What would make scientists realize they were on a flat world?

Question

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.

Answer 1

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.

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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.

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 :-).

Answer 2

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.

Answer 3

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.

Answer 4

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:

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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.

Answer 11

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).

Answer 12

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.

Answer 13

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.

Answer 14

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.

Answer 15

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.

Answer 16

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.

Answer 17

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.

Answer 18

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!

Answer 19

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.

Answer 20

[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.

Answer 21

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.