The difference in the moon’s shape is not real but apparent! Yes, the moon never changes its actual shape, but it seems to be changing conditions because of the sunlight reflecting on it while rotating around the Earth. During a 29 day cycle of rotation around Earth, the moon experiences low to complete illumination due to sunlight. Hence, the apparent change in the moon’s shape from Earth is due to the moon, sun, and Earth's alignment. And when we see from Earth, the shape of the moon changes every night based on the amount of sunlight reflecting on it.