  Due to stellar rotation, the observed radial velocity of a star varies during
the transit of a planet across its surface, a phenomenon known as the
Rossiter-McLaughlin (RM) effect. The amplitude of the RM effect is related to
the radius of the planet which, because of differential absorption in the
planetary atmosphere, depends on wavelength. Therefore, the
wavelength-dependent RM effect can be used to probe the planetary atmosphere.
We measure for the first time the RM effect of the Earth transiting the Sun
using a lunar eclipse observed with the ESO HARPS spectrograph. We analyze the
observed RM effect at different wavelengths to obtain the transmission spectrum
of the Earth's atmosphere after the correction of the solar limb-darkening and
the convective blueshift. The ozone Chappuis band absorption as well as the
Rayleigh scattering features are clearly detectable with this technique. Our
observation demonstrates that the RM effect can be an effective technique for
exoplanet atmosphere characterization. Its particular asset is that photometric
reference stars are not required, circumventing the principal challenge for
transmission spectroscopy studies of exoplanet atmospheres using large
ground-based telescopes.
