Inferring the dynamics of stellar streams via distance gradients
  We present a simple result in which the distance gradient along a stream can
be used to derive the transverse velocity (i.e. proper motion) along it, if the
line-of-sight velocity is also known. We show its application to a mock orbit
to illustrate its validity and usage. For less extended objects, such as
globular clusters and satellite galaxies being tidally disrupted, the same
result can be applied in its small-angle approximation. The procedure does not
rely on energy or angular momentum conservation and hence does not require a
Galactic model in order to deduce the local velocity vector of the stream.
