Dipole selection rule - removal of apparent inconsistency 

There is now an apparent inconsistency in the article as to why no spectrum is observed in molecules with no dipole. The intro says that a permanent (non-zero) electric dipole moment is required to have a spectrum, but the spherical top section says that spherical tops have no spectrum because there is no change in dipole moment with rotational quantum number. A reader may wonder why change in dipole is not mentioned in the intro. 

I understand that this inconsistency is only apparent, because the statement in the intro refers to a molecular frame of reference in which a dipole is constant (at least in each quantum state), whereas the statement about spherical tops describes the rotation of the dipole vector with the molecule as a change in the lab frame of reference. But this is not mentioned in the article, and I think it would be unnecessarily complicated to explain properly. Instead I will change the language in the spherical top section to conform to the intro.