In the first part of a three episode series on the emerging conflict involving Galileo, the Aristotelian natural philosophers of the Italian universities and the Catholic Church, we examine the factors that would lead to the initial confrontation of 1615 and 1616. These will include a debate that led to a fundamental revaluation of hydrology and a priority dispute on the discovery of sunspots.  Finally, we discuss the piece of information that may have led to Galileo's open support of the Copernican model o the solar system.