AbstractMartin Bylica became the favorite court astrologer to the Hungarian king Matthias Corvinus in 1468 and then enjoyed royal patronage throughout the remainder of Corvinus’s reign. Initially, Bylica’s success at the Corvinian court was based on his performance before the Hungarian diet, where he used an astrolabe to solve a technical astrological problem: rectifying a natal horoscope. Once he had gained the king’s patronage, Bylica solidified his position at court by emphasizing the various layers of his astrological expertise—academic credentials, personal experience, the use of instruments, and his collection of horoscopes. Bylica’s expertise was based not on privileged access to the natural world, but on his ability to offer expert interpretations of that world. Because of his success, Bylica’s career at Corvinus’s court also reveals how the king distinguished between competing claims to astrological expertise and the place of that expertise in his political program.