The ancient tradition of making celestial maps can be traced back, by way of Arabic sources, to classical ones. Dürer's two maps, this one and another of the Southern Hemisphere, derive from an Arabic type that depicted each hemisphere separately. His direct source was two richly decorated charts of the stars made in Nuremberg in 1503. Dürer's own additions to that design include the portraits of early astronomers in each corner: Aratus Cilix, Ptolemeus Aegyptius (Ptolemy), M. Mamlius Romanus (Marcus Manilius), and Azophi Arabus (Al-Sufi).