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Galileo was the first to discover the moons of Jupiter. (Michael Benson / Kinetikon Pictures / Corbis (Jupiter) / Scala / Art Resource, NY (Galileo))

Galileo's Vision

Four hundred years ago, the Italian scientist looked into space and changed our view of the universe

Like many figures whose names have endured, Galileo wasn't shy about seeking fame. His genius for astronomy was matched by a genius for self-promotion, and soon, by virtue of several canny decisions, Galileo's own star was rising.

In Tuscany, the name Medici had been synonymous with power for centuries. The Medici family acquired and wielded it through various means—public office, predatory banking and alliances with the powerful Catholic Church. Conquest of territory was a method favored in the late 16th century, when the head of the family, Cosimo I, seized many regions neighboring Florence. The family took a keen interest in science and its potential military applications.

The Medicis may have needed scientists, but scientists—and especially Galileo—needed the Medicis even more. With a mistress, three children and an extended family to support, and knowing that his questioning of Aristotelian science was controversial, Galileo shrewdly decided to court the family's favor. In 1606, he dedicated a book about a geometric and military compass to his student Cosimo II, the family's 16-year-old heir apparent.

Then, in 1610, on the occasion of his publication of The Starry Messenger, which detailed his telescopic findings, Galileo dedicated to Cosimo II something far greater than a book: the very moons of Jupiter. "Behold, therefore, four stars reserved for your illustrious name," wrote Galileo. "...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." (Galileo chose the name "Cosmian stars," but Cosimo's office requested "Medicean stars" instead, and the alteration was duly made.) "The Starry Messenger was a job application," says Owen Gingerich, an astronomer and science historian at the Harvard-Smithsonian Center for Astrophysics—and, sure enough, Galileo got just what he had been seeking: the Medicis' patronage.

He could hardly have hoped for better patrons, as the Franklin exhibit made clear. It included scores of intricately wrought instruments from the family's collection. The fanciful names of the ingenious contraptions hint at their function and describe their forms: nautical planispheres, gimbaled compasses, horary quadrants, armillary spheres. One of the oldest surviving astrolabes, an instrument for calculating the position of the Sun and stars, was on exhibit, as was a set of brass and steel compasses believed to have belonged to Michelangelo, another Medici beneficiary. (Galileo's telescope and the rest of the collection have since returned to Florence.)

Though capable of measuring the world in various ways and to various ends—determining the caliber of projectiles, surveying land, aiding navigation—some of the instruments were never used, having been collected for the very purpose to which museums put them today: display. A few, such as a compass that collapses into the shape of a dagger, demonstrate the era's alliance of science and power. But they also illustrate its blending of science and art—the gleaming artifacts rival works of sculpture. They tell, too, of a growing awareness that, as Galileo said, nature was a grand book ("questo grandissimo libro") written in the language of mathematics.

Not everyone took pleasure in—or even believed—what Galileo claimed to have seen in the sky.

Some of his contemporaries refused to even look through the telescope at all, so certain were they of Aristotle's wisdom. "These satellites of Jupiter are invisible to the naked eye and therefore can exercise no influence on the Earth, and therefore would be useless, and therefore do not exist," proclaimed nobleman Francesco Sizzi. Besides, said Sizzi, the appearance of new planets was impossible—since seven was a sacred number: "There are seven windows given to animals in the domicile of the head: two nostrils, two eyes, two ears, and a mouth....From this and many other similarities in Nature, which it were tedious to enumerate, we gather that the number of planets must necessarily be seven."

Some who did deign to use the telescope still disbelieved their own eyes. A Bohemian scholar named Martin Horky wrote that "below, it works wonderfully; in the sky it deceives one." Others nominally honored the evidence of the telescope but scrambled to make it conform to their preconceptions. A Jesuit scholar and correspondent of Galileo named Father Clavius attempted to rescue the idea that the Moon was a sphere by postulating a perfectly smooth and invisible surface stretching above its scarred hills and valleys.

The Starry Messenger was a success, however: the first 500 copies sold out within months. There was a great demand for Galileo's telescopes, and he was named the head mathematician at the University of Pisa.

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