The seven medieval or classical planets were the Sun and Moon and the five non-earth planets of our solar system closest to the Earth: Mercury, Venus, Mars, Jupiter, and Saturn. Notice these form the seven days of the week: Sun-day, Moon-day, Saturn-day, and in Latin: 'Martis' (Mars, Tuesday), 'Mercurii' (Wednesday), 'Iovis' (Jupiter, Thursday) and 'Veneris' (Venus, Friday).
Each of these heavenly bodies was believed to have certain attributes, and Ward says that these attributes were de liberately used by Lewis to furnish elements of the stories of each book:
- in The Lion the Witch and the Wardrobe the child protagonists become monarchs under sovereign Jove
- in Prince Caspian:The Return to Narnia they harden under strong Mars
- in The Vouage of the Dawn Treader they drink light under searching Sol
- in The Silver Chair they learn obedience under subordinate Luna
- in The Horse and His Boy they come to love poetry under eloquent Mercury
- in The Magician's Nephew they gain life-giving fruit under fertile Venus
- in The Last Battle they suffer and die under chilling Saturn
I think I've got some rereading to do!