This most sumptuous of all Renaissance instructive manuals explained the use of the astrolabe (for calculating the altitude of stars) and other instruments used for computing planetary positions. The author, court astronomer to Emperor Charles V, also provided new observations on the comet of 1531 (Halley's Comet). Only about forty copies of this work survive; very few still have the seed pearls that were originally attached to the string markers on each of the eighteen disks.