check out my model showing the sun, earth n moon. u can see as the moon moves around the earth, its path is at a angle w earth's path around the sun. so moon's reflection of sunlight as seen from earth changes every night. it is called lunar phases: going crescent moon to full moon n back. eclipse happens when earth is between sun and moon and blocks part of the sun light. earth's shadow covers part or all of the moon. it is called lunar eclipse.