The Moon Is a Harsh Mistress

Nora Jemisin is a rising star in the world of fantasy. I’m enjoying her work, though not without reservations. I’ve just finished reading the Broken Earth trilogy, and now I’m starting on the two-volume Dreamblood story.

The Broken Earth story (The Fifth Season, The Obelisk Gate, and The Stone Sky) is an odd mix of fantasy and science fiction. At first it seems to be SF — a distant-future story in which plate tectonics have gotten out of hand. Eventually we learn that the iron core of the Earth is sentient and angry, the extent of the lead characters’ psychic powers becomes hard to reconcile with any sort of science, and some other characters are 40,000 years old and can travel through solid rock. So, fantasy. A couple of story elements are never properly explained, and the ending is slow and heavy-going, but by golly I made it all the way through.

I just wish somebody would sit Jemisin down and explain the basics of celestial mechanics to her.

The troubles in her future Earth are massive earthquakes and volcanic eruptions. The human race is hanging on, but it’s not easy. At some point in the distant past, a high-tech attempt to harness the energy at the Earth’s core went horribly wrong, and the Moon was catapulted away, out of its comfortable orbit. The characters in the first book don’t even know what the word “Moon” means.

That’s not a bad premise. But somewhere along in Book 2 we learn that the Moon is not quite gone. Its orbit has become highly eccentric, but now and again it returns. Jemisin is a bit vague about how often this happens, but one set of historical documents tossed into the mix suggests it may happen every 30 or 40 years.

This makes no sense whatever, and for two reasons. First, if the Moon showed up every 30 or 40 years, the word would still have a meaning. People would darn well look up and see it. Even if it wasn’t visible this year, the fact that it was due to return would be known. But that idea is explicitly denied in Book 1.

The more serious problem is, of course, that such an orbit is impossible. A 40-year orbit would send the Moon out beyond the orbits of Jupiter (12 years) and Saturn (30 years). At that distance, the gravitational pull of the Earth would be insignificant. The Moon would be in a free orbit around the Sun, and no longer tethered to the Earth at all. Its path might occasionally bring it close to the Earth, but such occasions would be infrequent and would occur very irregularly.

So now I’m starting Dreamblood, and on page 17 of The Killing Moon she does it again. This book is clearly going to be a fantasy, with no SF overtones, but that’s not an excuse. Now we have a moon that is, for the most part, only visible at night. Here’s what she wrote, in what we’re to understand as a sort of creation myth of the world of the story (though given that the story is fantasy, I suppose we can’t rule out the possibility that the moon and sun are actually sentient beings — I’ll have to get back to you on that after I’ve read further):

Now they [the sun and moon] live apart as husband and wife, she in the night and he in the day. Always he longs for her, and the days shorten and lengthen as he strains to rise earlier, set later, all for a chance to glimpse her. With time she has grown fond of him, for he has been humble and well-behaved since their marriage. Every so often, she rises early so he can gaze upon her. Once in a great while she lets him catch up to her, and he darkens his face to please her, and they join in careful lovemaking. And sometimes in the night when he cannot see her, she misses his foolish antics and pines for him, and darkens her own face.

The last two sentences describe eclipses, and that’s all right. But the clear implication of the first part of the passage is that for years at a time the moon is only to be seen at night, though occasionally it may rise a little before sunset or not set until shortly after dawn.

Here again, the orbital dynamics are flatly impossible. A moon cannot possibly orbit in this manner. It would have to be essentially stationary, positioned on the night-side of the planet and orbiting the sun with exactly the same periodicity as the planet — except that once in a while it goes for a little trip and drifts around to the sun-side so there’s a solar eclipse.

If you’re going to write a novel in which a moon is mentioned, you have an obligation to get it right. The moon is not a literary device, to be tossed around however you please. It’s a great big ball of rock, and it obeys some rather simple physical laws.

I do realize that some knucklehead may feel obliged to respond, “But this is fantasy! Why not be creative with how the moon works?” Yes, well, even in fantasy the normal laws of physics have to be followed. If water flows uphill, it’s because a wizard or a god is doing something to make it flow uphill. It doesn’t just flow uphill because the author thought it would be a neat literary twist.

If the moon is coming and going like a yo-yo on a string, then a character in the story has to be holding the string and making it do that. If water runs uphill for no good reason, then we can expect a rabbit with a pocket-watch to come scurrying along at any moment, and a game of croquet using flamingos as mallets won’t be far behind.

Violating the laws of physics for no good reason is just sloppy writing. And when I see a really good writer indulge in sloppy writing, it pains me.

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