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 disc; 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. Copernicus was right; 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 further 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, after all, which seemed wrong-an idea only, not a sensible reality. 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.
Again he rang like a bell. His flesh buzzed like struck bronze, his hair stood on end. How things worked; it had to be; and he rang. He danced. He circled his occhialino like the Earth circling the sun, spinning in a slow four-step as he made his little orbit on the altana, arms swinging, fingers directing the music of the spheres, which despite Kepler’s craziness seemed suddenly plausible; indeed an audible chord was now ringing silently in his ears.
Then came a knock on the door below. He halted his dance with a jerk, looked down the staircase on the outside of the house.
Cartophilus was there inside the gate, holding a shuttered lantern, looking up at him. Galileo rushed down the stairs and raised a fist as if to strike him. ‘What is this?’ he exclaimed in a low furious voice. ‘Is he here again?’
Cartophilus nodded. ‘He’s here.’
When she saw that it was not that I would not speak, but that, dumbstruck, I could not, she gently laid her hand on my breast and said, ‘It is nothing serious, only a touch of amnesia, the common disease of deluded minds. He has forgotten for a while who he is, but he will soon remember once he has recognized me. To make it easier for him I will wipe a little of the blinding cloud of the world from his eyes.’
-BOETHIUS, The Consolation of Philosophy
Galileo strode to the gate and hauled it open just as another knock pounded it. The tall stranger stood there looking down at him, his massive perspicillum case in a heap at his feet. He looked flushed, and his eyes were like black fire.
Galileo felt his blood pound in his head. ‘Already you have found me.’
‘Yes,’ the man said.
‘Did this servant you foisted on me tell you where I was?’ Galileo demanded, jerking a thumb toward the hangdog Cartophilus.
‘I knew where you were. Are you willing to make another night journey?’
Galileo’s mouth was dry. He struggled to remember more than that flicker of blue. Blue people-‘Yes,’ he said, before knowing he would.
The stranger nodded dourly and glanced over at Cartophilus, who trudged out the gate and hauled the case over the paving stones into the courtyard. Jupiter lay low in the sky above Scorpio, still tangled with the trees.
The man’s heavy perspicillum seemed more than a spyglass. Galileo helped Cartophilus set up the tripod and to lift the fat tube, which looked to be made of something like pewter, but felt heavier than gold. When they had the device set on its stand and pointed toward Jupiter, which aiming it seemed to do on its own, Galileo swallowed hard, feeling again his dry mouth, his nameless apprehension. He sat on his stool, looked into the strangely luminous glass of the eyepiece. He fell up into it.
Around him lofted a transparent glow, like talcum in sunlight. What is it, he tried to say, and must have succeeded; the stranger replied in his crow’s Latin, ‘Around Jupiter hums a magnetic field so strong that people would die of it, if unprotected. It has to be held off by a similar field of our own creation, a counterforce. The glow marks an interference of the two forces.’
‘I see,’ Galileo murmured.
So he stood on the surface of Europa-again. Some memory of his previous visit had come back to him, though vaguely. The stars trembled overhead as if he were still looking at them through his occhialino, the bigger ones fulgurous, shedding flakes and threads of light into the blackness around them.
The surface of Europa, on the other hand, was exceptionally sharp and clear. The flat ice extended to the horizon that circled them so tightly, opaque white tinted the colour of Jupiter, and stained blue or ochre in some areas; sometimes pocked or chewed at the surface, sometimes deeply cracked in radial patterns; elsewhere smooth as glass. Everywhere it was littered with small rocks, and here and there stood a few housesized boulders, pitted with holes and depressions. Most of the rocks were almost as black as the sky, but a few were metallic grey, or red, the same shade as the red spot low on the banded immense surface of Jupiter. That awesome globe loomed directly overhead, huge in the starry night sky even though only half lit. That was the thing that was twenty-five or thirty times bigger which he had been trying to remember. Its dark half was very dark.
Possibly the tight horizon and the thin air gave the landscape its unreal clarity. The thin air was cool, the sun nowhere to be seen. The two men cast sharp shadows on the ice under them. Galileo, constantly troubled at home by fogged or ringed vision, stared around avidly. Here everyone had hawks’ eyes.
‘This is a hot spot, in local terms,’ the stranger said in the breathy silence. To Galileo the ice looked everywhere the same, and cold. Their feet crunched as the stranger led him to one of the biggest boulders.
There proved to be a door in this rock, which was not a rock, but rather some kind of carriage