Once upon a midnight clear, I wrote a fantasy story set, largely, in a flat (and magical) world. I won’t go into any detail about the story, because this thread isn’t about the story so much as about the idea of a flat planet.
Let’s postulate the following:
The world in question is a disc of about the same surface areas as our Earth; the distribution of land and sea area is about the same as well. Its surface gravity is about the same as ours too, as its its the chemical composition of its atmosphere. It does not rotate; heat & light are provided by a sun that orbits it. Its crust is the same in chemical makeup as our world’s; beneath it, though, is a base made of, oh, adamantium, one hundred miles thick. The planar Earth is surrounded by a clear dome, outside which the sun orbits. Said clear dome is made of unobtainium, which is transparent to EM radiation but impenetrable to any material object.
In what ways, both obvious & subtle, will such a place would differ from our world?
The first thing that occurs to me is that, unless the weather is always the same in all parts of this world, the sky will never be completely overcast.
Gravity. It will pull straight down in the center, and almost horizontally near the edges. Because of this, there will be a central ocean surrounded by land . . . and lots of steps and ramps everywhere.
Because the distance to the center of mass increases as you move away from the center of the disk, surface gravity can’t be of the same magnitude everywhere. It also can’t be of the same direction everywhere either; objects will fall towards the center of mass of the system, and at the edges that’s not the direction that we would consider down.
I’m also pretty sure that you can’t have a star orbiting a planet, but not quite confident enough to say that definitively. The only way that could happen is if the planet is much more massive than the star, but it’s not clear how a small object could begin to produce fusion reactions while a larger object doesn’t. Since the star has to be (much more) massive than the planet, the center of mass of the star/planet system–which is what both objects actually orbit–will be much closer to the star, if not actually inside it.
No tides. Ships will fade into the distance instead of sinking over the horizon. Weather and ocean currents will be different; looking at Wikipedia it appears that you wouldn’t get hurricanes on a non-rotating planet, and apparently a lot of other feature of climate/weather patterns depend on the Coriolis effect.
You could perhaps get around the gravity problem by changing the size of the disc, so that the edge effects are far away from the action. I like the Alderson disc, a vast disc with a star in the middle ( like an old style phonograph record ). Apparently on a disc, gravity is “down” except near the edges, so you’d want one bigger than Earth to provide an Earth’s worth of living space.
As for the argument that you can’t have a star orbiting a planet, since the whole system is clearly artificial, the obvious solution is that the sun isn’t a star, but some sort of much less massive fusion device.
Unless you want to say “wizards did it” to every problem, this scenario simply can’t work.
How does the sun orbit this tiny planet? How does the disc remain stable if it doen’t rotate? How can it have any topography if there’s no mechanism to build mountains? How are you going to keep the glass shield free from meteorite dust build up?
So we’re clear, I’m not trying to get around the gravity or any other problems, as I’m not working on the story. My explanation when i wrote it was, of course, magic.
I was thinking the whole thing had to be artifically maintained; in fact, something I left out of the OP was the notion that, since this world in question is much, much less massive than Earth, its gravity is supplied by artificial-gravity generators, doubtlessly stolen from Jean-Luc Picard. The sun, likewise, is in orbit around the planar world because it is being artificially propelled.
I hadn’t thought about tides. That is a good one. If I write a sequel to the tale I’ll mention that. The ships-not-thinking bit was one of my first thoughts, though; it was how the visitor from our world realized that the people on the plane weren’t full of shit when they said their world was flat.
How much pseudo-scientific hand-waving do you want in the explanation?
If you permit artificial gravity, for example, you’ve solve a lot of the issues right there.
One thought about the star rotating around the planet: you could theoretically have a binary star/black hole system. The star and black hole would have to be the same exact mass and period of rotation so that they could rotate around each other with a fixed common center in which the flat world would lie. They couldn’t see the black hole and thus would think there was only the sun. I suppose a black dwarf or cold neutron star might also fit the bill. This kind of system would make planets and moons either impossible or highly erratic in their movements.
As far as weather goes, I’m not sure how much weather you’d have at all. The Earth’s rotation and its roundness both contribute significantly to weather and you wouldn’t have either on a flat world. You might have small weather effects like air rising in the day and sinking at night, but there’d be nothing to drive larger weather patterns.
We don’t know the properties of a clear unobtanium dome, but my guess is that it would be pretty cold being exposed to space. Rising humid air would probably condense on it and either turn to frost or form droplets that trickle down the sides. Either way, it would be pretty hard to see through and might mean that rain as we know it is impossible. (Even if the dome is a hundred miles high in the middle, condensation would still be a problem on the edges).
One difference is that the stars don’t move across the night sky. All stars are fixed except the “sun” and any other bodies orbiting the disk. So nightime navigation is easy. Since the stars never move you can pick a bright star in the direction you want to travel and always keep it in the same position and you’ll travel in a straight line.
You may want to read Terry Pratchett’s early (pre-Discworld) novel Strata. It’s about a team of engineers who visit an Earthlike but flat planet that turns out to be the product of technology (not magic, as in the Discworld series). IIRC, the first people they meet are some Vikings who are sailing off from their homeland in search of new land…and either nearly or actually sail right off the edge of the planet.
But won’t the non-round planet quickly become tidally locked with the sun? Which would mean that there would be perpetual dusk on the flat surfaces, and either fiery inferno or freezing wasteland on the edges.
I dunno about quickly. Eventually maybe, but eventually the Moon is going to crash into the Earth, doesn’t mean I’m worried. The total mass of water on the disk planet is probably a fairly insignificant value on a planetary scale and we don’t know how long this planet has been like this or how it’s powered.
Or heck, you could spin the whole thing like a top. That would cause the sky to rotate and produce no discernible tide.
You could have a dual-star system where one of them is a black hole and there’s a disk planet between the two.
Though I’m not sure if you can do that and get the bodies far enough away from one another to not have the planet be too hot.
The sun, likewise, is in orbit around the planar world because it is being artificially propelled.