Worldbuilding

Changing the Way Your World Moves

A simple orrery with the Earth, Sun, Moon, and inner planets. Orrery by Karen used under CC BY 2.0

This post is 3 in the series: Setting Your World Apart From Earth
Earth is a great planet. It’s functional enough as life-bearing planets go, and it boasts plenty of room – 510 million square kilometers, to be precise* – for all sorts of interesting stories. Yet sometimes you just have to change the scenery; escape your earthly troubles, take a nice little getaway somewhere with new, exotic troubles. But the world should be comfortably exotic; familiar and friendly, with just the right something to let us know we’ve left home.* Like, for example, some strange foreign dancing.

Over the previous two months, we looked at how a world’s cosmic neighborhood and chemistry can vary from Earth’s. Now we’ll see how worlds loop, spin, wobble, and otherwise dance through space. After that, we’ll look at how a planet’s mass and magnetism impact such things as a person’s weight and lifetime radiation damage

We’ll be looking at planets from an outside perspective – the world’s dance partner. However, its inhabitants will be less like its dance partner and more like its fleas. They’ll experience their planet’s dancing as changing days, seasons, and eons.

Planetary Dance Steps

Worlds – especially Earthlike worlds – tend to dance around a lot. You might be familiar with the quicker moves:

  • Rotation: A planet spins about its axis, resulting in days and nights for the surface-bound observer. Rotation is also responsible for certain winds – namely directional trade winds and hurricanes – due to the Coriolis effect.
  • Solar Orbit: The planet swings around its star, often at a tilt.* Earth, for example, likes to lean 23 degrees to the side. This tilt is the reason for our years and seasons: the North Pole tilts toward the Sun in the Northern Hemisphere’s summer, and half a rotation later, the South tilts toward the Sun instead.*
  • Lunar Orbits: If the world’s partner is anything like Earth’s moon, it’s going to be pulling hard enough to cause serious tides and make the planet’s orbit wobble. We dealt with that in more detail in a previous post in this series.

Less familiar are a planet’s long, slow motions. These are less analogous to dance moves, and more like different phases of a dancer’s life. Civilizations care about these motions, but your world’s individuals may not live long enough to notice anything change.

  • Change in TiltAxial Tilt Changes:* The Earth’s axial tilt is not fixed at 23 degrees. Small, short-term variations in axial tilt (a result of the moon and sun tugging on Earth’s equatorial bulge) are called nutation. On Earth, this cycle lasts 18.6 years. Nutation is barely noticeable here, though it could be made much more extreme on your world. The effect would be yearly seasons that cycled between very weak and very strong over the course of decades or centuries. More potent (for Earth) is the 41,000 year cycle of tilt change: the planet can lean as little as 22.1 and as much as 24.5 degrees. This long-term shift is the largest driving force behind this planet’s periodic ice ages. Less axial tilt results in weaker seasons and a colder planet, while more pronounced tilts create more dramatic seasons and keep things a bit warmer overall.
  • Axial Precession: Our axis of rotation points in different directions over time. For Earth, it’s a 26,000 year cycle. This motion changes the direction of “north” in the sky; right now, “north” points directly at Polaris, but in 14,000 years, it will point near the very bright star Vega instead. (Still not sure how this works? Let the video help!)

  • Changes in an Eccentric Orbit: Earth’s orbit around the sun is a reasonable approximation of a circle, but it’s not perfect; we’re 3.1 million miles (3.3%) closer to the Sun on January 3rd (perihelion) than we are on July 4th (aphelion). Due to pushing and pulling from bigger, dumber planets like Jupiter and Saturn, our orbit stretches and compresses. Sometimes it’s a near-perfect circle, and at others it becomes noticeably warped.* This is a complex cycle with “waves” that span between 95,000 and 400,000 years.*
  • Perihelion Precession:* Earth’s axial precession doesn’t just change the north star and shift the background stars; the axial tilt also changes in relation to the planet’s eccentric orbit around the sun. I mentioned that we’re closest to the sun on January 3rd, near the North’s Winter Solstice. This date is currently accurate but slowly changing. 10,000 years from now, the Sun will loom larger in the sky on the Summer Solstice,* boosting the strength of the Northern Hemisphere’s seasons.

These dance steps combine to yield 100,000-year Milankovich cycles on Earth; these are the crude on and off switches for our planet’s ice ages. This timescale is fairly arbitrary and can be set to almost any long-ish timescale for other worlds.*

Long-term changes like these are fun to play with. Worldbuilders can use them to bring about Game of Thrones style decades-long winters and summers or a distant past filled with long cycles of crushing ice and burning heat. They shouldn’t affect a character’s day-to-day life like the rising and setting of the sun, but their mark on your inhabitants’ civilizations will be profound.

Days and Other Quick Spins

Let’s zoom back in on the timeline and take a look at the quick stuff: rotations and orbits. If these cha-cha far enough from Earth-standard days and years, your world will start to feel very alien. And not just because the clocks and calendars will look different; expect different biology and weather as well.

Spin Me Faster

If your planet spins faster, it will experience more regular wind directions and climate zones, and more hurricanes (thanks to a powered-up Coriolis effect). But curiously enough, it will be less windy overall. This is because winds are powered, in part, by variation in temperature; if the night side doesn’t have much time to cool off and the day side doesn’t have much time to heat up, winds both local* and global* are much diminished. Winds caused by the circulation cells – which are created primarily by the heat differential between hot tropics and cool temperate zones – will not be diminished in the same way. These winds will be more stationary and unchanging in direction, due to a stronger Coriolis effect. Predictable, moderately strong winds are a wonderful boon to any pre-industrial oceangoing people, as sail-powered travel will be safer and more reliable than on Earth.*

TooFastGettingDizzy

If the rotation is stupidly fast, days will be mere minutes long,* gravity could be noticeably lower at the bulged-out equator, winds will be highly regular and powered purely by the pole versus equator temperature differentials, and early space-age civilizations will have an easier time launching things into stable orbits. Orbits require rapid movement around a planet – and if the planet is already spinning crazy-fast, just traveling upward might be enough.*

Taking it Slow

If days become long enough, they start feeling more like seasons. Powerful winds arise in response to the intense temperature differential between land and sea and between the side of the world experiencing a long day and the side experiencing a long night. Hurricanes are killed before they form because the Coriolis effect has shut down almost completely. Slow the rotation even further, and civilization can only thrive near the windy, lukewarm terminator: the part of the planet’s surface experiencing a drawn-out sunset or sunrise. Everything else will be uncomfortably hot or cold, unless the world’s oceans are large enough that all climates are maritime. On a very-slowly-rotating desert world, humans may be bound to an existence as restless nomads eternally chasing – or fleeing from – the sun. Stragglers who fall behind are doomed to burn or freeze.

These nomads would appear to have a much easier time if the terminator would just stop moving altogether; their planet could tidally lock one face to the sun, resulting in a hemisphere of permanent day, another of permanent night, and a ring of habitable twilight. The nomads could settle down and relax… But they’d still have the strong winds to deal with, including the potential for a permanent hurricane centered on the sun-facing side. At this point, their world becomes VERY non-Earthlike.

Axis of Rotation: How Tipsy Are We?

On Earth, our seasons are (mostly) powered by our axial tilt. It’s Summer in the northern hemisphere when the North Pole points toward the sun, and… you get the idea. As you might imagine, more tilting means stronger seasons. Keep tilting the world until it turns sideways, like Uranus, and poles point directly towards or away from the sun at the solstices. For these bizarre sideways worlds, days and years would essentially be the same thing – though equatorial regions would experience fairly normal days during the Spring and Fall. Poles would be chilly and dark for half the year, but they’d still receive more sunlight on average than the equatorial regions. If you want your world’s seasons to be as brutal as possible, set your world sideways.

Curiously, a lower axial tilt is a trigger for ice ages. The strong winter versus strong summer tradeoff at high latitudes on planets with shallow tilts keeps ice from running rampant.* In fact, this is the reason Earth’s gradual changes in axial tilt (the most important factor in the complex Milankovitch Cycles) cause ice ages.

Years: Dancing ‘Round the Sun

Your planet’s year is indirectly dictated by the power of its sun. If the sun is blue-hot, your planet needs to stay far away, in an orbit many Earth-years long*, to keep the oceans from boiling and every damn thing catching on fire. (Or just never having oceans or life to begin with.) If it’s a cool star (say, a red dwarf) your planet should hug it in a tight, fast orbit to extract enough warmth – probably so tight that it tidally locks. This will make your years much shorter – though if the orbit is circular there will be no seasons, only a subtle shift in the stars.

A planet can still have zones of Earth-like climate if it sits just outside of its Goldilocks Zone – straying a bit too close or too far from its star:

Turning up the Heat: Moving in Close

A world circling too close to a furnace of a sun can have habitable poles separated by vast oven-like deserts or hot-tub-like oceans with steam-cooker equators and Earth-tropical poles.

But these poles won’t feel quite Earth-like, even if the temperature is just right for humans. Poles don’t experience days like the rest of a planet. If the axis is tilted slightly, like on Earth, they’ll experience a half-year of sunlight and a half-year of darkness. If the axis is straight, like on Mercury or the Moon, they’re permanently bathed in twilight, like the terminator regions of tidally-locked worlds.

Plants and animals would be adapted to very different cycles of light and dark, cold and hot; specifically, there wouldn’t be any.* And speaking of hot, that’s all a traveler will find in any direction: hot, steamy, burning non-polar climates. The two civilizations at the north and south poles would exist in total isolation from each other; a Columbian Exchange on this poles-only world might have to wait until one polar civilization enters the Space Age.

The Cold Shoulder: Drifting Apart

What if your world is too cold? Much less interesting, I’m afraid. The equatorial latitudes are the largest – and most boring – region of a world; on a colder planet, they’d simply be season-less lands with uniform days and Earth-temperate or Earth-arctic climates. Everything looks the same East and West, everything is ice North and South. It’s likely that Earth was such a striped snowball in its distant past.

The “too cold” scenario only gets interesting if the planet is tidally locked, in which case it behaves much like the inverse of the too-hot, only-poles-are-habitable world. In the too-cold tidally locked world, the front-pole (the side facing the sun) would have a bullseye of warm and habitable climate at its center, with all directions leading to increasingly chilly and unforgiving lands.

Advanced Moves: the Ellipse

Worlds both too close and too far from their sun can be made Earthlike – or enough for a setting that feels strange without being totally alien. But what about a world that swings between those extremes? All planets follow orbits that are at least slightly elliptical; Earth is no exception.* Your planet could swing wildly about its star like a comet or (for a less extreme example) Mercury.

This orbital eccentricity is just a way to drive seasons in lieu of an axial tilt; instead of the north pole tilting towards the sun during the northern hemisphere’s summer, the sun would just loom larger during the summer and shrink away during winter. Summers would be short and fierce, winters long and slow. In the absence of an axial tilt, the seasons span the globe and affect both hemispheres. If the world is tilted like Earth, the eccentricity and axial tilt might exactly cancel each other for the Northern Hemisphere and strengthen each other in the South; imagine one hemisphere of unbearable extremes and another predictably mild.

Planetary Bulk: Gravitational Attraction

How a planet moves – its orbit, spin, and wobble – can be about the same regardless of how bulky it is. On the other hand, how a person moves – her running, jumping, and climbing – is going to be very different on a light or heavy world.

Most implications of raising and lowering gravity are straightforward. Crank up the gravity dial, and all physical objects become heavier and fall faster;* it’s that simple. Everything – mountains, cities, animals, plants – will tend to keep a low profile. Flying becomes more difficult. Larger plants and land animals become less viable,* and even human-sized beasts would require sturdier frames: thick skeletons that are both stronger support structures and more fall-resistant. Tall mountains are rarer: less stable and more likely to crumble or erode. Their tops are also surrounded by much thinner air because the air is compressed. Deep trenches become ultra-pressurized, whether under air or sea. Depending on how high the gravity, humans not accustomed to the extra weight can be exhausted by simple tasks and seriously injured by minor falls and stumbles. Don’t expect your characters to slam-dunk anything.

Conversely, lower gravity allows towering structures, soaring dragons, and sky-scraping trees. Falls are less dangerous and loads are lighter, but humans may encounter unique difficulties: weight-generated friction is reduced. That’s a problem. Bring the gravity down far enough, and humans can’t walk normally. Instead, we’re forced to bound awkwardly in slow, loping arcs. Additionally, wheeled vehicles don’t do well; without the friction applied by gravity, they spin out and have difficulty achieving traction.*

Gravity and Atmosphere

Higher gravity worlds are statistically more likely to have thicker atmosphere – but there still could be worlds with air so thick and gravity so low that humans can achieve unassisted flight. Heck, there’s one in our very Solar System.

In any case, despite the positive correlation between atmosphere and gravity, you can arbitrarily set gravity and atmosphere to whatever values you need for your story. Especially atmosphere, which is more a function of a world’s particular evolution than a strict consequence of its size or temperature. A magnetic field is actually more important in maintaining an atmosphere than a large size or more gravity – gravity certainly helps, but this is rarely the limiting factor for any Earth-like world. For example, Mars would still have a reasonably thick atmosphere if it had maintained a magnetic field.

Planet-sized Magnets: How do They Work?

The Earth's magnetic field would only look like this if you could actually see magnetic field lines and the solar wind. The Sun has scored a direct hit! Planetary shields holding at 95%. Image by NASA (Public Domain)

If a planet’s surface-dwellers experience Earth-like gravity, there’s a good chance it has a beating heart of iron. The Earth has just such a heart: its core is a massive geodynamo. Sloshing, spinning, molten iron in the hellish depths below the surface creates a powerful magnetic field,* roughly aligned with its poles.*

Does your world have a good heart – er, magnetic field? Excellent; the planet’s inhabitants aren’t going to die from cosmic radiation and solar flares. So that’s good. As a bonus, animals with magnetoception – and any human civilization that has invented a simple compass – can use it to navigate. A magnetic field like Earth’s tends to reverse itself at random intervals. Nothing to worry about: these intervals are measured in millennia and the magnetic field never fully shuts off.*

Of course, if your planet’s sloshing iron core should freeze (or simply not exist in the first place), the magnetic field goes poof and the locals could be in trouble.* A thick blanket of atmosphere can help compensate for the barrage of high-energy solar particles, but it’s quite certain that your world has been leaking atmosphere slowly over time. Oh, and any reasonably hi-tech civilization is going to have a higher hurdle to enter its Space Age and must shield their electromagnetic equipment. But as a consolation, surface-dwellers get to enjoy gorgeous aurorae at all latitudes – especially just before and after sunset. Radiation (specifically its interaction with your world’s upper atmosphere) is pretty!

Earthlike but not Earth: Some Final Reminders

In this three-post series, I’ve laid out examples of how to make an extraterrestrial setting memorable without being alien. Some of the most interesting settings arise from making just one significant change from Earth-normal. Whether it’s the speed of rotation, atmospheric gases, or size of the moon, the consequences can help remind Earthling explorers that they’re not in Kansas anymore.

Pointing out implications of subtle changes can help a setting feel more real and maintain the suspension of disbelief in a story or game. For example, if a crash-landed character notices multiple moons in the sky, that could serve as subtle foreshadowing for the tidal wave that sweeps his beach shelter out to sea when the moons align a month later. If a world is ruled from flying castles, it’s less absurd if people routinely carry full backpacks larger than they are, and warriors fight using flying kung-fu moves. These separate, seemingly improbable features are plausible for the same reason: this is a low-gravity world.*

And lastly, don’t forget to break the rules and forego scientific accuracy if the story requires it. If your world has a green sun*, is cube-shaped, has clouds people can walk on, or is orbited by a small galaxy, then just remember to have a character mention how impossible that is and move on. Or maybe you’re in a dreamscape or – what was that obscure genre I heard about – fantasy? Point is: scientific accuracy is not a set of shackles for your writing, it is an inspiration and a guide.

Now go make a better world.

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Comments

  1. NelC

    > If a planet’s surface-dwellers experience Earth-like gravity, there’s a good chance it has a beating heart of iron.

    You’re being poetic, here, right? Because you know in our universe, gravity has nothing to do with magnetism? I mean, a big lump of iron will have gravity, but so will a big lump of ice or hydrogen or rock.

    • Perry

      The quid here is not magnetism from iron, but its great density. An ice core couldn’t cope with increased temperatures due to high pressure (there’s a reason our core maintains its temperature), plus iron is a simple metal, abundant becaise of its chemical properties.

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