Full moon

[bigben]

Changed the background colour to check I had this correct, and it wasn't exactly in the example above but it does work in principle. 

Figuring out the transformation of the mask image is doing my head in. Creating a full moon facing the viewer is easy...  the yaw and pitch of the image is just the opposite of the heading and altitude of the planet (the texture is rotated with the planet). The result then has to be transformed to face the hemisphere at the sun.  Time for a paper and pencil 

... For phases smaller than a half moon it may be necessary to switch to a rectangular source image (equirectangular projection) for PTGui to create a mask to render just a wedge of the moon, but there may be other factors for why this may not be a good idea in that situation.

This is wrong (in a good way). I forgot that the planet only renders a single side, so the inside of the hemisphere isn't rendered for a crescent moon

[Dune]

As mentioned in Luno (Dandel0's post): Some experimenting with glow and moon and used a very high 'cirrus' cloud behind the moon, plus a rather large (4) sun behind the moon, which kind of worked as well for a glow around the moon, but with a dark side as well. Perhaps 2 layers of thin cloud is better.