The most mysterious book in your collection is titled In Medio. The angels say it's not part of any catalogue, and it isn't in your creator's hand. As far as you can tell, this is the only copy ever written. It describes the celestial spheres in the area of aether influenced by Skaia, which seems to have four facets that each govern a different Medium, or maybe there are four ways of perceiving the same Medium. They're hierarchical, like the Vessels, one begetting the next until finally the last begets the universe. Each facet of Skaia governs a bright kingdom and a dark kingdom, and all but the first govern four elemental spheres as well. It's all very arcane and abstruse. Later when you asked the angels, you found out that Skaia was in fact the Urworld, the planet of cardinality 1, its skies the very parchment of the Book of Eternity.
You started making these orreries partly to practice your mechanics and partly to give the Medium a concrete form in the hopes that its mysteries would feel a little less fathomless. It kind of worked until your master told you that your own arrival in the Medium was already written in the Book of Eternity. Apart from how much you hate being told what you're going to do in the future, that also makes it sound like the universe will beget Skaia again in an endless recursion. Your master is extremely cool and knows basically everything, but sometimes the stuff she says creeps you out.