The subject of the paper is the shift from an astrology-oriented astronomy towards an allegedly more objective, mathematically grounded approach to astronomy. This shift is illustrated through a close reading of Tycho Brahe's scientific development and the contemporaneous changes in his communicational strategies. Basing the argument on a substantial array of original sources it is claimed that the Danish astronomer developed a new astronomical discourse in pursuit of credibility, giving priority to observational astronomy and natural philosophical questions. The abandonment of astrology in public discourse is primarily explained by Tycho's social position and greater sensibility to controversial issues. Tycho's example suggests that the changes in rhetorical strategies regarding astrology (which happened earlier than changes in astrological belief) should be given more recognition in the history of astronomy.