Here Comes The Sun - Theory 2: Unstable Orbits
Set down a stool, and it sits there quiet happily. Try to balance a pencil on its tip, though, and you'll find the task is pretty much impossible (unless you stab it in the ground - but that's not the point).
A stool sitting on its legs quiet happily is a stable system. The pencil, on the other hoof, is an unstable system.
The second theory on how Celestia and Luna "Raise" the sun and moon, is that Equus is in an unstable solar system.
To really understand what this means, we first need to explore the concept of stability. It's quiet a bit more involved than you think. Let's take a look.
Let's start with another example. Get a big beach ball, and set a big heavy weight on it (doing this in your head works, too :P ). That's going to fall over, right? Just like balancing a pencil on its tip, balancing a, say, 40 kg (~90 lb ) weight on a beach ball is pretty much impossible.
But now, instead of a big heavy weight, let's put Pinkie Pie on top! If she's a healthy adult mare, she should weigh a lot more than that weight...
(Click images for source!)
...Yet she can stay on top, despite it being an unstable system, because of her balance. When she starts to fall a little one way, within a tiny fraction of a second, she counteracts it by moving a little to keep herself balanced and on top. She keeps herself very near a balancing point.
Note, though, that the further she falls from her balancing point, the more force she has to apply to keep herself up.
If you look carefully, she's off balance a bit. She's probably about to topple off to our left. However, if you push her to the right, oh, with say just a dozen or so pounds of force, then she'll go back to her balancing point.
Now finally, if she goes even further... (Pretend she's still touching the ball :P )
Here she's in quite a predicament. In order to get her back to her balancing point, you'll have to apply a lot of force! Back in figure 1, she only had to make the tiniest of little thoughtless nudges to keep herself balanced. In figure 2, it'd take a good push to get her back into balance. And in this figure 3, you'd really have to throw her back on the ball to get her back to her balancing point.
Orbits have a behavior that is a lot like a weight on a balancing ball.
Just like a weight set on a ball will want to topple over onto the floor, a planet in an unstable orbit will, over just a few dozen years, shift into a stable orbit.
The theory, then, is this: Equus (the planet our ponies live on) is in an unstable orbit.
Sometimes, planets can cruise around in interstellar space without orbiting any star. These are known as rogue planets. The theory is that at some point, a rogue planet, or perhaps even a small star, swung by the solar system Equus is in and knocked the whole thing out of whack with its gravity. What resulted, is that a large Jupiter-like gas giant (let's call it Zues) was knocked into a highly elliptical orbit.
Not drawn to scale, this is an illustration to get an idea of what this would look like:
The red path would be Zues' orbit, and the blue circle is Equus' orbit.
This orbit is unstable. The gravity of Zues will try to throw Equus into a different orbit. This means one of a few things could happen:
1. Equus is thrown into a stable, but highly elliptical orbit, one where the amount of heat the planet gets would vary so much that life on Equus would struggle to survive, crops would fail, mass starvations, droughts, storms, dust storms, etc. would ensue.
2. Equus is thrown either out of the system, or onto a trajectory that will take it too far away from its sun to sustain life, even if its still in an orbit.
3. Equus is thrown into the sun, or into an orbit with a perihelion (closest point to the sun) that's so close that all life is burned.
This situation is exactly the same as the weight sitting on the ball, in the sense that this system is unstable. Where the weight will inevitably fall to the floor, Equus will inevitably be thrown into one of those situations by Zues.
However, what Celestia does, is every year, at the Summer Sun Celebration, she has a controlled magic surge.
Using this magic surge, just like Pinkie can give tiny little pushes to keep herself balanced on the ball in Figure 1, Celestia nudges Equus back on-trajectory. As long as she does this every year, then it doesn't take too much to keep Equus on track, though some years the planetary alignment may be that she needs to do it an extra time at some other point in the year.
So, simply put, keeping Equus in a livable orbit is like Pinkie balancing on the ball, and Celestia is the one who uses her magic to keep the balance. Remove Celestia from the equation, and Equus topples into an unsurvivable orbit, just like the weight on the beach ball topples to the ground.
Thus, Celestia is the reason the sun rises every day. If she didn't do her work, then in just a few years, you would see your last sunrise and the day/night cycle would be way out of whack, if you were close enough to the sun to have anything you'd call a "day" to begin with.
(Also, as some have pointed out - eternal night isn't too bad for starvation; oh no, you wouldn't worry about starving, you'd freeze to death long before that was an issue XP )
To explain how Luna keeps the Moon going, we first need to cover something called "Tidal Braking"
First off, the tides happen because of the Moon and Sun's gravity.
The Moon orbits the Earth, but a lesser-known fact is that the Earth orbits the moon a little bit, too (they both orbit a common Center of Mass). As a result of this, there's a little bit of centrifugal force on the Earth. In the Earth's center, this centrifugal force balances perfectly with the Moon's gravity (that's what an orbit is - balancing centrifugal forces and gravity). But at its surface, it doesn't. The result, is a net tidal force ("net" meaning when you add all the factors together).
Now, tides do an interesting thing... Take a look.
As illustrated, the tidal bulge pulls on the Moon, pulling it ahead in its orbit. Energy from the Earth's spin goes into the Moon's orbit.
However, if the Moon were to orbit the other way, the opposite direction from the planet's spin, then the planet's spin would pull energy out of the Moon's orbit, and the moon would be slowed down in its orbit, which would cause its orbit to decay.
In this theory, that's where Luna comes in. Every so often, she has to push the moon along in its orbit a little to make up for speed that it has lost to Equus' tides.
Now, say Equus' moon is in a highly elliptical orbit...
At this point, every nudge is a critical one. Luna would be best off nudging the moon up at apogee (furthest point from Earth/Equus) and perigee (closest point to Earth/Equus), so twice a month. If the moon's perigee fell below a certain altitude, then the moon would be torn apart by Equus's gravity due to tidal forces.
In order to make the perigee go higher, she'd be best off giving the moon a little push at apogee (as anyone who's played some Kerbal Space Program would know! :D ).
However, if Luna only pushed it at apogee, then over a huge span of time, as the Moon is slowly dragged in its orbit, its apogee would come down, too, since Luna would only be pushing its perigee up. Eventually it'd level off into a circular orbit and slowly decay.
So to keep that from happening, and because it's easier to keep an elliptical orbit up, Luna should give the moon a little nudge at perigee and apogee, and thus keep it in orbit.
So because of her, the moon rises. If she didn't do her job, then the moon wouldn't rise.
...It would be torn apart by tidal forces and crash to Equus in pieces.
Also, I should note, that all this pushing and stuff is going to mean that their moon might have a hot, liquid core, so it would have a magnetic field, and perhaps even active geology and volcanism, which would be super really awesome and cool :3
This was my favorite theory for a long time. I wrote a large post about it here. However, recently I've come up with a new favorite theory... That'll be my next update. For now, thanks for reading! :D
EDIT: Oh, and I'm sorry I haven't answered anypony's questions yet on an ASK blog! Right after I finish these posts on How Celestia and Luna Raise the sun and moon, I'll answer some questions, m'kay?! :D
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