  In this paper, we demonstrate the severity of the degeneracy between the
microlens-parallax and lens-orbital effects by presenting the analysis of the
gravitational binary-lens event OGLE-2015-BLG-0768. Despite the obvious
deviation from the model based on the the linear observer motion and the static
binary, it is found that the residual can be almost equally well explained by
either the parallactic motion of the Earth or the rotation of the binary lens
axis, resulting in the severe degeneracy between the two effects. We show that
the degeneracy can be readily resolved with the additional data provided by
space-based microlens parallax observations. Enabling to distinguish between
the two higher-order effects, space-based microlens parallax observations will
make it possible not only to accurately determine the physical lens parameters
but also to further constrain the orbital parameters of binary lenses.
