The Places of the Sun, Mercury, and Venus: Diagrammatic Innovation in Medieval and Renaissance Planetary Order

Pre-modern celestial observers had difficulty understanding the positions of the sun, Mercury, and Venus in a geocentric planetary order. Prominent ancients – Plato, Aristotle, Archimedes, Cicero, and Ptolemy – were inconsistent in their statements regarding the placement of the inferior planets. The same misunderstandings carried over into a plethora of medieval and Renaissance Latin manuscripts. The modern historian faces the question of how these later astronomy authors of the ninth through the seventeenth centuries diagrammatically responded to the planetary uncertainty? The answer, in many instances, was with innovation. By discussing several of these celestial diagrams, this paper demonstrates that attempts to adjudicate planetary locations resulted in a variety of novel planetary configurations: circumsolar epicycles, double and triple intersecting orbits, and implied epicycle-on-deferent schemes. Of particular focus for this presentation were unusual epicycle-on-epicycle arrangements for Mercury and Venus, which permitted a large variety of planetary orders to ensue. These planetary arrangements evolved from the fifth-century writings of Martianus Capella and Macrobius. The former sought to reconcile the ancient misgivings by suggesting that Mercury and Venus traveled in circumsolar epicycles, while the latter was widely (but perhaps incorrectly) interpreted to suggest intersecting planetary circles. These diagrams greatly impacted medieval and Renaissance ideas on planetary arrangements, including those of Copernicus. He explicitly acknowledged the role of Capella – and other Latin writers – in developing a sun-centered system that relegated the earth to planetary status.