The Moon passes through different phases on different days. It is based on accurate astronomy calculations which in turn are dependent upon the relative position of the sun, moon and the earth. Moon phases are caused by the position of the Earth in relation to that of the Sun and the Moon. On an average it takes 27 days 7 hours and 43 minutes for the Moon to revolve around the Earth. We all know that the Moon reflects the light of the Sun. And thus the Sun always illuminates one half of the Moon that is facing the sun, except during lunar eclipses. During its transition around the Earth, when the Sun and the Moon are positioned on the opposite sides of the Earth, the moon is seen to be bright and completely round. This is the 'full moon' day; again, when the moon falls in between the earth and the sun, it is known as the 'Dark Moon' day. And in between the 'full' and the 'dark' moon days, the surface of the Moon appears to grow (wax) and then decreases (wanes) to form the next new moon. And in this way the phases of the Moon continue to change.