  We present high-precision radial velocity observations of WASP-17 throughout
the transit of its close-in giant planet, using the MIKE spectrograph on the
6.5m Magellan Telescope at Las Campanas Observatory. By modeling the
Rossiter-McLaughlin effect, we find the sky-projected spin-orbit angle to be
lambda = 167.4 \pm 11.2 deg. This independently confirms the previous finding
that WASP-17b is on a retrograde orbit, suggesting it underwent migration via a
mechanism other than just the gravitational interaction between the planet and
the disk. Interestingly, our result for lambda differs by 45 \pm 13 deg from
the previously announced value, and we also find that the spectroscopic transit
occurs 15 \pm 5 min earlier than expected, based on the published ephemeris.
The discrepancy in the ephemeris highlights the need for contemporaneous
spectroscopic and photometric transit observations whenever possible.
