The original 1609 edition is a typographical marvel. It contains intricate woodcut diagrams illustrating planetary positions and geometry. A plain text transcription cannot do justice to the visual complexity of Kepler’s arguments.
Why does the planet move in this way? I propose that there is a force (an anima motrix ) emanating from the Sun. This force is magnetic in nature. The Sun rotates on its axis, and this rotation sweeps the planets around. But the planets themselves have their own magnetic poles. One pole is friendly to the Sun, the other is unfriendly. As the planet orbits, these poles turn, sometimes attracting the planet to the Sun (perihelion) and sometimes repelling it (aphelion). This magnetic interaction stretches the orbit into an ellipse. astronomia nova pdf