We present LOUPE, a novel type of spectropolarimeter to measure the flux and state of polarization of sunlight that is reflected by the Earth from 0.4 to 0.8 ensuremathμm. LOUPE has been designed as payload of a lunar lander. From the moon, the Earth can be observed as a whole, during its daily rotation and at all phase angles, just as if it were an exoplanet. LOUPE will provide benchmark data for the development of instruments for Earth-like exoplanet characterization, and for the testing of numerical retrieval algorithms.