  Phase curves of asteroids are typically considered to depend solely on the
scattering properties of airless particulate surfaces and the size of the
object being studied. In this study, we demonstrate the additional dependence
of phase curves on object shape, rotation pole orientation, and viewing
geometry over an apparition. Variations in the phase curve of near-Earth
asteroid (159402) 1999 AP10 over its apparition from July 2020 - January 2021
are verified to be due to aspect changes over the apparition. This is achieved
through shape modelling of the asteroid and simulation of the phase curve over
the apparition. We present simulations of asteroid phase curves over a range of
geometries to understand the potential magnitude of this aspect effect, and
under which circumstances it can begin to dominate in the phase curves. This
dependence on aspect may introduce significant additional uncertainty in the
properties derived from phase curve data. We provide and demonstrate software
code to estimate the aspect-related uncertainty in near-Earth asteroid phase
curves through simulation and model fitting of a randomly generated sample of
ellipsoidal asteroid models over the observed viewing geometry. We demonstrate
how ignoring this effect may lead to misleading interpretations of the data and
underestimation of uncertainties in further studies, such as those in the
infrared that use phase curve derived parameters when fitting physical
properties of an asteroid.
