  A standard model for the visibility of pulsar radio emission is based on the
assumption that the emission is confined to a narrow cone about the tangent to
a dipolar field line. The widely accepted rotating vector model (RVM) is an
approximation in which the line of sight is fixed and the field line is not
strictly tangent to it. We refer to an exact treatment (Gangadhara 2004) as the
tangent model. In the tangent model (but not in the RVM) the visible point
changes as a function of pulsar rotational phase, $\psi$, defining a trajectory
on a sphere of radius $r$. We solve for the trajectory and for the angular
velocity of the visible point around it. We note the recent claim that this
motion is observable using interstellar holography (Pen et al. 2014). We
estimate the error introduced by use of the RVM and find that it is significant
for pulsars with emission over a wide range of $\psi$. The RVM tends to
underestimate the range of $\psi$ over which emission is visible. We suggest
that the geometry alone strongly favors the visible pulsar radio being emitted
at a heights more than ten percent of the light-cylinder distance, where our
neglect of retardation effects becomes significant.
