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.

If you start at noon, things would appear quite normal and stay so into twilight as the suns moves lower in the sky. But the moment the sun passes the horizon, it will be completely dark. There may be some slight glow from clouds for a minute, but if the atmosphere is as thin a layer above the coin as it is above Earth (quite likely with similar gravity), it will be covered in the coin's shadow very quickly.

Daybreak would arrive just as suddenly, with a slight glow on the horizon the only warning that the sun is about to rise. Then morning and afternoon would seem fairly normal again.

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

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 Trueman Show it look like this:

Answer 5

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 6

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 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

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 12

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 13

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 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

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 16

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 17

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 18

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.