On account of his mathematical skills, Pope Gregory XIII invited him to Rome, appointed him pontifical mathematician and made him a member of the commission for the reform of the calendar. He also placed him in charge of the painters whom the Pope had summoned to the Vatican to continue the decoration of the palace, most notably to make a number of maps of the regions of modern Italy in the newly constructed Gallery of Maps along the Cortile del Belvedere. This project, begun in early 1580 and completed about 18 months later, mapped the entirety of the Italian peninsula in 40 large-scale frescoes, each depicting a region as well as a perspective view of its most prominent city.
