“Cosmography” sounds like the kind of thing a pair of fantasy authors would make up, but it’s absolutely real.

When Alyc and I first came up with the concept for the Sea Beyond duology, our pitch to our editor handwaved something about a “Guild of Cartographers” being the ones behind the mapping and colonization of the Sea Beyond. Early in our research, though, Alyc — looking for resources on the cartography of the era — came across a book that had a profound effect on our plans: María M. Portuondo’s Secret Science: Spanish Cosmography and the New World.

Nowadays you might hear the term “cosmography” in the context of astrophysics. There, it has to do with very large-scale studies of the observable universe; naturally this is not what we mean when we use the term, nor what Portuondo is describing in her book. Cosmography in the older sense of the word is an idea that predates modern intellectual specialization — something we’d call “interdisciplinary” now. Its aim was nothing less than the holistic description of . . . basically everything? (Kosmos, of course, being the Greek word for the world or the universe.)

Cosmography included cartography, but it didn’t stop there. It began with geography, describing where things were, how big they were, and so forth, and hydrography, describing the coast, the rivers and ports, how to sail from where you were to where that place is. Cartography then played a role in pinning those things down, using math to represent them accurately on a two-dimensional surface. But the physical side was only the first half of the equation, because cosmography also included historical chronicles, ethnographic descriptions, and inventories of natural resources. If it was worth knowing about a place, then cosmography included it.

This was both an intellectual and a political undertaking. Intellectually, it was an outgrowth of the old belief that it was possible to sum up the entirety of the world in one magisterial work. Politically, it was a very useful tool, because you can’t govern what you can’t find or don’t know about. For us, there’s also a mystical dimension: the idea that to know a thing, to be able to represent it in another form, is to gain control over it in less tangible ways.

And as Estevan studies cosmography, he sees the ways in which this isn’t entirely a negative undertaking. After all, human curiosity is a grand thing, without which we’d all live much poorer and less pleasant lives. He himself is fascinated to learn — including about humans, absorbing information on their customs and culture in much the same way that the king’s cosmographers do about the islands they’re conquering. The difference is the use to which it’s put.

Along with who gets to use it. One really fascinating thing in Portuondo’s book is the shift that happened between the reigns of Philip II and Philip III. Under the rule of the former, cosmography was the titular secret science. Information on the New World was meant to be closely controlled, with the official map being kept inside an ark with three keys. (This map was also supposed to be regularly updated. Turns out that . . . really didn’t happen.) One demonstrated one’s political power by keeping things hidden. But when Philip III took the throne, both changing circumstances and his own temperament led to a massive shift; the official map, far from being locked up, was published. Now the growing colonial powers of Europe demonstrated their power by showing how great their cosmographic understanding was!

But also, cosmography was on the way out, at least under that name and formulation. The influx of information from the New World had gone from a trickle to a firehose, and it proved absolutely impossible for one person to keep up with it, let alone to write the kind of all-encompassing book that could explain the entire world. That world had gotten too large, too complicated, and the component disciplines of cosmography had advanced too far to cram them all under a single umbrella anymore. We still do all the things cosmographers did; it’s just that now, geography and hydrology and anthropology and history and so forth are separate disciplines, cooperating here and there, but no longer seen as facets of one particular thing.

At the moment of the novel, though, the idea is still hanging on. And it means that, like us, Estevan goes from thinking he needs to know about maps to realizing that cosmography is the key to . . . well, everything.