  Vorticity is central to the nature of, and dynamical processes in turbulence,
including turbulence in astrophysical fluids. The results of
\cite{Raymond20a,Raymond20b} on vorticity in the post-shock fluid of the Cygnus
Loop supernova remnant are therefore of great interest. We consider the degree
to which spectroscopic measurements of an optically-thin line, the most common
type of astronomical velocimetry, can yield unambiguous measurements of the
vorticity in a fluid. We consider an ideal case of observations in the plane of
a flow which may or may not contain vorticity. In one case, the flow possesses
vorticity in a direction perpendicular to the plane of observations. In the
other case, the flow is irrotational (zero vorticity) by construction. The
observationally-deduced vorticity (referred to as the {\em pseudovorticity}) is
inferred from spatial differences in the line-of-sight component of velocity,
and assumptions of symmetry. My principal result is that in the case of the
vortical flow, the pseudovorticity is a reasonable match for the true
vorticity. However, and importantly, the pseudovorticity in the case of the
irrotational flow field is also nonzero, and comparable in magnitude to that
for a vortical flow. The conclusion of this paper is that while astronomical
spectroscopic observations may yield a good estimate of the vorticity in a
remote fluid, the robustness of such an inference cannot be insured.
