  Limb-darkening is a fundamental constraint for modeling eclipsing binary and
planetary transit light curves. As observations, for example from Kepler,
CoRot, and Most, become more precise then a greater understanding of
limb-darkening is necessary. However, limb-darkening is typically modeled as
simple parameterizations fit to plane-parallel model stellar atmospheres that
ignores stellar atmospheric extension. In this work, I compute linear,
quadratic and four-parameter limb-darkening laws from grids of plane-parallel
and spherically-symmetric model stellar atmospheres in a temperature and
gravity range representing stars evolving on the Red Giant branch. The
limb-darkening relations for each geometry are compared and are found to fit
plane-parallel models much better than the spherically-symmetric models.
Assuming that limb-darkening from spherically-symmetry model atmospheres are
more physically representative of actual stellar limb-darkening than
plane-parallel models then these limb-darkening laws will not fit the limb of a
stellar disk leading to errors in a light curve fit. This error will increase
with a star's atmospheric extension.
