What major effects would having a flat or rather flattish earth instead of a planet on a game world? (Technological, social, physical etc.)

Mind you, I'm not looking for fully detailed scientific treatises. (Though if you have one, I'd be happy to take a look.) What I'm looking for primarily now is... things that PCs would have to face in your average but flattish DnD world, one in which truly significant obstacles (such as the problem of gravity, tides, the Sun etc) to the forming of such a world have been resolved by... well, gods and magic - yet in which less life-threatening but still major issues would still be there (such as the problem of the horizon etc.)

I know this is somewhat vague, so, for those who need a stricter guideline/handhold: try and base your answer on Terry Pratchett's Discworld. What social, physical etc differences would a visitor to that world encounter that would result purely/mostly from the shape and physics of such a world?


10 Answers 10


Noting that I don't like Pratchet's writings I have read, I've not read Diskworld, so my familiarity with it is very low, and so this is answered more generically...

So I'll base it upon a thick plate, and presume gravity is present and works close to normally, but always perpendicular to the world-plane.

  1. no coriolis forces on non-spinning worlds
    • storms don't inherently spin
    • thermal convection cells are localized
    • low albedo spots will trigger an overcast until they cool; high albedo ones will not.
    • horizontal tube storms may exist along mountains - the lack of coriolis force means that a ridge line that gets sun can heat up, create an updraft, which rises, cools, and falls to the outside, to be sucked back in, and spun again.
  2. Horizon - your horizon distance is infinite, but subject to terrain.
    • earth horizon can be approximated with d=3700(√h) where d and h are in meters. At surface, this is about 5 km for a man, and about 10km for a man atop a 4-5 story tower.
    • flatworld visibility is pretty much pure terrain and atmosphere.
    • meteorological visibility is 100km in clear air - a 1m black object can be made out at that range.
    • larger objects can be made out at further distances. A jumbo jet in clear air can be made out at up to 1000km... but it's 100x the size of the meteorological black dot.
    • Fog, Haze, Smoke, Dust, etc, can reduce visibility to under 1 km; hard rains can reduce visibility to 5m; snow or sand can reduce visibility to 1m or less.
  3. Warfare
    • you can't count on curvature of the earth to hide your ships - a ship is visible to between 50 and 100 km.
      • Marine assaults are almost always either predicted or under cover of bad weather
      • navigation fixes are easier - at sea, visual navigation is able to take one up to 300-400km off shore, and with even a small telescope, to the limits of haze. (3000-5000km)
    • ground forces become visible the moment they clear the terrain. An observation post can command a huge region.
    • Troops are much more easily signaled with semaphores and signal fire towers.
    • Empires will thus tend to border at major terrain features, especially lakes/oceans, mountains and woods.
    • target points can be used for navigation if built above tree-top level
      • forts will come in two types - command towers built high, and low buildings built strong near choke points.
      • Anything that lets you see further lets you be seen further - underground fortifications can only be seen by their entrances.
      • if magic is available, many magics to conceal regions and troops will be worked upon. If doable, expect borders to have pretty permanent weather and/or blur effects to hide troops.
  4. directions
    • Sun goes across sky in stable pattern:
      East = Sun-Goes-Up
      West = Sun Goes Down
      North - sun's Right
      South - Sun's left.
    • Sun goes up & down in one spot, no spin, or hovers over one spot and dims at night:
    • Sun goes up & down in one spot, disk has spin:
      trailing, widdershins, or anti-spinward.
    • No Sun at all, just diffuse sky-glow: the tallest mountain in sight will become the reference, with in/mountainward and out/away will likely be the prime directions.
  5. seasons & sundials - different climatology will prevail.
    • Diskworld style central "Bobbing Sun" - no seasons inherent, climate cools as distance from hub increases, and shadows increase in length as well
      • sundials still work, but are linear and point hubward. Length tells when in the day it is... but note that times will be measured by length - noon is shortest shadow, and hours will be measured from noon; one won't be able to tell if a given hour is before or after noon without multiple readings, in order to determine if sun is going up or down.
      • Seasonality can be created with a slanted sun path, or with periodic dimming, or with variable bob-height.
    • orbiting sun, slow spin disk - seasons by being out from under the line. Based upon the disk's spin, and 4 even seasons...:
      Summer 0°-22.5° off sun-path
      fall/spring 22.5°-67.5° off sun-path
      Winter 67.5-90° off sun-path.
      A full rotation is 8 seasons long. Sundials work pretty close to normally.
    • Orbiting sun, no spin. Unless it wanders, no seasonality, but relatively normal sundials. If it wanders, when it's closer, it's summer; when it's further winter.
    • Sun goes around the edge - far side is winter. No days unless it also bobs or dims.
  6. Some other considerations
    • Bobbing Sun - all large heat-gathering windows are sunward; windows on other walls are for either ventilation or outside view. Same spots inside in shadow at same point of day every day.
      • internal heat circulation will result from internal heat differentials, so shadowed corners will not be frozen, but the perpetually shadowed spots will be cooler.
      • the most expensive housing in a town will be that which is sunward, as rimward housing will have some of its sun blocked by sunward houses.
      • Cities will tent to arrange as amphitheaters for maximum light
      • rimward sides of ring mountains will be essentially frozen solid, and lit only by light reflected by the far plains. Habitation will be negligible. Growth will require either chemosynthesis, magic, or rapid transport predation. Nead constant high pressure. Spoke mountains will be have the same issue to a lesser extent in their ring-aligned valleys.
    • Orbiting Sun, fixed path and no rotation -
      • Mountains: far sides of mountains (north side of northern mountains, south side of southern mountains) will have the perpetual winter night issue.
      • Architecture: 3 sides of windows (pathward, east and west) are useful for heating; still, pathward exposure will be desired more than away-from-path
    • Wandering path orbiting sun or fixed path orbiting sun with rotating disk:
      • all areas will have some sun at least part of year.
      • During winter, outward mountains get perpetual darkness on outboard side.
      • during summer, all mountains get at least some sunlight.
      • housing arrangements much more normal
      • seasonal variation least as one approaches center of a rotating disk.

The things that we take for granted that would be different on a flat earth or discworld are:

  • The horizon would be weird – there wouldn't be one. Eventually things would just get so far away that perspective would make them too small or they'd just fade into the atmospheric haze.

  • What's on the other side of the disc? The sort of players I know would try to get there, if it exists in a meaningful way at all.

  • If the sun still moved from "East" to "West", it would move differently relative to the world. This can be handwaved away, but thinking about it can inform how your cosmology and "beyond the disc" works, which is likely to matter to players with powerful characters. Both possibilities also affect the way societies keep time and the technologies that would and wouldn't work (i.e. sundials).

    • If it was still a circular path, it'd be a different distance from you depending on where on the disc you stood if it had a circular path (e.g. farther when rising and nearer when setting to someone near the "West" edge). This would also mean that the shape of its path through the sky would vary by where you were on the disc.
    • It would have an entirely different sort of path and a cascade of differences due to that.
  • How do stars work? Astronomy is an important part of navigation in pre-modern cultures because your position can be determined by them due to their regular motion, so this would have a large impact on travellers, such as most PCs. Maybe people don't navigate by the stars because they're just not useful for it? Known paths and passes might be more relevant. Sea travel might be significantly stuck in the stage where every ship is a "coaster" that doesn't venture beyond sight of land.

  • Wind and ocean currents happen on Earth due to its spherical nature and the rotating heating-cooling effect of day. What causes wind on a discworld? What keeps the atmosphere "in" in the first place? Is there a void farther out, or is it all just air? Or does local reality stop beyond the "sky", keeping things bottled up (which might make wind mechanics be more familiar)? Again, relevant because players tend to like visiting the edges of things.

  • Seasons as we know them would happen for entirely different reasons, if at all. Are they predictable? Seasons vastly influence cultures and civilisations, so how they work can be an interesting way of tweaking life on the disc.

The setting of my current game is a disc, so I was thinking of these things recently. I solved a lot of them by just handwaving it away as "reality is just what the bodies of the Gods look like to mortals", because making these things work coherently is hard for a sphere-living creature like myself.


Just look at our own history. First thing would be to ask if there are irrefutable proofs that the world is flat? If you climb the tallest mountain in the world can you see the end of the disk?

If yes, then navigation and transportation would be a different thing. A single war between two extremities of the disk would be a major problem for the economy of everyone.

Whatever would be in the middle of the disk would gain some spiritual importance or maybe political. What's in the middle of the disc? Some crater with a glowing crystal? A giant pillar that actually make the world work like a spinning top?

If people just assume the world is flat because there's no other way to prove it, people would try to know if the world is actually flat. It also depends on how the world ends...if it ends with high mountains, maybe an individual climbed it and came back with a vision of the Otherside. He may have gained powers from it. Or even cooler than that...he might had become blind after seeing the other side of the world.

Don't worry about extremely detailed weather and explanation for a flat world..it's impossible (or really unlikely). Make it cool..mysterious and fill the cultures with superstition.


The world of Glorantha (found in Runequest mostly) is lozenge shaped. But the answer to of most of the OP's questions is that physics works differently. There are cracks where you can fall into Hell. The sun moves across the sky for historical reasons - it used to be fixed. Weather is determined by things like the actions of powerful elementals. Tides are determined by the moon for mystical reasons and by the fact that the ocean is slowly exiting the universe through an unfortunate hole in the middle of the world, caused by chaos. No idea why gravity exists, but it might well affect dwarves more strongly than humans, because they are associated with stone. There isn't a complete in-game explanation because nobody understands that much of how it all works.

As far a seeing a great distance, it works but the sky has to be clear enough, and there is still terrain that can get in the way. You can't really ever see across on ocean or something, but you might see an approaching army hundreds of miles away.

Anyway, my point is that for some kinds of games, like fantasy in particular, you probably shouldn't have a modern understanding of physics inform how things work. You get to say how much of the physics of our world apply, and it can be more fun to make it different. Maybe read up on what various historical cultures thought the physics of our world was, and decide some of that is right. Maybe make it fit the campaign. Maybe just have things for the sake of being strange and cool.

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    \$\begingroup\$ I recall speculation that there is a horizon because light, being an attribute of the sun god Yelm, is naturally attracted to the sky and so curves upwards, but this is only noticeable at great distances. Likewise Gold used to be the lightest of metals, but became heavy when the Sun God was slain in mythic times and descended to the underworld. Also the middle world is very small compared to the size of the sky dome, so although the sun is closer to the eastern lands in the morning than it is in the evening, this is a relatively small effect. \$\endgroup\$ – Simon Hibbs Jul 19 '12 at 15:20

If the planet (the disk, actually) orbits the sun, and not otherwise (as in Discworld), a major effect would be no real nights to speak of, since the sun wouldn't actually set. You could alternately argue for "no real days" or "two different environments, one always in the dark, another always in the light", which suggests many interesting possibilities.

Assuming the disk rotates around the sun, and also around its axis, I could assume huge amounts of magic (as described in the Discworld books) to keep it from collapsing into a ball or a "bullet". The other explanation I can think of right now and still have it make sense is that the world would have to be made of a material hard enough (or magical enough) to withstand gravity. Which would mean that digging it would be very difficult.

Pratchett thought of 8 different seasons for the disk in Colour of Magic. It was -at least in the Spanish translation- a very complex paragraph, and I haven't read anything about it in any other DW books.

A good treatise on "flat" worlds is Ringworld, by Niven. It's true it doesn't actually feature a disk, but many of the problems are common. For example, the strength of the material required, the need for huge walls to keep the atmosphere in, the huge plates floating in the sky between the central sun and the ring...

@SevenSidedDie makes a good point on how in a universe not ruled by our laws of physics, things could be explained differently, but to give an accurate explanation in that case is a harder task than I can accomplish here and now.

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    \$\begingroup\$ …Strong materials, or science as we know it just not applying in that reality. \$\endgroup\$ – SevenSidedDie Jul 17 '12 at 22:01
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    \$\begingroup\$ Pratchett actually invested some time in the first book in practical worldbuilding, before deciding that sociocultural worldbuilding was more important for his purposes in later books. \$\endgroup\$ – TRiG Jul 18 '12 at 0:19

One point that no one has brought up yet is that it's likely that people will use a different set of cardinal directions than we do on a globe. Using the example of Discword, since the direction that the sun rises and sets in are different in different parts of the year, the traditional North, South, East, and West based on where the sun rises and sets is not really a feasible navigational tool. Instead, they refer to Turnwise (towards the direction of the spin), Widdershins (opposite the direction of the spin), Rimward (towards the Rim), and Hubward (towards the hub). Of course, this doesn't apply if your flat planet doesn't spin like the Disc does. If it's circular and not spinning, there still would likely be the concepts of Rimward and Hubward as standard directions, though.


As to how the sun moves and how seasons might work... In Discworld itself, the world slowly rotates (completing a rotation every two 'years'), while the sun's orbit is fixed. The sun therefore rises and sets at a slightly different point of the rim each day. When the sun is on a path that passes directly overhead, it is summer on your slice of the disc; 90 degrees away in either direction it is winter. The central region should logically have a somewhat cold, but less varying, climate - due to the sun never coming particularly near, and having a more uniform distance. Alternate summers might have a very different character in the more extreme rimward regions, due to the sun coming closest in the morning one year, and in the evening the next. Very close to the rim, summer temperatures might be extremely hot, due to the proximity to the sun at sunrise or sunset, while winters might not be much harsher than in other regions.

Discworld's moon's orbit is incredibly complicated, and it has a physical light and dark side rather than being lit on one side by the sun.

Such a world would not have "time zones" - there is only one sunrise, and one sunset, and it happens at the same time for everyone. As an aside, Discworld itself has time zones due to slow propagation of light (on the order of hundreds of miles per hour). Logically, if this were the case, these would change throughout the year: you would have a twelve hour day in the winter, a short day in the summer where the sun sets near you, and a long one in the summer where it rises near you [the seasons would still work because each area still gets the same total insolation]. High noon would be at the same time year round, with central regions getting it before rimward ones, and civil time zones might be standardized on this basis.


There might be no problems at all.

Remember that people all over this world once thought the world was flat. Living on Earth, it's hard to tell the world isn't flat in everyday life.

Let's say the world is a very large flat plane -- a disk or something like that. Gravity pulls everything down in the same direction. The plane itself is held up by something reaching infinitely far down. It could be dirt and rock all the way down, or an endless stack of turtles, or infinite stilts -- anything will do. The sun, moon, and stars are all weightless and circle around the world, very far from the plane. Air fills the universe up to the level of the plain and just a little bit higher (to be the sky).

You can't see an infinite distance. Air itself isn't completely transparent, which is why distance mountains look bluish. At a great enough distance, things will just look sky blue. Mountains and other terrain features will also get in the way.

The sun isn't any closer to different parts of the world. Rather, it is closer, but so imperceptibly that it makes no difference. Remember, the distance from the earth to the sun is much bigger than the width of the earth. Assume the same for your flat world.

Seasons could still happen. The sun's path around the world could tilt north/south over the course of the year, just like the sun appears to do in our world. If the center of the sun's orbit is actually below sea level, and its orbit tilts far enough, you could have long periods of dark like at the poles on earth.

The sea of air could have swirls and eddies in it. As the sun heats the great sea of air around the world, it could have pressure differences just like on earth, leading to winds. Cooling happens as the air is in the shade of the world.

Magnetism could still direct compasses. If you're already assuming gravity points down the same way for everyone, it's not too hard to assume that magnets always have a force making them point north.

At the edge of the world, as long as you have mountains keeping the sea in, you won't have any problem with losing the world's water. Then again, it might be that the regions near the edge are completely dry, because their water has drained away. And it's only a matter of centuries till the whole world has gone dry.


With that big a change I wouldn't assume any other laws of physics HAVE to apply. Perhaps the disk rotates making the sun move across the sky from left to right at a single level, but the sun only lights half the disk (Making it dark when the center point was between you and the light source). Heck, maybe the world is just "Light" with no directionality or periodic darkness--or perhaps the light comes from a disk above your disk that you may even be able to reach...

Winds and currents could work any way you want them to, of course. If the place was created deliberately, there may be no default topology (why bother creating mountains?)

As you dig down gravity may get stronger, lighter or stay the same--your choice--heck half way through the "Disk" you could be in 0 gravity and when you make it to the other side, maybe a whole new world!

Why guess what would happen when you can specify what you want to happen?


A quite important and not necessarily obvious point regarding disk shaped worlds is that gravity attenuates differently. There's a novella by Charles Stross named Missile Gap which deals with cold war earth being mysteriously laid out flat on a disk, with the following key difference:

  • Gravity falls off according to the inverse square law around a sphere.
  • To a first order approximation, gravity falls off linearly as you depart the surface of a disk.

The consequence of this is that rockets simply don't have the power to leave the disk and ICBMs are incapable of reaching their targets.


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