The workshop gang then sang, in their usual four-part harmony, one of their rudest love songs, replacing all the usual girls’ names with “Venus.”

That was Kepler: a good source for jokes. Now, reading Kepler’s defense of Galileo’s spyglass discoveries, Galileo felt an uneasiness that sharpened the further he read. Lots of people would read this book, but much of Kepler’s praise was so harebrained it cut both ways:

I may perhaps seem rash in accepting your claims so readily with no support from my own experience. But why should I not believe a most learned mathematician, whose very style attests to the soundness of his judgment? He has no intention of practicing deception in a bid for vulgar publicity, nor does he pretend to have seen what he has not seen. Because he loves the truth, he does not hesitate to oppose even the most familiar opinions, and to bear the jeers of the crowd with equanimity.

What jeers of the crowd? For one thing there hadn’t been that many, and for another, Galileo did not bear them with equanimity. He wanted to kill every critic he had. He liked fights in the same way bulls are attracted to red—not because it looks like blood, or so they say, but because it has the color of the pulsing parts of cows in heat. Galileo loved to fight like that. And so far he had never lost one. So equanimity had nothing to do with it.

Then further on in Kepler’s sloppy endorsement, he asked what Galileo saw through his perspicillum when he looked at “the left corner of the face of the Man in the Moon,” because it turned out that Kep ler had a theory about that region, which he now propounded to the world—that a mark there was the work of intelligent beings who lived on the moon, who must therefore have to endure days the equal of fourteen days on Earth. Therefore, Kepler wrote:

They feel insufferable heat. Perhaps they lack stone for erecting shelters against the sun. On the other hand, maybe they have a soil as sticky as clay. Their usual building plan, accordingly, is as follows. Digging up huge fields, they carry out the earth and heap it in a circle, perhaps for the purpose of drawing out the moisture down below. In this way, they may hide in the deep shade behind their excavated mounds and, in keeping with the sun’s motion, shift about inside, clinging to the shadow. They have, as it were, a sort of underground city. They make their homes in numerous caves hewn out of that circular embankment. They place their fields and pastures in the middle, to avoid being forced to go too far away from their farms in their flight from the sun.

Galileo’s jaw dropped as he read this. He was growing to dread the appearance of the word accordingly in Kepler’s work, a tic that always marked precisely the point where sequential logic was being tossed aside.

Then a few pages later, worse yet: Kepler spoke of the difference Galileo had noted through his spyglass between the light of the planets and that of the fixed stars: What other conclusion shall we draw from this difference, Galileo, than that the fixed stars generate their light from within, whereas the planets, being opaque, are illuminated from without; that is, to use Bruno’s terms, the former are suns, the latter, moons or earths?

Galileo groaned aloud. Just the sight of Bruno’s name in the same sentence as his own was enough to churn his stomach.

Then he came to a passage that made him go chill and hot at the same time. After Kepler’s congratulations for discovering the moons of Jupiter, and his ungrounded assertion that there must be a purpose for these new moons—and a false syllogism stating that, since the Earth’s moon existed for the pleasure of the people on Earth, the moons of Jupiter must exist to please the inhabitants of Jupiter, Kepler concluded that these inhabitants—

—must be very happy to behold this wonderfully varied display. The conclusion is quite clear. Our moon exists for us on the earth, not for the other globes. Those four little moons exist for Jupiter, not for us. Each planet in turn, together with its occupants, is served by its own satellites. From this line of reason we deduce with the highest degree of probability that Jupiter is inhabited.

Galileo threw this craziness to the floor with a curse and stalked out into his garden, wondering why his hilarity had so quickly turned to dread. “Kepler is some kind of idiot!” he shouted at Mazzoleni. “His reasoning is completely deranged! Inhabitants of Jupiter? Where the hell did that come from?”

And why was it so disturbing to read it?

The stranger … the man who had told him about the occhialino that afternoon in Venice … who had appeared after the great demonstration to the Venetian senate, and suggested he take a look at the moon—had he not said something about coming from Kepler? Quick flashes of something more—a blue like twilight—Had the stranger not come knocking at the gate one night some time ago? Had Cartophilus not joined the household soon after? What did all that mean?

Galileo was not used to having a vague memory of anything. Normally he would have said that he remembered basically everything that had ever happened to him, or that he had read or thought. That, in fact, he remembered too much, as quite a bit of what he recalled stuck in his brain like splinters of glass, stealing his sleep. He kept his thoughts busy partly in order not to be stuck by anything too sharp. But in this matter, clarity did not exist. There were blurs, as if he had been sick.

Cartophilus was picking up Kepler’s book from the floor of the arcade, dusting it off, looking at it curiously. He glanced at Galileo, who glared at him as if he could drag the truth from the old man by look alone. A nameless fear pierced Galileo: “What does this mean!” he shouted at the wizened old man, striding toward him as if to beat him. “What’s going on?”

Cartophilus shrugged furtively, almost sullenly, and put the book on a side table, closed so that the page Galileo had been reading was lost. Inhabitants of Jupiter!

“We have to keep working on the move to Florence, sire,” he said. “I’m supposed to be packing the pots.” And he left the arcade and went inside, as if Galileo were not his master and had not just asked a question of him.

GALILEO’S RETURN TO FLORENCE, as he was now calling his decision, continued to draw fire in Venice and Padua. Priuli was now terming it a breach of contract as well as a personal betrayal, suggesting to the Doge that it would be appropriate to ask for some salary to be returned.

With the mood turning so hard against him, it was a great comfort to know that Fra Paolo Sarpi would remain as steadfast a friend and supporter as he had always been. Galileo had referred to him as “father and master” in his letters to him for many years. Having Sarpi on his side was important.

One day, Sarpi was passing through Padua and dropped by the Via Vignali to visit Galileo and see how his combustible friend was doing. He brought with him a letter to Galileo from their mutual friend Sagredo, who was returning from Syria and had found out by mail about Galileo’s decision to move to Florence. Sagredo, concerned, had written; Who can invent a visario which can tell the crazy person from the sane, the good neighbor from the bad?

Sarpi, it quickly became clear, felt much the same. Galileo sat down with him on the back terrace overlooking the garden, by a table of fruit and some jugs of new wine. Relaxing in this little hole in the city under the stucco walls surrounding them was something they had done many times before, for Sarpi was no ordinary priestly mentor. Like Galileo, he was a philosopher, and he had made investigations of his own in the same years Galileo had worked on mechanics, and found things such as the little valves inside human veins, and the oscillations of the pupil, and the polar attraction of magnets. Galileo had helped him with this last, and Sarpi had helped Galileo with his military compass, and even with the laws of motion.