Galileo’s Fall of the Planets to the Copernican System of the World

Abstract

The First Day of Galileo’s Dialogue on the Two Great Systems of the World is on the nature of the heavens and circular motion, in which the criticism of Aristotle and his followers is relentless, a refutation of the Aristotelian division between the celestial and terrestrial regions, the former eternally unchanging with only uniform circular motions, the latter always changing with only nonuniform rectilinear motions. Galileo calls uniform circular motion the cornerstone of the Aristotelian universe, upon which is built all the properties of the heavens, absence of heaviness or lightness, incorruptibility, and freedom from all change except motion in place, while heaviness and lightness, corruptibility, and such belong to terrestrial bodies with natural rectilinear motions. His first argument, intended to strike at the heart of the division, is that in a perfectly ordered world only circular motion is natural, and rectilinear motion is, as Copernicus held, not natural, the motion of a body out of its natural place to return to its natural place where it can have only a circular motion or be at rest, as on the surface of the earth, whether the earth is rotating or at rest. Galileo’s argument is based upon De revolutionibus 1.8–9, even to rectilinear motion as a tendency of bodies out of their natural place to come together and form a sphere, but extended and turned against Aristotle in a way that would have surprised Copernicus very much. He also suggests several times that the motion of a falling body may not be rectilinear, which Copernicus never mentions, and the distinction of rectilinear and circular motion is radically transformed in the Second Day, where it is shown that apparently accelerated rectilinear motion may result, or does result, from uniform circular motions, thereby eliminating rectilinear motion from nature entirely.