  The merger of two neutron stars (NSs) or a neutron star and a black hole (BH)
produces a radioactively-powered transient known as a kilonova, first observed
accompanying the gravitational wave event GW170817. While kilonovae are
frequently modeled in spherical symmetry, the dynamical ejecta and disk
outflows can be considerably asymmetric. We use Monte Carlo radiative transfer
calculations to study the light curves of kilonovae with globally axisymmetric
geometries (e.g. an ellipsoid and a torus). We find that the variation in
luminosity in these models is most pronounced at early times, and decreases
until the light curves become isotropic in the late optically thin phase. The
light curve shape and peak time are not significantly modified by the global
asymmetry. We show that the projected surface area along the line of sight
captures the primary geometric effects, and use this fact to provide a simple
analytic estimate of the direction-dependent light curves of the aspherical
ejecta. For the kilonova accompanying GW170817, accounting for asymmetry with
an oblate (prolate) ellipsoid of axial ratio $2$ ($1/2$) leads to a $\sim 40
\%$ decrease (increase) in the inferred ejecta mass compared to the spherical
case. The pole-to-equator orientation effects are expected to be significantly
larger (a factor of $\sim 5 - 10$) for the more extreme asymmetries expected
for some NS-BH mergers.
