  HD179821 is an enigmatic evolved star that possesses characteristics of both
a post-asymptotic giant branch star and a yellow hyper-giant, and there has
been no evidence that unambiguously defines its nature. These two hypotheses
are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We
have obtained the two-epoch Hubble Space Telescope WFPC2 data of its
circumstellar shell, which shows multiple concentric arcs extending out to
about 8 arcsec. We have performed differential proper-motion measurements on
distinct structures within the circumstellar shell of this mysterious star in
hopes of determining the distance to the object, and thereby distinguishing the
nature of this enigmatic stellar source. Upon investigation, rather than
azimuthal radially symmetric expansion, we discovered a bulk motion of the
circumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to a
translational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star.
This finding implies that the distance to HD 179821 should be rather small in
order for its circumstellar shell to preserve its highly intact spherical
structure in the presence of the distorting ISM flow, therefore favoring the
proposition that HD 179821 is a post-AGB object.
