Galileo was born into a continent wracked by cultural ferment and religious divisions. The late 1500s saw the last years of the Italian Renaissance, a revival of arts and letters that sought the recovery and reworking of classical art and philosophy from ancient Greece and Rome. In the 15th and 16th centuries, Renaissance Italy was a center of artistic and intellectual ferment, a home for the great geniuses of the revived humanistic spirit–Machiavelli, Da Vinci, Petrarch, Michelangelo, and many more. But the popes were also enjoying the peak of their influence: not just the leaders of the Roman Catholic Church during these years, the popes served as secular leaders as well, controlling much of central Italy around their seat in Rome, and the decadent spirit of the age infected their Church. The Renaissance Popes, a colorful group who acted as great patrons of the flourishing artistic culture, presided over an era of corruption and worldliness within the Church, and their personal morality brought the reputation of the Papacy to historic lows.

By the time of Galileo's youth, the Church was purging itself of the excesses of the Renaissance–but facing a crisis in the north. Martin Luther, a former monk, attacked Catholicism for having become too worldly and politically corrupt, and for obscuring the fundamentals of Christianity with pagan elements ranging from the cult of the saints to the adoration of the Virgin Mary. His reforming zeal, which appealed to a notion of an original, "purified" Christianity, set in motion the Protestant Reformation, which split European Christianity in two. In response, Roman Catholicism steeled itself for battle. The Catholic Counter-Reformation, called into being by the Council of Trent, emphasized orthodoxy and fidelity to the true Church. A new religious order, called the Jesuits, or Society of Jesus, arose to act upon these principles, and now stood at the vanguard of the battle with Protestant heresy. The Counter-Reformation reinvigorated the Church, and incited a great wave of intellectual, artistic, and religious energy. But it also put an end to the liberality and leniency of the Renaissance; its emphasis on religious orthodoxy, rigidly enforced by the Inquisition, would soon clash with the emerging scientific revolution.

Galileo, with his study of astronomy, would figure at the center of this clash. Conservative astronomers, working without telescopes, had always ascribed to the theory of geocentricity, which held that the earth ("geo," as in "geography" or "geology") lay at the center of the solar system, and the sun–and the other planets–revolved around it. Indeed, to the casual observer, it seemed common sense that the sun "rose" in the morning and "set" at night, in its circling pattern around the earth. Ancient authorities like Aristotle and the Roman astronomer Ptolemy had championed this viewpoint, and the notion also coincided with the Catholic Church's view of the universe, which placed mankind, God's principle creation, at the center of the cosmos. Thus buttressed by common sense, the ancient philosophers, and the Church, geocentricity seemed secure in its authority.

Ptolemy had worked out a geocentric system of brilliant geometric precision–but this system of interlocking orbits grew ever more complex as astronomers strained to make more modern observations fit a mistaken theory, and in the 16th century, the theory began to fall under attack. The first to question it was Nicholas Copernicus, a Polish astronomer, whose work On the Revolution of Heavenly Orbs (published after his death, in 1543) proposed a heliocentric system, in which the planets–including earth–orbited the sun ("helios"). This more mathematically satisfying way of arranging the solar system did not attract many supporters at first, since the available data did not yet support a wholesale abandonment of Ptolemy's system. But by the end of the 16th century astronomers like Johannes Kepler (1571-1630) had begun to embrace it, and once Galileo began to observe the heavens through his telescope, the fate of the Ptolemaic system was sealed.

But so too was the fate of Galileo–for the Catholic Church, desperately trying to hold the Protestant heresy at bay, could not accept a scientific assault on its own theories of the universe. The pressures of the age set in motion the historic confrontation between religion and science, which would culminate in the Church's disastrous trial of Galileo in 1633.

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