So, after months of preparations, Galileo moved to Florence, leaving behind not only Marina and little Vincenzio, but also all his private students, and most of the servants and artisans as well, even Mazzoleni and his family. “I won’t be needing a workshop anymore,” Galileo explained brusquely. “I’m a philosopher now.” This sounded so ridiculous that he added, “The grand duke’s mechanicians will be available to me, if I need anything.”

No more compasses, in other words. No more Padua. He was saying good-bye to all of it, and didn’t want any part coming with him. “You can keep making the compasses here,” he told Mazzoleni, then turned his back and left the workshop. The compasses were what Mazzoleni had been hired to manufacture in the first place. They wouldn’t sell very well without the course Galileo gave in their use, but there were some instruction manuals left, and it was better than nothing. Besides, there was artisanal work all over Padua.

So the big house on Via Vignali was emptied, its people dispersed. One day in the fall it was handed back over to the landlord, and that whole little world was gone.

In Florence, Galileo had hastily rented a house that was a bit too near the Arno, but it had a little roof terrace for his night viewing—what the Venetians called an altana—and he figured he could find a more suitable establishment later. And a new acquaintance, a beautiful young Florentine nobleman named Filippo Salviati, assured him that during the year of his lease he could spend as much time as he liked at Salviati’s palazzo in town and at his villa, the Villa delle Selve, in the hills west of Florence. Galileo was pleased. He found the river vapors in Florence unpleasant, also the nearby presence of his mother. Since his father’s death, he had kept the old washtub in a house he rented in a poor part of the city, but he never visited her, and didn’t want to now. Better to spend his time out at Salviati’s, writing books and discussing philosophical matters with his new friend and his friend’s circle of acquaintances—men of high quality. When Cosimo wanted him, he could ride into the city quickly, and there would be no need to avoid his mother, or to fear running into her by accident.

Fra Paolo, who knew of this fear, had suggested that Galileo try to effect a reconciliation with her, but he didn’t know the half of it; indeed, he didn’t know the hundredth part of it. Galileo had recently gotten a letter from her welcoming him back to “his hometown,” and asking him to drop by and visit her, who was so lonely for him. Galileo snorted as he read this; along with everything else stuck in his memory in his pincushion of a brain, there was something new to add. In their departure from Via Vignali the cook had found a letter left behind by a servant she had fired, one Alessandro Piersanti, who had earlier worked in Florence for the old firedog. Giulia had written to him;

Since your master is so ungrateful to you and to everyone, and as he has so many lenses, you could very easily take three or four and put them at the bottom of a small box, and fill it up with Acquapendente’s pills, and then send it to me. Then, she went on, she would sell them and share the proceeds with him.

“Jesus Christ!” Galileo had shouted. “Thief on the cross!” He had thrown the letter down in disgust. Then he picked it up and saved it in his files, just in case it might be useful someday. It was dated January 9 of that year—which meant that the very week that Galileo was discovering the Medicean Stars and changing the skies forever, his own mother was conspiring to steal his spyglass lenses out of his house and sell them for her own profit. “Jesus son of Mary. Why not just steal the eyes out of my head?”

That was his mother for you. Giulia Galilei, suborner of servants, thief of the heart of his work. He would reside out at Salviati’s villa as much as he could.

Though exhausted by the move and the many sleepless nights that year, he still stayed out every clear night, looking at the stars and keeping track of Jupiter’s four moons. The Florentine nights were at first smokier than in Padua, but as the fall of his anno mirabilis moved toward winter, they turned cold enough to clarify the air. In December one of his former students, Benedetto Castelli, now a priest, wrote to suggest that if the Copernican explanation were indeed correct, then Venus was orbiting the sun also, in an orbit closer to the sun than Earth’s, so that one might therefore be able through an occhialino to see it go through phases like the moon’s, as one would be seeing either the side facing the sun or the dark side, or in between.

This thought had already occurred to Galileo, and he was irritated that he had forgotten to write it down in the Sidereus Nuncius. Then he remembered: Venus had been behind the sun the previous winter when he was writing the book, so he had been unable to check to see if the idea was right, and had thought it better to keep the notion to himself

Now he turned his best occhialino toward Venus as it appeared in the sky after sunset. In the first days of viewing it was low, a small full disk. Then as the weeks passed it rose higher and became larger, but was misshapen—possibly gibbous. Finally it was revealed in the glass to have the shape of a little half-moon, and Galileo wrote Castelli to tell him so. Eventually, when it began to sink again toward the horizon at its first twilight appearance, it was clearly horned. Galileo’s latest spyglass had a very fine objective lens that he had ground himself, and in the eyepiece the image of Venus gleamed, distinctly crescent, a miniature of the new moon that had set just an hour before.

Standing up straight, looking at the brilliant white point, feeling the moon just under the horizon and still shedding its light into the night air, suddenly it all fell into place for him. The ball of Venus and the ball of the Earth both rolled around the sun; the ball of the moon rolled around the Earth; the balls of Jupiter’s four moons circled the ball of Jupiter, which slowly circled the sun. Saturn was farther out and slower, Mercury quickest of all, there inside Venus, where it was difficult to spot. Perhaps a good enough glass would see its horns as well, for certainly it too would go through phases. So close to the sun, when visible at all it would have to be pretty near quarter phase. Farther out from Earth, Mars rolled between Earth and Jupiter, close enough to Earth to explain the strange back-and-forth aspects of its movement, a shift of perspective created by the two orbits.

The whole system was a matter of circles going around other circles. Copernicus had been right. His system had called for Venus to have phases, and there they were; while the Ptolemaic theory, advocated by the Peripatetics, would specifically reject these phases, as Venus was supposed to be going around the Earth, like the sun and everything else in the sky. Venus’s phases were a kind of proof, or at least a very suggestive piece of evidence. Tycho Brahe’s weird and unwieldy formulation, which had the planets circling the sun but the sun circling the Earth, would also save these particular appearances, but it was a ridiculous explanation in all other respects, in particular, simple parsimony. No, these phases of Venus were best explained by Copernicus.

They were the strongest indication Galileo had seen—not exactly proof, but powerfully suggestive. All those years in Padua he had taught both Aristotle and Copernicus, and even Tycho, thinking that all of them merely saved the appearances without in any sense explaining what was going on. The Copernican explanation required that the Earth be moving, which seemed wrong. And the foremost advocate of Copernicanism, Kepler, had been so long-winded and incomprehensible that no one could be convinced by him. And yet here it was, the truth of the situation—the cosmos revealed in a single stroke as being one way rather than another. The Earth was spinning under his feet, also rolling around the sun. Circles in circles.