Was out early this morning, and I noticed an interesting effect around the crescent moon. It's one of those days where the non-crescent part is still slightly visible. What I noticed was that the crescent part seems to "bulge" from the rest of it, making an imperfect circle when seen as a whole. What causes this? Is this just simple light dispersion because of the distance, or an atmospheric effect perhaps? I never really noticed before, but I now live at high altitude in the country rather than the city :)

Much appreciated in advance!