Why is ozone a polar molecule?

Can someone explain why ozone (O3) is a polar molecule? I thought that polarity was based on the differences in electronegativity between atoms and the geometry of the atom. I thought ozone would be a nonpolar molecule, since despite the bent shape, all of the atoms are oxygen, so they would be pulling the electrons equally. Apparently this is not true; why is it polar, and where would the dipole moment be pointing?