Seeing and Believing: Galileo, Aristotelians, and the Mountains on the Moon

Galileo’s telescopic lunar observations, announced in Siderius Nuncius (1610), were a triumph of observational skill and ingenuity. Yet, unlike the Medicean stars, Galileo’s lunar “discoveries” were not especially novel. Indeed, Plutarch had noted the moon’s uneven surface in classical times, and many other renaissance observers had also turned their gaze moonward, even (in Harriot’s case) aided by telescopes of their own. Moreover, what Galileo and his contemporaries saw was colored by the assumptions they already had. Copernicans assumed the moon was a terrestrial satellite, thus Galileo saw its mountains and Kepler “saw” the dwellings of its inhabitants. Aristotelians assumed the moon was a perfect sphere, so they saw differences in density and rarity in the lunar body. Theory corrected the results of observation, so Galileo’s lunar observations, like those that had come before, proved nothing. Yet this failure contained the germ of Galileo’s success, since the Siderius Nuncius gave observation a rhetorical force it did not have before. Observers on all sides set out to see for themselves what Galileo reported. Hence, all parties now had to answer to what they saw, whatever they believed. Thus, the Siderius Nuncius ultimately changed the grounds upon which natural philosophical argument and debate was carried out. In this new empirical arena, the Galilean science would eventually prevail.