We all know how Celestia and Luna keep the celestial bodies in motion. There have always been myths to this effect, the ruler of a civilization being regarded as a God and keeping the Sun and Moon in motion, but in Equestria, they actually do. There have been many theories as to how this works, so lets examine a few.

 

A common one is simply that the Alicorn sisters just keep them in motion with...

 

 

Pretty straight forward, and honestly, it would make sense in universe. But I've never liked it; it would require massive amounts of energy to move things of such mass over such distance enough to significantly effect things. Also, if the Sun and Moon have to be moved, what about all the other stars? We know that the stars move, but I have a hard time believing that either the Sisters are controlling the entire universe, or that each star has a planet in orbit around it inhabited by being powerful enough to keep it going. Also, the planets never would have formed if the random chunks of rock floating around in space hadn't been orbiting around each other and eventually been pulled together through sheer gravity.

And anyway, if they have that kind of power, Discord and Chrysalis should have been total pushovers

 

Another is that the Sisters keep the planet spinning and take turns out of tradition (and because I'm sure it's tiring). Again, the energy required to keep a planet rotating would be massive. Not to mention the fact that planets naturally rotate anyway.

 

And here's and interesting one I found from one of our members. To boil it down, the Sisters enact minor changes on the Moon and Planet to keep the orbits from getting all messed up:

http://mlpforums.com/topic/27530-how-celestia-and-luna-raise-the-sun-and-moon/

 

That last one is the closest to my personal thoughts.

 

So, minor changes in the tilt of a planet can have huge impacts on the climate of the planet. In fact, if the Earth didn't have such a stable orbit, the seasons and therefore climate would be so messed up that life couldn't exist in it's current form. In fact, most life would probably be microscopic, since smaller, less complex creatures thrive in more extreme environments. The tilt can also make the day or night last for far longer than normal in certain places (for example, the poles have "days" and "nights" that last for months during certain seasons). This would screw up a lot, such as the temperatures (Equestria being at freezing temperatures for months at a time does not bode well for the ponies). The tilt would also effect how the inhabitants of the planet see the celestial bodies, such as the Sun and Moon.

Anyway, I think that the Sisters make minor changes to the tilt of the planet. They don't actually move the Sun or Moon at all, they just keep the planet stable. It's just much easier to say that they raise the Sun and Moon than to explain it all to the average pony. Also, it probably keeps some tradition from the ancient times, when the Unicorns kept everything going. A scientifically primitive culture such as that would certainly not know what was actually happening, and would likely just say "the Unicorns keep the Sun and Moon rising and falling".

Another thing I've thought of... It should be noted that, at least for us, the Moon and the Earth stabilize each other, so maybe the Sisters combine their power to keep the Moon in orbit. Our Moon is actually slowly moving away from us (at a rate of a ~4 centimeters per year). Depending on the orbit, the pony Moon could be moving away at a much faster rate, not so much that it's destroying the solar system, but enough that it could be problematic in a few hundred years or so. The Sisters are enacting small changes to the Moon's orbit prevent it from getting out of hand.

Source: Science Time! --- The Sun, Moon, and Planet this all happens on

So... yeah. I made another one. ~CD~ is a pretty cool guy for suggesting I make it a thread instead of a blog post and then just blogging it anyway because the site design encourages spamming shit like this being able to post the same thing multiple times without the mods deleting it.