Miguel Angel Granada

The Defence of the Movement of the Earth in Rothmann, Maestlin and Kepler: from Heavenly Geometry to Celestial Physics

Intensive and continuous discussion on the physical problem of Copernicanism (the possibility of a circular motion of the Earth) begins in the 1580s, coinciding with Tycho Brahe’s proposal of a geoheliocentric system of the world. Rejecting Brahe’s criticism of the motion of the Earth as well as his geoheliocentric system, Christoph Rothmann develops in his correspondence with the Dane a defence of the Earth’s motion focusing mainly on both geometrical considerations concerning harmony, proportion, economy of means (simplicity and rationality) and an appeal to God’s omnipotence. At the same time, he introduces, following Copernicus, some physical considerations concerning “gravity” in order to establish the physical possibility of the Earth’s motion.

The same line of thought is present in Michael Maestlin’s public espousal of Copernicanism in 1596. In his collaborative contributions to Kepler’s Mysterium cosmographicum, Maestlin opposes tacitly to Brahe and he tries, independently to Rothmann and adopting some of the systematic results established a priori by Kepler, to strengthen the case of the Earth’s motion by both giving quantitative expression to the greater simplicity of the Copernican order and appealing to God’s omnipotence.

With Kepler, however, the problematic changes completely. From the Mysterium cosmographicum onwards, he does not limit himself to merely amplify the geometrical arguments in favour of the Copernican order and the Earth’s motion with the aprioristic reasons provided by the polyhedral hypothesis and with his belief in the inherence of geometrical archetypes in God’s essence. Besides it, Kepler begins the construction of a physical demonstration of the Earth’s motion, fully introducing mechanics in astronomy with his program of a celestial dynamics founded on his initial intuition of the Sun’s motive force and on his allegiance to the Aristotelian concept of an inertia to rest in physical bodies.