Rome celebrated the coming of the new century with a great jubilee: a million pilgrims converged on the city, and the ailing pope, Clement VIII, heard the confessions of heretics and blessed the progress of the Counter-Reformation from the balcony of his palace. Meanwhile, Galileo was facing new family problems; since his father's death early in the previous decade, he had become the head of the family, and its principle breadwinner. In this capacity, he supported his brother Michelangelo's fumbling attempts to become a professional musician, which eventually landed him a post in the Polish court, but not before bleeding considerable sums of money from Galileo's purse. In 1604, the Polish appointment came to an end, and again Galileo was forced to support his brother until another job came along, this time in the German principality of Bavaria. Meanwhile, Galileo's sister Livia, formerly a nun, had left her religious order in the hopes of attracting a husband, and when she found one, it was Galileo who had to pay her dowry. Moreover, in the first decade of the 17th century Galileo's mistress Marina gave birth to three children; Galileo was carrying a heavy financial burden on his professor's salary.

Meanwhile, in 1604, a new star had appeared in the sky, posing a remarkable challenge to the Ptolemaic system, which followed Aristotle in holding that the heavens were fixed and eternal. The star became known as Kepler's Nova, after the iconoclastic German astronomer, who became famously associated with the star by arguing that it shone from a high and distant location, rather than from a place close to the moon, as more conservative scholars argued. In a famous series of lectures in 1604 and 1605, Galileo took Kepler's side in the debate, ruffling the feathers of his traditionalist rivals at the university. But more mundane concerns soon tugged at him: during his annual summer visit to Florence, his mounting debts caught up with him, and his creditors threatened to take him to court. The financial pressure abated only when Galileo agreed to tutor Prince Cosimo, son of Tuscany's Grand Duchess, and grandson of the first Cosimo de Medici, who had ruled Tuscany at Galileo's birth. With this new pupil, and the consequent support of the Grand Duchess, Galileo was able to obtain a necessary salary increase, and rescue his sinking financial fortunes.

But by the end of the decade, Galileo was growing dissatisfied with Padua. He felt under-appreciated and overworked: he earned only half the salary of the typical Aristotelian professor, and he taught a growing collection of pupils. These pupils hailed from noble European families, and Galileo had to meet their needs not only out of his obligations as a teacher, but out of political obligations; to refuse to attend to these students' needs would be to lose the backing of powerful families. But his teaching duties took time away from his private studies, and crucial questions were goading Galileo.

He began to yearn for a new appointment, ideally in his native Florence, and he continued to curry favor with the royal family of Tuscany in the hopes of obtaining their patronage. But then, in the summer of 1609, news arrived in Italy of the invention of the telescope, by a Dutch spectacle-maker named Hans Lippershey. Intrigued, Galileo began assembling his own device, at first thinking only of its value in a military campaign and the profits to be made from selling it to the Venetian state. But hearing that the Dutchman himself was coming to Venice, he threw himself into experiments with lenses, and by late August he had produced his own telescope, which was presented as a gift to the Venetian State. The grateful Senate voted to double his salary. In the autumn months that followed, Galileo continued to tinker with his lenses, and eventually increased the magnification of the telescope four hundred times. Then, fatefully, he turned the device toward the sky and began making observations. The world of science would never be the same.

Galileo's telescope opened a window on the heavens that previous astronomers could have only wished for: suddenly the moon, the backdrop of stars, and the planets were revealed as having properties never before imagined. The moon, a regular and polished disc to the naked eye, was revealed to have a mountainous surface, covered with craters, basins, and peaks, much like the earth itself. This notion came as a shock, both to conventional science and to the Church; how could supposedly perfect heavenly bodies resemble the earth? Meanwhile, thousands of new stars suddenly sprang into visibility, filling out constellations and rendering the galaxy a glittering carpet of lights. And then, in January 1610, Galileo's telescope revealed four of the nine moons of Jupiter; these moons "move... around another very great star [Venus]," he wrote, in the same way as Mercury and Venus, and peradventure the other known planets, move around the sun."

Here, at last, was concrete evidence for Copernicus's system, which had long been confined to the realm of mathematical models. If Jupiter had attendant moons, just as Earth did, then it was reasonable to assume that Jupiter and Earth were similar bodies–planets orbited by satellites, and orbiting, themselves, around a larger sun.

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