A booklet published by the Palais de la Découverte to accompany the exhibition of space photographs "À la découverte de la Terre" (Paris, May-October 1974), produced with Kodak-Pathé and the support of NASA, the Institut Français du Pétrole, the BRGM and the Cosmos Club de France. It combines technical notes on imaging emulsions and sensors (multispectral films, the ERTS-1 satellite, Skylab's EREP mission), a captioned gallery of photographs of the Earth taken from space, and the text of Albert Ducrocq's lecture "Why the Earth?", a synthesis of comparative planetology on the formation and evolution of the terrestrial worlds. Special issue no. 4 of the Revue du Palais de la Découverte, in its 2nd edition (October 1975).