Why is heaven "up"?

In my reading into all things Tolkien, I repeatedly found references to a little book by C.S. Lewis titled, The Discarded Image: An Introduction to Medieval and Renaissance Literature. The way the various authors mentioned this book intrigued me and so I looked for it in bookstores for more than a year, but never with success. Finally, I decided to simply order it online, and I am glad I did.

This book is a sort of summary of his introductory lectures on the mindset of medieval man, of how he looked upon the world. Lewis describes medieval man as

an organizer, a codifier, a builder of systems. He wanted 'a place for everything and everything in the right place'. Distinction, definition, tabulation were his delight.... There was nothing medieval people liked better, or did better, than sorting out and tidying up. Of all of our modern inventions I suspect that they would most have admired the card index [of happy memory] ([Cambridge University Press, 2013], 10).

It is an excellent and thought-provoking read that overturns the current common misconceptions of the medieval worldview. Take, for example, the medieval view of the universe.

It is a commonplace today to mock the medievals for speaking of heaven as "above" us and hell as "below" us. We find such a notion quaint and childish, altogether lacking in scientific reasoning, but Lewis demonstrates how this modern characterization is quite incorrect.

The medievals based their understanding of the universe on the understanding of the world as handed down by none other than Aristotle, no fool he. As Lewis summarizes:

So far as he could find out, the celestial bodies were permanent; they neither came into existence nor passed away [the first supernova was not observed until A.D. 185]. And the more you studied them, the more perfectly regular their movements seemed to be. Apparently, then, the universe was divided into two regions. The lower region of change and irregularity he called Nature. The upper he called Sky. Thus he can speak of 'Nature and Sky' as two things. But that very changeable phenomenon, the weather, made it clear that the realm of inconstant Nature extended some way above the surface of the Earth. 'Sky' must begin higher up. It seemed reasonable to suppose that regions which differed in every observable respect were made of different stuff. Nature was made of the four elements, earth, water, fire, and air. Air, then (and with air Nature, and with Nature inconstancy) must end before Sky began. Above the air, in true Sky, was a different substance, which he called aether. Thus 'the aether encompases the divine bodies, but immediately below the aetheral and divine nature comes that which is passible, mutable, perishable, and subject to death' [Metaphysics, 1072b]. By the word divine Aristotle introduces a religious element; and the placing of the important frontier (between Sky and Nature, Aether and Air) at the Moon's orbit is a minor detail. But the concept of such a frontier seems to arise far more in response to a scientific research than to a religious need (4-5).

From this conception of the universe, from their understanding of the world, this was  - and remains - a perfectly logical way of describing what was known.

Because this conception was all we could know for centuries, when we discovered more about the nature of the universe we simply continued speaking of heaven as being above us. We view such a notion as infantile only because we do not know medieval man as we should.