Despite enjoying the honors bestowed upon him by the Venetian Senate, Galileo continued to negotiate for a new appointment in Florence, where his former pupil, Prince Cosimo de Medici, had become Grand Duke Cosimo II. When, in March 1610, he published his discovery of the lunar surface and the moons of Jupiter in a Latin treatise entitled Sidereus Nuncius, or "The Starry Messenger," he went so far as to dedicate the work to Cosimo, and even named the newly discovered moons the "Medicean Stars," after the Medici family. Galileo was soon rewarded for his efforts at wooing the powerful family: in June of 1610, he gained appointment as "First Mathematician of the University of Pisa, and First Mathematician and Philosopher to the Grand Duke," as well as a sizable annual salary, and exemption from the obligation to teach classes. He abandoned Venice and Padua for Florence and Pisa without a backward glance–ending his decade-long relationship with Marina Gambi, the mother of his children for the sake of his ambition–and while there was great rejoicing in Tuscany, the Venetians cursed his duplicity and arrogance.

Galileo was arrogant, certainly, but by the end of 1610 he seemed to have reason to be. Sidereus Nuncius had gained fame as the wonder of Europe, as philosophers and scientists marveled at the new vistas opened by Galileo's telescope, and kings and princes clamored to have the Italian astronomer name his ever-increasing discoveries after them. And the discoveries kept coming. In July 1610, the scientist had glimpsed the rings of Saturn–although he mistakenly believed them to prove Saturn's existence as in fact three stars in a row, rather than a planet. Then, in December of the same year, he found that Venus, like the moon, went through phases; this provided key evidence for the Copernican system, since it suggested the Venus orbited the sun just as the moon orbited the earth. In April of 1611, Galileo announced the existence of sunspots, confirming the observations of a German astronomer, Johannes Fabricius, and then, by charting them over a period of months, he concluded that the sun actually rotated.

As Europe's intellectuals gaped, the Catholic Church squirmed. They were long accustomed to the comforting ideas of Aristotle and Ptolemy, which assumed a crystalline, unchanging universe–a creation befitting an all-powerful God. The flux and change that Galileo now revealed bespoke a more chaotic system, a less-than-godly lack of organization. But for a time it seemed that disagreement would not necessarily lead to condemnation. Galileo made a triumphant journey to Rome in 1611, where he stayed with the Jesuit astronomers, who confirmed his observations in almost every particular. He received the blessing of Pope Paul VI, and more importantly, one of the Pope's advisers actually agreed to look through the telescope. This was Cardinal Robert Bellarmine: a great intellectual, Bellarmine had friends in common with Galileo, and had studied astronomy in his youth.

However, the cardinal had also crusaded ardently against heresy, and sixteen years earlier had condemned the celebrated Giordano Bruno to death for, among other heresies, preaching a form of Copernican philosophy. Thus the support of this man encouraged Galileo in his hope that the Church might come to recognize not only his discoveries, but the world-view they hinted at–the heliocentric (sun-centered) philosophy of the Copernican system.

For by this point, Galileo considered heliocentricity as providing the only model that fit all his facts. Copernicus's model, with Kepler's adjustments (which included changing the orbits of the planets from circles to ellipses) not only made mathematical sense, but it explained all of his new data. The phases of Venus occurred because the sun lay between the earth and Venus during part of the year; the surface of the moon suggested that Earth was just one among many similar planets; the moons of Jupiter implied that many other planets had satellites orbiting them even as they themselves orbited the sun. The evidence convinced Galileo, and in the two years after his 1611 journey to Rome, while he occupied himself with continued observation of sunspots and non- astronomical work on the properties of floating bodies, he began to speak of the Copernican system as fact, not theory.

This was his critical–and dangerous–move. The Church, and especially Bellarmine, could accept the Copernican system as a hypothesis, but not as truth. As "fact," it would challenge scripture, which referred to the sun "rising" and the earth as "unmoving"–and besides, from their point of view, there was insufficient evidence to consider the theory proven. Still, the debate remained muted until 1615, and Galileo was busy with other matters, as he suffered from a variety of intestinal ailments, squabbled with rivals in Germany over who had first discovered sunspots (he claimed he had preceded Fabricius), and published his "Treatise on Floating Bodies," which he sent to Bellarmine in Rome. The cardinal approved of the treatise's assertions; indeed, the work, which dipped into experimental physics in its discussion of how ice behaved in water, received favorable reviews almost everywhere. Galileo had reached a peak of fame as one of Europe's truly great minds.

Popular pages: Galileo Galilei