A friend of mine posed a question to me that I can not answer. He was wondering what would happen to the surface if the earth was hollow, and if all of the mass of the earth was in the crust. He said to ignore the fact that it would probably collapse in on itself. And assume that it stays in shape. My thought is that you would have a rotating focal point of gravity due to the slight ellipticity of the earths crust. Your thoughts? A question of my own. Would the effect gravity be affected on the scale of the solar system.