Web Master:
Hey Diana, try placing the hanging hook closer to the edge of the eaves, also hanging the basket lower will help it get more direct sun. You'll also find that during the months on either side of the solstice that the angle of the sun is lower and you could therefore make that work to your advantage.
However Petunias will still grow well with bright shade so don't fret if they don't get full sun during the middle of the day when the sun is at its highest point, they should still get some in the rest of the day - and that should be enough.
This photo of Petunia in baskets comes from London where the high buildings all around tend to keep these plants in shade for a lot of the day - yet they manage to do quite well as the overall light is fairly good.