The projection argument in Galileo and Copernicus: Rhetorical strategy in the defence of the new system

Abstract

Summary The anti-Copernican argument that the diurnal rotation of the earth would have catastrophic centrifugal effects on terrestrial objects has a curious history. Copernicus, who falsely ascribes it to Ptolemy, seems to have introduced it in De Revolutionibus as a foil for his own arguments, to create a rhetorically favourable context of discussion. Galileo, who tried to deal with the complexities of the argument (as Copernicus had not), managed to produce a rejoinder perhaps unequalled in scientific literature, a reasonably complex geometrical argument with rhetorical appeal to the literate mass audience. It is also fallacious, as (I argue) Galileo very probably knew. Various specific points are made concerning Galileo's rhetorical strategy. I also explain why Galileo (unlike contemporary commentators with Newtonian biases) would not have thought his conclusion is open to simple counterexample.

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