The moon has different phases that can be seen from Earth as the moon orbits our planet. The moon appears to change shapes monthly, from a small sliver to a full circle, because the portion of the moon's surface that is illuminated by the sun varies depending on the positions of the moon and sun in the sky. Over the course of about 29.5 days, the moon completes one full cycle of phases from new moon to full moon and back to new moon again.
