To understand medieval cosmology it is probably best to forget everything you have learnt about the universe.
Instead, think of the earth. Everyone knows it is a sphere because of the way that you can see further from the tops of hills, ships disappear over the horizon and the earth’s shadow on the moon is curved.
The Earth is a sphere, made of a mixture of the four elements, earth, air, fire and water. Naturally earth is heaviest and tends downwards, fire is the lightest and goes upwards as you can see in a fire.
Around the earth is a sphere with the moon embedded in it, and beyond that are more spheres with the other planets, in order of Mercury, Venus, the Sun, Mars, Jupiter and Saturn.
Beyond Saturn is the sphere of the fixed stars, effectively they are glowing dots painted on the inside of the sphere or small bright lights embedded in it. Beyond that is the primum mobile, the tenth sphere, driven by god which then drives the other interior spheres.
So basically the entire universe is just a bit wider than the orbit of Saturn, with the sun being, according to Roger Bacon, perhaps only 35,000 miles across.
Image that – no planets around other stars, no quasars, no galaxies, nothing at all. Beyond the primum mobile, there is only heaven.
When I first got my head around the idea, it made me feel horribly claustrophobic. For the last 4 generations or so we’ve known that the universe is far larger than our ancestors did, and reversing my perception is a bit weird.
Of course I expect that most normal people in the medieval time didn’t know more than the fact the earth was a sphere. You wouldn’t need to know much more than that, and that the seasons are caused by the planets rotating around the earth. One idea that was apparently discredited yet still known was that the earth might actually rotate. Aristotle, Ptolemy and the Biblical authority thought it stood still, but the idea was still around, although not taken seriously.
All these ideas were of course inherited from Greek philosophy, especially Aristotle.
Just to add to the complexity, the ancients had seen clearly that the movements of the planets could not be explained by simple circles, and invented more complex systems, the one current right up into the 16th century being the Ptolemaic one (after the 2nd century AD astronomer Claudius Ptolomaeus who I think came up with the idea of the circles in circles). In this the planets were moved by a number of spheres, which rotated at different speeds, and thus accounted for the non-circular path of the planet through the sky.
Why the obsession with circles? Well they were perfect, were they not? The only way for something to keep moving continuously would be as a circle coming back upon itself. Any other form of movement and it would crash into things surely?
The other important thing to at least be aware of is the zodiac, with the constellations having an effect on people’s health and lives. Belief in astrology fluctuated but it was an ever-present background concept in medieval times, with surgeons and physicians concerned with the time of operation or what to prescribe depending on the positions of the stars and planets. So certain medicines or operations would be bad at specific times of the year and others would be good.
Further exploration of medieval cosmology would take a long post. It is mind blowing trying to consider the differences between then and now though, and a good exercise if you are trying to re-enact the period. Then you can blow people’s minds by telling them about it all.