  Gravity darkening induced by rapid stellar rotation provides us with a unique
opportunity to characterize the spin-orbit misalignment of a planetary system
through analysis of its photometric transit. We use the gravity-darkened
transit modeling code simuTrans to reproduce the transit light curve of
Kepler-13Ab by separately analyzing phase-folded transits for 12 short-cadence
Kepler quarters. We verify the temporal change in impact parameter indicative
of spin-orbit precession identified by Szab\'o et al. (2012) and Masuda (2015),
reporting a rate of change $db/dt = (-4.1 \pm 0.2) \times 10^{-5}$ day$^{-1}$.
We further investigate the effect of light dilution on the fitted impact
parameter and find that less than 1% of additional light is sufficient to
explain the seasonal variation seen in the Kepler quarter data. We then extend
our precession analysis to the phase curve data from which we report a rate of
change $db/dt = (-3.2 \pm 1.3) \times 10^{-5}$ day$^{-1}$. This value is
consistent with that of the transit data at a lower significance and provides
the first evidence of spin-orbit precession based solely on the temporal
variation of the secondary eclipse.
