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13 March 2013 @ 12:54 pm
The Killing Moon  
So i just finished up N. K. Jemisin's "The Killing Moon", which was pretty good. It did take a little while to get into, in large part because it's modeled after Egyptian culture and it took me a little while to get the hang of the names.

One thing that's bugging me a bit though is the orbital dynamics of the world.

The planet has two "moons", the Dreaming Moon and the Waking Moon. The description makes it sound like the Dreaming Moon is actually a gas giant. However i'm having a tough time figuring out how to make that work.

Things that are definitely said in the book:

Dreaming moon goes through phases
Waking Moon is visible every morning
Dreaming moon is generally not visible in the sky during the day, though it does happen occasionally
Dreaming moon occasionally eclipses the sun
Days seem to be of about "normal" length
Years seem to be of about "normal" length.

Things i _think_ it said, but i'm not 100% sure of

Waking moon is also visible every evening
Waking moon emerges from behind Dreaming moon

I think i may be misremembering that first item though. If the Waking moon is always visible during a certain part of the day it seems like there must be some kind of tidal locking going on? And the only way that could work if it was visible during both morning and evening is if it was on the opposite side of the Dreaming moon. I couldn't figure out a way to get the angles to work if the Dreaming moon actually was a moon and the Waking moon was at L2, and it's even less possible if Dreaming moon is the primary in the system.

In fact, a quick check of Google Books seems to indicate that it is always visible in the pre-dawn, but not in the evening.

Setting aside the presence of magic the people seem to be normal humans, and 16 is considered adulthood, which is consistent with any number of real societies at that tech level. Age is measured by how many "floods" you've seen, which just as with Egypt seem to happen on a very consistent basis.

So either there's a year about the length of our year, or there's a very long year with multiple seasonal cycles that last as long as our year in it.

Given the length of the day the world is clearly spinning in relation to the sun at something approaching 24 hours.

The world could be tidally locked to Dreaming moon and in a 24 hour orbit, except i'm 90% sure it refers to the Dreaming moon rising and setting.

Oh hey, it looks like there are some sample chapters online! Including the ones with _most_ of the relevant astronomy bits:

The relevant bits:

"With some concern he saw that Dreaming Moon had reached zenith, her bright expanse glaring from the sky’s center like a great striped eye; the night was half over."

"He looked up. Beyond the window rose the rooftops of the city, and beyond those the glowing curve of the Dreamer sank steadily toward the horizon. Waking Moon peeked round its larger curve."

"Waking Moon was a small and homely thing who rarely strayed far from her sister’s shadow, fearful of being alone."

"Now they 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. She is always bright again when he returns."

So Dreaming moon definitely has phases and rises and sets. It's not clear if Waking moon is always "peeking round" the edge of Dreaming moon, or if it moves in relation to Dreaming moon.

It's possible i suppose that the orbits of the world and the Waking moon are synchronized, so it would always be hovering right at the horizon of Dreaming moon from the world's perspective. That wouldn't really work with Lagrange points, but i believe Lagrange points are specifically for one large body, one medium body, and one or more small bodies. Maybe it works differently for one large body and two medium bodies? Except in that case the "waking" moon would be visible all night as soon as Dreaming moon cleared the horizon. Plus they said it "rarely" strays far from Dreaming moon, not "never", though that could just be poetic license.

So somehow Waking moon comes out from behind Dreaming moon during the later part of the night, but never shows up in the evening? It must be inside the orbit of the world if it's always near the planet, but i can't think of how it could only emerge during a certain time of day according to another moon.

And also, if Dreaming moon rises and sets that means that the world is _not_ tidally locked to the gas giant. I wonder how that's possible? How many moons of gas giants _aren't_ tidally locked?

I dunno, this is all making my head hurt =P
Current Mood: geekygeeky
Steuard: lakesteuard on March 13th, 2013 09:18 pm (UTC)
No time for a full reply, but I'm wondering about a partial tidal locking like Mercury's, where the sun rises and sets but in a very specific ratio to the length of the year.

Actually, I think your first quote makes this surprisingly simple, though I don't quite get it: the characters' world must be at L1! If "Dreaming moon at zenith" always means "night is half over", then the planet is always opposite the Sun in the sky, and that is only true at L1. Is the other moon maybe at L4/5 but maybe with some odd sort of orbit?

But then how would there be eclipses or phases of Dreaming moon? I'm still not clear on this.
Steuard: lakesteuard on March 14th, 2013 02:26 pm (UTC)
So here's my best guess: this is all about Lagrange points of the Sun/Dreaming moon system.

The habitable planet is somehow at or near the L1 point, which has issues: L1 is unstable, so by all rights this planet shouldn't be there at all. (When we put spacecraft at L1, they need to frequently use engines to stay on course.)

The Waking moon is in a fairly large orbit around L4, allowing it to move closer and farther to the Dreaming moon over time (while always being clearly on the "sunrise" side of the sky).

This doesn't explain everything. I don't see evidence for phases of the Dreaming moon, but rather eclipses. (Or maybe just dimming: could it be that the planet at L1 occasionally lines up just right and shadows the Dreaming moon? I'm imagining it's not big enough to completely block the sun anywhere, but rather just create a penumbra or antumbra that dims the whole thing.) I also don't know what's occasionally eclipsing the Waking moon: the obvious candidate is the Dreaming moon itself, but what orbital geometry would make that visible from L1? Maybe the planet's orbit is fairly wide, too, though that just makes the L1 instability worse. It's not remotely clear to me why the days would "shorten and lengthen" in response to (relative) motions of the Waking moon: what could possibly couple rotational motion of a planet to orbital motion of its neighbors? And why on earth would the Dreaming moon ever "rise early"? (Some particularly big deviation from L1, I guess... but again, that makes the instability issue worse.)
DonAithnen: happydonaithnen on March 16th, 2013 07:13 am (UTC)
So i found a couple more relevant bits.

"A Gatherer can endure without dreamblood for several eightdays - as much as a full turn of the Waking Moon"

So the Waking moon does have some kind of multi-week cycle.

And more importantly, it seems there's an afterwards to some versions of the book which mine doesn't have. And one part of it says:

"Yes, the Dreaming Moon is a gas giant. The world of the Dreaming Moon is one of its moons; Waking Moon is another. The Gujaareen are aware of this, as their astronomy is about as developed as that of ancient Egypt, but the habit of referring to the Dreamer as a moon is something that long predates these discoveries, so it stuck. I did actually try to work out the astrophysics, for which I thank the instructors and my fellow attandees at NASA's Launch Pad astronomy workshop for science fiction/fantasy authors and other creative type, which I was privileged to go to back in 2009. Not much of that made it into the duology - maybe one day it will, if I write a story about the Teachers - but it was fun to play with,"

So i guess even when i get around to reading the second book there won't be any straight answers, though perhaps there might be a few more hints as to what's going on.