On the full moon day, the moon appears like a shining disc but it goes on waning till it disappears on the new moon night. And then it again starts increasing in size and becomes a full bright disc, on the full moon day. Changes in the moon’s size are called the ‘phases’ of the moon. Do you know why it occurs?
The fact is that the moon neither increases nor decreases in size. Its size simply appears to be changing because it changes its position with respect to the sun. We all know that the moon is the only satellite of the earth and revolves round the earth. It travels around the earth about 384,400 km away. This orbit takes 27 days and eight hours. It is illuminated by the sunlight. Only one side of the moon faces the earth. We do not see its other side. When the moon comes in between the earth and the sun, its bright side is not visible to people on the earth and only its dark side faces the earth. As a result the moon cannot be seen. This is the new moon day. As the position of the moon changes due to its motion from east to west with respect to earth, some illuminated part of its surface is visible to us. A week after the new moon, almost half of the illuminated lunar disc becomes visible to us. The bright part of the moon’s surface goes on increasing gradually and on the full moon day the whole disc, seen from the earth, is fully illuminated. On this day the earth is in between the sun and the moon. In the next fifteen days the bright portion of the lunar surface goes on decreasing and on the fifteenth day it disappears completely. In short, the changes in the apparent shape of the moon depend upon how much of its illuminated surface is visible to us.
The cycle continues like this and we see the different phases of the moon. On the full moon day it appears just after the sunset and disappears only at the sunrise. The time between new moon phases is called the synodic month.