Galileo's cosmological view from the Sidereus Nuncius to Letters on Sunspots

The Copernican question is a thread that runs through Galileo's entire research. This paper analyses Galileo's position towards Copernicanism in the period between the publication of the "Sidereus Nuncius" (1610) and "Letters on Sunspots" (1613). It also intends to show that one of the main reasons for Galileo's decision to give up his project of an expanded edition of the "Sidereus" was the discovery of sunspots. It was especially this new discovery that convinced Galileo of the need to write another book, one entirely devoted to the Sun in order to highlight the value of this new and extraordinary telescopic observation. "Letters on Sunspots" contains Galileo's strongest endorsement of Copernicanism, as well as his first attempt to found a new science of motion based on Copernican cosmology.