CS Lewis is best known for his "Chronicles of Narnia" and his books of Christian apologetics, such as: "Mere Christianity" and "The Screwtape Letters". However, he was also an academic: a fellow and tutor of Magdalen College, Oxford, and professor of Medieval and Renaissance Literature at Cambridge University. This is his last academic book, published the year after he died.
Lewis here outlines (what we would call) the Medieval Geocentric Cosmology and Natural History (especially Human Nature). This is very different from our modern scientific theories of Cosmology and Biology. Lewis' point is that, if we do not understand the worldview of medieval writers, we cannot fully understand their writings.
Two points made by Lewis are worth noting. It is sometimes thought by moderns that the medieval universe was small and closed in. In fact the Earth was, by cosmic standards, a point; it had no measurable size. The Sun and stars were known to be larger than the Earth. The stars were known to be far more than a hundred million miles away. (This is small by modern standards, the Sun being about 93 million miles distant, but can any of us claim that we truly appreciate the difference between a hundred million miles and the six million million miles of a Light Year?)
A second point is that it is often claimed that when Copernicus put the Sun rather than the Earth at the centre of the Universe, this was a demotion for the Earth. The Medievals believed that the Heavens were the place of purity, made of Quintessence, a fifth element not found on Earth. The Earth itself was the "offscourings" of the Universe, the dregs, the "cosmic dust-bin" (p 63). In making the Earth one of the planets, Copernicus could be regarded as promoting the Earth into the Heavens, surrounded by the Aether, the place of purity.
If you have read CS Lewis' novel "Out of the Silent Planet" you will know that in the final chapter (Chapter 22) Lewis refers to a twelfth century writer, Bernardus Silvestris. It turns out that Bernardus was a real person and Lewis refers to his writings on a number of occasions. What is more, Bernardus does use the word "Oyarses" from which Lewis got the "Oyarsa" of Mars (Green and Hooper, "CS Lewis, a Biography" chapter 7)
Lewis says that the Medieval Model of the Universe delights him but he does not recommend a return to the old Model. What we should remember is that our Model of the Universe is still a Model and is likely to be superseded in its turn. In fact our modern Cosmology depends on two theories (Quantum Theory and General Relativity) that are supported by every experimental and observational test and yet are thought to be incompatible with each other. That is why modern cosmologists are trying to supersede our current Model with their search for a Theory of Everything or a Theory of Quantum Gravity.