Robert, Welcome to IIS. Does this explain it?
When the moon appears to us halved, the great circle parallel to the circle which divides the dark and the bright portions in the moon is then in the direction of our eye;
that is to say, the great circle parallel to the dividing circle and our eye are in one plane.
From here....
http://perseus.mpiwg-berlin.mpg.de/G...positions.html
Michael.