to Florence when the University at Padua closed between terms, procured recommendations as a mathematical mentor to the royal household. As young Prince Cosimo’s tutor, Galileo gained status with the boy’s powerful parents: the much beloved Grand Duke Ferdinando I (who had started his career as a cardinal in Rome before being called home to the throne at the sudden death of his lecherous, murderous older brother, Francesco) and his devoutly religious French wife, Grand Duchess Cristina of Lorraine. By dedicating the tract on the geometrical compass to Cosimo, Galileo hoped to pave his way to an appointment as court mathematician— a prestigious position that would not only lighten his financial burden but also bring him home to his beloved Tuscany.
“I have waited until now to write,” Galileo said with all requisite deference in his first letter to Cosimo in 1605, “being held back by a respectful concern of not wanting to present myself as presumptuous or arrogant. In fact, I made sure to send you the necessary signs of reverence through my closest friends and patrons, because I did not think it appropriate—leaving the darkness of the night—to appear in front of you at once and stare in the eyes of the most serene light of the rising sun without having reassured and fortified myself with their secondary and reflected rays.”
No formal contract bound the prince and the scientist at that point. If and when Galileo’s tutorial services were required, he was summoned, as in the following invitation written by the chief steward of the grand duke and duchess, dated August 15, 1605, and sent from Pratolino, one of the seventeen Medici palaces, a few miles north of Florence: “Her Most Serene Highness wishes that you should come here not only that the Prince may receive competent instruction but that your health may be restored. She hopes that the excellent air on the mountain of Pratolino will do you good. A pleasant room, good food, a comfortable bed, and a hearty welcome await you. Messer Leonido will see that you are provided with a good litter whether you wish to arrive this evening or tomorrow.”
The grand duchess again sent her horse-drawn conveyance to fetch Galileo for the wedding of Prince Cosimo, in 1608, to Maria Maddalena, the archduchess of Austria and sister of Emperor Ferdinand II. The nuptials spread along both banks of the Arno, where spectators on grandstands watched a reenactment of Jason’s capture of the Golden Fleece, sumptuously staged on a specially constructed island in midriver, with special effects including giant sea monsters that spit real fire.
In January 1609, when Grand Duke Ferdinando lay ill, Madama Cristina implored Galileo to review her husband’s horoscope. Galileo’s early career experience teaching astronomy to medical students had familiarized him with astrology, since doctors needed to cast horoscopes, to see what the stars foretold of patients’ lives, as an aid to diagnosis and treatment, as well as to ascertain reasons for particular illnesses and determine the most propitious times for mixing medications. Galileo had prepared many horoscopes, including one for his daughter Virginia at her birth in 1600, probably for the novelty of playing with astronomical positions, as he never expressed any faith in astrological predictions. In fact he remarked how the prophecies of astrologers could most clearly be seen after their fulfillment.*
Nevertheless, Galileo courteously replied to the grand duchess’s request by return post. Despite his forecast of many more happy years for Ferdinando, the grand duke died of his illness just three weeks later. And so it happened that Galileo’s summertime student, not quite nineteen years old, was suddenly enthroned as His Serene Highness Grand Duke Cosimo II, sovereign of all Tuscany.
Cosimo’s accession gave Galileo the perfect opportunity to petition for the coveted court post, as he had created it in his dreams. “Regarding the everyday duties,” Galileo wrote in his application to Florence, “I shun only that type of prostitution consisting of having to expose my labor to the arbitrary prices set by every customer. Instead, I will never look down on serving a prince or a great lord or those who may depend on him, but, to the contrary, I will always desire such a position.”
Portrait of Galileo at age forty-two, by Domenico Robusti
But he did not obtain the position just then. He continued his teaching at Padua and his research, which focused on establishing the mathematical principles of simple machines such as the lever, and determining how bodies accelerate during free fall—one of the most important unresolved questions of seventeenth-century science. “To be ignorant of motion is to be ignorant of Nature,” Aristotle had said, and Galileo sought to end the general ignorance of Nature’s laws of motion. Later that year, however, in the summer of 1609, Galileo was distracted from his motion experiments by rumors of a new Dutch curiosity called a spyglass, or eyeglass, that could make faraway objects appear closer than they were. Though few Italians had seen one firsthand, spectacle makers in Paris were already selling them in quantity.
Galileo immediately grasped the military advantage of the new spyglass, although the instrument itself, fashioned from stock spectacle
lenses, was little more than a toy in its first incarnation. Seeking to improve the spyglass by augmenting its power, Galileo calculated the ideal shape and placement of glass, ground and polished the crucial lenses himself, and traveled to nearby Venice to show the doge, along with the entire Venetian senate, what his contrivance could do. The response, he reported, was “the infinite amazement of all.” Even the oldest senators eagerly scaled the highest bell towers of the city, repeatedly, for the unique pleasure of discerning ships on the horizon—through the spyglass—a good two to three hours before they became visible to the keenest-sighted young lookouts.
In exchange for the gift of his telescope (as a colleague in Rome later renamed the instrument), the Venetian senate renewed Galileo’s contract at the University of Padua for life, and raised his salary to one thousand florins per year—more than five times his starting pay.
Still Galileo continued to refine the optical design in subsequent attempts, and when autumn came with its early dark, he chanced to focus one of his telescopes on the face of the Moon. The jagged features that greeted him by surprise there spurred him to improve his skill at lens grinding to build even more powerful models— to revolutionize the study of astronomy by probing the actual structure of the heavens, and to disprove Aristotle’s long unquestioned depiction of all celestial bodies as immutable perfect spheres.
In November 1609 Galileo fabricated lenses with double the power of the glass that had dazzled the doge. Now equipped to magnify objects by a factor of twenty, he spent half of December drafting a series of detailed drawings of the Moon in several phases. “And it is like the face of the Earth itself,” Galileo concluded, “which is marked here and there with chains of mountains and depths of valleys.”
From the Moon he journeyed to the stars. Two kinds of stars filled the heavens of antiquity. The “fixed” stars outlined pictures on the night sky and wheeled around the Earth once a day. The “wandering” stars, or planets—Mercury, Venus, Mars, Jupiter, and Saturn—moved against the background of the fixed stars in a complex pattern. Galileo became the first to distinguish them further: “Planets show their globes perfectly round and definitely bounded, looking like little moons, spherical and flooded with light all over; fixed stars are never to be seen bounded by a circular periphery, but have rather the aspect of blazes whose rays vibrate about them, and they scintillate a very great deal.”*
Moon drawings by Galileo in 1609
He pursued this new nocturnal fascination through the winter, plagued by the cold and the difficulty of keeping the instrument steady against the trembling of his hands and the beating of his heart. He needed to wipe the lenses repeatedly with a cloth, xor else they become fogged by the breath, humid or foggy air, or by the vapor itself which evaporates from the eye, especially when it is warm.” Early in January, he fell on the most extraordinary discovery of all: “four planets never seen from the beginning of the world right up to our day,” in orbit around the planet Jupiter.
Beyond their enormous astronomical significance, the new Jovian planets held special meaning for a friend of the Florentine court. Cosimo I of glorious memory had created a classical mythology for the Medici family when he became duke in 1537—even before he catapulted to grand dukedom in 1569. Cosimo fashioned himself an earthly embodiment of the cosmos, as his name implied. By this coup, he convinced the Florentine citizenry that it was Medici destiny to usurp power from the other prominent families who had long governed in uneasy coalition. As the head of his de facto dynasty, Cosimo I identified with the planet Jupiter, named for the king of the Roman pantheon, and he filled the Palazzo della Signoria, where he lived and ruled, with frescoes stressing this Olympic theme.
Galileo had given Venice his telescope. Now he would offer Florence the moons of Jupiter.
He quickly set down his discoveries in a new book, titled Siderius Nuncius, or The Starry Messenger. As he had done with his earlier work on the geometric compass, he dedicated the text to young Cosimo II. On this occasion, however, Galileo took the time and gave himself enough space to properly extol his prince:
Your Highness . . . scarcely have the immortal graces of your soul begun to shine forth on Earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name, and not of the common sort and multitude of the less notable fixed stars, but of the illustrious order of wandering stars, which, indeed, make their journeys and orbits with a marvelous speed around the star of Jupiter, the most noble of them all, with mutually different motions, like children of the same family, while meanwhile all together, in mutual harmony, complete their great revolutions every twelve years about the center of the world. . . .
Indeed, it appears that the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others. For as these stars, like the offspring worthy of Jupiter, never depart from his side except for the smallest distance, so who does not know the clemency, the gentleness of spirit, the agreeableness of manners, the splendor of the royal blood, the majesty in actions, and the breadth of authority and rule over others, all of which qualities find a domicile and exaltation for themselves in Your Highness? Who, I say, does not know that all these emanate from the most benign star of Jupiter, after God the source of all good? It was Jupiter, I say, who at Your Highnesses birth, having already passed through the murky vapors of the horizon, and occupying the mid-heaven and illuminating the eastern angle from his royal house, looked down upon Your most fortunate birth from that sublime throne and poured out all his splendor and grandeur into the most pure air, so that with its first breath Your tender little body and Your soul, already decorated by God with noble ornaments, could drink in this universal power and authority.
In the continuing paean of the remaining paragraphs of this dedicatory note, Galileo took it upon himself to name the planets the Cosmian stars. But Cosimo, the eldest of eight siblings, preferred the name Medicean stars—one apiece for him and each of his three brothers. Galileo naturally bowed to this wish, though he was thus forced to paste small pieces of paper with the necessary correction over the already printed first pages in most of the 550 copies of The Starry Messenger.
The book created a furor. It sold out within a week of publication, so that Galileo secured only six of the thirty copies he had been promised by the printer, while news of its contents quickly spread worldwide.
Within hours after The Starry Messenger came off the press in Venice on March 12, 1610, the British ambassador there, Sir Henry Wotton, dispatched a copy home to King James I. “I send herewith unto His Majesty,” the ambassador wrote in his covering letter to the earl of Salisbury,
the strangest piece of news (as I may justly call it) that he hath ever yet received from any part of the world; which is the annexed book (come abroad this very day) of the Mathematical Professor at Padua, who by the help of an optical instrument (which both enlargeth and approximateth the object) invented first in Flanders, and bettered by himself, hath discovered four new planets rolling about the sphere of Jupiter, besides many other unknown fixed stars; likewise, the true cause of the Via Lactea [Milky Way], so long searched; and lastly, that the moon is not spherical, but endued with many prominences, and, which is of all the strangest, illuminated with the solar light by reflection from the body of the earth, as he seemeth to say. So as upon the whole subject he hath first overthrown all former astronomy—for we must have a new sphere to save the appearances—and next all astrology. For the virtue of these new planets must needs vary the judicial part, and why may there not yet be more? These things I have been bold thus to discourse unto your Lordship, whereof here all corners are full. And the author runneth a fortune to be either exceeding famous or exceeding ridiculous. By the next ship your Lordship shall receive from me one of the above instruments, as it is bettered by this man.
In P
rague, the highly respected Johannes Kepler, imperial astronomer to Rudolf II, read the emperor’s copy of the book and leaped to judgment—despite the lack of a good telescope that could confirm Galileo’s findings. “I may perhaps seem rash in accepting your claims so readily with no support of my own experience,” Kepler wrote to Galileo. “But why should I not believe a most learned mathematician, whose very style attests the soundness of his judgment?”
The copy of The Starry Messenger that had the greatest impact on Galileo’s life, however, was the one he sent to Cosimo, along with his own superior telescope. The prince expressed his thanks late in the spring of 1610 by appointing Galileo “Chief Mathematician of the University of Pisa and Philosopher and Mathematician to the Grand Duke.” Galileo had specifically stipulated the addition of “Philosopher” to his title, giving himself greater prestige, but he insisted on maintaining “Mathematician” as well, for he intended to prove the importance of mathematics in natural philosophy.
In negotiating his Tuscan future, Galileo requested the same salary he had recently been promised by the University of Padua— the figure of one thousand to be paid now in Florentine scudi instead of Venetian florins. Rather than plead for more money, he made the base pay stretch farther by seeking official release from responsibility for his brother’s share of their sisters’ dowries.
