THE properties of the Moon’s rarefied atmosphere, which can be traced through observations of sodium and potassium(1-4), provide important insights into the formation and maintenance of atmospheres on other primitive Solar System bodies(5-7). The lunar atmosphere is believed to be composed of atoms from the surface rocks and soil, which might have been sputtered by micrometeorites(8), by ions in the solar wind(9), or by photons(10,11). It might also form by the evaporation of atoms from the hot, illuminated surface(10,11) Here we report the detection of sodium emission from the Moon’s atmosphere during a total lunar eclipse (which occurs when the Moon is full). The sodium atmosphere is considerably more extended at full Moon than expected--it extends to at least nine lunar radii--and its brightness distribution is incompatible with sources involving either solar-wind or micrometeorite sputtering. This leaves photon sputtering or thermal desorption as the preferred explanations for the lunar atmosphere, and suggests that sunlight might also be responsible for the transient atmospheres of other primitive bodies (such as Mercury).