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For a fantasy, I need to know how a world similar to Earth would exist in a geocentric model.

1) I would assume the sun would have to be a lot smaller. I'm okay with artificial stars (hand-wave that stuff away with magic). But I would like to know if that makes an Earth-similar planet impossible. (I don't need other planets in this model.) For example, how would it effect...

  • seasons and climate
  • length of day, month, or year
  • sunrises and sunsets
  • gravity
  • constellations and/or navigation
  • any huge effect I don't have enough science to anticipate

2) How would I manipulate my universe's model to make it more Earth similar if those things are completely off? It doesn't have to be exact, but I need a temperate climate with pretty normal seasons and climate zones.

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    $\begingroup$ I think Isaac Arthur covered this in one of his videos but I can't seem to find it. $\endgroup$
    – BMF
    Oct 6, 2019 at 15:15
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    $\begingroup$ Please remove your later questions (3 and 4) about moons. Only one question per post (1 & 2 are essentially the same question restated). Post those moon questions separately (though I’m pretty sure they’re already answered on this site). $\endgroup$
    – SRM
    Oct 6, 2019 at 16:19
  • $\begingroup$ Aside from the answers (especially AlexP's is good), read up on Discworld. It tackles a few of these issues in it's own way, even if it has a few different issues than your target planet. For example, it has a small sun and moon orbiting the world. $\endgroup$
    – Gloweye
    Oct 7, 2019 at 7:50
  • $\begingroup$ I'm voting to close. To mix science-based and magic strict laws must be given. Magic can do anything and there is no science-based magic. Unless you supply HOW, they are unmixable. $\endgroup$ Oct 7, 2019 at 12:21
  • $\begingroup$ Hi @Caern, you have edited this question but in the process you have now added in multiple discussion based questions into it. We usually stick with a single question per post format, and I would recommend you ask those questions in their own respective posts. Please make sure to also separate your questions from your internal thoughts or musings to help create a clear and concise question. $\endgroup$
    – Shadowzee
    Oct 9, 2019 at 3:26

6 Answers 6

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The first thing we must understand is that from a purely kinematic point of view, the heliocentric and geocentric models are both equally correct within the accuracy limits of astronomical instruments available before the, say, 16th century. There was no way for an astronomer who lived before Tycho Brahe to bring serious arguments in favor of one or the other. (That's why Galileo Galilei had such trouble with the astronomical establishment of the day -- he simply did not have any good arguments to bring in favor of his pet theory.)

The problem is that from a dynamic point of view, for a geocentric model to be correct it is necessary to abolish the law of universal gravitation: and this means, of course, that the universe in which a geocentric model is correct from a physical point of view has vastly different physics from ours. Whether "a world similar to Earth would exist" in an universe with vastly different physics than ours is not something that anybody but you can answer.

How would it effect...

  • Seasons and climate:

    We don't know. The law of universal gravity doesn't work in your world, so we have no idea how wind works, how the water cycle works, the lot. By the way, how does fire work in a world where the law of universal gravity does not operate?

  • Length of day, month, or year:

    Those are purely kinematic phenomena, and from a purely kinematic point of view the heliocentric and geocentric models are both equally correct within the accuracy limits of astronomical instruments available before the Renaissance.

  • Sunrises and sunsets:

    The sun will rise and the sun will set. We have no idea how the atmosphere works, or how thick it is, because the law of universal gravitation doesn't work in that world. So we don't know if, for example, the sun will appear red at sunset.

  • Gravity:

    Our kind of gravity doesn't work in an universe where the geocentric model is correct. It must be some different force which is called gravity. How it works nobody but you, the author, can say.

  • Constellations and/or navigation:

    No effect whatsoever. The funny thing is that up to this day celestial navigation, as an application of practical astronomy, is done assuming a geocentric model. See celestial sphere for how this works.

    Of course, satellite-based navigation systems won't work, because the law of universal gravitation doesn't work.

  • Any huge effect I don't have enough science to anticipate:

    The main huge effect is that only the author can say how that world works, because it most definitely it doesn't work like ours. What keeps water in the ocean, what keeps people on the ground? Does hot air rise? Why? Are there tides? Why?

Note that you do not have to make Sun any smaller or bigger -- whether we adopt a heliocentric or geocentric system has no impact on the distance between the Earth and the Sun.

Everything also applies for the Moon. A Moon may or may not exist; if it exists, it is not universal gravitation which makes it orbit. What is it that keeps the Moon in orbit only the author can decide.

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    $\begingroup$ Galileo also kinda screwed up and insulted the pope. His book was written as a dialogue, like Plato's stuff. He argued his position as a narrative, one character put forth heliocentrism, another put forth geocentrism, and a third just asked questions. The geocentrist was named Simplicio, and used arguments similar to those made by the Pope. The Pope took this as a personal insult, and THAT's what started Galileo's inquisition problems. Not heliocentrism in general. $\endgroup$
    – Ryan_L
    Oct 6, 2019 at 18:26
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    $\begingroup$ @Ryan_L: Yes, he also had trouble with the political establishment of the day. He also had trouble with the administrative establishment of the University of Padua. Luckily for him, everybody agreed that he was a great scientist, so in the end he never spent even one day in prison and he was allowed to retire to his villa and continue to write. His Discourse on Two New Sciences was published five years after his famous trial, and it was openly available in Rome in the finest bookstores. $\endgroup$
    – AlexP
    Oct 6, 2019 at 21:53
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    $\begingroup$ Good point for the introduction, as many people today imagine geocentrists as fanatics who willfully disregarded the truth which lay before their eyes, while in fact no definite proof could be given with the instruments available at that time. However, I have to correct you in one point: gravity. Geocentrists had a very simple explanation: the tendency of every object is that they strive to move towards the center of the Universe, which is the same as the center of the Earth. The fact that heliocentrists didn't have an explanation for gravity was considered a significant flaw in their theory. $\endgroup$
    – vsz
    Oct 7, 2019 at 6:11
  • $\begingroup$ @vsz: That was a purely qualitative explanation. As soon as physics developed to a level where quantitative calculations were expected that explanation vanished. (And, anyway, the geocentric model evolved away from things revolving around the center of the Earth at some point in the Hellenistic era. In the "standard" Ptolemaic geocentric model planets moved on an epicycle carried on a deferent circle, the center of which (the eccentric) was not the center of the Earth, but rather a point placed symmetrically with the center of the Earth with respect to the equant.) $\endgroup$
    – AlexP
    Oct 7, 2019 at 9:27
  • $\begingroup$ How about if the planet is "Earth similar", but it just happens to be perfectly positioned in the centre of a binary star system? That might make it appear to be geocentric, although I guess this would certainly affect some of the other points of the question. $\endgroup$
    – Craig H
    Oct 7, 2019 at 11:30
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It would be perfectly possible for an extremely advanced civilization, perhaps humans of the future, to create a geocentric solar system.

They could take a rogue Earth-sized planet in interstellar space and create a giant sun satellite orbiting the planet with gigantic fusion power generators generating power for thousands of giant lamps aimed at the planet to heat it and warm it.

If they want the sidereal day of the Earth-sized planet to be similar to that of Earth (23 hours, 56 minutes, 4.0905 seconds) they will have to select an Earth-sized planet in interstellar space that rotates with a similar period and/or slow down or spreed up the rotation of the planet. If they do that the stars at night will seem to circle with the same speed as on Earth.

The giant artificial sun satellite will have to orbit at such a distance that the solar day (the time between two successive noons or midnights at the same location) will equal 24 hours. So that means that the time it takes for the giant artificial sun satellite to make one orbit combined with the time it takes for the planet to rotate once (the sidereal day) will equal 24 hours, a solar day on Earth. I'm certain there are some users at this site who can easily calculate the distance for you.

Of course there is the problem that the "moon" should orbit the Earth-sized planet at the same distance that the Moon orbits the Earth in order to have a month of the same length and similar tides.

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries.[183] However, in the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun.[184] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively.[185] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System.[186]

https://en.wikipedia.org/wiki/Moon#Before_spaceflight1

So the size and distance of the Moon was measured reasonably accurately about 2,000 years ago. And a fake moon orbiting a fake earth in an artificial geocentric solar system would have to orbit the fake earth at a similar distance to that of the real Moon.

Which could be a farther distance than than the proper distance for the giant artificial sun satellite to orbit. Which would be bad because on Earth eclipses are caused by the nearer Moon passing in front of the farther Sun.

There are many other things to consider when designing a possible artificial geocentric solar system. But presumably some users on this board can do it for you.

A possibly simpler way to create an artificial geocentric solar system would be to find an Earth-sized rogue planet in interstellar space and build a gigantic artificial geodesic spherical structure around it and fit the inner surface of that spherical structure with countless gazillions of lamps. The lamps would be programmed to turn on and off in patterns to simulate the movements of the Sun, the Moon, the visible planets in the Solar System, and the stars.

So if it is scientifically possible for an advanced civilization to create an artificial geocentric solar system, a possibly artificial or natural geocentric solar system might exist in a science fiction story set in some parallel universe where the laws of science are different. And of course a natural geocentric solar system might exist in a fantasy story filled with magic.

As I remember, in J.R.R. Tolkien's legendarium the world was originally not only geocentric but flat, until a great cataclysm where the God of the story changed the Earth into a sphere and made the solar system heliocentric.

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Not much ...

The three key bodies for life on Earth are Earth, Sun, and Moon. The Moon orbits Earth. The difference between the Earth orbiting the Sun and the Sun orbiting the Earth is one of reference frames, which are somewhat arbitrary.

The only real difference is explaining the other planets and their moons.

  • In a heliocentric view, Earth is a planet like the others, orbiting the Sun. No special cases are necessary.
  • In a pure geocentric view, Earth is unlike the other other planets, and you need complicated explanations for their paths.

Seasons, days, months, etc. are unchanged. Celestial navigation is greatly complicated on a global scale by the weird apparent paths of the stars.

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    $\begingroup$ The Sun's motion is also weird for a geocentric model, as it tilts up and down arbitrarily throughout the year (or you don't have seasons). Not hard to handwave, but it doesn't naturally follow from a geocentric model (the Moon doesn't tilt up and down because it essentially is geocentric). $\endgroup$
    – MichaelS
    Oct 7, 2019 at 7:08
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You don't have to change much at all, if you don't force yourself to follow Newtonian mechanics to explain planetary motions.

For one, universal gravitation was not mainstream until Newton built his system around it. Until then, it seemed possible that other heavenly bodies might not exert any kind of gravity at all. If you use this approach, then you don't have to worry about the size of these other bodies, or what they might do to your Earth (Although you might want to give the moon gravitational pull if you want tides).

People also once theorized that the other heavenly bodies moved in fixed tracks around the Earth; if you are willing to assert that some divine being fixed a track (perfect circles, for instance), then you can make them go wherever you want, as fast as you want, and you don't have to give a scientific justification for it.

If you're doing fantasy, there's no reason you should feel committed to being Newtonian about everything. Heavenly bodies were once thought to follow different laws than earthly ones; you could make that a reality in your world. This would also give you a lot of freedom to shape them as you like.

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    $\begingroup$ Yes, before Newton, the established view why things have weight (and how things don't fall off the spherical Earth) was that all objects have the tendency to strive to move towards the center of the Universe, because that center is pulling them via some unknown force. This was viewed as a strong argument against heliocentrism, because they couldn't explain how things can have weight if the Earth wasn't the center of the universe. $\endgroup$
    – vsz
    Oct 7, 2019 at 6:14
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Assuming that Newtonian mechanics still apply the factor that determines whether the Earth goes around the Sun or the Sun around the Earth is the mass of the sun. If the sun radiates equally in all directions then its total power output must be proportional to its distance from the Earth.

Assuming that the mass of the Sun is small compared to that of the Earth, an orbital period of 30Ms (about 1 year) gives a radius of about 2Gm corresponding to a sphere with a surface area of about 5.5E19 m^2. For a power density of 1.361 W/m^2 the Sun must produce 7.4E22 W. If it burns at this rate for about one billion years then it will consume about 2.5E22 kg of matter. For comparison, the mass of the Earth is about 6E24 kg, so if you postulate a light-weight machine with a store of fuel which it converts to energy and radiates in all directions then this arrangement could be plausible.

However, if you have a machine converting fuel to sunlight then why assume that it will waste most of it? If it could focus its entire output on the Earth then it could reduce its power consumption by a factor of about 430,000 and at the same time make the mechanism more easily accessible for the maintenance crew.

Unfortunately the radiation pressure would push the Sun away from the Earth, so perhaps it should radiate an equal amount in the opposite direction. This would produce as a biproduct an interesting galactic lighthouse.

From the perspective of a fairly primitive civilisation this should be almost indistinguishable from a heliocentric system. Weather would be similar, although I am not sure what effect the absence of the Sun's magnetic field would have. If you want a moon like Earth's then you must make arrangements to illuminate it. Maybe spread the beam in the plane of the Moon's orbit, or even provide a separately focused beam. Comets would be interesting, as they would now be orbiting the Earth rather than the sun, and they would abruptly disappear as the passed out of the beam.

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  • $\begingroup$ "Assuming that Newtonian mechanics still apply " - why would you assume that? Geocentric models worked with Aristotelian mechanics, not Newtonian. It's actually the discovery of Newtonian mechanics what nailed the coffin of geocentrism. $\endgroup$
    – vsz
    Oct 7, 2019 at 8:10
  • $\begingroup$ Why not assume that Newtonian mechanics apply? In a fantasy, you can choose which parts of physics you want to ignore. :) And when you want to ignore them. $\endgroup$
    – P.Péter
    Oct 7, 2019 at 9:08
  • $\begingroup$ You can take the Discworld approach, where the world works by magic, or the Ringworld approach, where exploring outside the plot can be part of the fun. If you find a serious anomaly the author may have to write a sequel to explain it. In between it can be unclear what is an intended change to the rules of the universe and what is simply carelessness on the part of the Author. I could happily accept that Anne McAffrey's dragons could teleport but flying was incompatible with the rules of physics obeyed by other objects in the world. $\endgroup$ Oct 7, 2019 at 13:47
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You have to limit the solar system to the planets visible by naked eye - or else starting from neptune they would need to rotate around your fantasy earth with more than the speed of light - which is impossible. Stars even increase this dilemma. Your true problem lies in "epi-cycles" ... the visible paths that planets take on our nightsky. Even Ptolomaeus wrote 14 books to show and explain those epi-cycles: every planet has its own set, except for the inner planets Venus and Mercury. Seasons are a problem because the geocentrist model needs the sun to actually move "up and down" between the tropics of cancer and capricorn to give the same path over the sky as can be viewed. And finally what forces would govern the movement of those bodies?

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