The sensor (1) for determining an orientation of the sensor in a gravity field comprises a ball (2) and a rolling surface (R) describing a generally concave shape on which the ball can roll inside the sensor. A further surface (F) is arranged opposite said rolling surface, and a light emitter (E), a light detector (D) and another light emitter or detector is provided. A region (R) within which the ball (2) can move is limited by at least the rolling surface (R) and the further surface (F). And the light emitters (E) and detectors (D) are arranged outside the region (R) for emitting light through the rolling surface (R) into said region and detecting light exiting the region (3) through the rolling surface (R) or for emitting light through the further surface (F) into said region (R) and detecting light exiting said region (R) through the further surface (F). Corresponding measuring methods and manufacturing methods are described, too.