Go here for the introduction to this interview.

In Planet Narnia, you discuss how following the Copernican revolution astrology and astronomy became separated. It appears that a naturalistic worldview dismissed the "non-scientific" understanding of the planets. You also note that "even where astrology is explicit in Lewis's work, it has received surprisingly little attention" (245) from Lewis scholars. Do you think this quick dismissal or avoidance of astrology among Christians today is related to the naturalism or materialism promoted by science?

I think that's one of the reasons, yes.  Christians who are too anxious to achieve scientific 'respectability' have bought into the naturalistic paradigm, - the idea that nature is just so much raw material to be chopped up and studied without reference to its connection with us, its fellow creatures, or its Creator.  Three other reasons spring to mind.  

1.  An unbalanced notion of human 'dominion' over nature, - where dominion is mistakenly glossed as 'domination'

2.  An excessive focus upon the distinction between 'nature' and 'grace'.  Lewis criticised this in Karl Barth.  Lewis preferred Richard Hooker's line of thinking, in which 'nature hath need of grace' but, also, 'grace hath use of nature'.  In an incarnational religion, such as Christianity, one can't draw a hard and fast line between matter and spirit.  Christ has honoured human nature by taking it upon himself, and by being resurrected in bodily form.

3.  An unbalanced reading of scripture, focusing only upon the condemnations of astrology, and not recalling that there is a much more positive view of star-lore in the Bible as well: e.g. Judges 5:20, Job 38:31, Psalm 19:1-3; Matthew 2:1-10; 24:29, etc.

You say of C.S. Lewis that, "He was not prepared to write off a view of the cosmos, as his schoolmasters had written off paganism, simply because it had been shown to be factually inadequate; ideas could be entertained for their beauty, not just their truth." (29) This seems to suggest that something can be beautiful without being connected to truth. How can something false be beautiful?

Let us distinguish two kinds of falseness.  There is a bad falseness, the falseness of a lie, but there is also a good falseness, the 'falseness' of a story.  Both can be beautiful.  

The former kind of falseness is dangerous because of its beauty, its power to attract.  It's because temptations to sin can be attractive that they are hard to resist.  The devil can appear 'as an angel of light'.  Lewis pointed out how the idea of 'the beautiful but evil fay' has all but disappeared from the modern imagination, which is one of the reasons why the beauty of Jadis in 'The Magician's Nephew' is so emphasized.  She is evil, merciless, false, but she is beautiful.

But there is a second kind of falseness, which is better called not falseness, but fiction or metaphorical thinking.  When Jesus told parables he was not relating historical events; rather, he was using his imagination (his power of thinking metaphorically) in a good way.  We don't need to believe that he had a particular fatted calf in mind when he told the story of the prodigal son.  Parables, though 'false' as histories of particular events, are still true and good and - yes, beautiful - as stories.

It's in this sense that Lewis approached scientific models of the cosmos.  Models of the cosmos are products of the human imagination.  (Imagination, Lewis thought, was essential to rational thought.)  They come and they go.  Once upon a time people believed in the Ptolemaic story of cosmological arrangements.  Then they believed in the Newtonian story.  Then in the Einsteinian.  None of these models is the sum total of truth about the universe.  Each gets in a certain number of facts and leaves out others.  In that sense, none is fully true, but each is useful (some much more useful than others), and all possess degrees of beauty.  Lewis thought the Ptolemaic model especially beautiful because of its orderliness, its comprehensiveness, its perfectly graded hierarchy in which great and small are equally at home.  For more on why he thought this, read his book, 'The Discarded Image'.  

Lewis said that, "the characters of the planets, as conceived by medieval astrology, seem to me to have a permanent value as spiritual symbols—to provide a Phanomenologie de Geistes which is specially worth while in our own generation." Is that medieval phenomenology still relevant for today's postmodern generations?

It's important to quote the next sentence.  Having said that the planets are especially worth while in his own generation, Lewis goes on to say: 'Of Saturn we know more than enough, but who does not need to be reminded of Jove?'  He says this because he thought his own generation had been 'born under Saturn' (so to speak).  Saturn was the planet of calamity and death and disaster.  Lewis thought that his own generation had been born under Saturn because his own generation was that generation which was doomed to grow up (and in many cases, not grow up) during the First World War.  Lewis had been a teenage officer in that conflict and was severely wounded in the Battle of Arras in 1918.  He described much of the poetry of the 1920s and 1930s as 'Saturnocentric', - fixated upon Saturn and associated pessimism, cynicism, and despair.  That was a natural and understandable response to the tragedy which was the Great War, but Lewis thought that the Saturnine shadow cast over his own generation was a historical accident and not an eternal truth about the universe.  He thought that Jupiter (Jove) was a much better representation of the heart of spiritual reality, because Jovial qualities (kingliness, magnanimity, sacrifice, festivity) were a good way of symbolising the Christian God. 

In today's postmodern generation, we have not the same cause for being Saturnocentric, though there is still plenty of cynicism and despair around.  Perhaps our current generation is more likely to be fixated with Venus (sexuality) or with Luna (doubt), than Saturn.   

Whichever planet currently 'dominates', Jupiter remains a valuable summary of spiritual qualities, - qualities which are eternally relevant, Lewis would argue, because they convey important aspects of the divine nature.   

In Spenser's Images of Life, Lewis coined the term "donegality." You've adopted this term to help you describe the planetary themes you found in the Chronicles of Narnia. For those not familiar with the word, what does it mean, and how is it helpful for this study?

Lewis thought that many places, like many books, had an indefinable quality, - hard to put into words, but unmistakable.  London has its peculiar 'Londonness' and Donegal (in Ireland) has its 'Donegality'.  In 'Planet Narnia' I take this word, 'donegality', and apply it to Lewis's technique of conveying atmosphere in the seven Narnia Chronicles.  I believe that what Lewis was attempting the Chronicles was a new thing in imaginative literature, and that 'donegality' is a helpful term because it encapsulates this new thing.  By donegality I mean to denote the spiritual essence or quiddity of the Narnia Chronicles as intended by Lewis and inhabited unconsciously by the reader. The donegality of a story is its peculiar atmosphere or quality; its pervasive and purposed integral tone or flavour; its tacit spirit, a spirit that the author consciously sought to conjure, but which was designed to remain implicit in the matter of the text, despite being also concentrated and consummated in a Christologically representative character, the more influentially to inform the work and so affect the reader.