Astronomical Applications Department, U.S. Naval Observatory pm AAS poster Page 2
stars make up 162 wide visual double star systems for which nonlinear orbital motion should be
negligible over the almost two centuries for which WDS measures are available. For each of
these pairs, we computed relative proper motions (B component minus A) in RA and Dec from
the WDS measures of separation and position angle. We then compared these relative proper
motions to those derived from the Hipparcos and Tycho-2 catalog data for each component.
The pair-by-pair differences in relative proper motion among the three catalogs indicate that
Tycho-2 is significantly better matched to the WDS data than Hipparcos. Furthermore, this kind
of three-way catalog comparison can provide unique estimates of the external proper motion
errors in each catalog separately. This analysis shows that the underlying mean errors in the
proper motions of individual stars are about 2.5 mas/yr for Tycho-2 and 3.5 mas/yr for
Hipparcos, which is not inconsistent with the tabulated errors for the sample of stars studied here.
A broader survey is planned.