Abstract During the Copernican revolution the supporters of the Ptolemaic theory argued that the tower experiment refuted the Copernican hypothesis of the (diurnal) motion of the earth, but was in agreement with the Ptolemaic theory. In his defence of the Copernican theory Galileo argued that the experiment was in agreement both with Copernican and Ptolemaic theory. The reason for these different views of the same experiment was not that the two theories were incommensurable, as Paul Feyerabend argues, but that Galileo (...) introduced a new theory of motion which he used as an auxiliary hypothesis in his discussion of the tower experiment, while those defending the Ptolemaic theory used the old Aristotelian theory of motion. Already before the Copernican revolution the Aristotelian theory of motion was criticized by philosophers in Paris, who suggested the impetus theory of motion. The later versions of this theory had the consequence that the tower experiment no longer refuted the hypothesis of the (diurnal) motion of the earth. Thus the impetus theory removed an old and important objection to the heliocentric theory. Galileo's inertial dynamics had the same function in the discussion of the tower experiment. (shrink)