arXiv2026
While the Jacobian of a planar harmonic mapping keeps a constant sign, the signed and the unsigned area elements agree up to that sign, and the two natural moment hierarchies coincide. Once the Jacobian changes sign the critical circle acquires a caustic as its image, part of the plane is covered several times with opposite orientations, and the hierarchies part company: integrating a radial weight against $J_F$ lets overlapping sheets cancel, giving the degree-weighted moments $J_n$, while integrating against $|J_F|$ lets every sheet count positively, giving the multiplicity-weighted moments $J_n^{\pm}$. These are the classical degree and area formulas for the same map, the second weighted by the multiplicity function (Banach indicatrix). It is the second family, the one the caustic threatens, for which a closed form is no longer automatic. For the two-monomial map $F(w)=w+iC\bar w^{k}$ on an annulus we show that it has one anyway: $J_n^{\pm}$ is the total variation of the very same explicit primitive whose endpoint difference gives $J_n$. Crossing the caustic therefore costs exactly one extra evaluation point, the critical radius $r_*=(k|C|)^{-1/(k-1)}$, and it is the same point for every order $n$: the weight $|F|^{2n}$ is nonnegative and so contributes no sign change of its own. On the degree-weighted side $J_n$ has a boundary-only closed form for every $n\ge0$, the zeroth being the signed area of the image counted with degree, and the complex-weighted moments vanish unless $(k+1)\mid(m-\ell)$. Neither weighted integral is a new object, and the caustic of this family has been located before in the literature on harmonic trinomials; what is new is the closed evaluation of the multiplicity-weighted hierarchy across it.