The goal is to devise a plausible geocentric system in appearance. However, for simplicity, we will need only to examine the one planet case. And more crucially, this one Earth-like body is to be the only immutable part of the design. Clearly we can't have something so massive as an actual star revolve around Earth. So we may explore what kind of luminous moons for this earth could work, natural, artificial or even theoretical.

By retrofitting geocentrism I mean giving it a distinct "geocentric vibe." Even though in the strictest sense, it won't be considered a solar system at all. I won't want to take away any answer leeway, but in all likelihood this will have to be a rogue planet that has some type of very luminous object that orbits it. It won't be required to be naturally occurring, it can be an artificial object and made of theoretical materials. The quality metric will be to veer more closely on the observable spectrum as opposed to the theoretical.

Lastly, by "geocentric vibe" I mean that we have days and nights and an overall convincing appearance and behavior/motion. Heat and gravitational fields to fend off asteroids, though important, will be out of the remit for our purposes.

All we require is that object that orbits the sun delivers on the luminosity and longevity fronts (rising and setting every day demands a long-term power supply).

enter image description here


How and of what materials, theoretical or otherwise, might we use for this captive luminous body, to have an astronomically correct "geocentric system"?


Perhaps a stumbling block in using the term "geocentrism" is how we think about the "sun". For our purposes, it doesn't have to be a star. It just has to be a scientifically plausible orb that produces similar perceived luminosity and orbital movements when viewed from Earth. Again, the challenge here will be explaining what stuff comprises the orb and how it is powered.

  • $\begingroup$ After reading all that, you're asking how we can engineer a suspension-of-disbelief sun in an arbitrarily geocentric system that can sustain an Earth-like planet? True geocentricity means the sun orbits Earth, which makes it the size of the moon(ish). Otherwise what we have is a binary system with a star and a planet having a barycenter outside the star. Would that be geocentric enough? $\endgroup$
    – JBH
    Nov 4, 2022 at 2:48
  • $\begingroup$ Why not use just set this in an entirely different universe with Aristotelian laws of physics (or something similar, modified for consistency as needed)? $\endgroup$ Nov 4, 2022 at 2:58
  • 1
    $\begingroup$ You can't expect a science based answer to allow geocentrism. Read the tag: based on real world science. $\endgroup$
    – L.Dutch
    Nov 4, 2022 at 4:02
  • $\begingroup$ Emphasis on geocentric feel. Note that "geocentric" is in quotes. It's for effect only. Tried to emphasize it doesn't have to be in the strictest sense, just scientifically plausible design that would conform to a geocentric narrative if and when inhabitants found out how to to traverse space and observed this "thing" for the first time. $\endgroup$ Nov 4, 2022 at 5:36
  • 1
    $\begingroup$ Why doesn't the world of astronomers prior to Copernicus meet your requirement? It certainly had a geocentric feel as far as they were concerned. $\endgroup$ Nov 4, 2022 at 10:58

8 Answers 8


Cherenkov radiation moon

Now I'm no expert on radioactive materials and processes and whatnot, but perhaps you could have a 'star' orbit your (probably)rogue planet world by having its moon be a long-forgotten relic of the ancient world, one that is riddled with huge reactors that sit on the floor of a shallow ocean covering the moon and was used in an age long-forgotten as the world's primary source of power and heat in the absence of their star that they found themselves without for whatever reason. The reactors at the bottom of said moon's ocean are still operational and still produce a lot of heat, and light(in the form of cherenkov radiation), giving you something like a blueish star in the sky if it were close and large enough to be perceived as such.

This of course assumes your world is some sort of super-earth and that the moon is a smaller earth-like mass world that is capable of holding on to liquid water, but it might allow you to have something of a geocentric system. If your world is large enough to keep an earth-like moon in its gravity well you may even be able to have more than just the moon following it around, so you need not be stuck with just the one moon.


The glib answer would be "have the inhabitants be sufficiently egocentric." Physicists sometimes look if their observations become easier to explain with a preferred frame of reference.

If we are both in deep space, and I am "standing still" while you move away from me at 1 mph, that's almost indistinguishable from you "standing still" and me moving away from you at 1 mph, in the opposite direction. On Earth, if I am "standing still" (relative to the surface of that rotating ball of rock) and you are moving up into the air at 1 mph, the switch would have to assume that you are "standing still" and both I and the planet are moving down at 1 mph. That's an equally viable viewpoint, but it complicates all sorts of calculations and observations.

At a slightly more complicated level, you get scenarios like "what happens when I jump up and down in a train carriage moving at 100 mph on a curved track?"

It is natural to start theories with the simple assumption that Earth is standing still. You can have observations fit that starting point by making the sun (as a ball of hot matter, much larger than Earth) rotate around Earth just as easily as you assume that Earth rotates around the sun. The problem with this theory is that the sun and moon would rotate around Earth, and lots of other planets rotate around the sun, and many of those planets have moons. and stars seem to shift in seasonal patterns.

So take this system:

  • One sun.
  • One planet, with several moons.
  • In intergalactic space, too far to make up stars from galaxies without very good instruments.

For all practical purposes, the geocentric frame is just as viable for the occupants as the heliocentric frame.

  • 1
    $\begingroup$ Great point with that "one planet". A lot of pressure for the heliocentric model came from the need to explain movements of other planets. Have no other planets, and the need to explain them will go away! $\endgroup$ Nov 4, 2022 at 15:55
  • $\begingroup$ I don't think even a background of stars would be a problem. It just acts as an outer sphere with little points of light on it. In the Earth setting, the background of stars just makes it easier to see the complexity of the other planets' apparent paths. $\endgroup$
    – David K
    Nov 6, 2022 at 12:34
  • $\begingroup$ +1 Without other planets, particularly without inner planets where you observe retrograde motion, and before Newton and gravity there's no scientific or astronomical need for a heliocentric model. $\endgroup$ Nov 6, 2022 at 15:13

Since you state

Emphasis on geocentric feel. Note that "geocentric" is in quotes. It's for effect only. Tried to emphasize it doesn't have to be in the strictest sense, just scientifically plausible design that would conform to a geocentric narrative if and when inhabitants found out how to to traverse space and observed this "thing" for the first time.

you might want to give a look at the Tychonic system

It is conceptually a geocentric model, or more precisely geoheliocentric: the Earth is at the centre of the universe, the Sun and Moon and the stars revolve around the Earth, and the other five planets revolve around the Sun. At the same time, the motions of the planets are mathematically equivalent to the motions in Copernicus' heliocentric system under a simple coordinate transformation, so that, as long as no force law is postulated to explain why the planets move as described, there is no mathematical reason to prefer either the Tychonic or the Copernican system.


The Witenagamot Decrees

...that from the earliest times, natural philosophy has observed the nature of the Earth and all philosophers have calculated and conjured and taught the same doctrine, namely, that there lies at the centre of our world the coalesced and petrified quantum of all ponderous and heavy elements. Which, due to the forces exerted from within and without alike, rotates upon a sensible axis which is called the irminsul or worldtree. This quantum is known to be round, and as a being of polydimensional space, a globe to be clear, and that we call Earth.

Around this globular quantum of ponderous elements arise ever more fluid and energetic elements, each rising in ever more rarified strata from the densest of aethers to the uttermost vacuum, beyond the marches of which lie the Realms of Empyrean.

enter image description here

The geologers and geomancers concur with the physicians, the metaphysicians, the alchemists, the astronomers and the astrologers on the natures of the Realms of the World, the Elements proper to each Realm and also the Powers or Forces that act upon them.

The ancient wise knew that a limb falls downwards from a tree, that rain falls from the clouds, and that steam rises from soup and smoke rises from fire. The logical conclusion is that two forces act upon all elements, and we call these Gravity and Antigravity. Gravity is that force which tends to draw all things down towards the centre of the quantum, whilst antigravity is that force which tends to draw all things upwards towards the empyrean, away from the centre.

Other forces, the lateral force, and the rotational force also act upon all elements, in strength according to the density of of the elements acted upon. We can observe: the rotational force acts most strongly upon the densest of elements, the transplumbians and the deep chthonic rocks, the rotation of which around the irminsul, or axis, causes great turbulence, earthquakes, volcanic eruptions, and the tearing apart of the very Earth itself. Whilst high above the surface of middle earth, in the highest over heavens, the rotational forces only barely act upon the slow stars of the Zone of Weak Lights, and their motion is languid, hardly to be observed at all by mere mortals, yet the astrologers vouchsafe the minutest of motion over the course of many myriads of years!

Observing the Four Forces, we therefore conclude that the Model of the Earth's Oecumene is one of a stratified and hierarchical nature. The most ponderous of elements, the heavy transmercuric and transplumbic metals and the densest of chthonic stones aggregate, due to the force of Gravity, into a globe. So much matter of this sort has been so aggregated that it might take a man many years to walk around its girth and come back again to the place where he started! These elements whirl around the irminsul at a rapid velocity, which churns up the waters and silts of seas and lakes and rivers alike. And it is the waters and silts, the lighter metals and less ponderous stones, the various minerals, salts, earths, humours and vitreols that comprise the surface of the Earth upon which we live.

Above the solid and liquid boundary of Earth rise the many layers of air. The gaseous and energetic elements of the Zone of Winds and Spirits find their home above our heads! The densest of these elements are the various gasses: such as hydrogen, oxygen, nitrogen, carbonics, steams, vapours, clouds, and so forth. These heavier gasses, are acted upon also by Gravity, which tends to draw them downwards, and also by Rotation, which, due to their intermediate and volatile nature, causes them to rotate around the irminsul at an extremely rapid velocity and in an agitated state.

Higher still in the atmosphere lies the Zone of the Mighty Lights and is the home of the Sun and the Moon. We call them luminaries, and the silly Heliocentrists call them "planets"; but in reality, observation carries us to the truth. And that is that the luminaries are ethereal bodies composed of highly energetic elements of varied hue and intensity. We know from telescopic observation of those distant strata of our atmosphere that venereon is a dull and pale yellow colour, helion is a bright phosphorescent colour, and selenon is a cooler, whitish grey colour. Helion is the most energetic of elements in the radiation of heat. This we know, for it is remnants of helion trapped within the materials of earth, such as wood and oil, that cause them to burn with such ferocious heat. The Sun, therefore, is a brilliant agglomeration of helion, burning with such intensity that its heat easily penetrates the thicknesses of the lower atmosphere and even warms the waters of the sea.

The heliocentrists argue with the utmost futility that day and night require the Earth to rotate around irminsul and that the Sun lies stationary at the center of the Oecumene. What nincompoops! How is it --- yet they can not explain --- that highly energetic elements such as helion and transarienon are somehow more attracted by Gravity than the ponderous elements of Earth! Nay, they have it wrong. Day and Night are most logically explained by the afore mentioned forces of Rotation and Lateration: being composed of highly energetic elements, the luminaries of Sun and Moon simply move rapidly over the face of the Earth, each with her own velocity and timing, such that both Sun and Moon appear to "rise" and "set" when it is simply the case that the luminaries move towards our eastern horizon, then pass overhead as does any cloud of denser vapour, and then move at last towards our western horizon, which is nothing more than Antipodeans' eastern horizon!

Beyond is the Zone of Weak Lights. The two bands of this zone are divided by the astrologers into the swift stars, which those heliocentric buffoons call "planets" and give names such as Ares and Venus and Ceres and Saturn and Mercury and Varuna and Ninurta. These superethereal bodies are not, as some would have us think, solid bodies, but they are conglomerations of highly energetic, though weakly visible phosphorescent elements: notably lumenon, metalumenon and elektron. So energetic are these elements that their light is able to penetrate all the wide and dense layers of atmosphere above the Earth!

It might here be noted that it is above the Zone of Winds and Spirits that the effects of Antigravity are felt much more strongly than Gravity. The lighter and more rarified elements of the highest altitude Zones of our atmosphere, being less dense than the clods of solid and liquid matter, hearken more towards Antigravity's pull. It is here that the high and twinkling stars shine forth, tiny pin pricks of nearly invisible elektron and metalumenon!

The two highest strata of our atmosphere, and the strangest, to be sure, are the Zone of Dark Matters and the Zone of Vacuum. Dark Matter is very strange indeed. It is so high above the surface of Earth that it can not be seen, for not even the weak light of the highest stars may penetrate and illumine its ebon surfaces. Yet it can be known by the astrologers for it is the source of the strange energy called Antigravity. Just as Gravity arises from within the depths of the most ponderous of elements, deep in the very heart of Earth, so Antigravity arises from its uttermost altitudes where all is dark and obscure.

Beyond the highest layers of the darkest matter lies the strangest stratum of all, and that is Vacuum. Here, there is nothing. No mote of even the tiniest speck of elektron or weird strand of Dark Matter penetrates this layer. It is as if Antigravity itself is repulsed! No higher strata of atmosphere are known to exist beyond Vacuum, and beyond its indeterminate borders lies the Empyrean, the extracelestial abode of the Creator, which lies outside the confines of the world.


The moon is a giant nuclear pile

Here goes. If anyone is kind enough to fix my formatting, I'll be grateful. The moon has mass around 1 x10^23 kg, and radius 1.75 x10^6m. Earth has radius 1.3 x10^7m.

At 3.84 x10^8m distance, earth covers 0.0338 rad angular distance from the moon. (theta = arcsin (radius/distance)).

This means it covers 0.00564 of the moon's night sky (areafrac = 1-cos(pi x theta)). And the sky is only half the angular area that the moon would send radiation to.

So for every watt received at the atmosphere of earth, the moon would have to emit 2 / arcfrac watts = 354.6 W.

In real life we receive 1.73 x10^14 watts of solar energy at the surface, with about 50% having been reflected into space. So earth received 3.46x10^14W total.

That means the moon would have to emit 354.6 x 3.46 x10^14W = 1.227 x10^17W, or 3.869 x10^24 J in a year.

A kg of uranium emits about 8.2 x10^13 J/kg when fissioned. So the burn rate of uranium is 3.869 x10^24 / 8.2 x10^13 = 4.72 x10^10 kg per year.

If this moon lasts 10^12 years, it has mass mass 9.44 x10^22 kg. This is pretty darned close to the real life mass of the moon.

This means that this moon of pure uranium, density 19000 g/m^3, has volume 4.97 x10^18 m^3. That gives it a radius of 1.06 x10^6, about half that of the real moon (which makes sense, uranium is roughly 6 times denser than moon rock).

In other words, the moon looks and acts very much like the moon does now, but it's a bit smaller and is as bright as the sun.

Now, of course, that moon will explode (right, physics crowd? I'm only a chemist / programmer).

But that's with it lasting 10^12 years. Shorten that lifespan by a factor of up to 1000 and substitute in inert moon rock. Now the moon lasts for a billion years. And it looks and acts just like the sun does now.

This assumes that with some proportion of U-235 between pure and 1 in 1000, the moon becomes a stable nuclear pile. And that's where I use my hands; I either hand wave the problem away, or I hand away the problem to a physicist.

Alternatively, the ratio could be even lower, but the life of the sun is then shorter.

PS What the heck are gamma rays and xrays? You're making things up, nothing to see here.


A red dwarf and a radioactive moon

I'm going to lay out a solar system that will satisfy your requirements. It should naturally lead people to geocentrism, it won't have planets 'obviously' (for some values of 'obvious') orbiting the sun, and they might need to develop serious telescopes to figure out what's actually going on. (which won't be a primary driver for the development of optics, because there doesn't seem to be much of a mystery to solve)

The radioactive moon

First let's talk about the 'sun' (that is, the moon orbiting the planet that appears to be a sun). The 'easiest' way I can think of is to have it be highly enriched in radioactive materials. Dump enough, sufficiently long-lived radioactives (but not fissiles) onto the moon as it's forming, and it'll glow (from the decay heat) for quite a long time. This doesn't even have to be dangerous to the inhabitants of the planet.

As long as the temperature of the surface of the 'sun' is similar to the temperature to the surface of our sun, they'll put out similar spectra of light, and if they occupy a similar area of the sky (our sun and moon do, though our moon is in too high an orbit to have the required orbital period), the amount of heat and light transferred to the planet will also be similar.

Sean OConnor did some calculations regarding lifetime and energy production, assuming a stable nuclear reactor. I'm assuming simple radioactive decay, so it's only mostly applicable, and I may do those calculations myself later.

The red dwarf

The next problem is the sun of the solar system, which the people shouldn't see as a sun. There are various options here.

Red dwarf

A red dwarf doesn't put out much light, put your planet in a wide orbit, and it'll look like just another star, though it's one that appears to move around the planet, like a 'planet'. This 'planet' won't have phases, which will keep your scientists from having a mystery to solve there.

Brown dwarf

A brown dwarf is even lighter than a red dwarf, and never got started on 'proper' fusion. As a result, these things have temperatures on the order of 1000 K, and don't emit much light. It'll be hard to see, even if you're fairly close, but you'll still have an anchor for the whole system. The lack of active fusion probably reduces the protection the parent star provides though, however much that is.

Rogue planet

Alternatively, you could dispense with the whole star thing, and just have a planet with its luminous 'star' moon go it alone through the galaxy. This does mean your planet is now exposed to the interstellar medium, though I don't know how much of a problem this will necessarily be.

Changes your scientists might notice

A hot ball of rock&metal and a hot ball of gas emit similar amounts of photons, in similar ranges, but the difference in chemistry is visible to physicists. Absorption and emission lines mean they will definitely notice that the 'sun' has a very different composition than all those stars they see in the night sky. This might be a mystery they eventually solve, or it might cause them to lose interest in further study of the stars.

Alternative: Just don't have planets

If you have just a sun, but no other bodies in the solar system, many of the hints that helped people on our planet figure out heliocentrism will be gone. If you want to go one step further, you can even remove the moon.

  • $\begingroup$ I just quantified the glowing for a long time / emitting as much energy as the sun postulate. $\endgroup$
    – user86462
    Nov 11, 2022 at 6:51
  • $\begingroup$ Nice work. I may build on that further, assuming just decay and comparing various elements&isotopes, when I have the time. $\endgroup$
    – AI0867
    Nov 11, 2022 at 12:03

A civilization could build an oversized florescent light bulb.

Think some big fusion power plants to supply power to a large mostly hollow structure. Phosphor material at the surface to glow appropriate colors. Some novel structures to light up the phosphors efficiently. It would need significant fuel supply and maintenance to deal with whenever something breaks/keep lumen levels stable etc.

I definitely have not considered the material requirements for a moon sized light bulb.

This could be done by a civilization that was fleeing there soon to be super nova solar system. They want a 'sun' to take with them. Biggest problem would probably be supplying the fuel to keep the fusion plants running. To help could possibly turn off the bulb on side facing away from the home world.

If the civilization had moved their home world to orbit a gas giant. Turned the gas giant into a rocket. Then they would have the gas giant to get fuel from. But then it would not be geocentric. But it being a gas giant it would not be lit up so maybe that is OK for the questions scenario?


Have a high tech civilization on a Rogue planet

Sometimes planets get tossed out into the void, away from all stars. Your planet can have the same issue. It could be a chance encounter with a rogue star or black hole, or enemy action, or even intentionally fleeing their star system. Regardless, they are now shooting through the void of space.

If you want the civilization to be lower tech, you could have their civilization fall but their tech remain.

Have a moon with powerful fusion engines and massive LEDs and a gas giant

They can convert their moon into a giant torch. They'll need a lot of fuel, so have a fairly distant gas giant that was ejected with the planet. Their advanced machines can draw in hydrogen from the gas giant, feed it to the moon, and use fusion engines to generate a massive light.

If you want to show this, you could have the 'sun' have a tail that leads to the gas giant.


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