A work of fiction doesn't have to be scientifically accurate. It just has to make sense. All it has to do is maintain an internal logic and consistency strong enough that you, the reader, aren't inadvertently thrown out of the world. If you're frequently frustrated by detail accuracy in fiction, that's likely your problem, not fiction's. Chill out. Breath deep. Smell the flowers. Experience some imagination and wonder.
I fully endorse all the sentiments outlined above. And yet. And yet. There are some fictional details that drive me crazy. Like the seasonal shifts in George R. R. Martin's A Song of Ice and Fire series, where winter and summer last for years—sometimes decades—and nobody knows exactly when the seasons will change. It's not that I feel a burning need to prove to Martin that this can't work. Instead, it makes me ravenously curious. I keep wondering whether, given what we know about astronomy, there's any way that this could actually work somewhere, in a galaxy far, far away.
A couple of weeks ago, io9's George Dvorsky put together a little round-up of five possible scientific explanations that would make Westeros' magical reality make more sense. I chatted about Dvorsky's list with Attila Kovacs, an actual astronomer who has a postdoc position at the California Institute of Technology. They've got differing perspectives on how unpredictable and ridiculously long seasons might work. Thanks to both these sources, I feel like I better understand our universe, and can read Martin more comfortably.
Dvorsky's list starts with planetary tilt. Specifically, what would happen if the planet Westeros is on had a particularly wobbly tilt.
Earth's seasons are caused by the tilt of its axis of rotation - a 23.4° offset of the axis to be exact. The direction of the Earth's rotational axis stays nearly fixed in space despite the fact that we're also revolving around the Sun. As a result, depending on the Earth's location during its orbit, the northern hemisphere is tilted toward the sun, causing us to experience summer. Half a year later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun, resulting in — yes, you guessed it — winter. The seasons are, of course, reversed for the southern hemisphere.
The seasons themselves are the result of shifting daylight exposures. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface. The less sunlight, the colder it is. Makes sense. It's important to note that the Earth's axis of rotation is extremely stable. If it wasn't, the Earth's tilt would be very wobbly, resulting in inconsistent and unpredictable seasonal lengths like the ones portrayed in Game of Thrones.
But thankfully we have the Moon. Or more specifically, we have a very large moon. The Earth's moon is disproportionately large compared to other planetary satellites in the solar system. And without it, there might not be any seasons, or the seasons could be very different than what we're used to. The Moon has the effect of stabilizing the tilt of the Earth's rotational axis. Without it, Earth would be a wobbly mess.
Kovacs, though, says Dvorsky has this backwards. Our Moon isn't a stabilizer at all.
Rotational axes of planets are almost impossible to nudge (IO9's #1), unless by a powerful tidal force — such as the one exerted by our large and close Moon (short of a catastrophic collision with another planet). IO9 has this completely upside-down. Earth's rotational axis would be extremely stable were it not for the Moon. Because of the Moon, it is constantly changing — precessing — with a 26,000 year period.
Basically, Earth does experience some erratic, hard-to-predict changes to its orbit which probably result in changes to observable weather/climate patterns. In fact, it's a big part of some theories on why Ice Ages happen. It's just that, here, unlike on Westeros, those changes happen over thousands of years, not tens or dozens. Instead, Kovacs offers two potential causes for unwieldy seasons that weren't mentioned in the io9 piece at all. First, he says, you could get a very irregular orbit—and thus, irregular seasons—just by having there be two suns.
IO9's list is missing my favourite explanation, that of a disrupted planetary orbit, a.k.a the 3-body problem. Earth goes around the Sun on a nice regular orbit, only because the effect of all other planets on Earth's motion is tiny, so one really only needs to consider the Earth orbiting the Sun (2 bodies) or the Moon orbiting Earth (2 bodies again). However, things get hairy with more large bodies close by — such as with planets orbiting binary stars. Around binary stars, most orbits would be chaotic. So much so, that in the long run planets would tend to be either ejected or collide with one of the stars. But, perhaps, Westeros got lucky, and stayed around long enough by slim chance... And, the second object in the binary could be a brown dwarf (essentially a very large planet, that is just short of becoming a star itself), which would explain why it still only has one real sun still...
And, here is one more possibility, just for fun: What if Westeros' sun has a variable energy output? It could have structural instabilities (resulting from changes in its stellar structure, or from recently swallowing a large inner planet). Or, it could have a close binary companion from which it accretes material at an unsteady rate...
Of course, Kovacs' "three-body problem" explanation has implications for the seasons on Tatooine, as well. But that's a whole 'nother issue.