Let B be the intersection of the sphere S^2 with the first octant \{(x_1,x_2,x_3)|x_1\geq0,x_2\geq0,x_3\geq0\} of \mathbb{R}^3. The book says B is homeomorphic to the ball B^2. But I'm not sure why that is, can anyone explain this please. Thank you very much.