The spheres began as a mathematician's answer to a philosopher's demand. Plato is said to have set the problem: to save the appearances of the wandering planets, their stations, retrogressions and varying brightness, using only uniform circular motion, the sole motion perfect enough for heaven. Eudoxus answered with nested concentric spheres, each planet carried by several, their combined rotations reproducing the observed wander. Aristotle took the geometry and made it physics: the spheres became real bodies of aether, the fifth element, incorruptible and naturally circular in motion, counted at fifty-five, each moved by its longing for the unmoved mover beyond.
Ptolemy's Almagest then made the scheme calculable, at the cost of elegance, with eccentrics and epicycles that strained the concentric picture while keeping its authority; his planetary order, Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, became the canonical staircase. What resulted was not one theory but a civilization's furniture. The seven-story heaven organized the week's days, the alchemists' metals, the angelic hierarchies, the astrologers' dignities and the mystics' ascents; Islamic astronomy refined its parameters and Latin scholasticism baptized its movers. Lewis called the finished medieval model the greatest work of art the Middle Ages produced, and its genius was integration: physics, theology and imagination agreed, for once, about the shape of everything.