  This paper presents a cross-calibrated catalog of Hipparcos and Gaia
astrometry to enable their use in measuring changes in proper motion, i.e.,
accelerations in the plane of the sky. The final catalog adopts the reference
frame of the second Gaia data release (DR2) and locally cross-calibrates both
the scaled Hipparcos-Gaia DR2 positional differences and the Hipparcos proper
motions themselves to this frame. This gives three nearly independent proper
motion measurements per star, with the scaled positional difference usually
being the most precise. We find that a linear combination of the two Hipparcos
reductions is superior to either reduction on its own, and address error
inflation for both Hipparcos and Gaia DR2. Our adopted error inflation is
additive (in quadrature) for Hipparcos and multiplicative for Gaia. We provide
the covariance matrices along with the central epochs of all measurements. Our
final proper motion differences are accurately Gaussian with the appropriate
variances, and are suitable for acceleration measurements and orbit fitting.
The catalog is constructed with an eye toward completeness; it contains nearly
98\% of the Hipparcos stars. It also includes a handful of spurious entries and
a few stars with poor Hipparcos reductions that the user must vet by hand.
Statistical distributions of accelerations derived from this catalog should be
interpreted with caution.
