I had this exact idea a while back

or maybe they’re both enjoying to fuck with ponies.

Discord at least moves the Sun and moon rather than rotating the planet, seeing as though the stars stay still. I assume the same is true for Celestia and Luna. Especially as they can lasso the moon.
 
I think that their solar system is a Tychonic solar system.

Equestria is set in a Ptolemaic vice Copernican cosomology. The Earth really is the center of their Universe, and the Sun, Moon, and stars are set in crystal spheres that are moved by heavenly beings–or in this case, Princesses Celestia and Luna.

@FanOfMostEverything  
Well, the obvious response to “who” is the Grand Fausticorn. Going by the gamemaster theory (which is actually something being studied by some quantum physicists), any work created spawns its own universe OR any work created is a tap into an existing universe.
 
And I like your points about Discord.

I wanna make a Comstock joke, but I’m half-certain that boat has passed. :)

@Neo987  
My headcanon is similar, save that Discord is actually a regulatory device designed to keep the synthetic plane from breaking down. If entropy has self-awareness and a sense of humor, it won’t want to make the plane decay. Celestia and Luna, meanwhile, are akin to Avacyn; powerful entities made to keep the world safe for mortals.  
Who made Equestria? Think about it. Who knows how it was made? Who would think to preserve a plane by incarnating the chaos that threatened it? Who would keep the world safe through the power of laughter? Who, even now, violates natural laws with a casualness that borders on disdain? Time shenanigans are, of course, involved.

Heresy! All who deny the powers of the Goddesses must be punished!

>Luna don’t physically raise and lower the sun/moon
 
Of course they don’t.They use magic.
 
Magic.
 
What part of fucking magic is difficult to grasp?
 
I’m not really much of a fan of these kind of theories,since it genarally show Celestia and Luna in bad light.

@FanOfMostEverything  
I’m guessing you haven’t seen the S4 animatic.

Doesn’t really work, if you’re going so far as explaining weather as well. The tidal force and temperature variance created by the rotation of the planet in relation to the sun, regardless of source of said rotation, would contribute to naturally occurring weather effects.
 
And jump start is a poor way to describe it. That implies the planet stops moving, perhaps at dawn/dusk for each shift, requiring action to restore rotation. During this pause, the temperature on the tidally locked planet would begin to shift slightly, freezing the dark side and baking the light side, until rotation restarted, while the clashing temperatures on the equator would create a superstorm ring. Then consider the force of rotation. Suddenly starting the planet rotating at the speed necessary to counteract the effects of a tidally locked planet would cause catastrophic physical damage.
 
Personally, I see it in a planeswalker perspective. It’s a crafted universe and planeswalkers (Princesses) and certain powerful entities (Discord, and possibly Sombra and Chrysalis) have access to its fabric to change physical laws and control or delegate control of appearance, climate, and general habitability, except in locations that have degraded/corrupted with age or powerful events (The Everfree Forest, which apparently grew to consume the site of the old castle ruins, former seat to the elements, and likely location of Nightmare Moon’s sealing) that need to be repaired (if they can be). If you’re versed in Magic: the Gathering lore, look at Serra’s Realm from creation to collapse, or even Phyrexia in general, for an example of what I mean.
 
tl;dr  
Don’t drag real world physics into a fantasy setting, EVER! Stick with magic and phantasy (or quantum physics, which is damn near fantasy until proven).

There are times when I think ‘Geez…stop over analyzing, it’s just a kids show’ But crazy fan theories like this one always brings a smile to my face. There’s nothing harmful about these headcanons

I just think ponies are control freaks.  
Everfree Forest is the pretty much the only place without control as animals care for themselves and clouds move on their own.

@web$  
True, but at the same time, it comes down to relativity. In their minds, it would seem like they are moving the celestial objects and do not realized what they are actually doing.

@web$  
Yes, because a millennium-old fable must adhere to strict standards of scientific accuracy.

or maybe it’s a little girls show and it doesn’t need to make perfect sense.

“Insane?” Aww, but this was always my headcanon, more or less.

@Lucky424  
But then it breaks the point that Princess Luna was Nightmare moon…  
As stated in the show “Luna would not lower the moon to allow space for the sun, causing eternal night” :o
 
But yes, this does make sense, perhaps a little TOO much sense O.o

Holy shit… that makes a surprising amount of sense!!
 
 
@Background Pony Maybe because they have to do it every twelve hours for the correct rotation speed, Celestia does it in the morning when the sun is ‘rising’, and Luna at night when the moon is ‘rising’?

@Background Pony  
Perhaps it relates more over into the direction in which they rotate the planet. As it would seem in my mind is that Equestria is a single mass land, much like Pangaea was to earth long ago. And thus, trying to rotate the planet for a full 360 would be a bit unnecessary. So each one rotates the planet to the relative direction in which is needed to show the sun or moon.

Then why do they have cuteymarks related to Sun adn Moon? According to this, they both do the same and it’s not related to Sun or Moon.