Some published settings -- such as the Greyhawk 'verse commonly seen in conjunction with Dungeons & Dragons -- treat the prime planet's host solar system as geocentric. How could this be reconciled with what we now know about orbital mechanics and gravitation, though?

The only theories I have come up with so far are:

A heliocentric system that 'fakes' a geocentric system convincingly

In this case, Oerth (Greyhawk's prime planet) is a large-ish planet that is tidally locked to a highly luminous host star, so that one side of the planet is permanently in the broiler, and the inhabited side is dark save for what light their star gives them. What the inhabitants perceive as the star, then, is a large, highly reflective moon that orbits the planet; the remaining 'planets' to the inhabitants then become smaller moons, and they'd have a few small rocks to serve as actual moons close-in to the planet (captured asteroids, perhaps?)

This wouldn't work, though, if there were gas giants roaming around -- they'd have to orbit the true host star, which'd provide a way for the inhabitants of the prime world to detect what's really going on.

A very large planet and a very small star in a tight binary couplet

In this case, Oerth would be an enormous rockball with high gravity and inertia, and be orbiting in a tight couplet with a very dim, small star such as a red dwarf. Moons would be as normal as could be in such a deep gravity well, while the rest of the planets would be further out from the star-planetoid (brown dwarf?) binary couplet.

However, this raises two questions:

  1. Greyhawk's published canon states that the sun there is the third object out from Oerth -- with the inner two presumably perceived as planets; they could be very small moons or captured rocks instead, though, and I suspect people wouldn't catch on, at least at first...

  2. How do you keep the gravity of this megaplanet from turning the inhabitants into Flat Stanley?

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    $\begingroup$ Playing around with formulas for surface gravity a bit tells us that a planet with half the density of earth (which is just a bit lower than the lowest density for any known terrestrial planet) and 4 times the radius would have 32 stronger gravity but only 2 times more surface gravity. $\endgroup$ – overactor Nov 18 '14 at 9:40
  • $\begingroup$ Good point that you can get a large total gravitational pull on a planet with a relatively weak surface gravity. $\endgroup$ – Shalvenay Nov 18 '14 at 12:46
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    $\begingroup$ Isn't this just perception? Both bodies orbit each other and each of those bodies are orbitting the larger galatic bodies as well. If we remove all other planets from the solar system, can we not say it's geocentric? (failing this answer I'd like to submit a magical chariot pulling the sun across the sky). $\endgroup$ – Twelfth Nov 18 '14 at 18:27
  • $\begingroup$ If a planet gets more massive than a star, it needs to become a star or collapse into a neutron star or black hole, if the planet is greater than 3 solar masses. $\endgroup$ – Oldcat Nov 18 '14 at 20:01
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    $\begingroup$ The calculations apply to any object - it's a calculation of nuclear force vs gravity. A cold object just makes the collapse happen faster due to the absence of radiation pressure. $\endgroup$ – Oldcat Nov 18 '14 at 23:18

TimB's answer is basically what I would write, except for a very small loophole that I'll include here.

Any celestial body does not actually orbit another body, but a point in space called the system's barycenter, the center of mass. You can take a look at some of the animations here to get a good idea of what I'm talking about. In a system such as the Earth-Sun system (where one body is much smaller than another), the barycenter is inside the larger object, like here:

enter image description here
Image in the public domain.

In a system where the objects are closer in mass (such as a pair of neutron stars), the orbits will be more like this:

enter image description here
Image in the public domain.

Now put a small body in the center of the system (you can use this simulator, accessible directly here). Use three bodies, with two of the same mass, and put the third - the smallest - in the center. What happens to the planet?

Absolutely nothing. The forces on it are in equilibrium.

This equilibrium would not last forever - in fact, it would only last for a very short while - but in this case, the stars would revolve around the planet.

There's actually a second possibility I hadn't considered: a binary planet.

Take two very massive Hot Jupiters (let's say about 18 Jupiter masses each). put them close together (I mean really close together) and set them spinning about one another. Now take a brown dwarf1, about which the stars revolve. Perhaps it's low-mass, around 25 Jupiter masses. Have the planets go in motion around it. In the resulting system, the barycenter will be closer to the planets, meaning that, technically, the star orbits the planets! It'll be more like this system.

enter image description here
Image in the public domain.

1 Okay, brown dwarfs aren't truly "stars", but they're close enough.

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  • $\begingroup$ That's a very neat solution to the 3-body problem! $\endgroup$ – Caleb Hines Nov 18 '14 at 16:43
  • $\begingroup$ @CalebHines Not a very stable one, unfortunately. . . $\endgroup$ – HDE 226868 Nov 18 '14 at 16:45
  • $\begingroup$ 25 solar masses is not low mass for a star..it is very HIGH mass. $\endgroup$ – Oldcat Nov 18 '14 at 23:20
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    $\begingroup$ @Smithers I don't know how long-term this is. But if it's unstable in the geological scale, we're equally in trouble, because there will be no possibility for life to develop. $\endgroup$ – HDE 226868 Dec 4 '14 at 21:50
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    $\begingroup$ Still not a problem, travel between "realms"/worlds/planes/universes/etc is common enough in the genre to allow for live to have been planted there after developing elsewhere. $\endgroup$ – Smithers Dec 4 '14 at 22:04

Unfortunately the answer here is "magic"...or alternatively "sufficiently advanced technology to be indistinguishable from magic".

As you correctly point out any known star will weigh massively more than the planet. In order to provide the desired result the sun must be considerably smaller than that and there is no way to get a natural star sufficiently small and light to orbit a planet without the planet being so massive it would crush all inhabitants.

An artificial sun would need to be constructed using mechanisms unknown to us. It would have similar size and mass to a moon and could then be placed in orbit around the planet and turned on.

It could potentially be fusion powered using something other than gravity to keep the fusion contained. Alternatively something more mystical like a portal to the plane of fire or something more pseudo-scientific like a wormhole connected to the heart of a star.

Your "fakes geocentric" option would also immediately fall down as soon as someone moved away from the planet into space (in a magical world that's easy to do) or started exploring and moved around on the surface until they were starting to see the real sun "rise". Even if you had a small continent on the far side of the planet from the sun surrounded by a huge ocean you can guarantee that someone would explore that far. Again magic would make that sort of exploration much easier than in our world.

Additionally the tidally locked and super-heating process would generate some really extreme weather conditions that locals would be able to study. While they may not be able to determine what is going on they would certainly know that something is.

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  • $\begingroup$ That takes answer #2 off the list, as I suspected -- Flat Stanley 1, Greyhawk residents 0. It still doesn't cover the other case I brought up, though. $\endgroup$ – Shalvenay Nov 18 '14 at 12:45
  • $\begingroup$ I liked #1, it's a clever solution. It also has some problems though, I've added a brief discussion of some of them into my answer. $\endgroup$ – Tim B Nov 18 '14 at 13:32
  • $\begingroup$ Note that the sun already orbits the earth by the way. Any bodies in space are always pulling on each other. It's just that the Sun effects the Earth much more than the other way around due to the difference in mass. $\endgroup$ – Tim B Nov 18 '14 at 13:34

Geocentrism only requires that we be a little bit wrong about gravity. And the atom proves how easy this could be the case.

How do we know that heliocentrism is the "correct" view for how the universe works? Quite simply, it's because it predicts orbits better than the geocentric models did. But that only became the case once Kepler discovered that orbits were not circular (as all geo- and heliocentric models had assumed until then), and then he applied the results of his calculations (using Tycho Brahe's observations) to Copernicus' heliocentric model. He could just as easily have applied them to Ptolemy's or Brahe's geocentric models instead, though; would have given us a more complex model, but that alone doesn't prove it's wrong.

So what does the atom have to do with this? The Bohr model is still taught today in schools despite everyone knowing full well these days that it does not accurately represent atoms. What it does do, however, is provide a simple and convenient model that explains covalent bonds, energy emissions and absorptions as electrons "jump orbits", etc. Which is why it's still taught in schools today. Just like Kepler's heliocentric model, it's the simplest explanation of how, when, and why these things will happen.

And yet we know it's wrong. We know electrons don't orbit atoms in such neat little circles, but rather dash around haphazardly in a chaotic cloud. (Thanks, quantum mechanics!)

So the Bohr model proves that you can have a simple model for how things work even though it represents those things completely wrongly; why can't the same be said for Kepler's heliocentric model?

Because we also know gravity does work the way Kepler's model requires. And our understanding of gravity is observed consistently throughout the universe, right? Well, no, actually -- astronomers have had to "invent" the concept of "dark matter" (stuff with gravity that we cannot see or touch but trust us it's totally there!) to account for all their calculations about gravity being wrong!1

So for the geocentric concept of the universe to be right, we only have to be wrong about a model that we've stood behind for a mere fraction of the time -- the Ptolemaic model was the model for the universe for around 15 centuries, and successfully resisted the Copernican heliocentric model because the latter was no more accurate; it was Kepler's heliocentric model that finally figured it all out, and only usurped it around 5 centuries ago.

We're wrong about the Bohr model despite it having its benefits, so why can't we be wrong about Kepler's model, too?

Depending on your world, you have a couple of ways to go if you want a geocentric universe:

  1. A magical force that produces the spheres-on-spheres (actually ovoids, as Kepler's proven) necessary for the Ptolemaic model
  2. "Dark matter" is real -- and it's actually the aether that made up the aforementioned spheres-on-spheres
  3. Ptolemy was right about the spheres-on-spheres, but not their composition, and they are in fact binary systems of planets/moons/the sun with a companion dark matter body, all orbiting around Earth because... well I haven't quite figured this part out yet...

Disclaimer before the comments start: I actually do not believe the conclusions postulated above. This is merely an attempt to answer the question as asked by presenting a somewhat plausible explanation for a geocentric universe given modern scientific concepts by first showing how science has progressed in the past and extrapolating that it could continue in the future by disproving what we "know" today just like it's disproven much of what we "knew" in the past. I've treated a few scientific concepts with a certain light brevity that would not fly in any scientific circles solely because presenting what is effectively an anti-scientific model as "reality" requires first putting science in the backseat, at least a little bit.

This is, after all, all for fiction.

1 This is, amazingly, only a slightly simplistic representation of how "dark matter" came to be conceived!

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    $\begingroup$ This neatly handles the issue with some historical examples of the concept, then defines the small (relative to the inclusion of magic and Things That Should Not Be) changes to the universe required. Well done! $\endgroup$ – Smithers Dec 4 '14 at 21:58

If you don't have the ability to measure stellar aberration, the Tychonic system (Sun orbits the Earth, planets orbit the Sun) is indistinguishable from a heliocentric system.

A true Ptolemaic system (everything orbits the Earth in circular orbits) is impossible except through magic, or at least sufficiently advanced technology, because the Sun needs to be sufficiently massive to sustain nuclear fusion.

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  • $\begingroup$ Good insight from a worldbuilding standpoint -- unfortunately, I suspect it won't quite work for Greyhawk itself, but it'd be good for many other fantasy-type universes that need a plausible pre-Keplerian/Newtonian cosmology. $\endgroup$ – Shalvenay Nov 19 '14 at 2:31

I think you can do this in a helio-centric system; as long as everything orbits the 'earth' (Moons) and the 'earth' orbits the sun. In other words as long as there are no planets (well visible ones at least)

When you observe planets, in out solar system, due to parallax effects they will seem to reverse their direction as they orbit. This is what led Kepler to work out that the solar system is a elliptical helio centric system.

So if there are no visible planets, either too small, or are too far away, to observe; then you would have a pretty convincing geocentric appearing system.

The only caveat is, you would not have any (slow moving) planets to hang myths and legends or have astrologers able to assign cause to (blame on) "Mercury/Mars/Venus is in Retrograde"

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If this is worldbuilding: I like a lightweight (but maybe large? in volume), magically powered sun that orbits a planet (2e: Continual Light, FTW!). I'd make this one of the dark planets, floating thru the interstellar void, brought to life. ie: No sun to muck things up.

If we're trying to justify Oerth, then we need all the details.

"Greyhawk's published canon states that the sun there is the third object out from Oerth"

So, what's the order? Oerth (center), next out are Y and Z, and then the Sun orbits them all? Moons are totally the way to go in this case.

Tidally locked Oerth gives tons of problems. The broiler side will have massive effects on your atmosphere. And reflected sunlight is going to be difficult to make the cold side anything but an icebox. Plus the always lit areas near the border, where you can't see the sun directly, but the sky stays lit up... forever.

Easiest way to do Oerth, is just to say that gravity doesn't run the orbits. Gravity can weaken a lot faster, and something else propels them in orbit (magic). Which could be used to explain the flying and other anti-gravity spells, and tie in with Spelljammer gravity effects as well.

Now, the question is, why is canon correct? Just because someone reports something, doesn't mean they didn't lie, or that they got [stufff] correct. Just retcon away :D

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My first solution would be that gravity works differently in this world. Barring that, I'd be inclined to alter physics elsewhere and postulate an element easier to fuse than hydrogen, allowing a lower-mass star.... Actually, that's not necessarily an alteration of physics in itself. We have isotopes easier to fuse than H1. It's just that the natural distribution of elements and isotopes doesn't lead to stars made primarily of something other than H1 and He4.

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  • $\begingroup$ This is a good idea actually. If elaborated, it could be a nice answer. $\endgroup$ – P.Péter Oct 7 '19 at 10:31

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