Give an account of phases of moon. 

Moon does not emit by light itself. It reflects the light of sun falling on it. Thus, that part only of the moon appears bright which is towards us. The moon along with earth is revolving around the sun. As a result the relative positions of the earth and the moon are continuously changing. Thus, when the earth lies in between the moon and the sun, the full face of the moon is visible to us. This day is called ‘Purnima' or Full moon day.On the other hand, when the sunlight falls on that side of the moon which is on the other side of the earth, moon is not visible to us. This day is called 'Amavus' or New moon day. The next day only a crescent of moon gets illuminated and thus a crescent of the moon becomes visible in the western horizon immediately after the sun set. The size of the illuminated portion of the moon goes on increasing till on the fifteenth day, the full moon appears. Then again the bright portion of the moon gradually decreases till on the fifteenth day, it becomes new moon day. The different bright portions of the moon visible to us are called phases of moon.