The invention concerns a timepiece including a dial (12) fitted with an aperture (16) behind which is mounted a moon phase disc (18) driven in rotation by a moon phase train (24). According to the invention said disc (18) is mounted so as to move in rotation about a fixed sun pinion (36). Moreover, two planetary wheels (44a, 44b), secured in rotation to representations of the moon (19a, 19b), are mounted so as to move in rotation on the disc (18), at least indirectly meshed with the sun pinion (36), so as to form a planetary train with the sun pinion (36).