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Planes are flat, right? And the Material Plane is infinitely large?

I typically describe the sky material plane in a manner similar to that of the Earth: there's a sun in the sky and clouds and things, and at night the sun goes down and there's a moon that has phases and stars. I feel like I have some in-book support for a moon, given werewolves and certain magic items and such, and Pelor is literally the God of the Sun (and there is a Sun domain), but recently I had a player point out that the Sun must smash its way through the whole of the Underdark and there should be a massive tunnel to go exploring in. That idea was cool, so the party is now doing that, but I don't really think that such a catastrophic geography is good for every campaign. Is there any other reasonable explanation as to the location of the sun at night? Is there any explanation given in official material?

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    \$\begingroup\$ Is there a permanent hole in the world that the sun passes through each day when it goes down? I hope you've thought about what it would be like past this point, where the sun is never overhead. Or does it go down differently each day, just obliterating a random continent? \$\endgroup\$ – DCShannon Nov 17 '14 at 5:31
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    \$\begingroup\$ I hope the party doesn't take more than a day to explore the tunnel... \$\endgroup\$ – Lily Chung Nov 17 '14 at 23:45
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This question is campaign setting specific. Fortunately, thanks to the Spelljammer campaign setting, the answer is actually known for some of the major published prime worlds.

Krynn (Dragonlance) and Toril (Forgotten Realms) are both planets that orbit around their respective suns and spin on their axies as they do so; At night, their suns are in the same places they are during the day, just illuminating the other side of the world.

Oerth (Greyhawk) is unusual in that it is part of a geocentric planetary system; Its sun is actually its third satellite. It's still a planet, though, and it being night still just means the sun's currently illuminating the other side.

That second point actually answers the general case: The Greyhawk campaign setting's geocentric model is specifically called out as a departure from the norm of a system's primary body being a sun that everything else orbits around. It seems clear that the default assumption is that most campaign settings take place on spherical planetary bodies that orbit suns unless explicitly stated otherwise (as it is in the case of your homebrew setting).

Oh, and to clarify something, "a flat surface" is just one of the many meanings of the word "Plane." It can also mean a device for smoothing wood, a kind of heavier-than-air flying machine, or a level of existence or thought. It's that last definition that we're dealing with here, so just because the Prime Material Plane is infinite doesn't mean that any of the surfaces it contains are.

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In a fantasy setting, the sun does not need to be a 100% physical entity, nor obey any laws of physics that get in the way of a good tale. Your physical fireball that must pass through the earth at some mystical location is nicely self-consistent (ignoring real-world physics of it) and pretty cool, although I don't think anything like it is referenced in D&D source material.

Before humans understood enough to explain the sun's behaviour, many mythologies sprung up around how it functioned, including where it might go at night. Most of these myths have the world as a finite place, but you can still adapt them to explain the route of the sun, and what might happen to it in at night. If you run an epic level mythologically-charged campaign, you could easily borrow from these ideas.

A lot of the myths have the sun as a physical object or simple creature that can be stolen, tamed or bound. Many more of the myths explain the sun's disappearance as a re-enactment or divine compact explained by a kind of just so story (one deity steals the sun and it gets returned but under some agreement where it must only be in the sky at certain times etc). The sun is obviously of huge importance to humans, so pretty much every civilisation has had a go or three at understanding and explaining it. There is a collection of some myths in Sun Lore of All Ages, and there may be other analyses too.


This, or variations of it where you start with a "target" cosmology and try to figure out more detail, would be a really nice question for https://worldbuilding.stackexchange.com/ by the way.

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For an infinite, flat material plane, the sun can still appear to go around overhead without actually going through the plane itself.

To the people of such a world, such a sun would always be in the same place in the sky at the same time. So if you teleported a million miles away, the sun wouldn't move. Very handy for navigation and time-keeping. (Of course, that's assuming it goes around in a completely consistent arc.)

If you want to get in to the geometry of it, you just need to make your world four (or more) dimensional. The people on it still only experience three dimensions, but if you take a 2D plane in a 4D space, you can have the sun go around the plane without ever passing through it.

You can achieve a similar effect by having the sun be infinitely far away. If the sun is infinitely far away and the plane is infinitely big, do the two intersect? Well, you'll have to travel infinitely far to find out. (Mathematically, I think the answer can go either way depending on the exact details.)

I don't know of any real world cultures that thought the world was infinitely big, so I can't compare this to any real mythologies. But, if you play Minecraft, this is how this sun there looks.

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    \$\begingroup\$ Your first proposed geometry doesn't really make sense; unless the sun is infinitely far away already, teleporting a million miles will make it move in the sky. Note that using Minecraft as a conceptual model for this is a bad idea: in Minecraft, every player actually has their own personal sun, invisible to everyone else, that moves with them and obeys the world clock (verifiable by inspecting the decompiled code), rather than there being one sun that determines the time as in the RPG scenario in question. \$\endgroup\$ – SevenSidedDie Nov 18 '14 at 22:33
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    \$\begingroup\$ You should explain further about how the 4th dimension would help. An analogy to a 2D Mario game with a 3D sun might be helpful, where the 3D sun revolves through the 2D plane (towards and away from the Mario game's player). It would still very thoroughly illuminate Mario's world - possibly even the underground - though it would eventually have to pass through a spot beneath somehow. I have no idea how this would look to the inhabitants, though... \$\endgroup\$ – doppelgreener Nov 18 '14 at 22:42
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    \$\begingroup\$ Would an infinitely-large sun an infinite distance away that travels around an infinite plane need to travel an infinite number of times faster than its own light in order to appear to be in the right place at the right time? \$\endgroup\$ – GMJoe Nov 19 '14 at 4:43
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    \$\begingroup\$ 2) You are correct that a Sun whose plane of rotation included a vector with a 4th dimensional component would allow the Sun to orbit the plane without passing through the ground- almost. The issue with this is two-fold: first the plane is also infinitely deep and the Sun necessarily appears in it (as a cave though), and second the Sun would do a rather awful job of illuminating the world (what with it being gone most of the day). The Sun would only be present in the Plane's 3-space when it's 4th dimensional position vector is 0, which occurs twice around its circle, at opposite ends. \$\endgroup\$ – Please stop being evil Nov 19 '14 at 6:31
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    \$\begingroup\$ Please integrate these comments into your answer, Joshua. \$\endgroup\$ – Brian Ballsun-Stanton Nov 25 '14 at 5:57

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