We measure wind velocities on opposite sides of the hot Jupiter HD 189733b by modeling sodium absorption in high-resolution HARPS transmission spectra .
Our model implicitly accounts for the Rossiter-McLaughlin effect , which we show can explain the high wind velocities suggested by previous studies .
Our results reveal a strong eastward motion of the atmosphere of HD 189733b , with a redshift of 2.3 ^ { +1.3 } _ { -1.5 } km s ^ { -1 } on the leading limb of the planet and a blueshift of 5.3 ^ { +1.0 } _ { -1.4 } km s ^ { -1 } on the trailing limb .
These velocities can be understood as a combination of tidally locked planetary rotation and an eastward equatorial jet ; closely matching the predictions of atmospheric circulation models .
Our results show that the sodium absorption of HD 189733b is intrinsically velocity broadened and so previous studies of the average transmission spectrum are likely to have overestimated the role of pressure and thermal broadening .