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Institute of Advanced Study

Colours From Earth

Abstract

More than one thousand planets have been discovered outside our solar system and the search techniques at our disposal are becoming sufficiently sensitive to find the small, rocky planets that may resemble Earth. How can we begin to characterise such objects by examining their surface and atmospheric properties and, eventually, their likelihood of harbouring life? Observing such small planets with geometrically-thin atmospheres will be extraordinarily difficult and so, in order to design optimum strategies, it is useful to examine and assess analogous observations of our own Earth. The Moon can be used as a convenient ‘mirror’ to give us views of reflected sunlight from a complete Earth hemisphere: a technique that was understood by Leonardo da Vinci early in the sixteenth century. Also, during a total lunar eclipse, the darkened, ‘copper-coloured’ Moon allows us to sample the long ‘tangential’ path that sunlight takes as it grazes the Earth’s limb, allowing us to read the signatures of the atmospheric gases. Both of these techniques are described in this article.

Insights Paper