

Earth's optical and near-infrared transmission spectrum has been measured from ground-based observations of a recent lunar eclipse in 2008. The data, published in a paper titled 'Earth's transmission spectrum from lunar eclipse observations' in the most recent (11 June) issue of the journal Nature, should prove important reference material as the search for Earth-like planets continues.
Enric Palle of the Instituto de Astrofisica de Canarias and colleagues observed light reflected from the Moon towards the Earth during a lunar eclipse on 16 August 2008. This allowed them to measure the Earth's spectrum as if observed from an astronomical distance during a transit in front of the Sun.
The transmission spectrum of a planet provides information about the chemical composition of its atmosphere, and the team find 'fingerprints' of the Earth's upper atmosphere and of the major atmospheric constituent, molecular nitrogen. Some biologically relevant atmospheric features that are weak in the reflection 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.
Ground-based measurements of solar eclipses are also yielding unique observations, Jay Pasachoff says in a Nature review article. A new generation of observations is reaching spatial, temporal and spectral resolutions that are inaccessible from space, directly complementing satellite studies. Observations of the Sun during total eclipses have led to major discoveries, such as the existence of helium and the corona's high temperature.
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