Explain to me again, how we know the Earth to be banana shaped.

[knasser]

You could even set up the size and position of the outer sun so that, from the viewpoint of the underside, it would become just perfectly eclipsed when it was directly overhead on the far side of the ring.

It still wouldn't shed any significant light on the inside, but at that point you'd be just barely able to see the corona and any major prominences - just a hint, tantalising to any surfacers down there and infuriating to the Drow, of what real sunlight looks like.

That way, you can still have the same day-cycle for the Underdark as for the world above. Also, you could have solar flares being an occasional calamitous event for the Underdark, briefly illuminating it with something approaching true daylight.

This is starting to give me chills. Imagine two suns - a dark one and a light one. Some creatures naturally see by the light of the dark sun, others by the light. Each sun is invisible to the others. It would be a mystery throughout much of the campaign as to why some creatures could see at night but grew blinder by day and some the other way around. Or what that dark, cold flame that burned but never shed any light actually did - (it illuminates for those that see by the light of the dark sun). Only during the campaign would come the shocking discovery that there are actually two suns in the sky.

 

[TBeholder]

There'd (almost) always be the "Great Arc of the Sky" (far side of the ring) at night, that would only briefly go dark in a regular pattern. (And how much detail you could see of it would depend on the dimensions of the ring. Additionally, days and nights wouldn't need to be the anywhere near same or even regular length.

Since that part also is in atmosphere, there should be a lot of atmospheric fog.
Inhabited Island (a common spherical planet, but with weird atmospheric optics) and Sigil (toroidal city with a lot of smoke) the upward curve is visible, but then it fades out.

 

[Igwilly]

Well, right now, I’m thinking about how to run a game in which the Earth is flat. This demands to determine “gravity” as being directed to the ground (below or above, depending on the side). I believe some side-effects are:
1.Time zones: Basically, in the real world, if one starts to move on the east-west axis too much, the time marked in your clock will be different from the one marked in the sky, precisely because of the Earth’s shape: different places are in different moments of the day (as the sky). If the world is flat, however, every place will mark the same time in the clock. Day and night would occur simultaneously everywhere, and your clock would not need to be adjusted.
2.Stars: In the real world, one of the keys factors as to the discovery of Earth’s shape is the sky that appears on the night: if one moves too much on the north-south axis, the sky will not be the same anymore; known stars and constellations are not there, and there’ll be new and strange stars and constellations. This happens because of the Earth’s shape. If the world is flat, however, the sky at night will be the same for everybody, and such change should not occur.
3.Maps: in our real-world, flat maps are inaccurate no matter how they are made: Earth is round and the map is flat; there’ll always be error. Such a thing would not occur if the Earth is flat: all maps would (supposedly) be of perfect or near-perfect accuracy, with no distortion.
4.Horizon: if the Earth is flat, the horizon line will be limited only by human vision: nothing would simply disappear by going far away, until your sight is not capable of detecting them anymore. This could make observation on-land much more precise. Perhaps your king will see his enemies approaching the kingdom much early. That could change something in the world.
5.There is the danger of falling over the edge, if one moves far away from the center. Honestly, there’s no way to determine an always-applicable effect for this: this is of your craft; you should decide what happens when someone falls over the edge. There’s the interesting question as to what, if anything, exists on the other side of the planet. This can be a source of great mystery.
I should note that I did see RPG settings using this proposition:
•13th Age: basically, right in the core book, at chapter’s start on describing the world, it states that the world of 13th Age is flat, with the Underworld being underground and the Overworld being high above the clouds.
•2e Spelljammer: I believe one of the possible shapes for planets is the flat one.

Now, if you ask me, I almost never use real-world astronomy/physics laws in my world. The problem is not the general proposition, though, but there’s one thing that always bugs me in RPGs: stars. Basically, real-world stars have such power/energy that would make most fantasy gods look like weak blades of grass. I think their size and their nature as giant nuclear reactors are the main factors here: an Earth-sized ball of eternal fire is fine to me.
I also had this idea: all stars are man-sized crystals which provide life for planets both physically and metaphysically, giving light/heat and the magical/metaphysical substance which mortal souls are made. They may be actually doors to the Otherworld, the source of life, which follows a different set of rules about the star-planet dynamics. Just an idea.

 

[tomBitonti]

You could also have the stars be on an plane that rotates, independent of land.

Except close to the edges, or far from the surface, a flat plane has gravity that decreases linearly with distance. The field eventually turns into a least square field, basically, as the plane becomes more like a point because of distance. The field all ways decreases linearly from an infinite plane.

Thx!
TomB

 

[Celebrim]

Except close to the edges, or far from the surface, a flat plane has gravity that decreases linearly with distance. The field eventually turns into a least square field, basically, as the plane becomes more like a point because of distance. The field all ways decreases linearly from an infinite plane.

This assumes that:

a) Gravity works according to the same laws that govern this universe.
b) Gravity is what makes things fall down.

Neither can be assumed to be true of a fantastic universe.

Some alternatives:

a) All worlds are finite in size, and have some body beneath them that causes them to float (a magic turtle, made of a substance impervious to gravity, for example). At some great distance below all worlds is a very vast gravitational body, exerting a field that from the perspective of the individual worlds is perpendicular to the ground.

b) Gravity does not exist. The apparent gravity that people experience is actually the world, and its surrounding crystal sphere(s) and associated bodies, accelerating continually in a single direction without upper bound.

c) Gravity does not exist. Earth spirits abhor the ideal of things not being 'grounded', and so continually pull on anything they observe above them. The strength of this pull does not depend on either the distance from the surface or the orientation of the person. In other words, there is no 'center of gravity', only a nearby spirit pulling you down to the ground were you belong. The world could be disk shape, or any other shape, and gravity would generally work in the same way you'd intuitively expect. A scientific investigator in such a world could prove this by noting slight deflections from true 'down' on the slopes of very large mountains (as the spirits here consider 'down' nearer to the perpendicular of the slope rather than the perpendicular to the plane of the world as a whole). A second way to prove this would be that gravity would very quickly and very suddenly drop to zero once you gained enough altitude that you were outside of the reach of the spirits. This would be very different than the experienced microgravity of an orbiting body. In this world, objects could achieve escape velocity at very low speeds and hang in place above the world without orbiting.

'C' is in fact the physics of my homebrew world. It's also the reason why gravity on the elemental plane of earth is always perpendicular to the surface you are on, allowing you to walk around the inside of a hollow sphere. It's also how magic related to gravity works, such as the 'fly' spell. You basically trick or bargain with the earth spirits to leave you alone. An anti-gravity field is simply tricking or coersing the earth spirits (they are pretty simple mindless things), into tossing everything away from the ground rather than pulling it down. This doesn't require massive expenditures of energy on the part of the spell-caster, as the average mortal spellcaster's mind simply can't conjure up the sort of energy required to perform those feats on their own. Transmutation magic is simply tweaking the anima that makes reality have a certain character on a local scale so that it has a different character, which you can do if you know the right words much like a programmer can program a computer. Essentially, wizards 'hack' reality using its own interfaces, as if they were the legitimate authority in the universe with a right to do so. Divine magic is similar, except that instead of hacking the system, you have some bargain with the legitimate authority in the universe to act on your behalf. So a Wizard would trick the local gravity spirits into thinking he was their boss, where as a shaman would call up their boss and have him (or her, or it) issue a legitimate order.

 

[knasser]

Many great replies here. Thank you. This whole thread reminds me of a scene from Rome. Ideally, I would love my players to have conversations like this in the game:

[video=youtube;erPu_t7tgQ4]https://www.youtube.com/watch?v=erPu_t7tgQ4[/video]

I decided that my world exists in a great sphere, the stars are holes out into the Astral and the Sun and the Moon and the planets themselves all revolve around the world at the centre.

 

[Beleriphon]

•2e Spelljammer: I believe one of the possible shapes for planets is the flat one.

Or cubes, or dodecahedrons.

 

[Tony Vargas]

Eratosthenes calculated the size of the earth, assuming that it was spherical and that the sun was very large and very far away, such that its rays were essentially parallel.
But, if you change the assumptions you change what you're calculating. Assume the sun is smaller than the earth and not so far away, and thus its rays /not/ parallel, and the exact same measurements can be used to determine the distance to the sun (much closer than IRL, obviously).
The history of science is a great thing to mine for cool ideas. Spontaneous generation is a favorite of mine. Look up the physics of Aristotle for some ideas, too. Like why and how things fall....

Well, right now, I’m thinking about how to run a game in which the Earth is flat.
]Time zones: ). If the world is flat, however, every place will mark the same time in the clock. Day and night would occur simultaneously everywhere, and your clock would not need to be adjusted.

It'll still be the same number of hours into the day on Flat Earth, but the angle of the sun in the sky will change as you travel east-west. Maybe quite a bit depending on how big the world is and how far away the sun. For instance, at noon, the sun is directly overhead, at the center of the world (Jerusalem, if you're a Medieval European). But, if you're half way to the western edge the sun's already past that relative point.

Assuming the sun is traveling in a nice circular path. If it vaults to it's cruising altitude first thing in the morning, and drives straight across the world, it'll be even more pronounced.

Anyway, yes, you'll have time zones, but they won't mean the same thing: it's noon and the proportion of morning to afternoon that'll change. Maybe barely noticeably (smallish earth, sun far away), maybe almost comically - sun is small and it's orbit just barely misses each edge of the relatively large earth (or, even more so if the sun cruises at one altitude all day).

2.Stars: In the real world, one of the keys factors as to the discovery of Earth’s shape is the sky that appears on the night: if one moves too much on the north-south axis, the sky will not be the same anymore; known stars and constellations are not there, and there’ll be new and strange stars and constellations. This happens because of the Earth’s shape. If the world is flat, however, the sky at night will be the same for everybody, and such change should not occur.

They might appear distorted, again, if you can travel pretty close to an edge, and the celestial sphere is not far beyond that edge...

3.Maps: in our real-world, flat maps are inaccurate no matter how they are made: Earth is round and the map is flat; there’ll always be error. Such a thing would not occur if the Earth is flat: all maps would (supposedly) be of perfect or near-perfect accuracy, with no distortion.

Accuracy is still hard in a medieval world. But some engineering-obsessed Lawful Empire like Rome or Nerath might have very accurate maps indeed.

4.Horizon: if the Earth is flat, the horizon line will be limited only by human vision: nothing would simply disappear by going far away, until your sight is not capable of detecting them anymore. This could make observation on-land much more precise. Perhaps your king will see his enemies approaching the kingdom much early. That could change something in the world.

It doesn't take many nor very high hills to screw that up. Then there's the nature of vision. One old theory about vision (I am not making this up, I swear) was that the eyes emitted rays, and you could see what the rays touched. Air and Light are transparent to those rays, and darkness (and a lot of other things) opaque to them. You need light to see because it displaces darkness, not because you see reflected light.

Anyway, under that model, the range of vision could be quite limited. Rather like 60' darkvision, really. See so far, and no further. LoS notwithstanding.

5.There is the danger of falling over the edge, if one moves far away from the center. Honestly, there’s no way to determine an always-applicable effect for this: this is of your craft; you should decide what happens when someone falls over the edge. There’s the interesting question as to what, if anything, exists on the other side of the planet. This can be a source of great mystery.

There may be nothing on the bottom side. Or there may be spiders and other things that can walk on ceilings (that's a new spin on Drow and their reverence for Lolth). Or y'know, very hungry piercers with no one to drop on...