Evidence #1: Sun orbiting the planet

This evidence shows that what makes the sun, stars, and moon all appear to move throughout the sky is literally what it appears to be. The sun orbits the planet. I know this because in season 4 episode 2, you can see that the stars don't move symmetrically with the sun when Celestia brings morning on the summer-sun celebration. Logically, if the planet was spinning, the stars and sun would move at the same speed across the sky. The sun has to be moving in relation to the planet, so I assume that this is crucial evidence to finding the nature of celestial bodies.

Evidence #2: Nature of the sun and moon

In season 4 episode 1/2 (don't read this if you don't like spoilers for these episodes), when Princess Celestia and Princess Luna disappear and aren't able to stabilize the movement of the sun and moon, the sky appears to be half day and half night. What doesn't make sense though, is that in our universe if the sun and moon were in the same sky it would appear day. What is going on here?!

I propose that the moon has it's own energy release, kind of like how the sun provides energy, the moon has some type of energy release as well. What can this energy be though? Whatever it is, it would make sense to conclude that it is an opposite energy of light. This lunar energy is able to overcome and/or combine with light energy. In season 4 episode 1, Twilight Sparkle experiences a flash-back to the time where Princess Luna became Nightmare Moon. What's fascinating about Luna's transformation though, is that she had the moon eclipse the sun...

my prediction is that it would be hard to create an eclipse, given the fact that the sun and moon are opposites. Somehow, the energies repel each other, and when Luna eclipsed the sun she was probably able to take in all of the moon's energy without the sun's energy interfering.In our universe the moon is simply another body of mass, but apparently not in the ponyverse.

Evidence #3: Stars and the moon

All the way back to season 1 episode 1, the myth of Nightmare Moon returning involved the stars aiding in her escape. But wait! Aren't stars individual suns in our universe? As Nightmare Moon returns, you can see how four stars start to circle the moon, and pass behind it, and thus releasing Nightmare Moon. To find out how this is possible, first we need to look at how Celestia banished Luna in the first place. Luckily, season 4 episode 1 contains the scene in which Luna is banished with the elements of harmony. Here, you can see how exactly Luna was banished in the first place.

So far, my conclusion is that Celestia used the opposite energy of the moon, light, and focused it by using the elements of harmony. This energy overcame Nightmare Moon, and flung her towards the moon. It appears that her body dissipated inside the rainbow beam that Celestia used to banish her, so I assume that light in the ponyverse has a whole extra energy embedded in it to engulf the lunar energy of Luna. At the moment the light energy hit the moon with Luna, you can see that a shadow of Nightmare Moon appeared imbedded on the moon. I therefore assume that Luna, since she had already taken in much of the lunar energy into her body when she eclipsed the sun, she became "connected" with the moon as she slammed into it.

That doesn't quite answer how the stars aided her escape though, but I do have an idea. The stars must be individual star systems, in one form or another, and must have combined their energy towards the moon. I bet that every star in the ponyverse has many magical properties, and a balance in each one. They effect each other too, and connect energies towards other star systems. I assume that enough stars were able to connect energies to equestria's star system, and "popped" open an escape for Nightmare Moon. The moon she was banished in just needed light energy to orient in order to separate Nightmare Moon's lunar energy from the moon.