Appended to a seventeenth-century translation of Marcus Manilius’s Astronomica is a canonical list of astronomers and astrologers since antiquity. Included in this canon was the fourteenth-century Byzantine polymath Nikephoros Gregoras. When I first read this it surprised me. Gregoras was one of the most important intellectuals of the late Byzantine period who enjoyed access to the highest echelons of political power in Constantinople. He is best known for his history of the late Byzantine Empire as well as a handful of commentaries on classical texts, but he is not typically associated with important works on astronomy or astrology. Here, however, he was remembered for his work on that quintessential medieval astronomical instrument, the astrolabe. I followed the tendrils of this note back through the seventeenth and sixteenth centuries to their source in a fifteenth-century translation by the Italian humanist Giorgio Valla—Valla’s own copy of Gregoras’s text survives with Valla’s marginalia throughout. Suddenly, Gregoras took on a new significance. The sixteenth century remembered him less as a historian than as an authority on astrolabes.