Ramifications of a terracentric system?

[CCamfield]

Seconded on the recommendation of Garfinkle's book. Fun stuff. Greeks versus the Chinese in 1000AD or so.

Some of the reasoning that was behind ancient models of the universe was based on the theory that the universe was fundamentally perfect. The earth was the centre of the universe, and the planets went around it, so (for instance) they must go around it in circles. That doesn't quite match observed phenomena, so they're also moving on these smaller circles at the same time, etc etc...

This isn't magic. It's a different set of assumptions and observations about how the world works. (And in Garfinkle's book, it's the truth, and he extrapolates lots of neat stuff from how the Greeks viewed things.)

But if you go back to that root assumption, that the universe is somehow perfect, well, D&D worlds usually do assume some deity or other created the world and universe. So why do the physical laws have to be the same as they are in our world? If the sun and planets do go around the world in perfect circles... why not? The sun can just be the source of light in that universe, not necessarily responsible for seasons and warmth on the planet.

 

[woodelf]

Our group is in the midst of a homebrew setting, which has been fairly collaborative in its creation. Recently, our GM stated that the solar system the campaign is in (this is a fantasy high magic game, not sci-fi, just to clarify) is a terracentric one; the main planet is in the center of the solar system, and the sun and other planets rotate around it.

What would be the major differences, if any, that a layperson would notice going from a heliocentric system like Earth to a terracentric one? Obviously, there are issues with real-world physics (like how a planet would have enough gravitational pull to keep other planets in orbit), but I'm not really interested in hearing "this won't work because of <insert long equation here>". I'm more interested in what Joe Everyday might notice.

Well, assuming no further changes (most of what i'm about to list could be "compensated for" if you wanted to):

• If everything is in the same plane, you'd have more-frequent [partial] eclipses; if not, you'd never have them (whether the eclipses are full or not depends on the relative sizes of the bodies, and their distances)
  
• Seasons are caused by the relative angle of the earth's axis and the solar ecliptic changing (due to the orbit), so unless either the sun's orbit wobbled, or the planet wobbled, you wouldn't have seasons. You could give the sun a highly-eccentric orbit (much moreso than most things in our solar system) to compensate for this, of course--or just have seasons happen for reasons other than celestial mechanics, as in most world mythologies (Demeter and Persephone come immediately to mind).
  
• No retrograde motion. The perception of retrograde motion of the other planets comes from the movement of Earth in its orbit, relative to their orbits. So, unless you introduce epicycles (as Aristotle, Ptolemy, and others did), the planets will always move in one direction in the sky.
  
• Similarly, planets will always be equally-bright, because their distance won't change.
  
• No precession of the moons' or planets' orbits. (This is the factor that turns the Moon's 26.something-day orbit into a 28-day cycle as seen from Earth.)
  
• Relating back to the tilt-of-the-Earth thing, the sun will always rise the same amount north every day of the year, instead of slowly progressing from the Tropic of Capricorn to the Tropic of Cancer, and back again.
  
• Unless you introduce a rotating "sphere of the heavens" or somesuch, the same stars will be visible every night from a given location, instead of changing over the year.

I think that's all the major stuff--there's lots of little stuff that a good astronomer might be able to pick up, but the above would be noticed by just about anyone, i'd think. Of course, without the frame of reference of our Earth for comparison, they'd not think anything of it.

 

[Umbran]

[*]Seasons are caused by the relative angle of the earth's axis and the solar ecliptic changing (due to the orbit), so unless either the sun's orbit wobbled, or the planet wobbled, you wouldn't have seasons.

As I noted earlier, this is incorrect. Assuming the planet sits in the center and spins to produce the day/night cycle, and the spin axis is tilted with respect to the plane in which the sun orbits, you will have seasons.

]No precession of the moons' or planets' orbits. (This is the factor that turns the Moon's 26.something-day orbit into a 28-day cycle as seen from Earth.)

You will still see prescession of the moon's orbit if the planet spins. In this case, the relative situation between the planet and moon here is no different from our real-world case - in both, the moon orbits the planet.

 

[Patryn of Elvenshae]

The fact is, for all intents and purposes, we *do* live in a terracentric / geocentric universe.

Why?

Because all you are doing is changing the axes about which movement is described. You can put the Sun (Sol) at the (0,0,0) point, with the "Galactic Up" vector in the same direction as the solar center to solar north vector, or you can rewrite the equations with the Earth at the (0,0,0) point with "Galactic Up" being in the same direction as the geocenter to magnetic north vector.

There are absolutely no changes from "current Earth" when moving to such a system.

Heck, there are no changes when moving to a Patryn-centric universe, which has *me* defined as the (0,0,0) point about which existence revolves, with "Galactic Up" defined as the direction perpindicular to the direction my eyes are looking. It's fun to watch the universe jump about as I move my head around!

Or, rather, it's fun to watch the universe jump about as my head remains in exactly the same place at all times!

It's all exactly the same.

 

[Umbran]

There are absolutely no changes from "current Earth" when moving to such a system.

Well, if the only thing you do is change the center of coordinates, you are correct. However, that means that the orbits of bodies in that set of coordinates are not simple ellipses around the central body.

If you also assume that bodies move in ellipses around the center, then you start seeing a few changes.

 

[niastri]

Our group is in the midst of a homebrew setting, which has been fairly collaborative in its creation. Recently, our GM stated that the solar system the campaign is in (this is a fantasy high magic game, not sci-fi, just to clarify) is a terracentric one; the main planet is in the center of the solar system, and the sun and other planets rotate around it.

What would be the major differences, if any, that a layperson would notice going from a heliocentric system like Earth to a terracentric one? Obviously, there are issues with real-world physics (like how a planet would have enough gravitational pull to keep other planets in orbit), but I'm not really interested in hearing "this won't work because of <insert long equation here>". I'm more interested in what Joe Everyday might notice.

Terracentric solar systems can only be seen after an incredible amount of observation.

The only way they ultimately proved that the solar system was true was the way that Mars moved in the night sky. It made a little circle once in a while(appearing to run backwards in a loop!) due to the difference in pace around the sun.

Even with proof of that sort, most didn't believe for a long time. Likely not to have any affect on a game, aside from comedy value as the egg head NPC's argue about the "proof" of helocentric vs. terrocentric theory's.

 

[woodelf]

As I noted earlier, this is incorrect. Assuming the planet sits in the center and spins to produce the day/night cycle, and the spin axis is tilted with respect to the plane in which the sun orbits, you will have seasons.

You will still see prescession of the moon's orbit if the planet spins. In this case, the relative situation between the planet and moon here is no different from our real-world case - in both, the moon orbits the planet.

Yep, on both counts. For some reason, it just didn't occur to me that you could have [meaningful] rotation of the central body in a geocentric system. Mental block on my part. Though, in my defense, i did acknowledge that every one of my points could be "compensated for" in some way--and rotating the central body is just teh sort of thing i meant (even if i hadn't thought of that particular way).

 

[MerakSpielman]

Terracentric solar systems can only be seen after an incredible amount of observation.

The only way they ultimately proved that the solar system was true was the way that Mars moved in the night sky. It made a little circle once in a while(appearing to run backwards in a loop!) due to the difference in pace around the sun.

Even with proof of that sort, most didn't believe for a long time. Likely not to have any affect on a game, aside from comedy value as the egg head NPC's argue about the "proof" of helocentric vs. terrocentric theory's.

The retrograde motion of planets has been observed for a LONG time, and was taken into consideration when Ptolemy worked out how the solar system works. In fact, The Ptolemic solar system could predict the future positions of planets (from earth's point of view) with greater accuracy than the Copernican solar system (the main probelm with the Copernican system is that he used circles instead of ellipses). The Ptolemic system withstood centuries of testing by direct observation.

 

[Quasqueton]

My campaign world is terracentric. The sun and moon revolve around the spherical earth---the sun brightly lights the day, and the [full] moon dimly lights the night (every night).

The earth is awake under the sun, and asleep under the moon, just as all creatures do (except for those unnatural creatures like goblins and orcs). When awake, the earth is warmer, and when asleep it is cooler.

The seasons have nothing to do with the sun (or moon). The earth goes through cycles of growth and dormancy in a regular pattern. During the growth seasons, the earth gives off more warmth, and the dormant seasons, the earth cools down.

Proof that the earth gives warmth, instead of the sun, is that the higher in altitude (via climbing mountains or using magic to fly) one goes away from the earth's center, the cooler the climate. If it were the sun warming the earth, well, then you'd find higher altitudes warmer---because you're closer to the sun.

And the idea that the earth rotates is absurd. Why, everyone on the surface would be thrown off. Everyone beneath the surface would be pressed against the ceilings of their caves and tunnels.

Quasqueton