were assumed to be riding on pure crystalline spheres. The spheres were invisible from Earth and interlocked, like the gears of a clock, to produce collective motion at a constant speed for all eternity. Plato and Aristotle had laid the foundation for the accepted model, and it dominated for two thousand years. That clockwork universe was the one German astronomer Johannes Kepler inherited. He accepted it, at first.
When the constellation Cassiopeia suddenly gained a new star (it was actually a supernova, the bright explosion at the end of a star’s life), Kepler recognized that the idea of the unchanging heavens could not be correct. A few years later, a comet tracked across the European sky. Shouldn’t it have cracked the crystalline spheres as it traveled, Kepler wondered? He began to doubt two millennia worth of accepted wisdom.
By 1596, when he turned twenty-five, Kepler had accepted the Copernican model of planets orbiting the sun, and now he posed another profound question: Why do planets that are farther away from the sun move more slowly? Perhaps the more distant planets had weaker “moving souls.” But why would that be? Just coincidence? Maybe, he thought, rather than many spirits, there was just one, inside the sun, which for some reason acted more powerfully on nearby planets. Kepler was so far outside the bounds of previous thought that there was no evidence in existence for him to work from. He had to use analogies.
Smells and heat dissipate predictably farther from their source, which meant that a mysterious planet-moving power from the sun might as well. But smells and heat are also detectable everywhere along their path, whereas the sun’s moving soul, Kepler wrote, is “poured out throughout the whole world, and yet does not exist anywhere but where there is something movable.” Was there any proof that such a thing could exist?
Light “makes its nest in the sun,” Kepler wrote, and yet appears not to exist between its source and an object it lights up. If light can do it, so could some other physical entity. He began using the words “power” or “force” instead of “soul” and “spirit.” Kepler’s “moving power” was a precursor to gravity, an astounding mental leap because it came before science embraced the notion of physical forces that act throughout the universe.
Given how the moving power seemed to emanate from the sun and disperse in space, Kepler wondered if light itself or some light-like force caused planetary motion. Well, then, could the moving power be blocked like light? Planetary motion did not stop during an eclipse, Kepler reasoned, so the moving power could not be just like light, or depend on light. He needed a new analogy.
Kepler read a newly published description of magnetism, and thought maybe the planets were like magnets, with poles at either end. He realized that each planet moved more slowly when it was farther in its orbit from the sun, so perhaps the planets and the sun were attracting and repelling one another depending on which poles were nearby. That might explain why the planets moved toward and away from the sun, but why did they keep moving forward in their orbits? The sun’s power seemed somehow to also push them forward. On to the next analogy.
The sun rotates on its axis and creates a whirlpool of moving power that sweeps the planets around like boats in a current. Kepler liked that, but it raised a new problem. He had realized that orbits were not perfectly circular, so what kind of strange current was the sun creating? The whirlpool analogy was incomplete without boatmen.
Boatmen in a whirling river can steer their boats perpendicular to the current, so maybe planets could steer in the sun’s current, Kepler surmised. A circular current could explain why all the planets move in the same direction, and then each planet steered through the current to keep from getting sucked into the center, which made the orbits not quite circular. But then who was captaining each ship? That brought Kepler all the way back to spirits, and he was not happy about it. “Kepler,” he wrote to himself, “does’t thou wish then to equip each planet with two eyes?”
Each time he got stuck, Kepler unleashed a fusillade of analogies. Not just light, heat, odor, currents and boatmen, but optics of lenses, balance scales, a broom, magnets, a magnetic broom, orators gazing at a crowd, and more. He interrogated each one ruthlessly, every time alighting on new questions.
He eventually decided that