Props Valroa

@Widdershins

Interesting, but...

We don't even know the layout of the Equestrian system. It could be a geocentric system where the sun and moon are only dozens of miles wide, orbiting around the planet in low orbits. In this case, it is not natural and created somehow. But I suppose you could do away with the 'its magic' explanation. Because this is a magical universe, things can be done differently than in our universe.

Not to scale, obviously.

Or it could be a real system, like pictured here. It really seems to match up so well! But it would mean there is not 1, not 2, not 3..4...but 5 suns.

Well, I was talking more of the general concept of A sun.

 There's two theoretical paths we can take there though. 

 A.) It's all a big hoax really, perpetrated by centuries of tradition. Luna & Celestia really just show up when their prospective object comes into play. Truth be told, maybe magically doing something to the Sun & Moon were important to some minor degree, but really. Having one being be in full control over the thing that allows all life, heat, tidal cycles, bodily cycles or just keeping the big thing from fallin' n' flattening somepony is a MIGHTY HUGE TASK. One that you'd think would require so much focus as to keep them from doing literally anything else, like eating.

 B.) Equestria is Flat. In purely concept at least. The original writers never gave thought towards connecting East & West or how literally far apart everything was so much as "put a new place to have fun in in that general direction." In a purely storytelling manor, it's unimportant facts to the story. Therefore, just holding the skymark major objects in place sounds pretty and makes the big alicorns feel important. Kids' show, ya know?

 I ascribe more towards the Hoax. Not an intentional one perhaps, but I can see that as the Sisters aged far beyond normal pony years, they kind of just shouldered the rumors and went along with it. Maybe they didn't really want full leadership over the free world... but hey, if they can do a job, and the idea that them getting assassinated out of power means that a giant ball of gas will come crashing down... all the better, at least.

@Widdershins

Theres also a 3rd option.

Celestia and Luna don't really control the moon and sun themselves, but they control their light, heat, etc with manipulation. Not actually moving the moon and sun, just their light and heat. Like an illusion.

Equestria can't really be flat.