I remember that, and I also absolutely love that idea, that the same force is being excerted with magic going the other direction, but all the same even though the only time physics in guaranteed to work our way in TES is when it is specifically shown to, it is a lot smaller than a moon would probably be and has also been hollowed out, so if weight and size still have a say in the force of impact it will be lessened. Add to that the "most likely." It is not neccessarily that big rock falling. it might be, but it also might not be. Hell, we don't even know if it's even still there. How did the city of Vivec fare in the Oblivion Crisis? For all we know, the entire prison was annihilated by the daedric siege machine.