  Spiral structures are a common feature in scattered-light images of
protoplanetary disks, and of great interest as possible tracers of the presence
of planets. However, other mechanisms have been put foward to explain them,
including self-gravity, disk-envelope interactions, and dead zone boundaries.
These mechanisms explain many spirals very well, but are unable to easily
account for very loosely wound spirals and single spiral arms. We study the
effect of light travel time on the shape of a shadow cast by a clump orbiting
close (within ${\sim}1\,$au) of the central star, where there can be
significant orbital motion during the light travel time from the clump to the
outer disk and then to the sky plane. This delay in light rays reaching the sky
plane gives rise to a variety of spiral- and arc-shaped shadows, which we
describe with a general fitting formula for a flared, inclined disk.
