Earthshine is electromagnetic radiation given off by Earth, both reflected from the Sun, black-body radiation due to the temperature of Earth and its atmosphere, including the effects of radiation emission and absorption by the atmosphere.
Earthshine is studied to determine the albedo of Earth its overall energy balance, the abundance of greenhouse gases, and the changes of these over time, as part of the study of Earth climate change. It also provides clues to biosignatures that might be found on extra-solar planets. Earthshine's spectrum shows molecular oxygen, ozone, and water vapor.
Earthshine can be studied from the ground through analysis the portion of the Moon facing Earth which is dark, e.g., when the Moon is close to new, from which much of the light is reflected earthshine.
Analogous planetshine, the same from other planets, is of interest, both for exoplanets and solar system planets. For exoplanets, some can be detected through transit spectroscopy and occultation spectroscopy, and direct imaging promises to provide more data.