  We have used a set of dedicated astrometric data from the Hubble Space
Telescope to measure the parallax and proper motion of the nearby neutron star
RX J0720.4-3125. At each of eight epochs over two years, we used the High
Resolution Camera of the Advanced Camera for Surveys to measure the position of
the B=26.6 target to a precision of ~2 mas (~0.07 pix) relative to 22 other
stars. From these data we measure a parallax of pi=2.8+/-0.9 mas (for a
distance of 360+170-90 pc) and a proper motion of mu=107.8+/-1.2 mas/yr.
Exhaustive testing of every stage of our analysis suggests that it is robust,
with a maximum systematic uncertainty on the parallax of 0.4 mas. The distance
is compatible with earlier estimates made from scaling the optical emission of
RX J0720.4-3125 relative to the even closer neutron star RX J1856.5-3754. The
distance and proper motion imply a transverse velocity of 180+90-40 km/s,
comparable to velocities observed for radio pulsars. The speed and direction
suggest an origin for RX J0720.4-3125 in the Trumpler 10 OB association ~0.7
Myr ago, with a possible range of 0.5-1.0 Myr given by the uncertainty in the
distance.
