Can someone explain Inertial Frame of Reference to me better than wikipedia?

So here I was on the porcelain thinking chair when I was thinking about a question on here someone asked about if the earth stopped rotating. I said something about the earth being an oblate spheroid thanks to it's rotational inertia and left it at that. But then I got to thinking how we know earth is rotating. I mean, motion is relative after all. We tend to think of stars as fixed points of reference but they're not really.

So lets say we took a rotating planet like earth that is a oblate spheroid thanks to it's inertia and plopped it into an otherwise empty universe. Without any frame of reference there would be no way to establish it was rotating, would it's rotational inertia simply vanish? Or would we assume there to just be some 'dark force' that acts against gravity keeping earth oblate.

I read though this which I think describes the answer as "We know it's rotating because we like how the math works better" but I'm not sure if I read it right.

http://en.wikipedia.org/wiki/Inertial_frame_of_ref...

K... go.