A lunar eclipse offers a unique opportunity to investigate the Earth's atmosphere. Even at its darkest, the Moon in umbra is still lit by solar photons refracted and scattered during their passage through the Earth's terminator. A recent ground-based observation of the 16 Aug 2008 eclipse [1] has shown that the spectrum of the light reflected from the Moon in umbra contains the signature of a few atmospheric trace constituents. The rapid variations in the Sun/Earth/Moon configuration as the eclipse progresses entail similarly rapid variations in some of the spectral signatures. To better interpret the observations of [1], we have built a model that traces the refraction-bent light rays through the atmosphere. In the talk we will present an overview of the model prediction capabilities and the major conclusions drawn from the observations. Some of the conclusions are also of interest in the interpretation of the transmission spectra of transiting exoplanets. Refs.: [1] Pallé et al. (2009), Nature, 459, 814-816.