We present measurements of internal proper motions at more than five hundred
positions of NGC 2392, the Eskimo Nebula, based on images acquired with WFPC2
on board the Hubble Space Telescope at two epochs separated by 7.695 years.
Comparison of the two observations shows clearly the expansion of the nebula.
We measured the amplitude and direction of the motion of local structures in
the nebula by determining their relative shift during that interval. In order
to assess the potential uncertainties in the determination of proper motions in
this object, and in general, the measurements were performed using two
different methods, used previously in the literature. We compare the results
from the two methods, and to perform the scientific analysis of the results we
choose one, the cross-correlation method, as the more reliable. We go on to
perform a "criss-cross" mapping analysis on the proper motion vectors which
helps in the interpretation of the velocity pattern. Combining our results on
the proper motions with radial velocity measurements obtained from high
resolution spectroscopic observations, and employing an existing 3D model, we
estimate the distance to the nebula as 1300 pc.