This paper analyzes sixteenth-century commentaries on Johannes de Sacrobosco's Sphaera in Portuguese and Spanish. Between the fifteenth and seventeenth centuries, there were more translations of the Sphaera into Castilian and Portuguese than into all other European vernaculars combined. A major reason for the interest in Sphaera translations in Spain and Portugal was that the basic astronomical and geographical knowledge contained in this text could be used for navigation. Because of their enormous interests in overseas exploration and colonization, Iberian monarchs supported the development of cosmography, a subject that combined mathematics, astronomy, and geography. The astronomical information in the Sphaera was also valued on the Iberian Peninsula for its applications to astrology. Finally, the Sphaera was critical to a Christian education because it taught readers to appreciate the wonders of God's creation. Spanish and Portuguese commentaries on Sacrobosco's Sphaera reflect this mix of practical, political, and spiritual concerns.