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 Ptolemyâ€™s â€œequantâ€ makes a planetâ€™s speed increase or decrease in direct proportion to its distance from the Sun. In Donahueâ€™s words, Kepler was arguing â€œthat Ptolemyâ€™s equant is really a flawed geometrical expression of [a] dynamic principle.
Kepler introduces Chapter 33 with the heading: â€œThe 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 â€œworthiness and eminence of the Sunâ€ alone. The Sun after all is the â€œsource of the light which forms the adornment of the entire [Solar System]â€ and â€œalso the source of the heat by which everything grows.â€ To these observations Kepler added that the Sun is â€œthe source of the worldâ€™s life (which is visible in the motion of the heavens).
But what is the power that moves the planets? Kepler called it an â€œimmaterial species of the solar body.
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Kepler’s discover, the new astronomy
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