  Of the 342 planets discovered so far orbiting other stars, 58 "transit" the
stellar disk, meaning that they can be detected by a periodic decrease in the
starlight flux. The light from the star passes through the atmosphere of the
planet, and in a few cases the basic atmospheric composition of the planet can
be estimated. As we get closer to finding analogues of Earth, an important
consideration toward the characterization of exoplanetary atmospheres is what
the transmission spectrum of our planet looks like. Here we report the optical
and near-infrared transmission spectrum of the Earth, obtained during a lunar
eclipse. Some biologically relevant atmospheric features that are weak in the
reflected spectrum (such as ozone, molecular oxygen, water, carbon dioxide and
methane) are much stronger in the transmission spectrum, and indeed stronger
than predicted by modelling. We also find the fingerprints of the Earth's
ionosphere and of the major atmospheric constituent, diatomic nitrogen (N2),
which are missing in the reflected spectrum.
