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Scenario: While poking around in an alien ruin, scientists discover a gateway which offers instant transportation to an Earth-like world.

The Observed World: The gateway leads to an area that is temperate (let's say it's similar to east coast of America, like Virginia/Maryland/Pennsylvania, for simplicity). The air is breathable, and there is an ocean visible within less than a mile. The gravity and day/night cycle almost exactly matches Earth. The planet has no visible satellites, natural or otherwise. There is flora and fauna, but no intelligent life.

The Actual World: The "planet" is actually an artificial construct—a flat (coin-shaped) world created through technological (rather than supernatural) means. The size is similar to what Earth would be, were its surface peeled open like an orange, and flattened into a disk. Gravity is artificially generated and regulated to mimic Earth (so you won't be pulled at an angle as you approach the edge). The atmosphere is held in by an invisible field which forms a dome over the livable side of the world.

The world orbits a yellow star similar to the Sun, and also rotates (like a spinning coin), so the sun will appear to rise and set.

Question: If a team of scientists are sent through the gateway with the purpose collecting flora/fauna/air/water samples, and observing the night sky (to determine the planet's location relative to Earth), what would tip them off that they're not on a typical spherical planet?

Particularly, what would stand out to someone with a good grasp of general physics, or astrophysics, even if they had no reason to suspect that the planet was anything other than a typical sphere?

I'm not looking for a mathematical proof, but rather something that visibly stands out and would make a scientist decide to perform such a proof in the first place.

Their available technology is modern-day: telescopes, laptops, quadcopter-mounted cameras, etc.

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    $\begingroup$ If they looked over the side and saw a turtle. $\endgroup$ May 28, 2016 at 11:40
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    $\begingroup$ No time an not enough physics to follow this up with an answer: what about Foucault's pendulum? en.wikipedia.org/wiki/Foucault_pendulum (They'd have to suspect something to set up the experiment.) $\endgroup$ May 28, 2016 at 20:00
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    $\begingroup$ Wouldn't star trails ( en.wikipedia.org/wiki/Star_trail ) be another give-away? I wonder if aiming straight away from the surface would produce straight lines, as opposed to curved lines when on a spherical world (when not on the equator). $\endgroup$
    – frozenkoi
    Jun 1, 2016 at 2:16
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    $\begingroup$ I wonder how you peel an orange and flatten it so that the result looks like a coin. $\endgroup$ Jun 1, 2016 at 9:11
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    $\begingroup$ @RemcoGerlich: Step 1? Believe in yourself. $\endgroup$
    – Liesmith
    Jun 1, 2016 at 10:43

21 Answers 21

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Daybreak and nightfall would be spectacular

A flat coin shape would have a day face and a night face with sudden transitions because unlike a sphere, it blocks all sunlight with its own shadow, there is no refraction around the sphere. The sunlight also passes through much more air when close to the horizon.

If you start at noon, things would appear quite normal and stay so into twilight as the suns moves lower in the sky. A few minutes before sunset the sunlight starts fading much more rapidly than it would on Earth. The effect would be like the sun sinking into clouds even on the clearest day, until the sun barely outshines our moon and the sky would be as dark as night while the sun is still above the horizon.

The moment the sun passes the horizon, it will be completely dark. There are no shimmering clouds or scattered glow, only pure darkness. The scattered light from the edge simply can't reach you through thousands of km of air.

Daybreak would arrive just as suddenly, with what looks like a moon rising in the night suddenly increasing in brightness until normal sunlight a few minutes later.

If the observer would be very close to one edge of the world, sunrise and sunset would be asymmetrical, with the closer one fading closer to the horizon.

No scientist is going to take long in figuring this one out.


EDIT: I've updated my answer for a bit more scientific accuracy, as many comments pointed out the effects should be noticeable even before the sun sets, and they are right. Below is the science behind the answer, that I could find online.

The air in our atmosphere reduces the intensity of light going through it by scattering, absorption and reflection. Even at the shortest path (straight down when the sun is at zenith) only about 75% of visible light makes it to the surface. This is a well-known and important effect in Astronomy and other fields of science and modelled as "Air Mass" (wikipedia). The lower the sun is in the sky, the higher the amount of air mass the light travels through. On Earth, the air mass is about 38x higher when the sun is at the horizon, resulting in a drop in light intensity in the environment from 100k+ lux to only ~400 lux on a clear day.

On a flat world with a similar atmosphere this would be about the same until the sun gets close to the horizon. Then the light needs to pass through much more atmosphere making it much darker. I drew this for a visual impression. The blue and purple areas show the atmosphere of a round and flat world respectively. The curve is exaggerated for clarity. It's roughly to scale for a 100 km high atmosphere, but only the lowest few km have enough density to matter.

diagram of sunlight at various angles on a flat vs curved world

It's easy to see that the difference in air mass is unnoticeable until about 85 degrees from zenith (5 degrees above the horizon). This is 20 minutes before sunset on Earth. 12 minutes before sunset (87 degrees) the difference is perhaps 20%, noticeable but easily dwarfed by variation due to atmospheric conditions (hazy sky). 6-7 minutes before (88-89 degrees ) it's already as dark as it should be as sunset and then in the next few minutes the sun and daylight will fade to perhaps only the strength of a full moon before setting.

To back this up with some numbers: The Wikipedia article has a graph with several models. Conveniently, the most basic one is the "plane parallel" which is a flat world. It's given only as a reference because it is invalid at high angles, but exactly what we want to compare. When looking at the graph, the air mass at sunset (90 degrees) is ~38 for most models, a value the plane parallel line already reaches between 88 and 89 degrees. The air mass then increases rapidly, approaching infinity because it doesn't account for the limited radius of the flat coin world in this question :-).

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    $\begingroup$ This for sure. The other things might go unnoticed, but the first day/night transition would be an dead give away. After, they could perform all kinds of experiments to confirm. $\endgroup$
    – Trenin
    May 27, 2016 at 18:07
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    $\begingroup$ You probably would see something of a twilight as the sun goes down. You go from very little distance through the atmosphere at noon (~16 km) to approximately the entire radius of the disk (~ 12000 km, if it's the same surface area as earth). That's a lot of light attenuation. Wikipedia indicates that at the clearest, max visibility is ~300 km. From the angles, you should reach that at about 12 min before sunset. So you'd get fading light for about a half hour before "true" sunset. Perhaps a bit longer, if the air is cloudy/dusty. $\endgroup$
    – R.M.
    May 27, 2016 at 18:44
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    $\begingroup$ No. There will be a difference but you aren't going to get the sudden effect unless you're near the edge already. Rather, the sun will fade in/fade out due to the huge amount of atmosphere it's going through. $\endgroup$ May 28, 2016 at 0:19
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    $\begingroup$ I guess nobody who ever thought the Earth was actually flat realized this... $\endgroup$
    – Michael
    May 30, 2016 at 5:43
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    $\begingroup$ Also the sun is very big[citation needed], so it takes up a significant portion of the sky. The sun's angular diameter is about 32 minutes, or half a degree. For that reason alone, sunrise/set will not be instantaneous, but will take at least 1/720 of a day, provided that the sun is the same size and distance as on Earth. (On Earth this is about 2 minutes.) $\endgroup$
    – Devsman
    May 31, 2016 at 18:46
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Apart from the horizon topic that was already covered by Separatix and Ctouw, they could quickly verify their observation by measuring the angle towards the sun at different points of the planet at the same time).

Those angles will, much unlike at home, be almost identical, since they are measured from a plane a large distance from the observed object (the sun), while comparably close to each other, even if they are on different continents.

Also, they will quickly notice that they won't have time zones, for exactly the same reason.

As a result, all programmers of earth will, almost immediately after that discovery, migrate to the new planet, and will forever be happy coders that don't have to deal with time zone handling any more.

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    $\begingroup$ This. Kill all timezones. And the people who think "daylight saving time" is a good idea. $\endgroup$
    – Tim B
    May 27, 2016 at 13:41
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    $\begingroup$ @TylerH Not according to the programmers that inhabit it. $\endgroup$ May 27, 2016 at 14:48
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    $\begingroup$ This assumes that they travel far enough to experience the lack of timezones, or split up and take measurements. I think OP wants something that will be a trigger like "Hey guys! I think we are on a flat planet because I just unexpectedly noticed XXXX" $\endgroup$
    – Trenin
    May 27, 2016 at 18:02
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    $\begingroup$ The no-timezones feature could be the reason this planet was created in the first place. $\endgroup$ May 28, 2016 at 0:38
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    $\begingroup$ You think time zones are bad? How about the fact that this newly discovered planet has no moon? There are 5 types of astronomical months, and at least 20 different ways of designating civil months. $\endgroup$
    – Michael
    May 30, 2016 at 5:46
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Horizon effects would be the first signal.

As a quick and dirty calculation, the distance to the horizon in miles is half your height in feet.

Given their visual range is going to be far greater than that, you have two options, either the world is absolutely vast (even though gravity is Earth normal) or it's flat.

They'll quickly realise something strange is going on, after that it's a matter of working out what. Spread out, do some triangulation and they'll find the answer.

Then the sunset will be all wrong.

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  • $\begingroup$ There will probably also some strange things going on with the weather considering the lack of Coriolis force and possibly strange weather phenomena at the edge of the disk. I am not planning to write an answer so if you find this point valid you could add it to yours ;) $\endgroup$
    – Jaywalker
    May 27, 2016 at 13:23
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    $\begingroup$ Due to my hobbies I am liable to notice within a few hours the hull-down effect just isn't there. Depending on where I'm let down I might well notice immediately. $\endgroup$
    – Joshua
    May 29, 2016 at 3:00
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    $\begingroup$ Why would they be expected to assume that the planet is the same size as Earth? Might it be a much larger planet (with much farther horizons) made of less dense material? $\endgroup$
    – Readin
    May 29, 2016 at 4:01
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    $\begingroup$ @Readin, first judgements are always based on experience, Earth gravity would initially imply Earth radius so you'd expect the same horizon line. $\endgroup$
    – Separatrix
    May 29, 2016 at 15:40
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    $\begingroup$ This was my first thought also, though one must concede there's a catch: If the terrain is hilly, or they're in a thick forest, or some other situation where you don't expect to see all the way to the horizon, you might not notice. If you're on the coast of the ocean or in a wide flat desert, would you notice if there were no obvious landmarks? I'm not sure. $\endgroup$
    – Jay
    May 31, 2016 at 13:48
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An Edge

Not sure why this has not been said, but when you go for a long enough walk and get to the perimeter of the disk planet, there is an edge.

In the Truman Show, the edge looks like this:

The Edge

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    $\begingroup$ Given the specified size of the flatworld you might need to travel quite a long way to find an edge. $\endgroup$ May 29, 2016 at 6:57
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    $\begingroup$ ROFL, by far the most humorous answer. +1 (Also, it works!) $\endgroup$ May 30, 2016 at 10:48
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    $\begingroup$ It's worth noting, that most current "flat earth theorists" say that the edges are all at water. That is, all the continents are surrounded by water, and far after the water comes the edge $\endgroup$
    – Cruncher
    May 30, 2016 at 13:01
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    $\begingroup$ Though it's the most blunt solution and requires the least scientific insight, this is for sure the most dramatic solution. $\endgroup$
    – Devsman
    May 31, 2016 at 18:52
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    $\begingroup$ You find out the gender of A'Tuin $\endgroup$ Jun 2, 2016 at 15:48
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There will be no horizon line or it would look way further than on actual Earth

The horizon line is caused by the Earth being a sphere, so that when you look straight in front of you, at a certain point you can't see things because they are hidden by the curvature of the Earth itself.

In the world you describe, you would see what is in front of you up to the edge of the "planet"; or, depending of its span, instead of having a horizon line, distant objects would progressively disappear into "distance fog". But you would see way further than on actual Earth anyway.

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  • $\begingroup$ Started writing my answer before Separatix posted, sorry for the duplicate. $\endgroup$
    – user20952
    May 27, 2016 at 13:26
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    $\begingroup$ Don't worry it happens all the time $\endgroup$
    – Jaywalker
    May 27, 2016 at 13:27
  • $\begingroup$ Being able to see further away could also indicate the new planet is (way) bigger than Earth, but still a sphere. $\endgroup$
    – Abigail
    Jan 15 at 14:44
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Triangles

Most of the methods posted involve the sun or the sky, but since the planet was created using some very advanced technology, it's possible these effects are hidden.

Instead, a simple and foolproof method is to measure a (sufficiently large) triangle.

Why?

You might think the angles of a triangle add up to exactly 180°, but this is only true on a flat surface. On Earth (and any sphere) the angles of a triangle actually always sum to more than 180° (up to 540°). There are similar distortions in area, and other properties.

So, as soon as they seriously consider any 3 points on the planet that are far enough apart, someone clever will notice something is unusual (180° triangles).

As a bonus, because this is a purely mathematical property of flat surfaces, there is absolutely no way to hide the flatness of the world.

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    $\begingroup$ If the triangles are line of sight, you get 180º even on the surface of the Earth. Measuring an angle surplus has to be done with lines that follow the contours of the surface itself, and will be rather confounded by any hills and slopes present. $\endgroup$
    – user2781
    May 27, 2016 at 17:46
  • $\begingroup$ If you're going to invoke godtech as a reason why other measurements might not work, you have to consider the possibility of being in a funky universe. Like if the EM and gravitational fields aren't divergenceless, you could get all kinds of wackiness, including a world that seems flat but is actually curved. $\endgroup$ May 27, 2016 at 19:43
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    $\begingroup$ Hurkyl, it of course has to be over a large enough distance that Hills and slopes are negligible. It wouldn't be something you'd notice walking down the street, but any attempt to map the area will reveal the flat nature (and it IS an exploration mission ). $\endgroup$
    – LaDeDaDo
    May 28, 2016 at 12:08
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    $\begingroup$ But why would they want to do the triangle test in the first place, if they were not already suspicious about the "planet's" form? $\endgroup$
    – vsz
    May 29, 2016 at 17:21
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    $\begingroup$ They're on an exploratory mission, so it makes sense they would try to draw maps. The person doing this would find it odd that the world perfectly maps to a flat plane (this isn't supposed to happen which is why there are all sorts of complicated projection schemes). Admittedly, the effects from the sun people mentioned would be noticeable before this. So if nothing hides those, this would merely be a confirmation of what they already figured out. As a historical note, people have done this sort of test as far back as ancient Greece. $\endgroup$
    – LaDeDaDo
    May 30, 2016 at 4:41
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Eratosthenes made the first estimate of the size of the spherical earth. He did so by measuring the length of the sun's shadows while at two different locations. One location was much further north than the other, and the length of shadow told him how much the earth surface curved between the two. His measurments were both performed on the summers solstice when the sun was at it's highest point in the sky. The known distance between the measurement points and his calculated difference in the sun's shadow angle allowed him to calculate the curvature (size) of the earth, within 5-15% of its true value.

So if you are limited to low tech, the way to measure a planet's curvature is if two people were to measure the angle of the sun's shadow at noon on the same day at two distant points (exactly north-south of each other), and if that angle is different at the two locations, that would imply that the earth is curved and probably spherical, or at least curved in the north to south direction. The time-zone answers given by other answers here would prove the earth is also curved in the east-west direction, but to use that method requires precise/reliable clocks for the two measurements to occur at the same time but east-west from each other.

Which is also one of the reasons why sailors historically needed precise clocks for navigation; this allowed them to know how far east-west they were when out of sight of land. What time noon occurs at depends on how far east-west you are. If you know the time, the sun's position tells you where you are (east-west). If you know where you are (east-west), you can use the sun to tell you what time it is.

Given your premises of scientists with higher tech, I would say one tip-off to them of the flat planet scenario would be if one scientist were to video-call another while they were located at widely different points on the flat planet, and one of them notices that the sun's shadows appears to fall at the same angle at the same time in both locations.

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    $\begingroup$ +1 for the plausible scenario for making that accidental discovery. $\endgroup$
    – frIT
    May 30, 2016 at 8:07
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    $\begingroup$ +1 for pointing out that east-west curvature could be different from north-south curvature. If ellipsoid / sphere isn't the only option, then cylinder is possible as well as flat. $\endgroup$ May 30, 2016 at 17:24
  • $\begingroup$ +1 for historic reference to how our civilization figured it out $\endgroup$
    – Atog
    Aug 12 at 23:05
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No clouds near the horizon
On a cloudy day when you look at the horizon where the sky meets the water over the ocean, you see clouds appearing to touch the water even though those clouds are in the air. On a flat world the clouds would not do this. They would get close to the horizon, but never touch.

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    $\begingroup$ Wouldn't perception of perspective make the things almost the same? You may see farther, but water and clouds will eventually look as if they're touching (since you can't really know which side of a cloud you're seeing). $\endgroup$ May 31, 2016 at 6:32
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    $\begingroup$ @KamenMinkov: I think you'd lose vision in haze at a short enough distance that you could tell the difference. For example if the clouds are at 1km and you can't see much beyond about 300km then there would still be a discernible angular "distance" between the sea and the sky except that air isn't transparent enough. But I might be wrong. $\endgroup$ May 31, 2016 at 9:01
  • $\begingroup$ This is the same phenomenon as a ship appearing to be low in the water when it is a few miles offshore. Take a sailing ship a bit further out and you see the sails but not the hull, though you can climb a hill or tower to peer slightly further over the horizon. Sailors knew the world was round long before learned men accepted it! $\endgroup$
    – nigel222
    Jun 17, 2016 at 11:01
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Atmospheric readings would be weird

On Earth, the atmosphere is a spherical layer around a spherical earth. However, you say that the atmosphere is a dome over the world. If the dome is a physical object, it would have to be close to round to be able to support its own weight. If it's held there by the gravity, most of it would spill over the edges. If it's an invisible force field, the easiest way would again be a spherical field.

Assuming the dome is round...

  • Either the entire atmosphere weighs just as much as Earth, which leads to a much thinner atmosphere;
  • Or the atmosphere is just as dense as on Earth, which leads a much larger atmospheric pressure;
  • Or the atmosphere changes density in a different way than on Earth, which means that incoming light is diffused differently.

Assuming the dome is a fixed-height force field, then the lower view angles would have to travel through far more atmosphere to reach you compared to on earth, again affecting diffusion.

Compasses would not work like they do on Earth

Earth's magnetic field is caused by molten metal deep inside the core moving around and generating electric currents. Because of the Coriolis effect, this field is roughly aligned with Earth's rotational axis.

However, on a flat disk, there probably is no molten core, and even if there was one, it wouldn't generate a magnetic field in the way that we know it on Earth, because the molten metal would flow differently and there wouldn't be as strong a coriolis effect.

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  • $\begingroup$ It isn't the atmosphere that's a dome. The dome is a field surrounding the entire disc. That will give the atmosphere the same exponential pressure gradient we have on Earth. (By the way, how can a dome over a disc be a fixed-height force field?) $\endgroup$ May 27, 2016 at 20:16
  • $\begingroup$ @MackTuesday Are you saying that the artificial earth-surface-normal artificial gravity works on the atmosphere above it, and hence causes the same type of atmosphere, rather than the atmospheric pressure being caused by the dome? That could work. $\endgroup$ May 28, 2016 at 0:08
  • $\begingroup$ @DewiMorgan -- I don't see why gravity would affect people but not the air, but let's say that's true. This hemispherical dome is big and holds a lot of gas. If the pressure is the same throughout, light will be maximally scattered across the visible spectrum, making the sun indistinguishable in a fantastically bright sky that dims near the horizon. $\endgroup$ May 28, 2016 at 1:40
  • $\begingroup$ "Artificial gravity" just means "a downward force not caused by gravity": spinning, etc. Assuming a hemispherical dome with a radius of about 8000km (based on it covering a disk with earth's surface area), and gravity reducing at an earthlike rate from the surface to about 2m/s/s by 8000km, that's enough height to encompass the exosphere and the atmospheric properties should be about earth-normal, I think. But if it's a dome filled with gas and an artificial gravity effect that only operates near the ground, or which applies with the same force throughout the dome, then it would get weird. $\endgroup$ May 30, 2016 at 20:59
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    $\begingroup$ Using rotation, acceleration is higher the closer to the hub you go, so "gravity" would go up, not down, as you rose. Still, just in case it's of interest, to get 9.8m/s of acceleration with a rotational period of 24hrs, you'd must rotate about a radius of 1.8 million km. With a "moon" or other massive object as a counterweight, it's feasible... handwaving away that the moon wouldn't move in the sky (paint it black?) and the whole "what kind of tethers could hold that weight" problem, and the atmospheric issues. But at least it's not handwaving away "artificial gravity". $\endgroup$ May 30, 2016 at 23:00
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First, why would it be spinning? Consider a coin spinning clockwise from your point of view. The leading (top left) edge moves with the spin, as does the bottom right. Someone standing at either point would experience higher gravity than someone in the centre. An observer at the trailing edge would experience negative gravity. It's the same concept as spinning cylindrical space stations to generate artificial gravity-it varies according to your direction of movement. A disk spinning like a coin would be obvious the moment you take a few steps orthogonal to the spin axis. If it's spinning like a flat top, centripetal force increases as you travel closer to the edges. Again that should be fairly obvious. Not to mention the sun travelling the other way at "night". If anything, the only way for this to work is with the classical flat earth model, where the disc is at rest and everything rotates around it.
So, now that we have a stationary flat disc, under a VR dome, how does the air and water circulate? Cold generators at the poles and heat generators at the centre? Pressure generators at the rim? As someone pointed out, if we assume godtech, anything is possible. Assuming a circular world however, geometry would indicate everything moves either to or away from the centre.

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    $\begingroup$ Also, everything would be perpetually sliding toward the edge. $\endgroup$
    – Devsman
    May 31, 2016 at 19:04
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    $\begingroup$ If it's spinning like a phonograph record, perhaps the density (and surface gravity) is adjusted at different radial distance to compensate for the centrifugal force. Something similar might work for spinning like a coin on a table; but you'd have to somehow keep the axis from podholing. $\endgroup$
    – JDługosz
    May 31, 2016 at 21:36
  • $\begingroup$ @JDługosz: You'd still see the sun going both ways $\endgroup$
    – nzaman
    Jul 13, 2016 at 13:09
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One example of a similar scenario I've read is "Missile Gap", a short story by Charles Stross. This follows the reactions of the Cold War superpowers to the entire planet being transplanted onto the surface of a disk with the mass of 50,000 suns just after the Cuban missile crisis. The main effect observed is the altered geography, which shifts the balance of power as ICBMs become no longer in range, and the near-uniform gravitational field, which prevents any further space exploration.

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    $\begingroup$ Wow, thanks for this! I hadn't expected such a specific situation to already have a short story...I'll definitely be giving it a read. $\endgroup$
    – Liesmith
    May 31, 2016 at 13:13
  • $\begingroup$ And termite-beings, and the motives of the aliens who built such a thing... $\endgroup$
    – JDługosz
    May 31, 2016 at 21:30
  • $\begingroup$ Minor note: it has no effect on the story (because the characters aren't aware of it), but the planet wasn't transported: the planet was scanned by Sufficiently Advanced Aliens and then copied on to the Alderson Disk where the story takes place. $\endgroup$ Nov 25, 2018 at 1:26
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Weird atmospheric effect.

We have a disc-shaped world which is either illuminated (albeit with varying angle) or in the dark. There is no circulation between Lightside and Darkside.

As a result, the atmosphere gets very little horizontal circulation, or none (this depends on how thoroughly the gravity generators compensate with height the centripetal force from the world's spinning: they would need to shoot upwards on the axis and shoot at an angle, and more powerfully, nearing the edges). Vertically, there would be only convection.

Now during the night the heat escapes into space, and the atmosphere cools off. During the day it warms up starting with the lower layers.

Under these conditions, light gets refracted in the atmosphere and gets bent upwards. This, combined with a horizon much farther than the Earth value of around 5 km, would cause the illusion of being at the bottom of a shallow cup.

At that point, I'd expect that the curiosity of pretty much any scientist regarding the actual shape of the world he's on would be quite aroused. Travelling some fifty kilometers with some device capable of measuring the Sun's angle with a high time precision would then quickly hint about what's happening. More subtle tests with the local intensity of the gravitational field would show it's artificial (actually, I suspect an artificial field with the needed characteristics of directionality just can't be produced. Perhaps, the disc might be made to orbit a massive black hole orbiting around the sun, so that it doesn't spin around its axis like a coin. The disc would need to be slightly rounded or the black hole very far, though).

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Other clues will show up before this one does, but it's worth noting anyway.

Gravity might be weird anyway.

The generator accounts for essentially all of the gravity, and somehow projects* it up like a floodlight so you feel gravity only from the surface within meters of you. This will make the strength of gravity independent of altitude within a cone thousands of kilometers high. The tighter the floodlight, the taller the cone. The wider the floodlight, the farther from the edge gravity stops pointing down.

If you decrease the width near the edge, you can make a roughly dome-shaped region above the disc where gravity is actually stronger along the "surface" of the dome than within.

If you don't want this stuff to be true, then the generator has extra magic that makes its field violate the inverse-square law.

*It's a static field so nothing's really being "projected", but the floodlight analogy holds up pretty well otherwise.

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  • $\begingroup$ The change in the earth's gravity as you go up is so small that by the time you notice it, you can already just see the shape of the world by looking down. In the international space station the strength of earth's gravity is still 90% of what it is on the surface, but in orbit you don't notice because you are constantly falling around the earth. A much weirder effect would be gravity suddenly ending when you leave the gravity generators 'projection'. $\endgroup$
    – JanKanis
    Aug 15, 2020 at 21:29
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Euclidean Geometry

On a sphere a triangle's interior angles add up to more than 180 degrees because of the curvature.

On a flat world, Euclid would rule supreme. An accurate survey would reveal that triangles have 180 degrees hence no curvature.

enter image description here

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  • $\begingroup$ this may sound esoteric but if scientists find themselves on a new world they will likely try to calculate this fairly quickly to find out how big a planet they are on. this is how we confirmed THIS planet is round. $\endgroup$
    – John
    Aug 12 at 13:35
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The Clouds

Due to effect of the horizon, there'd be clouds approaching it. Since this place is flat and not round, it cannot have a horizon. So there wouldn't be clouds on the horizon. (There wouldn't be a horizon :O)

Atmospheric Effect

There wouldn't be day and night at the same time. The entire disc (if you will) will be either be at day or night.

The edge

There'd be an edge. Since it's not round and there aren't any buildings like in Pennsylvania or Virginia, it'd be easy to see too.

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    $\begingroup$ Hi Aravind Suresh, and welcome. We like answers to add something not previously said in response to a question. This answer appears to merely repeat things that have already been said, though perhaps worded slightly differently. I would recommend that you add something not previously mentioned to this answer, such that it stands out more. That way, it is likely to be better received by the community. $\endgroup$
    – user
    May 29, 2016 at 16:06
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    $\begingroup$ Welcome to Worldbuilding Aravind! Several answers before yours seem to include all of the points you've listed here, so please check that your content in your answer has not been posted already before you post an answer to a question. $\endgroup$
    – fi12
    May 29, 2016 at 16:23
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There would be a few things, but one of the major ones would be an appeared incline as you moved away from the center. There is a great youtube video on this topic, and it's how I got this idea.

https://www.youtube.com/watch?v=VNqNnUJVcVs

The main point is up to about 1:53.

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    $\begingroup$ Could you talk more about this in your answer itself? We prefer answers to not be link-only - they should be able to stand on their own as an informative source without leaning on a link they're pointing to. $\endgroup$ May 31, 2016 at 8:30
  • $\begingroup$ This has been mentioned on other questions. The disk coukd be built to compensate by having more mass near the rim. $\endgroup$
    – JDługosz
    May 31, 2016 at 21:32
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My first thought was observing the horizon, but others have mentioned that.

Second thought: You say there is artificial gravity, which I presume means a uniform force over the entire surface. Then you say there is a dome to hold in the atmosphere. If by "dome" you're implying a curved surface, then the dome is closer to the ground near the edges than at the center. Which means there is a taller column of air near the center. Which means the air pressure must be higher. If the scientists travel far enough, the difference in air pressure will be noticeable.

If the scientists travel far enough, sooner or later someone should notice that there is a fairly uniform average temperature over the entire world, rather than cold poles and hot equator.

Which brings to mind that sooner or later someone is going to try to figure out their latitude and longitude and calculate the size of the planet, at which point they're going to figure it out geometrically.

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  • $\begingroup$ Hmm...that's a very good point about the air pressure being very different in the center column...thank you very much for pointing it out! $\endgroup$
    – Liesmith
    Jun 1, 2016 at 6:39
  • $\begingroup$ If the center column has higher pressure, then it would expand outwards, creating a wind that radiates from the center to the edges in all horizontal directions. Either that would cause the air pressure to equalize in horizontal layers, or loop back up to the top and fall down again, creating a massive warped donut-like wind system covering the entire planet. However, since the central column of air would push outwards at every elevation, there would probably be no way for air to cycle back to the top, meaning equal air pressure at equal elevations. $\endgroup$
    – Darcinon
    Jun 1, 2016 at 18:29
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    $\begingroup$ The pressure will be the same along the entire surface. As an analogy: back in the day when the water mains was pressurized by a water tower, do you expect the pressure to only exist at the bottom of the water tower, with other water pipes having close to no pressure because they only have the few cm of the diameter of the pipe as head? $\endgroup$
    – JanKanis
    Aug 15, 2020 at 21:42
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    $\begingroup$ @Jay The force caused by pressure in a fluid is the same in every direction. The pressure depends on the height of fluid (directly or indirectly) above the point where you're measuring. In the case of the glass in the dishpan things get more complicated because there is also the atmosphere applying pressure to the water, and the glass is blocking some of that pressure. $\endgroup$
    – JanKanis
    Aug 16, 2020 at 12:08
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    $\begingroup$ As a different experiment, if you have a rainwater tank with a tap in the side, you can easily verify that the water pressure at the tap is caused by the height of the water above the tap level, even though there is no water directly above the tap. If the tap can be rotated you can verify that the pressure is mostly independent of if the tap is pointing upward, downward or sideways. Any other water container with a tap on the side will also work. $\endgroup$
    – JanKanis
    Aug 16, 2020 at 12:10
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Most was said, but nobody spoke of seasons. If axial tilt is zero (the "coin" stands perpendicular to the orbital plane and spins like a top), there would be no season. However, if the planet is tipped and spins on a diagonal axis, it will be the same season everywhere. I agree, though, that other effects mentioned will catch the eye more quickly.

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[The portal outlet would have to be near the centre; if it is near the edge, space will immediately look different in different directions, to the visitors, because of the different thickness of the air. (...Unless the air is perfectly translucent... which would raise suspicions.)]

[The disc must spin around its diameter, not its centre; otherwise, there will be dramatic centrifugal force differences with very obvious effects near the centre.]

[Anyone who looked through a telescope from a high point would be able to see (not automatically implying notice) that the horizon was infinite. The aliens would presumably compensate for this by making the landscape universally hilly (or perhaps slightly curved (inwards or outwards), or completely flat). Regardless, at least one scientist would probably start to wonder...]

I am thinking that the immediate obvious difference will be in the types of animal and plant. (If they are imports from a spherical world, then they will be okay initially.) The seasons have to do with, not only the axial tilt of the Earth, but also the differential sun heating from north to south... which would not work with a flat surface. That is... there would be no seasons. The scientists would begin to notice this after a number of weeks, but the animals would be obviously relevantly very different from Earth animals from day one; they would neither migrate nor change with the seasons. There would be only one mode of life, for every single plant and animal... not to mention the terrain. No scientist studying the flora or fauna could possibly miss this -- at least subconsciously initially, and probably consciously within 24 hours. (From there, I would guess it would be only a few hours until someone suggested the obvious but fantastic explanation, and only a few hours more until someone started working on checking it.)

[Of course, if there is anything that would give the game away easily, the whole scenario is suspect.] One might expect the aliens to compensate for this. The obvious easy way would be to make the ground more reflective in the north and south, but I imagine that that (in concert with the atmosphere issues) would have undesirable effects. However, nothing along those lines would work anyway, as seasonality near the equator depends on the sphere thing.

The aliens have to either engineer a whole artificial seasons system or just leave the thing as it is.

In turn, then, the obvious inference is that the homeworld of the aliens does not have an axial tilt. It would still have cold poles, though. Thus, that difference remains -- no seasons, but latitude temperature variations. This gives us a variety of different animals and plants, every last one of which is suited to only one temperature.

...And I shall stop there.

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The planet's magnetic field would be a lot different/non-existant

I think this is plausible, but I'm not sure about the specifics.

The Earth has a magnetic field because its full of churning molten iron. This makes compasses work, protects us from some radiation from space, and sometimes makes auroras.

I assume a flat artificial planet would not have any volcanic or tectonic activity, and thus no magnetic field. (Maybe there is an artificial one?)

But without one, everyone would be getting blasted with a lot more radiation every second, and some instruments might pick that up.

If they don't have proper detectors, then they could still see the effects indirectly. Maybe over time a lot of people get skin cancer. Maybe the radiation causes a much higher rate of computer errors than they expect. Airplanes usually have triple-redundant processors for essential computations because there is a higher likelihood of this kind of thing happening at 30,000ft than on the surface.

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It's the same time everywhere!

The expedition arrives. One person begins taking pictures of the night sky, rapidly determining the location of the celestial equator. I'm assuming the pole stars are harder to find, because they will be directly in the plane of the coin. They conclude they are on the equator.

The rover teams branch out in different directions in motor boats, hover craft, ultralight aircraft, and optionally robotic walking vehicles. They use inertial navigation (accelerometers) as a substitute for GPS, generating an accurate map of the landscape.

But which way is North? They are going in all directions. If they move east or west, the star overhead at base camp will have gone overhead already or not yet arrived, giving a sense of the planetary radius. If they go north or south, the celestial equator will appear tilted and a pole star will become apparent. NONE of this happens. The planet has an infinite radius and the local time is the same everywhere! (Infinite until they fall off the edge, anyway)

First they may consider something is wrong with their measurements, it's a very large world, they're in a Mimas-like flat spot. But the further they go the more impossible the data looks: no way to tell latitude with a mariner's astrolabe or any more sophisticated instrument. No way to tell longitude by the time. The world... is flat!

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