This actually does bring up some interesting issues with the classification of objects in the solar system; issues that are still being worked out.
There are many dwarf planets that are rounded by their own gravity but are actually smaller than our moon like Pluto, Eris, and Ceres.
While there are differences between these bodies in terms of composition and location, the main difference between them in terms of classification is some orbit other bodies and some directly orbit the sun. Most are trans-neptunian objects like Pluto but Ceres is in our asteroid belt. There actually could be hundreds in the outer Kuiper Belt, scattered disk, and Oort Cloud.
So if you see a "moon" in Rebellion and it's not near another planet, it might just be a dwarf planet.
Mars actually has 2 misshapen"crappy moons" called Deimos and Phobos that are probably just captured asteroids that resemble the resource asteroids in Sins (not my observation, originally, someone else pointed this out). Our moon, by contrast was likely formed out of the debris from a collision with a Mars-sized planet, which is way more bad-ass.
Factoid: Did you know it took the Apollo 11 crew 3 days to get to Earth's Moon?
http://en.wikipedia.org/wiki/File:Speed_of_light_from_Earth_to_Moon.gif