  We present a measurement of the systemic proper motion of the Large
Magellanic Cloud (LMC) from astrometry with the High Resolution Camera (HRC) of
the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). We
observed LMC fields centered on 21 background QSOs that were discovered from
their optical variability in the MACHO database. The QSOs are distributed
homogeneously behind the central few degrees of the LMC. With 2 epochs of HRC
data and a ~2 year baseline we determine the proper motion of the LMC to better
than 5% accuracy: mu_W = -2.03 +/- 0.08 mas/yr; mu_N = 0.44 +/- 0.05 mas/yr.
This is the most accurate proper motion measurement for any Milky Way satellite
thus far. When combined with HI data from the Magellanic Stream this should
provide new constraints on both the mass distribution of the Galactic Halo and
models of the Stream.
