  A precise, accurate, and relatively straightforward in-situ method to measure
and control the ellipticity of light resonantly interacting with an atomic
vapor is described. The technique can be used to minimize vector light shifts.
The method involves measurement of ellipticity-induced resonances in the
magnetic-field dependence of nonlinear magneto-optical rotation of
frequency-modulated light. The light propagation direction is orthogonal to the
applied magnetic field $\textbf{B}$ and the major axis of the light
polarization ellipse is along $\textbf{B}$. When the light modulation frequency
matches the Larmor frequency, elliptically polarized light produces precessing
atomic spin orientation transverse to $\textbf{B}$ via synchronous optical
pumping. The precessing spin orientation causes optical rotation oscillating at
the Larmor frequency by modulating the atomic vapor's circular birefringence.
Based on this technique's precision, in-situ nature (which avoids systematic
errors arising from optical interfaces), and independent control of the most
important systematic errors, it is shown that the accuracy of light ellipticity
measurements achievable with this technique can exceed that of existing methods
by orders of magnitude.
