As a literary historian, Lewis had a particular interest in medieval cosmology. According to this old view of the cosmos, Earth was the centre of everything. It was circled by the seven planets in their spheres. Each of these seven planets was believed to possess particular characteristics and to exert special influences upon people on the Earth and even upon the metals in the Earth's crust. The place where Lewis writes most about this old cosmology is in his book, The Discarded Image.
Lewis's interest in this 'geocentric' view of the heavens was not confined to his academic life. He had an imaginative interest in it too. For instance, in 1935 he published a long poem about the seven heavens, entitled simply 'The Planets'. And in his trilogy of interplanetary novels, the planets again play a major part. In the first novel, Out Of The Silent Planet (1938), the hero, Ransom, travels to Mars. In the second novel, Perelandra (1943), he travels to Venus. And in the third book, That Hideous Strength (1945), Ransom stays on Earth but becomes a 'bridge' across which the planetary powers pass as they come down to Earth to bring about the end of the whole story. Lewis described the seven planets as 'spiritual symbols of permanent value'. He thought that they were 'especially worthwhile in our own generation'.