  We present the results of differential proper-motion analyses of the Egg
Nebula (RAFGL 2688, V1610 Cyg) based on the archived two-epoch optical data
taken with the Hubble Space Telescope. First, we determined that the
polarization characteristics of the Egg Nebula is influenced by the higher
optical depth of the central regions of the nebula (i.e., the "dustsphere" of
about 1000 AU radius), causing the nebula illuminated in two steps -- the
direct starlight is first channeled into bipolar cavities and then scattered
off to the rest of the nebula. We then measured the amount of motion of local
structures and the signature concentric arcs by determining their relative
shifts over the 7.25 yr interval. Based on our analysis, which does not rely on
the single-scattering assumption, we concluded that the lobes have been
excavated by a linear expansion along the bipolar axis for the past 400 yr,
while the concentric arcs have been generated continuously and moving out
radially at about 10 km/s for the past 5,500 yr, and there appears to be a
colatitudinally-increasing trend in the radial expansion velocity field of the
concentric arcs. There exist numerical investigations into the mass-loss
modulation by the central binary system, which predict such a
colatitudinally-increasing expansion velocity field in the spiral-shock trails
of the mass-loss ejecta. Therefore, the Egg Nebula may represent a rare edge-on
case of the binary-modulated circumstellar environs, corroborating the previous
theoretical predictions.
