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 rumours of a new Dutch curiosity called a spyglass, or eyeglass, that could make far-away objects appear closer than they were. Though few Italians had seen one first-hand, 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 travelled 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 revolutionise 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.’*
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, ‘or else they become fogged by the breath, humid or foggy air, or by the vapour 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 after 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, entitled Sidereus 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 properly to 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 marvellous 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 centre 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 splendour 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 Highness’s birth, having already passed through the murky vapours of the horizon, and occupying the midheaven 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 splendour 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 furore. 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 12 March 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
