For over half a century, scholars have laboured to show that C. S. Lewis's famed but apparently disorganised Chronicles of Narnia have an underlying symbolic coherence, pointing to such possible unifying themes as the seven sacraments, the seven deadly sins, and the seven books of Spenser's Faerie Queene. None of these explanations has won general acceptance and the structure of Narnia's symbolism has remained a mystery. Michael Ward has finally solved the enigma. In Planet Narnia he demonstrates that medieval cosmology, a subject which fascinated Lewis throughout his life, provides the imaginative key to the seven novels. Drawing on the whole range of Lewis's writings (including previously unpublished drafts of the Chronicles), Ward reveals how the Narnia stories were designed to express the characteristics of the seven medieval planets - - Jupiter, Mars, Sol, Luna, Mercury, Venus, and Saturn - - planets which Lewis described as "spiritual symbols of permanent value" and "especially worthwhile in our own generation".