Question

why gap between the new moon day and full moon day is of 15 days

Answer 1

Moon takes around 29.5 days for one full revolution around earth. And new moon day is when moon and sun are in same side, so the light reflected from moon does not come to the surface of earth. Full moon day occurs when moon and sun are in opposite side of earth, so the light reflected from moon will come to the surface of earth. Now this incident occur in exactly opposite position of the full path. So they divide the whole path in two portion of half each. And hence it the time difference between them is 15 day (14 day and 6 hour).