 One key effect of these principles is that the diurnal terrestrial rotation asserted by the Copernican system is unobservable. The Earth and all the objects on it naturally move in circles around the Earth’s axis once a day, but since human observers share this motion, it cannot be detected. We only notice departures from shared rotation, such as bodies falling or rising. Consequently, “all experiments practicable upon the Earth are insufficient measures” for proving its stability or its mobility, “since they are indifferently adaptable to an Earth in motion or at rest” (Galilei 1967, 6). This blunted standard objections to Copernicanism on the grounds that there is no evidence of terrestrial motion.