  We have obtained new images of the protoplanetary disk orbiting TW Hya in
visible, total intensity light with the Space Telescope Imaging Spectrograph
(STIS) on the Hubble Space Telescope (HST), using the newly commissioned BAR5
occulter. These HST/STIS observations achieved an inner working angle
$\sim$0.2\arcsec, or 11.7~AU, probing the system at angular radii coincident
with recent images of the disk obtained by ALMA and in polarized intensity
near-infrared light. By comparing our new STIS images to those taken with STIS
in 2000 and with NICMOS in 1998, 2004, and 2005, we demonstrate that TW Hya's
azimuthal surface brightness asymmetry moves coherently in position angle.
Between 50~AU and 141~AU we measure a constant angular velocity in the
azimuthal brightness asymmetry of 22.7$^\circ$~yr$^{-1}$ in a counter-clockwise
direction, equivalent to a period of 15.9~yr assuming circular motion. Both the
(short) inferred period and lack of radial dependence of the moving shadow
pattern are inconsistent with Keplerian rotation at these disk radii. We
hypothesize that the asymmetry arises from the fact that the disk interior to
1~AU is inclined and precessing due to a planetary companion, thus partially
shadowing the outer disk. Further monitoring of this and other shadows on
protoplanetary disks potentially opens a new avenue for indirectly observing
the sites of planet formation.
