  The Wigner derivative is the partial derivative of dihedral angle with
respect to opposite edge length in a tetrahedron, all other edge lengths
remaining fixed. We compute the inverse Wigner derivative for spherical
tetrahedra, namely the partial derivative of edge length with respect to
opposite dihedral angle, all other dihedral angles remaining fixed. We show
that the inverse Wigner derivative is actually equal to the Wigner derivative.
These computations are motivated by the asymptotics of the classical and
quantum 6j symbols for SU(2).
