Comparison of Pulsar Positions from Timing and Very Long Baseline
  Astrometry
  Pulsar positions can be measured with high precision using both pulsar timing
methods and very-long-baseline interferometry (VLBI). Pulsar timing positions
are referenced to a solar-system ephemeris, whereas VLBI positions are
referenced to distant quasars. Here we compare pulsar positions from published
VLBI measurements with those obtained from pulsar timing data from the Nanshan
and Parkes radio telescopes in order to relate the two reference frames. We
find that the timing positions differ significantly from the VLBI positions
(and also differ between different ephemerides). A statistically significant
change in the obliquity of the ecliptic of $2.16\pm0.33$\,mas is found for the
JPL ephemeris DE405, but no significant rotation is found in subsequent JPL
ephemerides. The accuracy with which we can relate the two frames is limited by
the current uncertainties in the VLBI reference source positions and in
matching the pulsars to their reference source. Not only do the timing
positions depend on the ephemeris used in computing them, but also different
segments of the timing data lead to varying position estimates. These
variations are mostly common to all ephemerides, but slight changes are seen at
the 10$\mu$as level between ephemerides.
