Kepler wanted to know how the Solar System works, not just to describe it with mathematical formulas. In Part III of The New Astronomy he searched for and identified the physical causes of planetary motion, illustrated in Keplers Discovery

First, Kepler proves in Chapter 32 that Ptolemy’s “equant” makes a planet’s speed increase or decrease in direct proportion to its distance from the Sun. In Donahue’s words, Kepler was arguing “that Ptolemy’s equant is really a flawed geometrical expression of [a] dynamic principle.

Kepler introduces Chapter 33 with the heading: “The power that moves the planets resides in the body of the Sun.

He explaines that even if he were not to engage in lengthy proofs but simply to assert a priori that the Sun is the center of the world this opinion would deserve an equal hearing based on the “worthiness and eminence of the Sun” alone. The Sun after all is the “source of the light which forms the adornment of the entire [Solar System]” and “also the source of the heat by which everything grows.” To these observations Kepler added that the Sun is “the source of the world’s life (which is visible in the motion of the heavens).

But what is the power that moves the planets? Kepler called it an “immaterial species of the solar body.

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Kepler’s discover, the new astronomy