When he published the Dialogo in 1632, Galileo called on a range of physical arguments to increase the plausibility of the Copernican system and to challenge the view that Aristotelian physics alone could account for the phenomena. In the "Second Day", Galileo introduced an ingenious explanation of vertical free fall as the product of uniform circular motions. The consequence of applying his concepts of circular inertia and relativity to reveal this interesting possibility proves very striking. However when Fermat later challenged Galileo’s explanation, his response indicated that he knew more about this curve than he had revealed in his Dialogo. Firstly Galileo stated that it was not a spiral as Fermat claimed, but parabolic. Secondly, his Dialogo theory was no more than a "poetic fiction" that had three unexpected consequences (not cited by Fermat and presumably not recognized by him). It was, in short, not a serious explanation at all, but a "caprice and a curiosity" as he had mentioned when presenting it.

The matter raises a range of interesting and important questions about Galileo’s strategy in advancing the case of Copernicanism. Would the recognition of these "unexpected consequences" strengthen Galileo’s claim that he knew, as he claimed, that this was no more than "a poetic fiction"? Was it used simply as a useful means of indicating that there is more than one way of explaining any given phenomena?

The paper will look at some of the unexpected consequences of Galileo’s explanation of circular fall and additionally at his research on the projectile trajectory that he had conducted over two decades earlier while in Padua. The conclusion presented will be that this was an instance of thought experiment, aimed at opening up the mind of the reader to new possibilities rather than at arriving at some definite outcome concerning the nature of reality.