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Inspired by this question, how might one determine the shape of the world given a cosmology with no cycles? The moon, sun, stars, and planets are all fixed. The Earth does not rotate. You can't simply walk around it because it is essentially tidally locked; one side is a barren, hellish desert, the other is a frozen wasteland.

Ash
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Ryan_L
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  • I'd be worried more about my house not blowing away in the continual hurricanes you'll have than knowing what shape my world is. –  Aug 22 '18 at 18:34
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    Is there a certain technology level you're looking for, or would "camera strapped to a satellite" be an answer? – Giter Aug 22 '18 at 18:34
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    @Hosch250 Forget the house, nothing can live in such a setting, there is no observer, not just not on that planet, not in a universe that can have such a world. – Ash Aug 22 '18 at 18:37
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    Ladies and gentlemen, while frame challenges are permissible on WB:SE, they're not our regular practice. Part of our job is willingly taking the OP at their word and offering an answer. In other words, whining that the premise is implausible is unconstructive (if they were constructive every question about magic should be dismissed... do you see my point?) – JBH Aug 22 '18 at 18:46
  • By 'determine the shape of the world', are looking for ways to determine any arbitrary shape(like a triangular world or something weirder), or are worlds in this universe still spheres? – Giter Aug 22 '18 at 18:53
  • @Giter Ideally your solution will narrow it down as much as possible. I accept that a satellite could trivially prove it but I don't think I should allow that for the sake of the discussion. – Ryan_L Aug 22 '18 at 19:07
  • No matter how barren or hellish there would be people who want to know what's beyond that. Some people would eventually circumnavigate in some way. Once aircraft were discovered people would start trying to use to figure out the world is essentially round. Even climbing mountains would make someone wonder. Note that the stars being fixed would require a radical rework of physical laws and make the likelihood of a planet and star existing doubtful in the first place. – StephenG - Help Ukraine Aug 22 '18 at 19:33

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Astronomy will still help, even if there is no motion. Imagine for a moment that there's no moon, no other planets, and no distant stars: just the planet you live on and its sun, and that the planet is tidally locked.

From one specific point (in the dead center of your planet's "hot side") you will see the sun directly overhead, all the time. At every other point on that hemisphere, you'll see the sun somewhere other than directly overhead. As an extreme example, at the very furthest extent of the hot side, you'll see the sun on the horizon.

If you can measure the angle between the sun and horizon (and your people should really develop this ability, since it's far and away the best way to navigate the hot side of such a planet), and you plot out the angles on a piece of paper, you'll eventually notice that they describe the behavior of a curved, not flat, surface. In fact you can measure the changes in angle to work out how curved the surface is, and comparing that angle to known distances will let you work out how big your planet is.

This was the method Eratosthenes of Cyrene used to determine Earth's circumference ca. the 3rd century BC. He had to make his measurements on one particular day in order to account for the apparent movement of the Sun, but your people wouldn't have to; they could run this sort of experiment any time they liked. They wouldn't even have to travel particularly far; the cities he used were only about 800km apart, and if you just wanted a proof of sphericity, not an accurate measurement, they could be closer.

Now you might point out that Eratosthenes's method only makes sense if you already think the Earth is spherical, and that may be true. The idea of a spherical Earth appears to be about 300 years older, and was one of several theories floated at the time, more or less because the Greeks thought it was suitably elegant and geometrical. What Eratosthenes and other astronomers of his era did was provide evidence to support an idea that was already popular.

Cadence
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  • What if the society is on the outer rim of a toroidal world? Correct me if I'm wrong, but a toroidal world with the sub-solar point in the center of the "donut hole" would show parallax just like a spherical world would it not? – Ryan_L Aug 22 '18 at 19:13
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    On a torus, they should be able to work out the difference in curvature between the direction that leads around the whole torus, and the direction that leads to the inner rim. Whether they could deduce the overall shape from that, I'm not sure. – Cadence Aug 22 '18 at 19:17
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Put a stick in the ground in two widely separated locations at the same time and measure the angles of the shadows. That's how it was done in reality.. and the calculation made was accurate to within 5%

Richard
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Step 1. Build a rocket

Step 2. Go up in the rocket

Step 3. Look and see what shape the world is, take pictures.

Mathaddict
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Someone is bound to circle the planets green belt eventually. Knowing that you can make it all the way around in a circle mean either your planet is round, or flat with a circle of habitable space.

Clay Deitas
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