World Building Ideas: A Tidally Locked World

[CockroachTeaParty]

I've been tossing around ideas for a new campaign setting, and recently wondered what the implications of living on a world tidally locked with its sun would be.

To clarify, one side of a tidally locked body always faces another. For instance, the same hemisphere of the moon always faces the earth, because it is locked in a tidal orbit.

What if a planet always faced the sun? There would be no real day and night cycle, since one side would always face the sun and the other would always face the darkness of space.

I'd imagine that the side that always faced the sun would be some manner of scorching desert, while the dark side would be a frozen glacier, but I'm no scientist, so perhaps that wouldn't be the case.

I imagine the strip between the dark and light sides would be habitable, not too warm and not too cold. Would it always appear to be morning/evening in such a place? If I used this setting in D&D, I could just handwave it and say that magic or divine intervention keeps the world habitable, but I'd like to get a bit of verisimilitude.

It would be cool if horrid monsters of cold and darkness lived on the dark side, and threatened to invade the habitable ring. Perhaps the world was not always tidally locked, and the ruins of ancient civilizations await brave explorers who dare to explore the scorching sands or frozen wastes.

Anybody with a better understanding of celestial bodies, gravity, orbits, or weather have any ideas?

[Frozen_Feet]

There actually is a long-running scientific speculation detailing such a world, orbiting a red sun. Don't remember the name, though.

The conclusion was: the side facing the sun will be barren and too hot for life. The dark side is too cold, and eternally frozen. However, between them is a ring-like, intermediary belt, a world of eternal eternal sun-set. Water flows from the glaciers of the Dark, where the heat of the Light is just enough to melt the ice - thus, life can appear on this "calm belt".

[CockroachTeaParty]

That's the general idea I had.

I don't know how exactly weather patterns and wind work, but what would the weather be like on such a 'calm belt'? Would there be constant winds from the different temperatures and pressure zones of the two extreme hemispheres?

How would civilizations living on such a belt tell the passage of time? Could they perhaps see the changing constellations near the darker side of the sky?

I think a cool campaign setting could be produced out of this... a world living in a delicate balance between life and death...

[Flickerdart]

Maybe they wouldn't have such a cultural importance placed on time, since nothing in their environment visibly changes. You wouldn't even have seasons, so they have absolutely no need to develop a calendar, or days, so a clock would be useless.

[CockroachTeaParty]

Yet they would still grow old and die. I think marking the passage of time is a cornerstone of any mortal race.

Perhaps if the world had a moon or two that orbited the planet? The moons might create 'shadow zones,' eclipses, and other phenomena that would mark the passage of time. Perhaps they have magical properties, or are linked to other planes of existence?

[Jack_Simth]

That's the general idea I had.

I don't know how exactly weather patterns and wind work, but what would the weather be like on such a 'calm belt'? Would there be constant winds from the different temperatures and pressure zones of the two extreme hemispheres?

Weather would be... interesting, probably stormy, as most of the habitable realm is a convergence zone.

The corollary to that, however, is that the terrain shapes the weather quite a bit. You're going to have some "sheltered" sections that get much less of one side of the planet's contribution than the others... and these will have much calmer weather, overall. Great places for civilization to flurish, small as they'll be.

In general, though, the sunward side hot air will pull moisture that reaches it, dragging it up and darkside, where it cools, rains, and snows. The snow builds up darkside, creating ice sheets that push water (in the form of ice) back at a (literally) glacial pace towards lightside... where they melt.

Depending on how the planet is internally set up, and how much water is out there for redistributing, though, you may have a periodic disaster: a buildup of ice on darkside will slowly cause the planet's center of gravity to shift... possibly making darkside and lightside slowly trade over time, possibly doing absolutely nothing until it hits a tipping point at which the crust slides and shifts thirty degrees all at once (thrusting much of the civilization into the uninhabitable extreme sides, leaving only a few small packets of what used to be the civilized world in habitable areas).

There's a lot you can do by whim, as it's based on things not necessarily observable about the planet in question.

How would civilizations living on such a belt tell the passage of time? Could they perhaps see the changing constellations near the darker side of the sky?

There's lots of things to base clocks on. Moons, stars, meteor showers, lifecycle of some plant or critter... not all that hard. Alternately: How do the denizins of the Underdark tell time? It's the same problem, really.

I think a cool campaign setting could be produced out of this... a world living in a delicate balance between life and death...

Could be. But then, interesting is what you make of it. Be warned: Sci-fi books that are too heavy on the Sci have a very much smaller market than the "soft" sci-fi books.

[Flickerdart]

Hm, moons would definitely be fun, since they would allow for passage under their shadow through the dead zone on the hot side.

If they measured time by people dying, they'd have colossal units of measurement. Someone would be so and so generations old, for instance.

[CockroachTeaParty]

How would directions work? Would there be any real way to tell North from South?

I could see directions such as 'sunward,' or 'shadeward,' to describe getting closer to the hot/cold sides of the planet. But what about moving up and down the habitable belt?

Some cool ideas here. I like Jack Smith's idea of impending disaster... perhaps a powerful guild of mages keeps the ice melting properly to prevent such a backup...

[TheMerchandise]

Depending on the size of the planet, there could be a gradient along the hospitable-ish area. You could have more desert-like or tropical climate on the one side, temperate in the middle, and cooler, dark stuff towards the shade end. That way you can have a variety of inhabitants and terrains.

Nations would be a tricky thing as well. Natural boundaries along the Ring depend on what you had in mind for this place, but they might cuts things in half left and right or be few and far between. The Ring might be a giant mess of tiny townships or it could have sprawling metropolises. IT'S UP TO YOU!

[Jack_Simth]

How would directions work? Would there be any real way to tell North from South?

I could see directions such as 'sunward,' or 'shadeward,' to describe getting closer to the hot/cold sides of the planet. But what about moving up and down the habitable belt?

North, South, East, and West are fundamentally arbitrary. North = Sunward, South = Darkward, east and west are respective directions around the twilight. If you want to use earth-terms, which may or may not have any bearing whatsoever. You'll have to learn a rather arcane art on Darkside, though: Navigation by the stars.... something those who don't dwell on Darkside don't have any use for. And almost nobody dwells on Darkside...

Some cool ideas here. I like Jack Smith's idea of impending disaster... perhaps a powerful guild of mages keeps the ice melting properly to prevent such a backup...

That's one plot-hook.

Another is that Darkside need not be void of life. On Earth, there's volcanic vents deep in the ocean, that have bacteria that digest the assorted energetic compounds coming out of the vent... which are in turn eaten by other sea life. No particular reason you can't replace "bacteria" with "fungus" and put that above ground.

Likewise, an area that can't produce food of any kind can still be inhabited by things that migrate - near darkside (and sunside), you'll get roving predators (intelligent and otherwise) that retreat to darkside (or sunside) to rest in relative safety. And, of course, you'll also have things that stay on darkside (or sunside) preying on said predators ... or stealing the leftovers they bring.

[Flickerdart]

Constant winds also means an easy source of power, so they wouldn't necessarily develop very complicated technology. Certainly no ships, but I could see a train-like contraption powered by the wind that goes around the habitable belt, for a more technologically advanced version of the setting.

[Frozen_Feet]

Obviously, you can circle in two directions, and depending on which way you go, Light and dark will switch sides. So if you go "north", light is on your right, and if you go south, it's on your left.

[TheYoungKing]

I think you should definitely imply that the world was not always that way.

I'm imagining secluded crystal spire cities occasionally interrupting the monotony of the Brightside/Darkside landscape. Arcanist arcologies, keeping some semblance of a lost empire alive.....

[Lycanthromancer]

The world might still spin, but it could spin with the 'poles' facing toward and away from the sun. If the axis was slightly irregular, the sun would wobble in the sky, and may even dip below the horizon, thereby still granting 'day' and 'night'. There most surely wouldn't be any seasons in such a world, however.

One thing, though, is that water would be constantly pulled away from the inhabitable strip due to tidal action (the sun's gravity working on the oceans, which in our world causes tides), so sailors who traverse the sea would be constantly pulled toward the sun or away from it, depending on which side of the equatorial regions they'd sail on.

Whatever water is pulled toward the sun would evaporate long before it reached the solar pole, and would be blown back toward the equator due to the temperature of the air expanding and being blown away. Likewise, water pulled toward the lunar pole would freeze well before it hit, creating massive cliffs and glaciers that drop down to an invisible, dark nothing. Explorers that brave climbing down the cliffs to the void below would likely come across a desert just as barren as the one on the other side of the world, since liquid water cannot exist here (and what little there used to be would've evaporated via sublimation). There may be underground hot springs and creatures living off of it, but that would be all that could really exist there (sans possibly elemental and undead creatures roaming the ruins of the ancient civilizations).

[Jack_Simth]

The world might still spin, but it could spin with the 'poles' facing toward and away from the sun. If the axis was slightly irregular, the sun would wobble in the sky, and may even dip below the horizon, thereby still granting 'day' and 'night'. There most surely wouldn't be any seasons in such a world, however.

One thing, though, is that water would be constantly pulled away from the inhabitable strip due to tidal action (the sun's gravity working on the oceans, which in our world causes tides), so sailors who traverse the sea would be constantly pulled toward the sun or away from it, depending on which side of the equatorial regions they'd sail on.

Whatever water is pulled toward the sun would evaporate long before it reached the solar pole, and would be blown back toward the equator due to the temperature of the air expanding and being blown away. Likewise, water pulled toward the lunar pole would freeze well before it hit, creating massive cliffs and glaciers that drop down to an invisible, dark nothing. Explorers that brave climbing down the cliffs to the void below would likely come across a desert just as barren as the one on the other side of the world, since liquid water cannot exist here (and what little there used to be would've evaporated via sublimation).

Not really a problem, of itself. Ice, under stress, flows like water - and with a source of more ice, but nothing to cause melting, it will do so under it's own weight, given time and buildup - it's how glaciers work. On the hot side, all water eventually migrates towards the cold side. On the cold side, if there's enough water to really go around, you get an ice sheet, flowing away from the freeze zone, both towards the cold-pole (until it meets at the cold pole, of course, at which point, almost all ice flow is towards the temerate zone) and towards the twilight zone. In the twilight zone, the ice heats up enough to melt into the middle. The twilight zone has a steady diet of melted ice... and the melted ice, of course, either pools in the twilight zone, or flows towards hotside, where it evaporates (cyclically).

If conditions are "right" for it to happen, ice will build up in the cold zone... unbalancing the world, and causing it to shift in either a rather spectacular cataclysm (the world suddenly shifts 30 degrees; you have MASSIVE worldwide earthquakes, cities turned mostly to rubble standing in the freeze zone to be slowly buried in snow and ice ... or worse, past the freeze zone, to be trapped between encroaching walls of inevitable ice, other cities shifted to the hotzone, to roast, burn, or even melt, depending on how far they go and how hot it gets there ... plus absurd amounts of ice shifted to the hot zone, where ice cannot long endure... causing massive flooding, clouds, wind, and storms as that ice melts, flows, and evaporates... then ultimately condenses) or in a very slow cycle of doom (if the world shifts, say, one degree per decade ... which with a roughly earth-sized world, is about seventy miles ... then cities won't survive more than a few centuries before they have to be abandoned to the ice/heat ... making for civilizations that are either at least somewhat nomadic, or are relatively short-lived).

Oh, yes - and you can't tell "periodic sudden shifts" from "the world used to be much less polarized" from the more visible traces.

In any scenario, though, the constant glaciers will tear up the land something fierce, so a map from a few decades or centuries ago will be just this side of useless ... which makes finding an ancient place quite the puzzle.

There may be underground hot springs and creatures living off of it, but that would be all that could really exist there (sans possibly elemental and undead creatures roaming the ruins of the ancient civilizations).

Quite reasonable. Oh, and don't forget the terminations of underground rivers, insulated from the heat, on hotside causing the occasional oasis.

[CockroachTeaParty]

Could it be possible to have one giant ocean around the habitable belt? Perhaps dotted with islands, and flanked by mountains/deserts sunside and glaciers darkside?

It sounds like realistically water would be moving around too much to really form large bodies of water, but maybe if there's a lot of water and a lot of rain/precipitation...

Also, I could definitely see a tropical zone closer to sunside, a temperate area, and a colder area closer to darkside. The idea of things living beneath the frozen ice of darkside, or the general 'underdark' of sunside would also be certainly possible.

I can see elementals, or elemental themes, being prominent as well. Fire for sunside, water (ice) for darkside, air for the crazy weather, and earth for the world itself, trying to resist the forces that want to tear it apart.

[Volkov]

That's the general idea I had.

I don't know how exactly weather patterns and wind work, but what would the weather be like on such a 'calm belt'? Would there be constant winds from the different temperatures and pressure zones of the two extreme hemispheres?

How would civilizations living on such a belt tell the passage of time? Could they perhaps see the changing constellations near the darker side of the sky?

I think a cool campaign setting could be produced out of this... a world living in a delicate balance between life and death...

It would be very windy due to enormous temperature differentials.

[Lycanthromancer]

Could it be possible to have one giant ocean around the habitable belt? Perhaps dotted with islands, and flanked by mountains/deserts sunside and glaciers darkside?

It sounds like realistically water would be moving around too much to really form large bodies of water, but maybe if there's a lot of water and a lot of rain/precipitation...

You wouldn't have that much water, actually, since the vast majority of the water would be in frozen deposits on the cold side. All of the liquid water would be around the equatorial zone, but it's all either being dragged sunward or shadeward, and it's mostly the sunward water that would return in a regular fashion (as ice-sheets can grow several miles thick, even in normal worlds; I'd hesitate to imagine just how many miles thick the ice shelf would be in sideways-world).

It would be very windy due to enormous temperature differentials.

I would expect frequent windstorms, even to the point of monsoons and tornadoes. This would leave civilization nomadic more than stationary; windstorms would demolish more permanent structures.

Also, the faster the world spins and the rougher the terrain, the more wind you'd have, I'd expect. Of course, the rougher the terrain, the more calm areas you might get.

[CockroachTeaParty]

Would plants wind up looking strange? Would trees have leaves only on the side facing the sun?

If the winds are especially crazy, I could picture winged races such as raptorans having greater power/importance, as couriers, scouts, etc.

[Jack_Simth]

Could it be possible to have one giant ocean around the habitable belt? Perhaps dotted with islands, and flanked by mountains/deserts sunside and glaciers darkside?

It sounds like realistically water would be moving around too much to really form large bodies of water, but maybe if there's a lot of water and a lot of rain/precipitation...

Well, here's the thing... how big of an ocean can be made, how fast water is moving, and whether you've got the sudden cataclysm, the slow turn, or no movement at all depends on quite a few things that are up to DM whim:

1) How much water is on the planet
2) The rigidity of the planet (crust, mantle, core, the interfaces between them, and any additional layers)
3) How much solar input is actually present (what color is the star, how far away is it, and how big is it? But those can be shorthanded to "solar input" as we've decided the planet is tidally locked already ... so it's just how much is hitting the planet, and how much is bouncing back off)
4) The mass of the planet

If the planet is sufficiently rigid, and outmasses the water enough that the water could all bunch up on one side without changing the gravitational arrangement any, then it won't shift at all, and you can have rather long-running civilizations where the enough glacial melt meets enough sunlight to grow crops. Bear in mind: If you go this route, it's normal, minus easy time, and plus rough weather.

If the planet is not up to the structural task of keeping the crust in place, and there's enough water to cause enough of a weight difference, then you'll get a periodic sudden shift of the crust. How often depends mostly on how much solar input there is (more sunlight means it happens more often... but bear in mind: if you go this route, the oldest civilization is either undead and in an inhospitable region, underground subsisting on magic/technology in some manner, is tiny and living off of some geothermal resource on coldside (rivers will move too often to form long-running civilizations, due to the effects of glaciation), was formed just after the most recent cataclysm, or was in a ridiculously lucky spot at the time of the last cataclysm, and didn't shift much).

If the planet is up to the structural task of keeping the crust in place, but doesn't sufficiently outmass the water to keep the center of gravity from shifting, then you'll have a very slow rotation of darkside ground to sunside ground as the world turns on a second (or third) axis. How quickly depends mostly on how much solar input there is. Bear in mind: If you go this route, the oldest civilization is either close to one of the borders (hotside/coldside), as above.

More water means more oceans; less water means less oceans, but all three other options are still viable in either case, due to other considerations.

AKA, pick your choice of sudden periodic disaster, slow habitat change cycle, or no particular migration, then build your world around it.

Oh yes, and with scenario 2 or 3, you could also have critters with a ridiculously long life cycle, that hibernate while their habitat is on coldside or darkside.

Also, I could definitely see a tropical zone closer to sunside, a temperate area, and a colder area closer to darkside. The idea of things living beneath the frozen ice of darkside, or the general 'underdark' of sunside would also be certainly possible.

I can see elementals, or elemental themes, being prominent as well. Fire for sunside, water (ice) for darkside, air for the crazy weather, and earth for the world itself, trying to resist the forces that want to tear it apart.

Could work quite well.

[TheMerchandise]

Would plants wind up looking strange? Would trees have leaves only on the side facing the sun?

I dunno if they'd be lopsided in that way, but there would be definite tilt. They might lean towards it so that all their leaves are facing that direction. Though there wouldn't be any trouble whatsoever distinguishing direction, it would be amusing to be able to tell which side is which simply by looking at the lean of the plants. And I certainly would expect that flying folk would be in high demand if they can bear the brunt of the winds. Wind power is a cool idea; whoever thought of that deserves a medal. An intarwebs medal.

[Lycanthromancer]

If there is a standard Underdark, or if the mantle of the planet is sufficiently fragile, the glacial sheets of ice could potentially crush the crust on the shadeward side. If it got thick enough, you'd end up with a gigantic ring of buckled crust, where the planet's shell simply collapsed. You'd end up with a massive ring-shaped crater around the leeward pole, possibly a few miles deep, filled with ice.

Bear in mind that this is only in case there's not a slow rotation beyond that swinging around the normal sunward pole.

[PersonMan]

You know, if the Dark/Sun sides are moving, why abandon cities? Personally, I'm imagining a city simply growing as it goes towards the inhospitable side, being built up towards the more temperate zones, with the scrapped ruins(or just abandoned areas) stretching deep into the deserts.

Interesting plot possibilities.
"Er, yeah, the ancient artifact we need is in the mayor's office."
"Then why not just get it?"
"The first mayor's office. Deep into the desert and by now probably collapsed and/or burned by heat and neglect, dozens of miles from here."
"Oh my...."

[TheMerchandise]

RE: plot possibilities-
Let's not forget the opposite being true. Why, what about the ancient temple that's been making its way across the shade for the past 3 or 4 decades? Dunno about you, but I have a hard time making my way through miles of ice. Blowtorch + couple of centuries maybe

[PersonMan]

RE: plot possibilities-
Let's not forget the opposite being true. Why, what about the ancient temple that's been making its way across the shade for the past 3 or 4 decades? Dunno about you, but I have a hard time making my way through miles of ice. Blowtorch + couple of centuries maybe

I just got a mental image of traveling through a city once protected my magical barriers against the ice, slowly collapsing as you go farther and farther, until the city was smashed beneath the ice.

Although, if the planet was small and this movement happened very quickly, you could have a "race against time" scenario to try and retrieve an artifact or whatever before the crust's movement drove it into the ice...

[Volkov]

You wouldn't have that much water, actually, since the vast majority of the water would be in frozen deposits on the cold side. All of the liquid water would be around the equatorial zone, but it's all either being dragged sunward or shadeward, and it's mostly the sunward water that would return in a regular fashion (as ice-sheets can grow several miles thick, even in normal worlds; I'd hesitate to imagine just how many miles thick the ice shelf would be in sideways-world).

I would expect frequent windstorms, even to the point of monsoons and tornadoes. This would leave civilization nomadic more than stationary; windstorms would demolish more permanent structures.

Also, the faster the world spins and the rougher the terrain, the more wind you'd have, I'd expect. Of course, the rougher the terrain, the more calm areas you might get.

Frequent? Try never-ending. The twilight zone would have at the very least hurricane force winds at all times. And it's almost certain that it will be F-5 tornado force winds at the least. Making a civilization in a world where one side is so hot that you quickly die from heat stroke, and on the other you freeze to death within moments and there isn't a plant to be seen, and the in-between zone has winds so fierce that the brick you are trying to lay hits you with enough force to instantly kill you is nigh impossible.

[Jack_Simth]

Frequent? Try never-ending. The twilight zone would have at the very least hurricane force winds at all times. And it's almost certain that it will be F-5 tornado force winds at the least. Making a civilization in a world where one side is so hot that you quickly die from heat stroke, and on the other you freeze to death within moments and there isn't a plant to be seen, and the in-between zone has winds so fierce that the brick you are trying to lay hits you with enough force to instantly kill you is nigh impossible.

Not necessarily. In order for winds to blow, they must push other air out of the way; and they're limited by how fast that other air moves. You'll have hot winds higher up, and cold winds lower down... but there's also a heat exchange between the two, and an inertia exchange between the two, and don't forget the impact of terrain.

But you won't necessarily get lethal winds very often.

[Hecore]

If you wanted a somewhat less dramatic world, you could make it a moon of a gas giant. Most of the heat is generated due to the interaction of gravity between the two worlds; the sunward side is still a desert (but more like the Sahara than Mercury) and the planetward side is more like an arctic ocean then a land of perpetual glaciers. The central belt would still be the best place to live, but there could still be a few scattered groups living in the two extremes.

At least this would solve the issues of massive earth/ice quakes and constant hurricanes.

[magic9mushroom]

A planet tidally locked to its sun will have a massive Hadley cell circulation. Air rises near the middle of the sunny side, sheds its water in huge perpetual thunderstorms, blows around to the dark side in the upper atmosphere, descends on the dark side, and blows near the surface back to the sunny side (meaning that the wind will always blow from shadeward).

@Lycanthromancer: Tides wouldn't actually pull water away from the middle. Because the tides don't change, the entire planet would elongate to compensate, meaning that gravity perfectly compensates for the tides (from the sun).

[Volkov]

If you wanted a somewhat less dramatic world, you could make it a moon of a gas giant. Most of the heat is generated due to the interaction of gravity between the two worlds; the sunward side is still a desert (but more like the Sahara than Mercury) and the planetward side is more like an arctic ocean then a land of perpetual glaciers. The central belt would still be the best place to live, but there could still be a few scattered groups living in the two extremes.

At least this would solve the issues of massive earth/ice quakes and constant hurricanes.

If it's too close and the gas giant has a powerful magnetic field, you will get a massive current of electricity flowing from the gas giant to the moon. Much like Io. This current will ionize the atmosphere and almost certainly fry everything it touches.