  Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*,
with the Event Horizon Telescope (EHT) rests on the assumption of a stationary
image. We explore the limitations of this assumption using high-cadence GRMHD
simulations of Sgr A*. We employ analytic models that capture the basic
characteristics of the images to understand the origin of the variability in
the simulated visibility amplitudes. We find that, in all simulations, the
visibility amplitudes for baselines oriented perpendicular to the spin axis of
the black hole typically decrease smoothly over baseline lengths that are
comparable to those of the EHT. On the other hand, the visibility amplitudes
for baselines oriented parallel to the spin axis show significant structure
with one or more minima. This suggests that fitting EHT observations with
geometric models will lead to reasonably accurate determination of the
orientation of the black-hole on the plane of the sky. However, in the
disk-dominated models, the locations and depths of the minima in the visibility
amplitudes depend primarily on the width and asymmetry of the crescent-like
images and are highly variable. In the jet-dominated models, the locations of
the minima are determined by the separation of the two image components but
their depths depend primarily on the relative brightness of the two components
and are also variable. This suggests that using time-independent models to
infer additional black-hole parameters, such as the shadow size or the spin
magnitude, will be severely affected by the variability of the accretion flow.
