Tag Archives: Astronomy

On Moons, Barycenters, and What you Put into your Work

Been writing a lot recent.

Not here, though, mostly in my books, which I am still trying to – you know – get published. I’ve been quiet because I have Sun-King out with both an agent and an editor and have quietly had my fingers crossed. I also lost my copy-editor, but that’s okay because my last few first readers are wrapping up (hopefully).

With the new prologue and the new chapter one, people’s reactions have greatly improved – and in rewriting it so has my skill (I think). It took a very rude awakening but I thank those deconstructive readers very much for really not holding back and giving it to me straight. It is what I needed and it worked. Thanks gentlemen who probably avoid this site like the plague.

Recently I’ve gone from one fantasy writing sin (too much conlanging) to the other (too much thinking and science). I’ve mentioned on reddit that I’ve always enjoyed small consistencies. To me a little nod to physics or reality is like looking at the fine details of a painting or the tiny, intricate shapes of a statue. It can really show a writer’s love or perhaps a second passion.

Last year I talked about moons. Namely I talked about moons in my books, the three that orbit the world my stories take place in. They go by many names in different languages and they are storied and worshiped. But they are also consistent.

The three moons in my world orbit in a 1:2:4 resonance, in the same positions as the inner-most Galilean moons (Io, Europa, Ganymede). But unlike the Jovian moons they are grey and dusty like our own moon.

It was important to me that the moons orbit correctly, that I know what phases they are in, how big they look in the sky, and how quickly they move. These facts would dictate to the writer what a character can and cannot be seeing. For example new moons rarely follow full moons. We also rarely think about how big the moon really is and how it grows and shrinks, and though I didn’t go as far as to make more advanced libation cycles  or eclipse cycles.

But I did use some simplified equations to figure out how far apart they are, how big they appear, when they appear, cataloging all this gave me a sense of being an astronomer in this world. How do I figure out these things? Where does the planet fit? Like we often think about the moon orbiting the Earth, but it isn’t so simple.

The Earth/Moon system orbit a barycenter or a center of mass. That center of mass is inside the Earth but it isn’t the center of the Earth – it is actually more than 4,600 km from the center, about 73% of the Earth’s radius. It is this point that traces the orbit around the Sun and it means that Earth doesn’t just rotate, it rocks back and forth as it “orbits” the barycenter. If this point was above the Earth’s surface, by most definitions the Moon wouldn’t be a satellite anymore, it would be a planet in a binary planet system. (There are other definitions that might already consider the Moon a planet rather than a simple satellite.)

This started going down a path that lead to at least one scene I really like and I hope other people enjoy- a scene that is unique to this system. And it’s as accurate as I am willing to get.

The system in Sun-King (and that series as a whole) has some fun tidbits hidden in it, but by far my favorite is the work put into the moons. I’d love to be able to sit on my deck and watch as two moons reach their zenith together, or seeing the nearest rise alone in its full glory.

On a tangent thought – what if the Earth/Moon system were both tidally locked to the other?  That is to say, what if one face of the Earth always pointed the Moon? What happens when people from the moonless side go to the mooned side? Could you imagine? What if the European explorers reached the Americas and there was this big, extra light in the sky?

Their little heads would have exploded.

Funny to think about.