Question

Why does the shape of the lighted part of the moon change when viewed from Earth?

Answer 1

Because of earth's rotation on its axis and also moon's movement around earth.

Answer 2

As we know our atmosphere or air have different different levels. Every level have another another gasses. and every gas have another another permeability. 

when a light come from lower permeability to higher permeability it change it's direction. 

so that when light come from moon to earth it change it's direction many many more times. so the shape of the light part of the moon changed in earth.


If you don't understand  how light changed with permeability then you can tell me i will describe it.