The transit method, employed by MOST, \emph{Kepler}, and various ground-based
surveys has enabled the characterization of extrasolar planets to unprecedented
precision. These results are precise enough to begin to measure planet
atmosphere composition, planetary oblateness, star spots, and other phenomena
at the level of a few hundred parts-per-million. However, these results depend
on our understanding of stellar limb darkening, that is, the intensity
distribution across the stellar disk that is sequentially blocked as the planet
transits. Typically, stellar limb darkening is assumed to be a simple
parameterization with two coefficients that are derived from stellar atmosphere
models or fit directly. In this work, we revisit this assumption and compute
synthetic planetary transit light curves directly from model stellar atmosphere
center-to-limb intensity variations (CLIV) using the plane-parallel
\textsc{Atlas} and spherically symmetric \textsc{SAtlas} codes. We compare
these light curves to those constructed using best-fit limb-darkening
parameterizations. We find that adopting parametric stellar limb-darkening laws
lead to systematic differences from the more geometrically realistic model
stellar atmosphere CLIV of about 50 -- 100 ppm at the transit center and up to
300 ppm at ingress/egress. While these errors are small they are systematic,
and appear to limit the precision necessary to measure secondary effects. Our
results may also have a significant impact on transit spectra.Comment: 12 pages, 14 figures, accepted for publication in ApJ after revision
