The relative change in positions of sun, earth and moon are responsible for the phases of moon. As the earth stands between sun and moon , it blocks sunlight to reach one side of moon and thus sunlight illuminates the other side of moon. Now due to the relative position of earth and moon , sometimes the illuminated part as well as the dark part of moon is perceived . This is what causes the different phases of moon. Thus, The phases of the moon occur due to " the sunlit portion or illuminated portion " and " shadowed portion or dark portion " of the disc of the moon as it revolves around the earth.