I've been designing a world and wanted to base it on a rather interesting cosmology proposed by the ancient Greek philosopher Philolaus. He was a very early opponent of geocentrism, believing that both the Earth and the Sun orbited around a third body he called the Central Fire. He claimed that the Sun's orbit around the Fire took a year and the Earth's took a day, and that the day/night cycle took place because the Earth was tidally locked to the Fire and we all lived on the other side. For a visual description, take a look at this.

The other body you see there is Counter-Earth (or Antichthon in Greek), but that can just be ignored for now and I'll ask about its revelance in another question if I want to include it. It should also be assumed that the "Central Fire" is in fact just another sun, though I'm interested in replacing it with a black hole which I'll ask about in a seperate question.

So anyway, as my physics and astronomical knowledge is rather intermittent (and completely devoid of anything mathematical), this seems like an important place to start with questions. I want to make the "Earth" of this system have living conditions as Earth-like as possible (at least in the view of the Sun-side inhabitants). The most obvious concerns are:

  1. Given that the orbiting sun would need to appear as Earth's, with presumably the same mass and at such a distance that it appears to be the same size, what does this mean for the central sun? More specifically, is there any possible configuration of mass or distance that allows it to hold the orbit of the other sun without making it impossible for the planet to be Earth-like (though obviously the face directed to the central sun will be scorched from 24 hour exposure). It's rather important for the model that one sun orbit the other, rather than them orbiting eachother, but I don't even know if this is actually even possible.
  2. What would the result of the above look like to an observer on the planet, both on the side facing the orbital sun and on that facing the central sun? Are there any important planet-based observations arising from the model, or more specifically the way it arranges the orbits of the planet and the outer sun.

In terms of how much I understand, I'm pretty competent with elementary ideas (I recently found out that all lightlike paths beyond a black hole's event horizon warp towards the singularity such that moving towards it is inevitable, and I know enough about relativity for this to make sense to me), but I'm not really capable of determining how they all interact together. Thanks in advance to anyone who can shed some light on this for me, it would be greatly appreciated (and hopefully interesting for you to consider).

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    $\begingroup$ Welcome to the site Kanderas. This is a generally well asked question but falls into a common trap that newer posters (and some older posters for that matter) often fall into. There are just way way too many questions here. I would recommend breaking this apart into component pieces. 1 might be identifying a stable stellar system 2 might be the impacts on your "earth" etc etc etc. Let just know if you have questions and please check out help center for the details on how to ask good questions (feel free to visit the Worldbuilding Chat as well). $\endgroup$
    – James
    Mar 25, 2016 at 14:04
  • $\begingroup$ Yeah, unfortunately I looked at the question guidelines after posting it. Part of the problem is I don't really know enough to see where to draw the line without impacting somebody's ability to answer. I was originally going to ask the question by presenting the model and flatly asking what was necessary to make it Earth-like. Then I thought that might be a bit broad and sought to delineate the pertinent aspects a bit, and in doing so it now seems the questions might all be a bit much. It's rather late where I am so I need to head to bed, but I'll see if I can't break it up somehow tomorrow. $\endgroup$
    – Kanderas
    Mar 25, 2016 at 14:22
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    $\begingroup$ Don't worry about impacting somebody's ability to answer, we're a wordy lot and will usually include relevant information that doesn't answer the question or include links to that information. I'd love to answer several aspects of your question but as @James pointed out, there are really multiple questions here and they should really be split into different WB questions. $\endgroup$
    – Jim2B
    Mar 25, 2016 at 14:40
  • $\begingroup$ @Kanderas If you have questions when you are reformatting/breaking this apart let us know. $\endgroup$
    – James
    Mar 25, 2016 at 15:04
  • $\begingroup$ For your first question along these lines, I would suggest focusing on the question about the seasonal cycle, which could be expanded to ask if we on Earth would experience any cycles lasting a year (including cycles related to how high the Sun appears to climb in the sky relative to the background stars). Offhand I can't see how we could detect the period of the Sun's orbit at all, since it seems like the Earth would be at exactly the same distance and relative orientation at its closest approach each day. $\endgroup$
    – Hypnosifl
    Mar 25, 2016 at 21:20

2 Answers 2


The problem is that the arrangement indicated in this diagram...


...simply cannot be reconciled with what we see in the sky every day. In that scenario, the sun would not only rise and set; it would approach and retreat every day. The sun would appear relatively small at sunrise, grow to an apparent much larger size at noon, then shrink again to sunset.

The tides associate with a daily approach and retreat from the sun would also be significantly different than what we see on our Earth. The difference might not be that great, but solar tides are about 1/3 as high as Lunar tides, so it would have some effect even if not very great.

Here's an alternative scenario: The Sun and the Earth are both in the same orbit around the "central fire", which is a black dwarf star. That is, it's an old, cold white dwarf which emits no visible light (altho it's probably still emitting some infrared... so you might want to stipulate this world's sun is a bit cooler or further away, to account for the difference. The Earth is in a "Trojan" position relative to the Sun's orbit; a Trojan position is a stable orbit. That is, it's 60 degrees ahead of or behind the Sun in the same orbit. The "central fire", the Sun and the Earth would be at the vertices of an equilateral triangle at all times.

Assuming this alternate Earth has the same axial tilt, the same seasons and the same year length would be preserved. You might stipulate the seasonal variation is slightly less... the planet is slightly more temperate with slightly less seasonal variation... if the "central fire" emits much infrared radiation.

Note that in this scenario, both sides of the Earth are equally inhabitable.

You could still have a Counter-Earth, but it would not be hidden from view; it would be visible in the night sky near dawn and dusk, just like Venus is in our world. Occupying the opposite Trojan position, either leading or trailing the Sun in its orbit, it would always appear in the same position relative to the sun. Unlike Venus, it would not be a "wandering star".

Here is a diagram: http://hyperphysics.phy-astr.gsu.edu/hbase/solar/imgsol/LagrangeJup.gif

In the scenario I describe, the Sun would replace Jupiter, and the "central fire" would replace the Sun. The Earth would be in either the L4 or L5 position, with Counter-Earth in the other position. (The L3 position, which would be preferable for a more traditional "planet hidden behind the sun" scenario, is unfortunately not a stable orbit.)

From everything I've read, the "Counter-Earth" scenario, with Earth's twin hidden behind the sun, simply isn't possible with what we understand about gravity and astrophysics. There's no stable orbit that would put two planets on exactly opposite sides of the Sun, and keep them there. The scenario you pointed to is an interesting variant, with Counter-Earth hidden on the opposite side of a one-face Earth, but again I don't see how to make that fit with being an alternative way of explaining what we see in the sky every day. Of course, if that's absolutely necessary for your scenario, you can hand-wave away the impossibility by using a near-magic technology (such as gigantic reactionless drives hidden deep inside Earth and/or Counter-Earth, forcing them to stay in a tidally unstable orbit).

An assumption I'm making here is that the Earth and Counter-Earth were moved into their current positions, probably within the last 100 million years or so, by a technology sufficiently advanced to appear as magic (Thank you, Arthur C. Clarke!). First of all, the Sun coalescing in that orbit would "clear the orbit" of the gas and dust needed to form a planet, so no other large bodies (such as the Earth or Counter-Earth) would be formed during the planet-forming stage of the evolution of that solar system. Furthermore, chaos factors over a very long time period would cause both Earth and Counter-Earth to be lost from their stable position re the Sun. That is, this is a stable orbital configuration over the course of perhaps millions of years, but according to current astronomical theory, some or most of the planets have moved in and out in their orbits over the lifetime of the solar system, so this arrangement probably wouldn't be stable on the time scale of billions of years.

Note this doesn't mean I'm stipulating the Earth and Counter-Earth can't be more than ~100 million years old. They could have been in orbit around other stars, with the planets cooling and life evolving for billions of years, and then moved into their current positions perhaps ~100 million years ago... or perhaps much more recently, if you'd prefer.

I leave the problems associated with moving a major planet that far without disturbing the planet's crust, and without freezing it and killing off everything above the level of a microbe, as trivial engineering challenges for a society (or group, or entity) with near-magical technology.

You haven't mentioned the existence of Earth's (or Counter-Earth's) moons. I think with the orbit I've suggested, a similarly large moon in a similar orbit would be possible for either or both planets, altho again those orbits might be less stable than they are in our universe, over extremely long time periods.

  • $\begingroup$ The Sun would approach and retreat, but only by a small amount, since as I noted in a comment, according to Kepler's third law the Sun's orbit would have to have a radius 51.097 times larger than the Earth's orbit. And of course people on outward-facing side could only see the Sun for the half of the Earth's orbit that was closest to the Sun at the midpoint. Working it out, if the distance from Earth to central fire is x then closest distance we see the Sun around noon would be 50.097x and greatest distance we see it at sunrise/sunset would be 51.107x. $\endgroup$
    – Hypnosifl
    Mar 29, 2016 at 11:38

Kanderas, I'm going to add a second answer here, because I didn't directly address your question; rather, I pointed out why that scenario couldn't be reconciled with what we see in our sky.

But what if we ignore that? How would things be different in your scenario, and what can we do to minimize those differences?

The main problem I see, if the scale on your diagram is correct, is that the day/night temperature difference will be much greater, with the sun advancing and retreating by a large degree every day. Such a large temperature difference isn't compatible with the stable, mild conditions necessary for complex life to evolve, nor in fact with retaining an Earth-like atmosphere. If the uninhabited side is baked by unending daylight, it will soon be heated beyond the boiling point of water, and so lifeless as the moon. Worse, on the very hottest parts, it might get so hot that even water vapor would be disassociated into hydrogen and oxygen. If this happens, the hydrogen would escape into space, and slowly over the eons the planet would become lifeless as Mars.

Contrariwise, if the "central fire" is just a massive object but not a sun radiating warmth, the dark side would freeze so hard that the atmosphere would freeze there, and it wouldn't be long until the entire planet's atmosphere (and hydrosphere -- the water, lakes, rivers, and oceans) would vanish away.

In either case, either a terrifically hot or extremely cold "dark side" would create eternal hurricane winds flowing into the cold side, or away from the hot side, all along the terminator and probably thousands of miles into the inhabited side. If you're getting the idea that in this case, "inhabited" means "not very inhabitable"... you're right. This isn't a scenario for a habitable planet.

But there are other assumptions we can make with much more temperate outcomes, so let's make them. Let's assume the "central fire" is a red dwarf star, emitting enough light (and infrared radiation) to keep the hidden side of our planet pleasantly warm, at a stable temperature that just happens to be the same as the average temperature of the inhabited side. Let us furthermore assume the Earth is in close orbit around this red dwarf, fitting your tidally locked scenario.

Now, what we need to do is greatly expand the distance between the Earth's orbit around that "central fire" and the Sun. This has many benefits, not the least of which it greatly minimizes the objection I made in my first response. If the Sun's orbit is quite distant, relative to the Earth's orbit around the red dwarf, then the growing and shrinking as the Earth approaches and retreats from the sun will be unnoticeable, or at worst barely noticeable. Since in our universe, the Sun is some 93 million miles away, we have lots of room to do this and keep it realistic.

In this scenario, we do lose our moon. A tight orbit around a red dwarf would be much too close for a stable large moon at a large distance, like Luna. A smaller one would be lost fairly quickly (on the geologic time scales) because of tidal dragging by the red dwarf.

Of course, we can expand the Earth's orbit around the red dwarf and keep Luna... and also make more room to squeeze in Counter-Earth in an even closer orbit. But then an observer on Earth would start noticing that the sun grows and then shrinks as the day passes.

If there is enough difference in the apparent size of the Sun to be noticeable, then there will be enough difference to cause a greater day/night variation. Even if the "dark side" of Earth doesn't freeze or roast, there still will be enough variation in day/night temperature to significantly affect evolution. Of course you can arbitrarily choose any evolutionary history you wish, since it's your world. But realistically, we'd tend to see much less variety in land animals. I'd suggest no land animals more complex than insects, and less variety in plants, too. Keep in mind that trees and grass are products of fairly recent evolution, so aren't likely to exist. Unless you plan to make the "people" in your story sea-dwellers, there's not going to be a lot of story-telling potential in this scenario.

The trick here will be trying to choose a "Goldilocks" distance for the Earth to orbit the red dwarf; far enough away to allow a stable Luna orbit, but close enough to minimize the variation in the Sun's apparent size. This would be far easier if you stipulate that Luna's orbit is artificial... that it's the product of some long-vanished alien race which buried a reactionless drive deep in the moon to keep it in that orbit.

Now, about using a black hole as the "central fire": That's a very bad idea, from the standpoint of scientific realism. Black holes have accretion disks pulling matter into them, and they emit very high energy radiation. (Even the thin interplanetary hydrogen and dust will be sufficient to create a weak but permanent accretion disk around the black hole.) They also have searchlight-like jets of high radiation and plasma emitted from their poles. Thankfully that wouldn't directly impact Earth, because Earth's orbit should be roughly perpendicular to those jets. But even the secondary radiation from the black hole and its jets would do really nasty things to the Earth's atmosphere, and remember that winds will flow from the inhabited side to the "dark side". Water will flow around the world, too.

So, my advice is to not try to use a black hole as the "central fire". It's just too nasty.

I hope you realize that in any variation, Counter-Earth is significantly closer to the "Central Fire", and thus has a significantly hotter surface. So any assumptions we use to make Earth relatively close to Earth-as-we-know-it will automatically rule out an Earth-like Counter Earth.

Seems a pity to have a Counter-Earth and have it be as lifeless as Venus or Mercury. Frankly, if it was me, I'd make this a fantasy world rather than a scientifically realistic one. Then you could have Luna without worrying about its unstable orbit, and Counter-Earth might be the home of ifrits and fire demons! Obviously the inhabitants of that Earth would find a different name for the other planet; it's hardly a "Counter-Earth"!

  • $\begingroup$ "Even the thin interplanetary hydrogen and dust will be sufficient to create a weak but permanent accretion disk around the black hole." How weak, though? Physicist Kip Thorne in The Science of Interstellar said that in that movie, the accretion disk was supposed to be weak because the black hole hadn't swallowed any other stars or large bodies in millions of years, so its temperature was around the temperature of our Sun's surface, and there'd also be no jets. And that was a supermassive black hole, a smaller "central fire" black hole might have an accretion disk with much lower luminosity. $\endgroup$
    – Hypnosifl
    Mar 29, 2016 at 11:43
  • $\begingroup$ Also, I found this post by astrophysicist Jean-Pierre Luminet where he comments on Thorne's statements about an "anemic" disk around the black hole in Interstellar, saying "In that sense, such a quiescent accretion disk could be relatively safe for humans. But I doubt that it could provide enough light and heat to the planet, like the Sun to us, just because an anemic accretion disk would also be optically thin (i.e. transparent), while the Sun’s photosphere is optically thick (i.e. opaque)." $\endgroup$
    – Hypnosifl
    Mar 29, 2016 at 16:12
  • $\begingroup$ ...Although, it's possible he was discussing a theoretical black hole that is not even being "fed" by the interstellar medium (one that had been ejected from its host galaxy, say), rather than a more realistic situation. Also, after some googling, it looks like the lowest rates of accretion are found in "advection dominated accretion flows" (ADAFs), I'll try googling some more and see if they give any equations for ADAFs being fed by the interstellar medium alone, in particular the luminosity as a function of black hole mass, which would determine how much energy an orbiting planet gets. $\endgroup$
    – Hypnosifl
    Mar 29, 2016 at 16:43
  • $\begingroup$ (a) There is unfortunately an extreme disparity between the claims of "scientific realism" in "Interstellar", vs. the actuality. Kip Thorne's scenario makes many extremely unrealistic assumptions which aren't mentioned in the movie. For example, Kip had to postulate "intermediate" black holes (IBHs) to justify the spaceship being able to get to that planet and back within a human lifetime. IBHs that were no part of the story in that movie. Let's just not use that; it's not at all realistic. A supermassive black hole also has different characteristics than what you need for the "central fire". $\endgroup$
    – Lensman
    Apr 2, 2016 at 2:56
  • $\begingroup$ (b) One problem with assuming a central black hole, even if you assume there's no appreciable accretion disk emitting radiation, is that means the "dark side" of Earth isn't heated. I noted the problem with letting it become a "deep freeze" in one of the two Answers I posted. $\endgroup$
    – Lensman
    Apr 2, 2016 at 2:58

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