There is a change in the shape of moon every day. There are days when the shape of moon appears perfectly round. Also, there are days when the moon cannot be seen at all even if the sky is clear. The day on which the whole disc of the moon is visible is called the full Moon day. Every night, the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day, the moon is not visible. This day is called the new Moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again, the moon grows larger every day. On the fifteenth day once again we get a full view of the moon. After fifteen days, the full moon changes into a new Moon.