Lunar Phases
This will be a little visualization exercise.
Imagine that you are above the North Pole looking down on the Earth and the Moon orbiting around it. The Sun, shining from your right, illuminates the half of the Moon that faces it, this is the day side of the Moon.
From the Earth, we see the Moon changing shape: as the Moon revolves around the Earth, we see varying amounts of its sunlit day side and dark night side. This causes the lunar phases.
