You all don’t know this, but this has been a long time in the making; I’ve had this headcanon mostly written since November, but it took me until now to actually manage to illustrate in a way I’m happy with. But I finally did, and now I get to post it. :)

-Intro-
During the reign of Princess Celestia and Princess Luna, there have been two main models of the universe in use, colloquially known as the “Traditional Model” and the “New Model”.

The traditional model is based heavily on even older models where the earth is placed in the center of the universe, the sun, moon and stars orbiting it. But these varied quite a lot from place to place; for example, some featured Venus and Mars while others had no mention of them. When the Princesses took the thrones over Equestria, they unified the different models –as well as provide some insight of their own provided to them by their connection to the sun and moon– into the Traditional Model.

But that was just inside Equestria. Outside of it, what later became the New Model was starting to form, even long before the Sister’s appearance. While most of these did not feature Sol in the center, as they had no idea it existed, many who studied the movements of Venus, Mars and the stars more closely realised that they did behave as though they orbited the earth. Then, once Sol was revealed, these theories and models got even more popular.

Still, these theories were often shut down; after all, first the Unicorns and later Princess Celestia and Princess Luna endorsed the models which placed the earth in the center, and they moved the celestial bodies, yes? So clearly they knew best.

But, eventually, as the methods for observing the universe improved, even Equestria had to admit that the Traditional Model simply did not work. In current time, the New Model is what is officially used and taught in schools –though, that is not to say the Traditional one still does not have its supporters. Many ponies much prefer the idea of the earth –and especially Equestria and her Princesses– being the center of everything. And so, it lives on. Much to the anger of Twilight Sparkle, who will someday have an aneurysm if she keeps trying to debate those who support it.

-The Traditional Model-
Earth-centric:
According to the old model, The earth, “Gaia”, is placed in the center of the universe. Around it, there are three orbits containing celestial bodies.

First Orbit:
The first and closest orbit has the Sun “Celestia”, the Moon “Luna”, and four stars; “Eiar”, “Theros”, “Phthinoporon” and “Kheimon”. This orbit is the only one controlled by people; at one point it was the Unicorns but other magic wielders throughout history has also performed this duty. They are currently being spun by Princess Celestia and Princess Luna, who were named after the heavenly bodies they control. To avoid confusion, “Luna” and “Celestia” shall refer to the celestial objects, while “Princess Luna” and “Princess Celestia” means the alicorn sisters.

Celestia and Luna always appear on opposite sides of Gaia, keeping the same speed as each other to perfectly be highest in the sky during midday and midnight respectively.

While the traditional model does have a lot of incorrect assumptions, it does get the structure of how the “sun”, moon and four “stars” orbit the earth correct. This is because when a first version of this model was proposed by a unicorn country, they were the ones controlling those celestial bodies. So it was not so much the unicorns mapping how they moved – but rather them writing instructions to themselves on how to move them.

They do get the classifications of the objects wrong though, if we use our terms. In which case, we would say that their earth, Gaia, has six moons; one larger one, Luna, almost identical to our Tellus’ moon, one burning one, Celestia, a magically created miniature clone of a star, and four very small ones.

Phases and Seasons:
Luna has phases, which are created by the moon being moved in a circle in and out of the shadow cast by the earth when Celestia’s light hits it. A new moon occurs when completely covered by Gaia’s shadow, a full moon when completely lit up by Celestia.

The northern and southern parts of Gaia have seasons. These are created by Celestia being moved more north during its summer and more south during its winter. Thus warming and cooling the temperature of the atmosphere there.

Again; this is correct to how their solar system works, since they are the ones controlling these parts. But from an outside perspective, it is quite odd.

To start with: their lunar cycles are a lie. In our world, the phases are created by the sun shining on one side of the moon. So a full moon happens when the moon is on the other side of the sun from us, a half-moon when it’s at a ninety-degree angle, and a new moon when it is on the same side. The very vast majority of you already know this.

On Gaia, it used to work somewhat similarly, back when Sol was still alive. Then, long long ago when it died, Celestia was created and people took over control of the movement of the sun and moon, they knew the moon should have phases. It had always had them until that point. So they figured out how it worked, by moving Luna to be at different angles to Celestia. Until, somewhere in history, that knowledge was forgotten. A new idea of the moon only being visible at night took hold, and the old way of the lunar phases was lost. So, they faked it via the earth’s shadow, just as Princess Luna continues to fake it to this day. This means their “new moon” isn’t completely in shadow and next to invisible to the naked eye like ours. Because theirs is actually a lunar eclipse. Therefore, it’s bright red.

The same happened for the seasons; people knew there should be seasons, and knew roughly how long they should be and how intense. So they figured out how to achieve that, via the method still used by Princess Celestia to this day.

Second Orbit
The second orbit contains two more celestial bodies; Venus and Mars. It is unknown what they look like; they do not glow like Celestia, nor does its lights reach them with enough intensity that they are clearly visible. They are only detectable by magic; from which it is determined that they mirror the movement of Celestia and Luna, except shifted a quarter day. So Venus is highest in the sky during dawn, and Mars during dusk.

This is where the traditional model starts to completely fall apart from reality. It is not completely the creators’ fault; it is much harder to find and study the movements of celestial bodies when they are almost completely dark, only being lit up by faint starlight and the light of a small moon far away. They were invisible to the naked eye, and so, astronomers did not know where to point their telescopes; not that the telescopes of the time were powerful enough to really help much in viewing the planets either way.

The reason they were discovered in the first place is due to magic. Powerful scrying spells revealed objects far far away, almost outside of the caster’s perception. But they could only detect them very very few times and those few instances always occurred during dusk and dawn; hence, the belief that they mirrored the movement of Celestia and Luna, only shifted. This part of the model was created before the rule of Princess Celestia and Princess Luna, though there were many variants: some placing Venus as dawn and Mars at dusk, some the opposite, and a few removed one of them entirely and believed that Venus or Mars guarded both. It wasn’t until the age of the Sisters that the model got unified into the Traditional Model.

Though, in actuality, the reason they could only detect them during dusk and dawn was because they always searched up –not completely illogical when searching for heavenly bodies– and those were the only times Celestia and Luna were not ‘in the way’ and interfering with the spell’s result.

Third Orbit:
Furthest away, the third and final orbit, containing what some believe to be ‘the old system’. The dead sun “Sol”, spinning on the opposite side of the night sky. The “old system”-belief is that Celestia and Luna did not originally exist; Sol used to be alive and light up the day, and the night sky only had stars to brighten it. Celestia and Luna were then created once Sol died.

Whether that is true or not, the current third orbit does currently contain a heavenly body made from darkness on one side, and the stars on the other. They mirror the movement of the sun and moon, with the stars always keeping pace with the movement of Luna and appearing behind it each night.

It is worth noting that while the stars can be clearly visible to everypony, the existence of Sol is only known because of the words of Princess Celestia and Princess Luna, who claims to have met its undead Spirit. Most ponies take them on their word.

Once again, the traditional model differs from reality quite a bit. Ponies just needed an explanation for what the stars were, and it fit nicely with what their princess demi-goddesses were telling them of Sol. That it is a dead sun (actually, it’s a black hole, but they do not know what that is yet), somewhere too far away to scry for. It makes sense, from a certain perspective, to assume that an old symbol of the day would orbit opposite of a symbol of the night. Just the current symbols of day and night orbit opposite of each other.

This third orbit is what differs the Traditional Model, created and popularised under the rule of Princess Celestia and Princess Luna from the ones it was based on which came before. Since Sol was not known until Princess Celestia revealed it.

-The “New” Model-

Four orbits around Sol:
In this model, Gaia is no longer placed in the center. Rather, Sol is considered the center of the universe, with four objects orbiting it. The size of Sol is unknown, but it is known to be massive compared to the others. Of these four objects, Mars is furthest away, followed by Gaia, then Venus, and lastly Mercury –the newest object to be found, and closest to Sol.

One reason this model was created and accepted so widely (eventually) started because of the discovery of Mercury. Up until that point, the traditional model was very neat and felt very “right” – Gaia in the center three orbits; Celestia and Luna, Venus and Mars, and Sol and the Stars. Then scrying spells improved, alongside telescopes and other methods for observing the universe. Mercury was found, and the neat version cracked. More attention was placed on the other objects, and with the improved methods it was found that Venus and Mars does not behave at all as if they are orbiting Gaia. It seemed more like they were orbiting something else… and then they found Sol. Or rather, they found something so massive it made their knees weak. Detected on the other side of Mercury, it was so giant their spells got confused and sent back either erratic information or nothing… and it was completely invisible. As that size, it should be impossible for their telescopes to miss it, yet… nothing. It took some time before they, with the help of Princess Celestia, realised it was actually Sol they were detecting.

Gaia, Celestia and Luna:
The smaller system of Celestia and Luna orbiting around Gaia is still the same as it is in the traditional model; Celestia and Luna being equal and opposite, one highest during day and the other during night. The phases of the moon and the seasons remain unchanged in how they function.

The only difference is the four stars Eiar, Theros, Phthinoporon and Kheimon. More specifically, their classification as “stars”. While they do glow as dots in the night sky, they are undoubtedly quite different in nature from their brethren. They are closer, can be moved the same as Luna and Celestia, and do not glow by themselves, merely reflecting the light of Celestia. As such, they are often referred to by other names, such as “Lunar Stars”, “False Stars” or “Orbit Stars”.

Again, since the Luna-Celestia-Gaia cycle is directly controlled by people, the people’s models of it are completely true to reality. Therefore, not much changed with this new model.

Stars:
According to the new model, the stars are placed extremely far away in a sphere around Sol. They do not orbit like the planets, but rather drift incredibly slowly in currents; moving at such a slow pace they only appear to have shifted a few hairwidths over hundreds of years.

So far, only the most powerful spells have even been close to actually perceiving them –multiple small ‘blips’ far far away on the other side of Mars.

There were many reasons for this mistake. For one; their solar system does not have a sun in the center, so they would not intuitively assume that suns –giant suns, visible from far away, not their own small burning moon– are a thing. And then they managed to catch ‘glimpses’ of the asteroid belt… so similar in nature to the four pseudo-stars they know so well. That it was the stars they were able to perceive was a natural assumption to make.