Giving Your World Fantastic Skies

This post is 1 in the series: Setting Your World Apart From Earth
Earth is consistently used as a setting for fiction – even speculative fiction such as scifi and fantasy. This may have something to do with all known authors and readers being from there; Earth is a friendly, familiar, ready-made planet to dump characters and plot onto. But some fiction – fantasy in particular – calls for more exotic locales: worlds with decades-long seasons*, regular, city-destroying storms*, or a giant planet hanging in the sky.*

Alas, far too many fantasy and soft scifi writers either do too little to distinguish their setting from Earth (pointy ears* and strange maps* can only take you so far), or they go too far without knowing it, tacking on impossibly alien features without considering the ramifications. Subtle changes to seas, satellites, suns, and spin can cause all sorts of side effects: coast-wrecking tides, clockwork-regular winds, chaotic seasons, and more. These are unfortunate oversights if ignored, but incredible opportunities if properly incorporated into the fabric of plot and setting.

In this series, I’ll be exploring the consequences of seemingly minor changes that can be used to set a world apart. To start, we’ll explore the heavens. What if all we want for our world is an exotic view? Surely tacking a few extra moons or suns onto the sky is just a harmless, hassle-free way to show readers that our world isn’t Earth…right?

Your Moons

Changing around the moon, or moons, is about the easiest way to tell your readers that they aren’t in Kansas anymore. Or in the same solar system, for that matter. It’s often a safe choice: you can declare your unearthly location with a change that is cosmetic in nature, yet cosmic in scope.

Yet the implications extend beyond a modest change in scenery. Different moons cause different biological cycles and tides. A planet with three large moons offers three free biological clocks* for its native species, and its oceans will experience tides that are both powerful and complex.

A small or nonexistent moon doesn’t just dim the night sky* and dull the tides,* it gives rise to shorter days*, a wildly variable axial tilt,* and a weaker magnetic field. Planetary magnetic fields require an internal geodynamo: molten iron (or another magnetic material) must be sloshing around in the core. This iron is more likely to stay molten and keep sloshing around if it’s subjected to the tidal forces of a large moon. This technically isn’t required, and Earth would probably still have a magnetic field without a moon, but large moons – specifically, large tidal forces – mean a greater chance at having a good internal geodynamo. This is especially true for smaller planets with less self-generated internal heating due to radioactive elements and leftover heat from formation.*

There’s no upper limit on the size of your moon, but at some point a planet has to ask itself: who is orbiting whom in this relationship? Should I give my moon full double-planet status? What if I’m deluding myself, and it’s really ME who’s the moon?

Your Planet

If an ominously large planetary disc looms in your world’s sky, implications extend far beyond the romantically radiant nights. Tides will bend continental plates and regularly inundate entire coastal plains. Or, if the world becomes tidally locked with its massive partner – the most likely scenario – the tides will disappear almost entirely.* Tidal locking sets your world’s day equal to its month,* freezes your world’s large moon/planet in the sky,* and provides you with four more poles to play with.

In addition to a North Pole and South Pole, your world now has a Front Pole,* Bright Pole,* Back Pole,* and Dark Pole.* If the other world is something as small as Earth’s Moon (which we’re going to tidally lock to in a billion years or so), then these “poles” are semi-arbitrary curiosities. But if you’re circling a huge, hot, magnetically active gas giant, your world will be split into four very different settings. The skies of the Front Pole will be ablaze with permanent aurorae as your moon-world eternally dredges through the gas giant’s magnetic field. Bright Pole will be a land under a heat lamp, with nights that stay hot even when they go dark, and eclipses that last hours or even days. “Moons” are kind of a big deal when they’re hundreds of times bigger than you.

Your Day Stars

Moons and planets aren’t the only shiny bits in the sky. Duplicating or recoloring the sun is a scifi staple, and a classic way to make a world seem alien and strange. But bathing your planet in otherworldly light might be overkill on aesthetic merits alone – and that’s before considering the strange tides, unstable orbits, and solar winds that can wreak havoc with your world.

If you must swap out the sun, you should at least know what you’re getting yourself (and your world) into. Is your sun less massive? While it will technically be smaller and dimmer, it will also loom larger in the sky (assuming the world is still in the Goldilocks Zone), and it can cause tides equal to or exceeding that of a large moon.

If your sun is a red dwarf, your world will be alien indeed. A vast swirling orb of red fire will dominate the world’s tidal-locked day side. However, it will be seen only rarely, through the clouds of the ferocious hurricane that swirls eternally around the day-pole. Life will flourish only along the twilight rim that separates day and night; the biosphere* will be adapted for the “dark” infrared radiation and shielded from sporadic X-ray flares. Due to the dominance of a close sun’s gravity, it’s much less likely that your world will possess a moon – yet other planets may swing close enough to take on the role.*

A bright, blue-hot sun has consequences that are less exotic, but still severe. Ultraviolet radiation will be incredible, and life will only be possible if the atmosphere boasts a remarkably thick ozone layer. Also consider that the year will be longer,* shadows sharper, and even small moons will eclipse the sun – the merciless, welding-torch-bright sun that is effectively a blue laser aimed forever at your eye. And while terraforming can put habitable worlds in some very strange places, nature doesn’t have time to build and evolve stable biospheres around massive blue-hot suns, as they tend to go crazy and explode after just a few million years.

Binary systems are more complex. They can be perfectly simple, cosmetic things if the stars are comfortably distant or cozy-close. The criteria is that a planet must be able to cleanly orbit either the center of mass of a close pair, or orbit just one star with the second star at a great distance. Or, why not both at once? There are real planets with four suns: two primary suns the world orbits, and two distant, secondary suns that appear as bright pinpricks – about as luminous as our moon. Daylight will change on a “monthly” cycle as the inner suns eclipse each other, and night will alternate between true darkness and pale twilight over the year, as the outer suns appear to drift nearer and farther from the primaries.*

Celestial Leftovers

There are other cosmic alterations that go beyond looks and change the mechanics of your world: things like the sun being part of a tight star cluster,* a solar system full of comets and debris,* a nearby galaxy hanging overhead,* or even a thick, luscious planetary ring.* But let’s bring it down a bit. The big stuff in your sky – the planets and moons – are important, but they’re nowhere near as crucial as the little stuff – the molecules that make up your atmosphere.

Ah, but that’s too much to discuss here. We’ll have to look at that in my next post.

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  1. Neil

    This is an exceptionally useful article, thank you!

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