Posted: Oct 29, 2016 10:29 am
As far as I know, phases are a result of the position of the moon to the sun from an earth perspective. When the earth is between the sun and the moon, there is a full moon. When the moon is between the sun and earth, it is a new moon and all the other phases are in between those two positions.
The distances between the moon and earth vary slightly over the cycles because the orbit isn't a perfect circle around the earth but not enough to make a noticeable difference.