I personally like the idea I came up with that their sun/was extremely small compared to our world’s one. Small and light enough to orbit their planet; and possibly not volatile enough to burn anything from that distance until Celestia came around.
As for which came first…
Personally, I have a… tenuous idea of how the world of MLP formed. With comes with various versions suited to various interpretations.
Version 1: The Pie Family
-Under this interpretation, Pinkie Pie’s family dredged up the raw materials for the world from some strange area a long time ago; similar to creation myths about drawing the world out of the sea. And theoretically they could build the world whichever way they wanted. But not having much of an idea of how to do so, they may have made the prototype sun that Celestia would eventually control, but at that time would have been small. Then much later, in the earliest recorded history, unicorns are moving the prototype sun around the planet to maintain healthy levels of effects of sunlight . And eventually, Celestia manages to bind herself to that sun (Which could later lead to my proposal of her increasing the sun’s size to have the planet orbit it.). Pinkie Pie’s presence in Equestria could have to do with Time travel, immortality or simply reincarnation.
-Theory Reliability: Far fetched, but possible.
Version 2: Pulled out of Sea of chaos
-Assume that Discord is not the first of his kind. Perhaps some other being of extreme chaos, maybe even another version of him, just happened to bring the world into existence. And their influence shaped a prototypical Equestria. Only for them to leave, and whatever magics where left in that place “stabilized” for a while, and those that knew how to use them used them to move the sun in the sky. Which had always “just been there”, but in reality was placed in the sky to be moved around the planet by the chaotic precursors; the fact that it would have moved around the planet being their idea of a joke; which may or may not have been cruel. Celestia eventually ascends and ties herself to the sun. And from there, we set ourselves up for the proposal if we still find it necessary.
Theory Reliability: Plausible
Version 3: Equestria as a Dream World.
-As an obligatory “Left field” idea, lets assume that Equestria is weaved into physical form from the understandings, myths, ideas, dreams, and more; of some outside force. And the main rule for how it works is for it not to have any contradictions, or other things that would have a logical break down. And as many ancient cultures have a myth where the sun goes around the earth, or is raised in the sky, that is reflected in Equestria. As such, that fact couldn’t be contradicted unless Celestia changed the dynamic herself when she “realized” there was a better way to have the dynamic work.
-Theory Reliability: Crazy… But awesome.
And for fun, let me discuss my fun musings on the Planet-centric model for Equestria… which I still support because it sounds so fun.
Personally the system that I like to think of with Equestria works with their sun naturally being a similar size and/or mass to both their planet and their moon. And the moon be at a similar distance away from their planet as their sun in the opposite direction. From there, each side of Equestria could naturally be locked to one or the other in normal direction; with the sun and Moon both orbiting in the same direction, following each other.
From there, the princesses can move the sun and moon relative the the planet which is already in spinning motion. And since the sun and moon would already be in motion, it would be a simple matter of accelerating or decelerating the speed that the sun and moon move at.
…The only problem would be that it might also cause the planet itself to spin faster, which could do anything from Time dilation to messing with the ratio of gravity to centrifugal force (which may explain how pegasus can fly with under proportioned wings.)
At some point, perhaps already happened, perhaps not, some pony may make observations that make them realize the planet is spinning. And then, the princesses will look at each other with a sheepish grin before putting the breaks on the planet’s rotation. And then the princesses should have no more need to move the sun and moon in the sky right?
…Unless it turns out they are both moving extremely fast round the planet. Causing an effect similar to what discord did (Theories on discord being the spirit of the planet itself, incoming!)
And then they go back to the old system if they happen to have figured out that the sun and moon spinning around them keeps them from colliding with each other, or that physics is much more forgiving when they are spinning.
Also related: in real science, the sun doesn’t just pull the earth, the earth pulls the sun just the tiniest bit. As does every other body in the sky. Gravity is a two way street.
… And its at this point I realize something…
Who cares the exact details, Celestia, Luna, Discord; they could all probably use whichever method they wanted to make it work. And may even switch between methods for various effects during the year. Hey, Discord is almost guaranteed to switch around which orbited which for comedic reasons, back and forth.
What’s important is this:
Celestia has the powers of the sun: who cares which way she uses them?
Luna has the powers of the moon: who cares which way she uses them?
Discord is chaos… Who cares… Just… who cares? It’s chaos! he don’t have to explain stuff.
Just… stay calm… and enjoy the ponies.
…excuse me for a second while I compose myself.
(This break down brought to you by a Homestuck running gag.)
Edit: It’s like some old Philosophy person’s musings once when, something about a man circling a tree, and a squirrel on the tree circling on the other side. When the question of if the man was also circling the squirrel or not came up; some chap said “This is stupid! The distinction doesn’t matter yet!”