  The atmospheric composition of transiting exoplanets can be characterized
during transit by spectroscopy. For the transit of an Earth twin, models
predict that biogenic $O_2$ and $O_3$ should be detectable, as well as water
vapour, a molecule linked to habitability as we know it on Earth. The aim is to
measure the Earth radius versus wavelength $\lambda$ - or the atmosphere
thickness $h(\lambda)$ - at the highest spectral resolution available to fully
characterize the signature of Earth seen as a transiting exoplanet. We present
observations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Earth
eclipses the Sun and opens access to the Earth atmosphere transmission
spectrum. We used HARPS and UVES spectrographs to take penumbra and umbra
high-resolution spectra from 3100 to 10400 Ang. A change of the quantity of
water vapour above the telescope compromised the quality of the UVES data. We
corrected for this effect in the data processing. We analyzed the data by 3
different methods. The 1st method is based on the analysis of pairs of penumbra
spectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of all
penumbra and umbra spectra. Profiles $h(\lambda)$ are obtained with the three
methods for both instruments. The 1st method gives the best result, in
agreement with a model. The second method seems to be more sensitive to the
Doppler shift of solar spectral lines with respect to the telluric lines. The
3rd method makes use of umbra spectra which bias the result, but it can be
corrected for this a posteriori from results with the first method. The 3
methods clearly show the spectral signature of the Rayleigh scattering in the
Earth atmosphere and the bands of H$_2$O, O$_2$, and O$_3$. Sodium is detected.
Assuming no atmospheric perturbations, we show that the E-ELT is theoretically
able to detect the $O_2$ A-band in 8~h of integration for an Earth twin at
10pc.
